University of Calgary
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Graduate Studies The Vault: Electronic Theses and Dissertations
2016
CMOS Laser Diode Drivers for Supercontinuum
Generation
He, Yuting
He, Y. (2016). CMOS Laser Diode Drivers for Supercontinuum Generation (Unpublished master's
thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/25339
http://hdl.handle.net/11023/3030
master thesis
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UNIVERSITY OF CALGARY
CMOS Laser Diode Drivers for Supercontinuum Generation
by
Yuting He
A THESIS
SUBMITTED TO THE FACULTY OF GRADUATE STUDIES
IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE
DEGREE OF MASTER OF SCIENCE
GRADUATE PROGRAM IN ELECTRICAL ENGINEERING
CALGARY, ALBERTA
MAY, 2016
© Yuting He 2016
ii
Abstract
There have been intense research efforts on developing compact and low-cost supercontinuum
generation (SCG) systems, which have various application areas including telecommunications,
spectroscopy, and optical coherence tomography. This research employs complementary metal–
oxide–semiconductor (CMOS) technology to design and implement two integrated laser diode
drivers for reducing the size and cost of SCG systems. A continuous-wave CMOS driver with a
maximum output current of 600 mA is developed for driving a laser diode in an erbium-doped
fiber amplifier (EDFA). A picosecond pulsed CMOS driver is designed and applied for gain-
switching a laser diode to produce optical pulses with a pulse width of 200 ps and a repetition rate
of 5.6 MHz. The gain-switched laser diode output is amplified by an EDFA and then launched into
a highly nonlinear fiber for SCG. The generated supercontinuum has an average power of 62 mW
and a spectral bandwidth of 806 nm.
iii
Acknowledgements
Firstly, I would like thank my supervisor Dr. Orly Yadid-Pecht for her invaluable guidance,
support and encouragement throughout my research work.
Secondly, I thank all the I2Sense lab students, post-doctoral fellows, my friends throughout
the ECE department and department staff for their help and support. Many thanks to Dr. Kartikeya
Murari, Dr. Yuhua Li, Michael Himmelfarb, Nikhil Vastarey, Pauling Cummings, Dr. J.P.E.
Hadden, Prasoon Ambalathankandy, Matthew Jackson, Dr. Arthur Spivak, Linhui Yu, Donuwan
Navaratne, Zhixing Zhao, Christopher Simon, Kathryn Simon and Shem Chenoo for their helpful
discussion and support regarding this thesis work.
Thirdly, I would like to thank my committee members Dr. Paul Barclay and Dr. Leonid
Belostotski for taking their time to serve in my committee.
Last but not least, I would like to thank the CMC Microsystems for the access to the design
tools, workshops and fabrication services through MOSIS. Special thanks to Dr. Shahriar
Mirabbasi from UBC for organizing the CMOS Electronics for Photonics training course and for
helpful discussions regarding the pulsed CMOS driver design during the course.
iv
Dedication
To my parents for their unconditional love and support!
v
Table of Contents
Abstract ............................................................................................................................... ii Acknowledgements ............................................................................................................ iii Dedication .......................................................................................................................... iv Table of Contents .................................................................................................................v List of Tables .................................................................................................................... vii List of Figures .................................................................................................................. viii List of Abbreviations and Symbols......................................................................................x
Chapter 1: Introduction ........................................................................................................1 1.1 Background and Review ............................................................................................1 1.2 Motivation and Objectives .........................................................................................4 1.3 Thesis Scope and Contributions ................................................................................6 1.4 Thesis Outlines ..........................................................................................................7
Chapter 2: High Current Continuous-Wave CMOS Laser Diode Driver ............................8 2.1 Introduction and Objectives .......................................................................................8 2.2 Characteristics of Laser Diodes .................................................................................9 2.3 Continuous-Wave CMOS Driver Circuit Design Methodology ..............................11
2.3.1 Current Reference Circuit ................................................................................12 2.3.2 Start-up Circuit ................................................................................................14 2.3.3 Current Source Circuit .....................................................................................14
2.4 Circuit Design and Simulations ...............................................................................15 2.5 Measurements ..........................................................................................................19
2.5.1 Measurement Setup .........................................................................................19 2.5.2 Measurements of the Output Current ..............................................................21 2.5.3 Measurements of the Optical Output Power ...................................................22
2.6 Summary ..................................................................................................................25
Chapter 3: Picosecond Pulsed CMOS Laser Diode Driver ...............................................26 3.1 Introduction and Objectives .....................................................................................26 3.2 Gain-switched Laser Diodes ....................................................................................26 3.3 Pulsed CMOS Driver Circuit Design Methodology ................................................28
3.3.1 Voltage-Controlled Ring Oscillator ................................................................29 3.3.2 Voltage-Controlled Delay Line .......................................................................31 3.3.3 Exclusive-OR Circuit ......................................................................................33 3.3.4 Current Source Circuit .....................................................................................34
3.4 Circuit Design and Simulations ...............................................................................35 3.5 Measurements and Analysis ....................................................................................38
3.5.1 Measurement Setup .........................................................................................38 3.5.2 Measured Results .............................................................................................39 3.5.3 Measurements of Tunable Output ...................................................................41 3.5.4 Analysis ...........................................................................................................43
3.6 Summary ..................................................................................................................44
vi
Chapter 4: Supercontinuum Generation in a Highly Nonlinear Fiber Using CMOS Laser Diode Drivers.......................................................................................................................45
4.1 Introduction and Objectives .....................................................................................45 4.2 Erbium-Doped Fiber Amplifier ...............................................................................46 4.3 Highly Nonlinear Fiber ............................................................................................49 4.4 Nonlinear Optical Effects ........................................................................................50
4.4.1 Chromatic Dispersion ......................................................................................50 4.4.2 Stimulated Raman Scattering ..........................................................................51 4.4.3 Four-Wave Mixing ..........................................................................................51 4.4.4 Modulation Instability .....................................................................................51
4.5 Supercontinuum Generation ....................................................................................52 4.5.1 Experimental Setup .........................................................................................52 4.5.2 Measured Results and Discussions ..................................................................54 4.5.3 System Performance Comparison ...................................................................57
4.6 Summary ..................................................................................................................60
Chapter 5: Conclusions and Future Work ..........................................................................62 5.1 Conclusions ..............................................................................................................62 5.2 Future Work .............................................................................................................63
References ..........................................................................................................................65
vii
List of Tables
Table 2.1: Design parameters of transistors in the proposed CMOS circuit ................................ 16
Table 2.2: Performance summary of the designed CW pumped laser diode ................................ 25
Table 3.1: Performance summary of the designed pulsed laser diode .......................................... 44
Table 4.1: Optical properties of the HNLF ................................................................................... 49
Table 4.2: Characteristics of the amplified laser diode pulses ...................................................... 53
Table 4.3: Performance summary of the seed laser module ......................................................... 57
Table 4.4: Comparison of the designed SCG system and the reference SCG system .................. 61
viii
List of Figures
Figure 1.1: Simplified SCG process in the spectral domain ........................................................... 1
Figure 2.1: Block diagram of a basic EDFA configuration ............................................................ 8
Figure 2.2: Typical output light versus injection current (L-I) curve of laser diodes ................... 10
Figure 2.3: Typical forward voltage versus injection current (V-I) curve of laser diodes ........... 10
Figure 2.4: Block diagram of the CW laser diode driver circuit design methodology ................. 11
Figure 2.5: Circuit schematic of the proposed CMOS laser diode driver ..................................... 11
Figure 2.6: Simulation results of transistors’ operation regin versus the resistance ..................... 17
Figure 2.7: Comparison of modelling current output and simulation current output at the resistance of the off-chip resistor range from 1100 Ω to 1950 Ω. ........................................ 18
Figure 2.8: Die micrograph of the fabricated CW laser diode driver ........................................... 19
Figure 2.9: Package of a complete driver circuit with two identical CMOS dies; Note that the designed driver circuit is only one small part of the whole CMOS die, there are unrelated circuits shared on the same die. ............................................................................................ 20
Figure 2.10: (a) Host PCB with a driver package and four identical potentiometers which controls three current output ports; (b)Laser diode package mounted on a PCB ................. 20
Figure 2.11: Comparison of the output current between the measurement result, simulation result and mathematical modelling result; Note that the resistance at x-axis represents the resistance of each one potentiometer on the PCB in the measurement. ............................... 21
Figure 2.12: Measured data of optical output power with respect to the injection current, and the fitting curve based on measured data .............................................................................. 22
Figure 2.13: Relation curve of the optical output power of the laser diode and the resistance of potentiometers ....................................................................................................................... 23
Figure 2.14: Stability test of the optical output power over a 400 minute period. ....................... 24
Figure 3.1: Evolution of the photon and carrier density during a gain-switching cycle [37] ....... 27
Figure 3.2: Block diagram of the proposed pulsed laser driver circuit design ............................. 28
Figure 3.3: Design methodology for generating pulse waves ....................................................... 29
Figure 3.4: Circuit design schematic of the VCRO ...................................................................... 30
Figure 3.5: Schematic of the VCDL circuit .................................................................................. 31
ix
Figure 3.6: Timing sequence diagram of the VCDL operation .................................................... 32
Figure 3.7: Design schematic of the XOR circuit ......................................................................... 33
Figure 3.8: Negative power supply operation of the laser diode .................................................. 34
Figure 3.9: Post-layout transient simulation of the proposed CMOS laser driver: (a) the output current waveform; (b) a sample current pulse. ..................................................................... 36
Figure 3.10: Output plots when tuning (a) repetition rate and (b) pulse width in simulations ..... 37
Figure 3.11: (a) Die micrograph and (b) PCB layout of the pulsed laser source .......................... 38
Figure 3.12: (a) Laser pulse waveform and (b) one Gaussian fitted laser pulse ........................... 39
Figure 3.13: Optical spectrum of the pulsed laser diode output ................................................... 40
Figure 3.14: Measured results of tuning the optical output pulses’ repetition rate (a) by adjusting control voltage Vvar and (b) by adjusting control voltage Vctr ............................... 42
Figure 3.15: Measured results of tuning the optical output pulses’ pulse width (a) by adjusting control voltage Vvar2 and (b) by adjusting control voltage Vb .............................................. 42
Figure 4.1: Supercontinuum generation system design block diagram ........................................ 45
Figure 4.2: EDFA amplification mechanism based on an energy-level diagram of erbium ions . 46
Figure 4.3: Block diagram of the EDFA module setup ................................................................ 47
Figure 4.4: Spectrum of the output light from the EDFA monitor output .................................... 48
Figure 4.5: Experimental setup diagram of the SCG system ........................................................ 53
Figure 4.6: Spectrum of supercontinuum output .......................................................................... 54
Figure 4.7: Supercontinuum output evolution at different pump power level .............................. 55
Figure 4.8: Temporal profile of the seed laser pulse, the EDFA monitor pulse and the supercontinuum pulse ........................................................................................................... 56
Figure 4.9: Output optical pulses from the commercial seed laser module: the left figure shows the pulse waveform; the right figure shows a single pulse shape. ........................................ 58
Figure 4.10: (a) Spectrum of optical pulses output from the seed laser module; (b) spectrum of the amplified pulses output from the monitor of EDFA ....................................................... 58
Figure 4.11: SCG results from two different systems .................................................................. 60
x
List of Abbreviations and Symbols
Abbreviation Definition ASE Amplified Spontaneous Emission CBL Current-Balanced Logic CDS Cadence Design System CFP Ceramic Flat Package CMOS Complementary Metal-Oxide-Semiconductor CW Continuous-Wave DFB Distributed Feedback DWDM Dense Wavelength-division Multiplexing EDA Electronic Design Automation EDFA Erbium-doped Fiber Amplifier FC Fiber Connector FS Fusion Splicing FWHM Full Width at Half Maximum FWM Four-Wave Mixing HNLF Highly Nonlinear Fiber MI Modulation Instability NMOS N-channel Metal-Oxide-Semiconductor OCT Optical Coherence Tomography OSA Optical Spectrum Analyzer PCB Printed Circuit Board PCF Photonic Crystal Fiber PDK Process Design Kit PMOS P-channel Metal-Oxide-Semiconductor RMS Root Mean Square SCG Supercontinuum Generation SMF Single-Mode Fibers SRS Stimulated Raman Scattering TEC Thermo-Electric Cooler VCDL Voltage-Controlled Delay Line VCRO Voltage-Controlled Ring Oscillator XOR Exclusive–OR ZDW Zero Dispersion Wavelength
xi
Symbol Definition Ith Threshold injection current of laser diodes ROUT Off-chip resistor IREF Reference current IO Output current 𝜇𝜇𝑛𝑛𝐶𝐶𝑜𝑜𝑜𝑜 Transconductance parameter of CMOS transistors Mi Transistor number i L Length of CMOS transistors Vth Threshold voltage Vgs Gate source voltage of CMOS transistors K An integer KI An integer L Optical output power I Injection current N Number of delay stages 𝑓𝑓𝑜𝑜𝑜𝑜𝑜𝑜 Oscillation frequency td Delay time of each delay stage CL Load capacitance Vosc Oscillation voltage amplitude Ictrl Control current Vvar Varactor control voltage Vctr Transistors’ current control voltage Vb Control voltage Rin Input impedance Vf Forward voltage If Forward current 𝑡𝑡𝐹𝐹𝐹𝐹𝐹𝐹𝑀𝑀 Duration time of FWHM a1, b1, c1 Coefficients of the Gaussian pulse Ppeak Peak power Pavg Average power fRR Repetition rate Ep Pulse Energy ω Frequency γ Nonlinear coefficient
1
Chapter 1: Introduction
1.1 Background and Review
A supercontinuum is a special type of light with high intensity, broad spectral bandwidth
and a high degree of spatial coherence. It is considered to be the combination of a lamp with
broadband spectrum and a laser with high brightness. This kind of light does not exist in nature. It
only occurs when high intensity laser light interacts with nonlinear media. The nonlinear process
that broadens the spectrum of the laser light to produce the supercontinuum is called
supercontinuum generation (SCG). A simplified SCG process in the spectral domain is illustrated
in Figure 1.1.
λ λ0 λ λ0 λ1 λ2
Nonlinear Process
Laser Light Spectrum Supercontinuum Spectrum
Figure 1.1: Simplified SCG process in the spectral domain
SCG was first observed by Alfano and Shapiro back in 1970s [1]. They focused powerful
picosecond pulses into a bulk BK7 glass to generate a white light supercontinuum covering a
spectral range of 400 nm to 700 nm. Since then, SCG has been observed when intense picosecond
or femtosecond laser light is incident on various nonlinear materials which can be solid, liquid and
gaseous [2], [3]. SCG in optical fibers was first observed in 1976 by Lin and Stolen [4]. They used
2
nanosecond laser pulses at kilowatt peak power levels to pump the conventional silica fiber in
order to generate a supercontinuum with spectral bandwidth of 180 nm and spectrum centered at
530 nm. Many subsequent research efforts have been made for SCG in standard single-mode
optical fibers [5], [6]. As the nonlinearity of standard fibers is relatively low, it requires pump
lasers with high peak power levels to generate significant spectral broadening in standard fibers.
However, laser light with a high peak power can damage the silica fibers, which limits the power
level and consequently the spectrum bandwidth of the supercontinuum.
The advent of photonic crystal fiber (PCF) greatly improved SCG technology. PCFs are
produced by the cladding of an optical fiber incorporated with photonic crystals that are dielectric
periodic structures on the scale of a wavelength of light. The PCF has the advantage of high
nonlinearity as well as designable dispersion, which enhance nonlinear effects and obtain much
broader spectra than the standard fibers [7]. The first octave spanning SCG was accomplished with
a PCF pumped by nanojoule pulses from a Ti:Sapphire laser in the year 2000 [8].
The successful SCG designs with PCFs have inspired similar work using optical tapered
fibers [9] or highly nonlinear fibers (HNLFs) [10]. Both the optical tapered fibers and HNLFs
present comparable enhanced nonlinearity, which makes them good alternative to PCFs. Optical
tapered fibers are produced by gently stretching optical fibers to a thin core diameter. Optical
tapered fibers have designable dispersion, a controllable tapering process and a reduced fiber
diameter, all of which enhance the SCG performance. SCG in a tapered fiber with a diameter of 2
µm and a length of 90 mm was reported to have a broad spectrum output of more than two octaves
(370 -1545 nm) [9].
HNLFs are produced by fabricating optical fibers with a narrow core and high material
doping level, which reduces the effective core area and thus enhances the nonlinearity. Compared
3
to the PCF and tapered fibers, HNLFs are the easiest to fabricate but exhibit the lowest
nonlinearity. As HNLFs are single-mode optical fibers, they can be coupled with standard single-
mode fibers (SMF) at a low coupling loss, which is an advantage of HNLFs based all fiber SCG
systems. SCG in a 200-meter-long HNLF pumped by a femtosecond fiber laser with 110 fs pulses
at 1550 nm was reported to have a spectrum spanning from 1100 nm to 2100 nm [10].
SCG in optical fibers has found numerous novel applications in the field of
telecommunication [11], optical frequency metrology [12], [13], optical coherence tomography
(OCT) [14] and spectroscopy [15]. For example, in the field of telecommunications, SCG systems
can be used in dense wavelength-division multiplexing (DWDM) systems. One can use optical
filters to slice the supercontinuum spectrum, so that thousands of single wavelengths of laser light
can be obtained and applied to a number of transmission channels. This approach can realize a
high transmission rate with only one light source.
4
1.2 Motivation and Objectives
Although SCG systems have great potential to be applied in these cutting-edge areas, most
commercial SCG systems nowadays are just used as a laboratory tool. Current commercial SCG
systems are still limited in performance by the availability of suitable wavelength ranges and
limited by size, cost, and power. There is high demand for the development of compact, low-cost
and reliable SCG systems that are practical and accessible for use in various application areas [16].
As the supercontinuum is generated when laser light interacts with a nonlinear medium, SCG
systems in general contain two components, which are a laser light source and a nonlinear medium.
Numerous experimental results using different lasers and nonlinear optical media for SCG have
been reported. PCFs, optical tapered fibers, HNLFs, dispersion-shifted fibers [17], and even silicon
waveguides [18] have proven to be successful nonlinear optical media. Mode-locked lasers [19],
Q-switched lasers [20], gain-switched lasers [21] and continuous-wave (CW) pump lasers [22]
with various wavelengths have been used as pump lasers in SCG. Compared to fiber media, pump
lasers often take up most of the space and cost, and cause the most reliability and maintenance
problems in SCG systems. Thus, a compact, low-cost, power-efficient and reliable pump laser
light source would be a turnkey solution for the next generation of SCG systems. The objective of
this thesis is to develop such a laser light source for use in SCG systems.
Among all types of pump lasers mentioned above, CW pump lasers are the most compact
and low-cost. However, CW pump lasers require the highest average power to obtain the same
broad spectral supercontinuum as pulsed lasers. Methods of pulsing lasers are mode-locking, Q-
switching and gain-switching. Ultrashort pulses can be produced by mode-locked lasers using
nonlinear polarization rotation [23], active Q-switched lasers using fiber Bragg gratings [24],
passive Q-switched lasers using standard small-mode-area saturable absorber fibers [25], or gain-
5
switched lasers using ultrashort current pulses. As gain-switched lasers do not require additional
optical components, they are the most compact and cost-effective among all pulsed lasers.
A laser diode is considered to be a good candidate for a compact and reliable pump laser. It
is a simple and cost-effective method to obtain tens-of-picosecond short pulses by gain-switching
widely available telecommunication-grade laser diodes. The drawback of gain-switched laser
diodes is the limitation of the maximum output power. To get a high pulse power, the output of
laser diodes can be amplified by compact fiber amplifiers, which are widely used in optical
communication systems. The combination of telecommunication-grade low-power gain-switched
laser diodes and fiber amplifiers offers a compact, reliable and low-cost approach to generate high-
power short pulses. This approach has been utilized for octave-spanning SCG in the report [17].
SCG systems using this approach have been implemented in the DWDM application [26].
The SCG method employed in this research uses amplified gain-switched laser diode pulses
to pump an HNLF. Based on this method, a laser diode is gain-switched by short current pulses
from a pulsed electronic driver. A fiber amplifier is utilized to amplify the power of optical pulses
produced by the gain-switched laser diode. In the fiber amplifier, a pump laser diode is driven by
a CW electronic driver. To make these electronic drivers compact and low-cost, complementary
metal-oxide-semiconductor (CMOS) technology is utilized to design and fabricate electronic
driver circuits. CMOS technology has the advantages including low power, low fabrication cost,
and high integration. Using CMOS technology for the design of laser diode drivers in
telecommunication is a subject of intense research [28], [29], however, there has not been much
implementation of CMOS laser diode driver circuits in the development of compact and low-cost
SCG systems. This research work aims to design and apply CMOS laser diode driver circuits for
developing such SCG systems.
6
1.3 Thesis Scope and Contributions
The scope of this thesis is to implement CMOS technology in the design of the laser diode
driver circuits for the purpose of developing compact and economical SCG systems. The method
of SCG employed in this thesis is applying amplified picosecond gain-switched laser diode pulses
to efficiently induce nonlinear effects in an HNLF.
Based on the scope, the main contributions of this thesis are:
1. Design and implementation of a CMOS laser diode driver circuit with a tunable CW
high current output. A CW pump laser with a maximum output power of 350 mW is
built using the CMOS driver circuit.
2. Design and implementation of a CMOS laser diode driver circuit with picosecond
current pulses output. A picosecond pulsed distributed feedback (DFB) laser diode is
built using the CMOS driver circuit. To the best of the author’s knowledge, this is the
first such design reported in the open literature.
3. Establishment and measurements of a SCG system using the designed picosecond
pulsed DFB laser diode in combination with an erbium-doped fiber amplifier (EDFA)
and an HNLF.
7
1.4 Thesis Outlines
The rest of this thesis is organized as follows.
Chapter 2 presents the design of a high current CW CMOS laser diode driver. It initially
introduces the design objectives and characteristics of laser diodes. Then, a circuit design
methodology is proposed and analyzed. Circuit simulations in Cadence Design System (CDS) are
presented with detailed simulation parameters. Experimental measurements of the CMOS driver
circuit output as well as the laser diode optical output are conducted. A summary is given to
conclude performance of the design.
Chapter 3 demonstrates the design of a picosecond pulsed CMOS laser diode driver. The
gain switching technique that is implemented to generate picosecond pulses is explained. A circuit
design methodology using CMOS analog logic circuits is demonstrated. Circuit simulations based
on the proposed methodology show the output performance. The CMOS driver circuit is packaged
and connected with a DFB laser diode experimentally. Measured results of the laser output are
reported and analyzed. A performance summary table is given at the end of this chapter.
Chapter 4 establishes a SCG system, which contains the designed picosecond pulsed DFB
laser diode, an EDFA and an HNLF. Characteristics of the EDFA and the HNLF used in the system
are explained. A brief introduction of nonlinear optical effects in the context of a SCG process is
described. The system setup is depicted and measured results are presented. For comparison,
measurements of a reference SCG system utilizing a commercial seed laser module are given. A
comparison between the designed SCG system and a reference SCG system is made in the
summary.
Chapter 5 discusses the results obtained and suggests future work.
8
Chapter 2: High Current Continuous-Wave CMOS Laser Diode Driver
2.1 Introduction and Objectives
Continuous-wave (CW) current sources provide power for a large group of laser diodes.
Many researchers and engineers develop stable CW laser diode drivers with printed circuit boards
(PCBs) and off-the-shelf components. However, there has been an increasing interest in using
commercial CMOS technology to design CW laser diode drivers, for reasons of a compact circuit
size, a low fabrication price and integrable packages.
CW laser diode drivers are required to drive pump laser diodes in erbium-doped fiber
amplifiers (EDFAs), which have proven to be effective fiber amplifiers for supercontinuum
generation (SCG) [17], [29]. A block diagram of a basic EDFA configuration is shown in
Figure 2.1. The gain medium is an erbium-doped fiber. The EDFA amplifies the incident light
using the simulated emission in the erbium-doped fiber, which is continuously pumped by high
energy light from a pump laser diode. The pump laser diode demands a CW laser diode driver to
provide electrical energy.
Figure 2.1: Block diagram of a basic EDFA configuration
975nm Pump Laser Diode
CW LaserDiode Driver
Erbium DopedFiber
Incident Light~1550nm
Amplified Light~1550nm
EDFA
9
Pump laser diodes used in the EDFA usually need to have high power output. A 975 nm
pump laser diode (Bookham LC95B74ET) is selected in this design. It has a maximum output
power of 350 mW in the CW mode, which corresponds to 600 mA injection current based on the
datasheet. Thus the CW laser diode driver requires to be able to output 600 mA current to the laser
diode. Apart from this requirement, a tunable output feature needs to be taken into consideration
for this driver design, as it can add to the flexibility of the laser output. With a tunable output
power from the pump laser, the fiber amplifier system is able to amplify the incident light to
different power levels.
The objective of the research described in this chapter is to implement CMOS technology in
the design of a high current CW laser diode driver circuit with tunable output feature.
2.2 Characteristics of Laser Diodes
In order to design drivers for laser diode, it is critical to understand characteristics of laser
diodes. Laser diodes, also known as injection lasers or diode lasers, are semiconductor lasers in
which the light is generated by injecting an electrical current. In other words, a laser diode emits
light in response to an injection current. The optical power of the emitted light is a function of the
injection current, which is commonly referred as the light versus current (L-I) curve of a laser
diode. Figure 2.2 shows the L-I curve of a typical laser diode. If the injection current is below the
threshold current Ith, the emitted optical power Pout is very small and is generated due to
spontaneous emission in the p-n junction. As the injection current is increased above the threshold
current, the laser diode starts lasing due to the stimulated emission. The efficiency of a laser diode
can be derived from the curve, which is defined as the slope efficiency. The slope efficiency is
denoted as ∆P/∆I (mW/mA) as shown in the figure. It indicates the incremental output power gain
over the increase of injection current.
10
Injection Current (mA)
Opt
ical
Out
put P
ower
(mW
)
ΔP
ΔI
Ith
Figure 2.2: Typical output light versus injection current (L-I) curve of laser diodes
The forward voltage is an important electrical characteristic of a laser diode. It represents
the voltage drop across the laser diode. The relation between the forward voltage and the injection
current of a typical laser diode is shown as the voltage versus current (V-I) curve in Figure 2.3.
This V-I characteristic is similar to the analogous characteristic of other types of semiconductor
diodes. Typical laser diodes exhibit a forward voltage in the range of 1.5 V to 3 V, which requires
CMOS driver circuits to have high voltage supplies and large transistor breakdown voltages [30].
Injection Current (mA)
Forw
ard
Volta
ge (V
)
1
2
Figure 2.3: Typical forward voltage versus injection current (V-I) curve of laser diodes
11
2.3 Continuous-Wave CMOS Driver Circuit Design Methodology
A block diagram of a CW CMOS laser diode driver circuit design methodology is shown in
Figure 2.4. The driver circuit is a current source. There are three sub-circuits including a start-up
circuit, a current reference circuit and a current source circuit. The start-up circuit ensures that the
current reference circuit is turned on. The current source circuit amplifies the reference current
generated from the current reference circuit. Based on this methodology, a CMOS laser diode
driver circuit is proposed and shown in Figure 2.5.
Figure 2.4: Block diagram of the CW laser diode driver circuit design methodology
M6
VDD
M5
M7
M3 M4 M0
M1 M2
RO UT
Laser Diode
IREF IO
Start-up CurrentReference
CurrentSource
Figure 2.5: Circuit schematic of the proposed CMOS laser diode driver
Start-up Circuit Current Reference CircuitTurn On
Current Source CircuitAmplified
12
2.3.1 Current Reference Circuit
This proposed current reference circuit shown in the center of Figure 2.5 is based on the
structure of beta-multiplier voltage reference [31]. Transistors M1-M4 are self-biased to operate in
the saturation region. The p-channel metal-oxide-semiconductor (PMOS) current mirror M3 and
M4 force the same current through each leg of the circuit. An off-chip resistor ROUT has been placed
between the source of M2 and ground. The size of M2 is made larger than that of Ml so that the
difference in the gate to source voltage of Ml and M2 is dropped across ROUT. Through adjusting
the resistance of the off-chip resistor, the reference current will be adjusted accordingly.
As the PMOS M3 current mirrors the PMOS M4, the current I1 passing through M3 and M1
is the same as the reference current IREF passing through M4 and M2, which is expressed as
The gate source voltage of M1, Vgs1 equals the sum of the gate source voltage of M2, Vgs2, and the
voltage across the resistor ROUT, which is shown as
𝑉𝑉𝑔𝑔𝑜𝑜1 = 𝑉𝑉𝑔𝑔𝑜𝑜2 + 𝐼𝐼𝑅𝑅𝑅𝑅𝐹𝐹𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂. (2.2)
As all transistors are operated in the saturation region, the current I1 and IREF can be described as
(neglecting the channel-length modulation)
𝐼𝐼1 = 𝜇𝜇𝑛𝑛𝐶𝐶𝑜𝑜𝑜𝑜𝐹𝐹12𝐿𝐿1
(𝑉𝑉𝑔𝑔𝑜𝑜1 − 𝑉𝑉𝑡𝑡ℎ1)2
𝐼𝐼𝑅𝑅𝑅𝑅𝐹𝐹 = 𝜇𝜇𝑛𝑛𝐶𝐶𝑜𝑜𝑜𝑜𝐹𝐹22𝐿𝐿2
(𝑉𝑉𝑔𝑔𝑜𝑜2 − 𝑉𝑉𝑡𝑡ℎ2)2, (2.3)
where 𝜇𝜇𝑛𝑛𝐶𝐶𝑜𝑜𝑜𝑜 is the transconductance parameter of CMOS transistors, W1 and L1 are the width and
length of the transistor M1, Vth1 is M1’s threshold voltage, W2 and L2 are the width and length of
the transistor M2 and Vth2 is M2’s threshold voltage.
Based on Equation (2.3), Vgs1 and Vgs2 can be rewritten as
𝐼𝐼1 = 𝐼𝐼𝑅𝑅𝑅𝑅𝐹𝐹. (2.1)
13
𝑉𝑉𝑔𝑔𝑜𝑜1 = 2𝐼𝐼1𝛽𝛽1
+𝑉𝑉𝑡𝑡ℎ1
𝑉𝑉𝑔𝑔𝑜𝑜2 = 2𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅𝛽𝛽2
+𝑉𝑉𝑡𝑡ℎ2,
where β1 = µnCoxW1L1
,β2 = µnCoxW2L2
.
(2.4)
Neglecting the body effect from M2, for the same CMOS technology, the two threshold voltages
Vth1 and Vth2 should be equal
𝑉𝑉𝑡𝑡ℎ1 = 𝑉𝑉𝑡𝑡ℎ2. (2.5)
By substituting Equation (2.1) (2.4) (2.5), Equation (2.2) can be rewritten
2𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅
𝛽𝛽1= 2𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅
𝛽𝛽2+ 𝐼𝐼𝑅𝑅𝑅𝑅𝐹𝐹𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂. (2.6)
The reference current can be derived by solving Equation (2.6)
𝐼𝐼𝑅𝑅𝑅𝑅𝐹𝐹 = (1 − 1√𝐾𝐾
)2 2𝛽𝛽1𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂2
where 𝐾𝐾 = (𝐹𝐹1𝐿𝐿1
)/(𝐹𝐹2𝐿𝐿2
). (2.7)
Equation (2.7) shows that the reference current is only dependent on the resistance of the off-chip
resistor and the device parameters of transistors. The device parameters of CMOS transistors are
fixed after fabrication of the design. The reference current can only be tuned by adjusting the
resistance of the off-chip resistor.
14
2.3.2 Start-up Circuit
When solving Equation (2.6), there is another scenario except the answer indicated in
Equation (2.7). That is the zero reference current scenario. In this scenario, the circuit is completely
off even after the power supply is on. This is possible because all transistors are self-biased.
To eliminate the zero reference current scenario, a start-up circuit shown in the left part of
Figure 2.5 is introduced into the design. This start-up circuit is able to initiate the current reference
circuit from a dead (zero current) operating point to its normal operating point [32]. When the
current reference circuit is at a dead operating point, the start-up circuit sets the drain voltage of
the transistor M1, and initiates M1 to draw current. Once the start-up transistor M7 provide a current
path between the supply voltage and ground, the transistors M1, M2, M3, and M4 operate normally
and the reference current will reach the desired amount. After the current reference circuit is turned
on, the gate source voltage of M7 drops to below the threshold voltage and no current flows through
M7. Thus, the start-up circuit has no impact on the value of the reference current.
2.3.3 Current Source Circuit
The current source circuit is shown in the right part of Figure 2.5. It is a single PMOS with
a bias voltage from the reference current circuit. This current source configuration extends the
output voltage to accommodate the required forward voltage of the laser diode.
The gate bias voltage of transistor M0 is connected to the gate bias voltage of transistor M4.
This mirrors the current along M0 and M4. Since M4 operates under the saturation region, M0 does
in the same mode. Thus, the output current Io is
𝐼𝐼𝑂𝑂 = 𝜇𝜇𝑛𝑛𝐶𝐶𝑜𝑜𝑜𝑜𝐹𝐹02𝐿𝐿0
(𝑉𝑉𝑔𝑔𝑜𝑜0 − 𝑉𝑉𝑡𝑡ℎ0)2. (2.8)
15
Since the gate-source voltage Vgs0 and the threshold voltage Vth0 of the transistor M0 are the
same as the gate-source voltage and the threshold voltage of the transistor M4, the amplification
ratio KI of the output current over the reference current can be expressed as
𝐾𝐾𝐼𝐼 = 𝐼𝐼𝑂𝑂𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅
= (𝐹𝐹0𝐿𝐿0
)/(𝐹𝐹1𝐿𝐿1
). (2.9)
By substituting IREF with the result from Equation (2.7), the output current can be expressed as
𝐼𝐼𝑂𝑂 = 𝐾𝐾𝐼𝐼(1 − 1√𝐾𝐾
)2 2𝛽𝛽1𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂2
. (2.10)
Equation (2.10) shows that the output current has the same output characteristics as the
reference current. The tunable output current is achieved by adjusting the resistance of the off-chip
resistor. Equation (2.10) also shows that the output current is independent of the supply voltage. It
is desirable for the current source to be insensitive of the supply voltage, as most power supplies
fluctuate. By utilizing the self-biasing technique, power supply sensitivity can be greatly reduced.
Self-biasing technique means the biasing voltages especially gate biasing voltages are not
connected to the supply power directly. All transistors in this proposed design are self-biased.
Thus, the output is insensitive to the power supply voltage.
2.4 Circuit Design and Simulations
CMOS circuits are often designed with electronic design automation (EDA) software.
Cadence Design System (CDS) is a ubiquitous commercial tool for CMOS circuit schematic
design, simulation, layout design and verification. To start a CMOS circuit design with CDS, an
industry-standard CMOS process with a foundry-certified process design kit (PDK) needs to be
determined. As the drain voltage of M0 requires a minimum 2 V output voltage to accommodate a
2 V forward voltage of the laser diode, a 0.35 µm CMOS process with a high transistor breakdown
voltage of 3.6 V is selected for designing the laser diode driver circuit.
16
To have a maximum output power of 350 mW from the pump laser, the laser diode driver
circuit needs to inject 600 mA CW current to the pump laser, according to the L-I curve of the
pump laser. This required output current is quite high for the CMOS technology due to the metal
electromigration issue. For each CMOS process, the amount of current carried on a metal wire or
bus is limited. A metal wire or bus carrying too much current causes a change in the metal
dimensions, spots of higher resistance and eventually failure [31]. This is termed as the metal
electromigration effect. The current density limit for the 0.35 µm CMOS process is 1.6 mA/µm.
To avoid the electromigration effect, the output current is distributed to 12 identical metal wires
on chip. Each metal wire has a width of 33 µm. This setup guarantees that the driver circuit can
deliver 600 mA current without experiencing the electromigration effect. These 12 metal wires are
connected together as a bus wire on a PCB, which allows a much higher current density.
The proposed CMOS circuit was designed and simulated with CDS. In order to create a large
tuning range of the output current, the size of the transistor M2 is made four times larger than the
transistor M1. As increasing the channel length can reduce the body effect, transistors of the
reference current circuit are set to a fairly large 3 µm channel length. Detailed design parameters
of transistors are listed in the Table 2.1.
Table 2.1: Design parameters of transistors in the proposed CMOS circuit
Transistor Parameter Value Transistor Parameter Value
M1 L=3 µm, W=10 µm M5 L=0.5 µm, W=20 µm
M2 L=3 µm, W=40 µm M6 L=5 µm, W=20 µm
M3 L=3 µm, W=60 µm M7 L=0.5 µm, W=20 µm
M4 L=3 µm, W=60 µm M0 L=1 µm, W=100 µm *216
17
For the transistor M0, the total width is 21.6 mm. A multi-finger structure is implemented in
the design. The multi-finger structure allows multiple identical transistors with short width
connected in parallel to replace a single transistor with long width. This technique reduces the gate
resistance and the circuit’s physical size. The supply voltage for this driver circuit is 3.3 V and the
laser diode is modeled as a –2 V voltage source in simulations based on the datasheet.
The model of the output current in Equation (2.10) is based on the fact that all transistors
M1, M2, M3, M4 and M0 are in the saturation region. With the help of simulations in the CDS, the
relation between the resistance and transistors’ operation region is shown in Figure 2.6. Transistors
M1 and M4 can only operate in the saturation region. Reg0, Reg2 and Reg3 represent the operation
region of transistor M0, M2 and M3, respectively. In this simulation, Y = 1 means the transistor is
in the triode region and Y = 2 means the transistor is in the saturation region.
Figure 2.6: Simulation results of transistors’ operation regin versus the resistance
1000 1200 1400 1600 1800 20000-Cutoff
1-Triode
2-Saturation
3-Subthreshold
4-Breakdown
Rout (Ω)
Y (O
pera
tion
Reg
ions
)
Reg0Reg2Reg3
18
The plotted result shows that all transistors are operating in saturation region (Y=2) when
resistance is higher than 1100 Ω. Under this condition, the relation between the output current and
the resistance as illustrated in Equation (2.10) is valid. By substituting design parameters K=4,
KI=1080 and β1=7.1667·10-4, the obtained output current is
𝐼𝐼𝑂𝑂 = 1.88371∙105 𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂2
. (2.11)
This expression of output current is obtained based on mathematically modelling behaviors
of transistors. To verify the mathematical model, a comparison between the modelling output and
the CDS simulation output is made in Figure 2.7. The comparison shows that the modelling current
output agrees with the simulation current output. In the range from 1300 Ω to 1400 Ω, the
modelling one has an accurate fit to the simulation one.
Figure 2.7: Comparison of modelling current output and simulation current output at the resistance of the off-chip resistor range from 1100 Ω to 1950 Ω.
1100 1200 1300 1400 1500 1600 1700 1800 1900 2000150
200
250
300
350
400
450
500
550
600
650
Resistance of the off-chip resistor (Ω)
The
outp
ut C
urre
nt (m
A)
Simulation OutputModeling Output
19
2.5 Measurements
2.5.1 Measurement Setup
Due to the size restriction of the fabrication process, the design has to be divided into two
identical CMOS dies. Each CMOS die provides half of the total current output, therefore each one
has six identical current outputs. The die micrograph is shown in Figure 2.8. As the complete driver
circuit is connecting two of these dies in parallel, two dies are placed into one package. A complete
driver with the package is shown as Figure 2.9. The package is mounted on a custom-designed
PCB, as shown in Figure 2.10 (a). Four identical off-chip potentiometers are also connected on the
PCB for tuning the output, as each potentiometer controls three current output ports. This driver
circuit with the host PCB can output the driving current to the laser diode with a mount PCB,
which is shown in Figure 2.10 (b).
When the CMOS driver circuit provides the driving current to the laser diode, the laser diode
outputs laser light. The output current from the CMOS driver circuit is measured by an ammeter
placed between the driver circuit and the laser diode. The power of the output light is detected by
an optical power meter (Newport 1916-R). The supply voltage VDD to the driver circuit is 3.3 V.
GND Io1 Io2 Io3 Io4 Io5 Io6
VDD
VDD
VDD VDDRout1 Rout2
W =
2 m
m
Figure 2.8: Die micrograph of the fabricated CW laser diode driver
20
Figure 2.9: Package of a complete driver circuit with two identical CMOS dies; Note that the designed driver circuit is only one small part of the whole CMOS die, there are unrelated circuits shared on the same die.
Figure 2.10: (a) Host PCB with a driver package and four identical potentiometers which controls three current output ports; (b)Laser diode package mounted on a PCB
21
2.5.2 Measurements of the Output Current
By tuning the four off-chip potentiometers with the same pace from 1100 Ω to 1950 Ω, a
wide range of the output current from 200 mA to 600 mA is realized. This result is compared to
the results from the simulation and the modelling. The comparison result is shown in Figure 2.11.
Results indicate that the measurement agrees to the modelling better than the simulation. From a
resistance of 1250 Ω to 1450 Ω, results from three different approaches have good agreement.
Figure 2.11: Comparison of the output current between the measurement result, simulation result and mathematical modelling result; Note that the resistance at x-axis represents the resistance of each one potentiometer on the PCB in the measurement.
1100 1200 1300 1400 1500 1600 1700 1800 1900 2000150
200
250
300
350
400
450
500
550
600
650
Resistance of the off-chip resistor (Ω)
The
outp
ut C
urre
nt (m
A)
SimulationModellingMeasurement
22
2.5.3 Measurements of the Optical Output Power
The optical output power from the laser diode is directly dependent on the amount of
injection current provided by the driver circuit. The measured relation between the optical output
power of the laser and the injection current is shown in Figure 2.12. Measured data shows that the
optical output power from the laser diode can be tuned from 100 mW to 350 mW. Based on
measured data, a linear fitting curve is also plotted in Figure 2.12. The fitting plot indicates that
the threshold current of this laser diode is 40.1 mA. There is a linear relation between the optical
output power L and the injection current I, which can be expressed as
𝐿𝐿 = 0.6433 · 𝐼𝐼 − 25.81. (2.12)
Figure 2.12: Measured data of optical output power with respect to the injection current, and the fitting curve based on measured data
0 100 200 300 400 500 600-50
0
50
100
150
200
250
300
350
400
The injection current (mA)
The
optic
al o
utpu
t pow
er o
f the
lase
r dio
de (m
W)
Measured DataFitting Curve
23
As the injection current is the output current of the laser diode driver circuit and the
resistance of the off-chip potentiometers determines the output current, the optical output power
of the laser diode is controlled by the resistance. The relation between the optical output power
and the resistance is shown in Figure 2.13. Since the current output is in a hyperbolic relation with
a square of the resistance according to Equation (2.11) and the optical output power is in a linear
relation with the injection current according to Equation (2.12), the fitting curve between the
optical output power and the resistance is a hyperbolic function of the resistance’s square. A
desired optical output power level can be obtained by setting the corresponding resistance of
potentiometers, based on the experssion of fitting curve
𝐿𝐿 = 4.6·108
𝑅𝑅𝑂𝑂𝑂𝑂𝑂𝑂2− 14.6. (2.13)
Figure 2.13: Relation curve of the optical output power of the laser diode and the resistance of potentiometers
1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000
50
100
150
200
250
300
350
400
The resistance of a potentiometer (Ω)
The
optic
al o
utpu
t pow
er o
f the
lase
r dio
de (m
W)
Measured dataFitting curve
24
As the output current from the driver circuit is relatively high, its thermal stablity needs to
be investigated. During the measurement, a fan with 0.8 W power placed between the driver PCB
and the laser PCB is turned on. The thermo-electric cooler (TEC) inside the laser package is also
turned on. With this setup and an output power of 309 mW, the stability test is carried out for 400
minutes. The test result shown in Figure 2.14 indicates that the optical output power has a 2 mW
(0.65% of the average output power) fluctuation over the 400 minute test. The fluctuation can be
caused by combinated effects of the thermal noise in the CMOS circuit, the laser diode and the
power meter. There is no trend of consistent power increase or decrease. This stable power output
performace makes the laser suitable for various applications including laser pumping and material
processing.
Figure 2.14: Stability test of the optical output power over a 400 minute period.
0 50 100 150 200 250 300 350 400305
306
307
308
309
310
311
312
313
314
315
Test time (minutes)
The
optic
al o
utpu
t pow
er (m
W)
25
2.6 Summary
A CMOS laser diode driver circuit with off-chip potentiometers is designed for driving the
pump laser diode. The driver circuit has compact size and a tunable output up to 600 mA. Using
this driver circuit, the pump laser is able to provide a stable output optical power up to 350 mW.
A detailed performance summary of this pump laser diode is given in Table 2.2. The laser driven
by the designed CMOS circuit is suitable to be used as a pump laser in erbium-doped fiber
amplifier systems. The advantages lie in its compact size, low cost and potential integration with
other CMOS-compatible platforms.
Table 2.2: Performance summary of the designed CW pumped laser diode
Parameter Value
Laser Diode Optical Output Power up to 350 mW
Driver Current Output up to 600 mA
Supply Voltage 3.3 V
Centre Wavelength 975 nm
Technology 0.35 µm CMOS process
CMOS Die Size 4 mm · 4 mm
26
Chapter 3: Picosecond Pulsed CMOS Laser Diode Driver
3.1 Introduction and Objectives
Picosecond pulsed laser source have application areas such as optical communications,
biomedical imaging [33] and supercontinuum generation (SCG). Gain-switching laser diodes with
short injection current pulses offers a compact, cost effective and power-efficient approach to
generate picosecond optical pulses [34]. Gain-switched distributed feedback (DFB) laser diodes
have been used as seed lasers for master oscillator power amplifiers in SCG [35]. DFB laser diodes
are a type of narrow spectral width laser diodes, which are widely used in optical communication
systems as transmitters. The combination of low-power gain-switched laser diodes and fiber
amplifier systems creates an attractive technological approach to the development of low-cost,
robust, and compact high-power short-pulse optical sources [36].
The objectives of the research described in this chapter are to design a picosecond pulsed
CMOS laser diode driver for gain-switching a DFB laser diode and to establish a compact
picosecond pulsed laser source as the seed laser of a SCG system.
3.2 Gain-switched Laser Diodes
Short optical pulses with pulse widths in the picosecond range can be conveniently generated
by directly driving laser diodes with large amplitude and fast speed current pulses. This technique
is known as gain-switching laser diodes, as the optical gain of laser diodes is modulated by
switching the driving current. A typical gain-switching cycle is shown in Figure 3.1.
27
Lasing threshold
Switching current pulse
Electron density Photon density
t
Figure 3.1: Evolution of the photon and carrier density during a gain-switching cycle [37]
The laser diode is biased below the lasing threshold. When the switching current pulse is
first injected into the laser diode, there is no stimulated emission due to the low initial value of the
photon density. The electron density increases rapidly with a constant injected current. Once the
electron density is above the lasing threshold, the stimulated emission occurs and the laser diode
emits light. With a significant electron compulsion by the stimulated emission, the electron density
decreases to the level below the lasing threshold in a short period. Thus, a short laser pulse is
generated during a gain-switching cycle. As there is no laser light emission at the beginning or the
end of the current pulse, the laser pulse has a shorter pulse width than the current pulse.
Gain-switched laser diodes are attractive lasers for generating picosecond laser pulses
because they are simple, compact and stable [37]–[39]. Compared to gain-switched laser diodes,
picosecond Q-switched lasers and picosecond mode-locked lasers are more expensive, more
complex and less robust. Moreover, the pulse width and pulse repetition rate of optical pulses
generated by gain-switched laser diodes can be easily tuned in a wide range. As the output pulse
width and repetition rate are controlled by the electronic driver instead of the laser resonator setup,
it is simple and compact to achieve a tunable output feature with gain-switched laser diodes.
28
3.3 Pulsed CMOS Driver Circuit Design Methodology
CMOS analog logic circuits are implemented to design the electronic driver for gain-
switching laser diodes at picosecond pulse levels. A block diagram of the proposed CMOS laser
driver circuit is shown in Figure 3.2. The design methodology for generating pulse waveforms is
shown in Figure 3.3. This design implements a logic-based pulse generation method with CMOS
technology. There are four sub-circuits: a voltage-controlled ring oscillator (VCRO), a voltage-
controlled delay line (VCDL), an exclusive–OR (XOR) circuit, and a current source circuit. The
VCRO generates a periodic square wave signal, which determines the repetition rate of the output
pulses. The VCDL sets the delay of the square wave signal at a certain time period, which
determines the pulse width. Electrical pulses are created by the XOR circuit when the delayed
signal is XOR-ed with the original square wave signal. A CMOS current source at the output stage
converts voltage output pulses to current output pulses. The peak current of the output pulses
depends on the size of the CMOS transistor and the supply voltage.
VDD
VCRO Buffer
VCDL
XOR Buffer
NMOS
Laser Diode
VSS_33
Figure 3.2: Block diagram of the proposed pulsed laser driver circuit design
29
Square wave
Delay square wave
XOR
Pulse wave
Figure 3.3: Design methodology for generating pulse waves
3.3.1 Voltage-Controlled Ring Oscillator
The two main categories of CMOS voltage-controlled oscillators are the ring oscillator and
the LC oscillator. Compared to the CMOS LC oscillator, the CMOS ring oscillator has the
advantages of small design area, wide tuning range and ease of integration, which are preferred in
this design.
A ring oscillator requires the connection of an odd number of inverters and feedback from
the output of the last inverter to the input of the first inverter [40]. The proposed VCRO circuit, as
shown in Figure 3.4, implements the varactor-tuned technique and current-starved inverters. A
CMOS varactor is a reverse-biased diode whose capacitance is controlled by the applied reverse
voltage. The varactor-tuned technique means tuning varactors to control the oscillation frequency.
Current-starved inverters are standard inverters with an additional transistor, which is used to
control the current charging the load capacitor. Using current-starved inverters can also increase
the tuning range of the oscillation frequency.
30
C1
M3
M2
M1
Vvar
VoutVin
Vctr
VDD
VSS
Figure 3.4: Circuit design schematic of the VCRO
The oscillation frequency of the ring oscillators is given by
𝑓𝑓𝑜𝑜𝑜𝑜𝑜𝑜 = 12𝑁𝑁𝑡𝑡𝑑𝑑
(3.1)
where N is the number of delay stages and td is the delay time of each stage. The delay time of
each stage is given by [41]
𝑡𝑡𝑑𝑑 = 𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜𝐶𝐶𝐿𝐿𝐼𝐼𝑜𝑜𝑐𝑐𝑐𝑐
(3.2)
where Vosc is the voltage amplitude of oscillation signals, CL is the load capacitance of each delay
stage, and Ictrl is the control current. By subtracting Equation (3.2) into Equation (3.3), the
oscillation frequency can be rewritten as
𝑓𝑓𝑜𝑜𝑜𝑜𝑜𝑜 = 𝐼𝐼𝑜𝑜𝑐𝑐𝑐𝑐2𝑁𝑁𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜𝐶𝐶𝐿𝐿
. (3.3)
In the proposed design the VCRO has five delay cells in the loop. Each cell consists of a
basic cascaded inverter pair with a varactor for controlling the load capacitance and an additional
n-channel metal-oxide-semiconductor (NMOS) transistor for controlling current passing through
inverters. The delay of each inverter is controlled by a current control voltage Vctr and a varactor
31
control voltage Vvar. Introducing these two voltage control elements in the design enables a wide
tuning range of the oscillation frequency.
3.3.2 Voltage-Controlled Delay Line
Transmission gate based delay lines have high power efficiency and full swing output.
However these designs suffer from the tuning range because the delay changes quadratically with
the number of cascaded transmission gates. Delay lines based on current-balanced logic (CBL)
feature low switching noise and high output voltage swing [42].
The proposed VCDL schematic based on CBL is shown in Figure 3.5. The control voltage
Vb sets the same amount of charging current through transistors M1 and M2, thus the current
balancing is ensured. The delay at the rising edge and the delay at the falling edge are identical
since transistors at first stage (M1, M2) are matched with transistors at the second stage (M3, M4)
and the load capacitance at node Vd2 is the same as at node Vdout.
M4M2
M1
Vdin
VDD
M3
Vvar2
Vdout
Vb
C1 C2
Vd2
VSS
Figure 3.5: Schematic of the VCDL circuit
32
The timing diagram of the VCDL is shown in Figure 3.6. When the rising edge of Vdin is
applied, the node Vd2 is first pulled down by discharging through transistor M2. Then, PMOS M3
at the second stage is turned on and pulls up the node Vdout by charging the varactor C2 and the
load capacitance. When a falling edge transition of Vdin is applied, the node Vd2 is first pulled up
by charging the varactor C1 and the load capacitance. Then NMOS M4 at the second stage is turned
on and pulls down the node Vdout.
Rising and falling edge delays are controlled by the current control voltage Vb and varactors’
control voltage Vvar2. This VCDL has wide delay tuning range with respect to control voltages.
t
Vdin
Vd2
Vdout
VDD
VCC
Figure 3.6: Timing sequence diagram of the VCDL operation
33
3.3.3 Exclusive-OR Circuit
The design of an XOR circuit is shown in the Figure 3.7. This conventional but effective
circuit can operate with a full output voltage swing [43], which is critical to have a high peak
current output. The upper part of this circuit is a complementary pull-up PMOS network while the
lower part consists of pull-down NMOS networks. Only when two inputs V1 and V2 differ, the
output voltage Vxor will be pulled up to VDD.
V1
VDD
V2
VDD
VDD
V2
V1
V2
V1
Vxor
V1
V1
V2
V2
V2
V1
VSS VSS
Figure 3.7: Design schematic of the XOR circuit
34
3.3.4 Current Source Circuit
NMOS-based current source circuits have been approved as good candidates for being
implemented in laser driver circuits [44]. This design applies a single NMOS based current source
at the output stage. Using only one NMOS at this stage maximizes the drive current swing and
accommodates the required forward voltage.
The current source circuit is connected with the cathode of a DFB laser diode (FUJITSU
FLD5F6CX-J). The anode of the DFB laser diode is connected to the laser package metal case. As
the package metal case is usually grounded for low-noise output performance, the anode of this
DFB laser diode is earth-grounded. The positive terminal of the power supply, which is connected
to VDD in the Figure 3.2, is at zero volts with respect to ground. Accordingly, a negative voltage
from the negative terminal is applied to VSS_33. This configuration shown in Figure 3.8 is often
called negative power supply operation of laser diodes.
I
VDD
VSS_33
Figure 3.8: Negative power supply operation of the laser diode
35
3.4 Circuit Design and Simulations
In this design, the 0.13 µm CMOS process with thick oxide transistors is selected with
concerns about the performance, cost, and design requirements. It is a low-cost mature process
with advanced features and high performance. Thick oxide transistors that allow a maximum of
3.6 V voltage supply are utilized in the design. Using these special transistors at the output stage
can increase maximum supply voltage, so that the output voltage can be increased to meet the
required forward voltage of the laser diode. Once the process is specified, a circuit schematic can
be created based on the methodology. Device parameters in the design are usually determined
based on design requirements and process specifications.
To conduct simulations of the CMOS driver circuit in Cadence Design System (CDS), a
circuit model of the DFB laser diode is established as a load circuit. According to the information
from the DFB laser diode datasheet, the voltage versus current characteristic is linear and the input
impedance Rin is matched to 25 Ω. So the relation between the forward voltage Vf and the forward
current If is: Vf = If · Rin + Vth. Based on the test data of the forward voltage (1.6 V) and forward
current (30 mA), the threshold voltage is 0.85 V. Therefore, a simple equivalent circuit model of
this DFB laser diode is derived as a 0.85 V reverse voltage in series with 25 Ω matched impedance
with rise time and fall time of 100 ps at 2.5 Gb/s modulation rate.
In order to withstand the high voltage swing for the laser (above 1.6 V), the 3.3V I/O
transistor with thick oxide (nfet33) is utilized as the NMOS current source. This nfet33 transistor
has a width to length ration of 800. All other transistors are thick oxide transistors (dgnfet/ dgpfet)
with a breakdown of 2.7 V. This ensures a desired high current output. All VDDs in the schematics
are connected to the ground. The negative supply voltage is -3.3 V for the VSS_33 of the NMOS
current source and -2.5 V for the VSS in all other blocks. Control voltages (Vctr, Vvar, Vb, Vvar2)
36
are tuned to obtain pulse train outputs with picosecond pulse widths at repetition rates of several
megahertz.
The critical characteristic of the driver circuit performance is the output current to the load
circuit, because the laser output power is dependent on the injection current. In simulations, the
output current from the proposed driver circuit is plotted. As this driver circuit is designed for the
seed laser in SCG, driving current pulses with a high peak current, a short pulse width and a low
duty cycle are aims of this design. The best simulation performance waveform indicates that the
pulse full width at half maximum (FWHM), the peak current and the repetition rate of drive current
pulses are 200 ps, 80 mA, and 5.8 MHz respectively. The output current waveform from the post-
layout transient simulation is shown in Figure 3.9 (a) and a sample current pulse is shown in
Figure 3.9 (b).
Figure 3.9: Post-layout transient simulation of the proposed CMOS laser driver: (a) the output current waveform; (b) a sample current pulse.
0 200 400 600 800 1000-10
0
10
20
30
40
50
60
70
80
90
Time (ns) (a)
Out
put C
urre
nt (m
A)
36.1 36.2 36.3 36.4 36.5-10
0
10
20
30
40
50
60
70
80
90
Time (ns) (b)
Out
put C
urre
nt (m
A)
37
The repetition rate and the pulse FWHM of output current pulses can be tuned by varying
the control voltages. The repetition rate is determined by the oscillator frequency of the VCRO.
Tuning the repetition rate has a negligible impact on the pulse width. Simulation results of tuning
the repetition rate are shown in Figure 3.10 (a). It is tuned from 5.8 MHz to 45.9 MHz by changing
the control voltages Vctr and Vvar. Tuning the pulse width is accomplished by changing control
voltages Vb and Vvar2 in the VCDL. The plot of simulation results is shown in Figure 3.10 (b). The
output with 200 ps pulse width and 5.8 MHz repetition rate is achieved when Vctr, Vvar, Vb and
Vvar2 are -1.5 V, -2.5 V, -2.5 V and -2.1 V respectively.
Figure 3.10: Output plots when tuning (a) repetition rate and (b) pulse width in simulations
-2.5 -2 -1.5 -1 -0.5 05
10
15
20
25
30
35
40
45
50
Control Voltage Vvar (V)
Rep
etiti
on R
ate
(MH
z)
Vctr=0 V
Vctr=-1.5 V
-2.5 -2 -1.5 -1200
400
600
800
1000
1200
1400
Control Voltage Vb (V)
Pul
ses
FWH
M (p
s)
Vvar2=-2.1 V
38
3.5 Measurements and Analysis
3.5.1 Measurement Setup
This laser driver circuit is fabricated with the 0.13 µm CMOS process. A die micrograph
shown in Figure 3.11(a) has a 0.3 mm2 CMOS chip area. This CMOS die is packaged in a ceramic
flat package (CFP) and interconnected with a 4 GHz DFB laser diode on a PCB as shown in
Figure 3.11(b). The size of this laser source PCB is 28 cm2. Since the gain-switching frequency is
in the RF frequency range, the connection between the driver output and the laser diode cathode
is designed as short as possible (less than 5 mm) to reduce the transmission line effect. Four
potentiometers are used on PCB to set control voltages Vctr, Vvar, Vb and Vvar2.
The VDD is connected to the earth ground. VSS is set as a -2.5 V power supply. In the
measurement, when VSS_33 is set to -3.3 V, the output power is quite low. In order to get the
desired power, VSS_33 is set to -3.65 V for all measurements.
W=
0.3
mm
(a) (b)
L=1 mm
W=
39 m
m
L= 69 mm
CMOSVb
Vctr Vvar VSS Vvar2 VSS_33
Iout
VDD
DFB
Vb
VctrVvar
Vvar2
VSS_33VDD
VCROVCDL
XOR NMOS
Laser Driver
VSS
Figure 3.11: (a) Die micrograph and (b) PCB layout of the pulsed laser source
39
3.5.2 Measured Results
A temporal waveform of this output light is captured by a 5 GHz bandwidth photodiode
(Thorlabs DET08CFC) and displayed on an oscilloscope with 8 GHz bandwidth and 25 GS/s
sample rate (Tektronix DSA70804B). The control voltages Vctr, Vvar, Vb and Vvar2 are set to be
-1.5 V, -2.5 V, -2.5 V and -2.1 V respectively. A part of this periodic waveform is shown as Figure
3.12(a). The repetition rate of this waveform is 5.6 MHz with a standard deviation of 2.8 kHz at a
count of 215 pulses. Figure 3.12 (b) shows a measured laser pulse with Gaussian fitting. The
Gaussian fitting model 𝑓𝑓𝑔𝑔𝑔𝑔𝑔𝑔 and the FWHM of a Gaussian pulse 𝑡𝑡𝐹𝐹𝐹𝐹𝐹𝐹𝑀𝑀 are expressed as
𝑓𝑓𝑔𝑔𝑔𝑔𝑔𝑔(𝑜𝑜) = 𝑎𝑎1𝑒𝑒
−(𝑜𝑜−𝑏𝑏1𝑜𝑜1)2
𝑡𝑡𝐹𝐹𝐹𝐹𝐹𝐹𝑀𝑀 = 2√2 𝐶𝐶1 , (3.4)
where amplitude a1, mean b1 and variance c1 are coefficients of the Gaussian fitting model. The
calculated average FWHM is 200 ps with a standard deviation of 25 ps at a count of 215 pulses.
Figure 3.12: (a) Laser pulse waveform and (b) one Gaussian fitted laser pulse
1.8 1.9 2 2.1 2.2 2.3 2.4-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
Time(µs)
Mea
sure
d V
olta
ge(V
)
1988.8 1989 1989.2 1989.4 1989.6 1989.8-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
Time(ns)
Mea
sure
d V
olta
ge(V
)
Measured PointsGaussian Fitting
(a) (b)
40
The laser output spectrum is recorded by an optical spectrum analyzer (OSA, YOKOGAWA
AQ6375), which is shown in Figure 3.13. The output average power is 7.0 µW. The peak power
and the pulse energy of Gaussian pulses can be calculated by [45]
where Ppeak is the peak power, Pavg is the average power, tFWHM is the FWHM of the pulse, fRR is
the repetition rate, and Ep is the pulse energy. Based on measured results, the pulse peak power is
5.9 mW and the pulse energy is 1.25 pJ. The center wavelength of its output light is 1548.1 nm.
The spectral width is 0.24 nm at 20 dB root mean square (RMS) level.
Figure 3.13: Optical spectrum of the pulsed laser diode output
1540 1542 1544 1546 1548 1550 1552 1554 1556 1558 1560-70
-65
-60
-55
-50
-45
-40
-35
-30
-25
Wavelength (nm)
Spe
ctra
l int
ensi
ty (d
Bm
)
𝑃𝑃𝑝𝑝𝑝𝑝𝑔𝑔𝑝𝑝 = 2𝑙𝑙𝑛𝑛2
𝜋𝜋𝑃𝑃𝑎𝑎𝑎𝑎𝑎𝑎
𝑡𝑡𝑅𝑅𝐹𝐹𝐹𝐹𝐹𝐹𝑓𝑓𝑅𝑅𝑅𝑅
𝐸𝐸𝑝𝑝 = 𝑃𝑃𝑎𝑎𝑎𝑎𝑎𝑎𝑓𝑓𝑅𝑅𝑅𝑅
, (3.5)
41
3.5.3 Measurements of Tunable Output
Simulation results show that the repetition rate and the pulse FWHM of the driver circuit’s
output current pulses can be tuned by adjusting control voltages. In measurement, the control
voltages Vctr, Vvar, Vb and Vvar2 can be adjusted by adjusting the potentiometers on the PCB. The
repetition rate of optical output pulses is related with the control voltages Vctr and Vvar, while the
pulse width is related with control voltages Vb and Vvar2. Figure 3.14 shows measured results of
tuning the optical output pulses’ repetition rate by adjusting the control voltage Vvar and Vctr. When
the Vvar is adjusted from -2.5 V to -0.5 V, the output pulses’ repetition rate is increased from
5.6 MHz to 10.4 MHz, as shown in Figure 3.14 (a). This measured result agrees with the simulation
result. When the Vctr is adjusted from -1.5 V to 0.9 V, the repetition rate is increased from 5.6 MHz
to 12.6 MHz, as shown in Figure 3.14 (b). However, when the Vctr is above -0.9 V, the amplitude
of the optical pulses starts decreasing. The maximum repetition rate with a stable output
performance is 13.2 MHz, when the Vctr is -0.9 V and Vvar is -2.1 V. Thus, the tuning range of the
repetition rate is from 5.6 MHz to 13.2 MHz.
Figure 3.15 shows measured results of tuning the pulse width of optical output pulses by
adjusting control voltages Vb and Vvar2. The minimum pulse width occurs when Vvar2 equals 2.1 V
and Vb equals 2.5 V. The tuning range of the pulse width is not wide in measurement. When the
pulse width is increased to above 350 ps, ultrashort pulse trains appear next to the main pulse, and
the output characteristics become unclear.
42
Figure 3.14: Measured results of tuning the optical output pulses’ repetition rate (a) by adjusting control voltage Vvar and (b) by adjusting control voltage Vctr
Figure 3.15: Measured results of tuning the optical output pulses’ pulse width (a) by adjusting control voltage Vvar2 and (b) by adjusting control voltage Vb
-2.5 -2 -1.5 -1 -0.55.5
6
6.5
7
7.5
8
8.5
9
9.5
10
10.5
Control Voltage Vvar (V)
Rep
etitio
n R
ate
(MH
z)
-1.6 -1.4 -1.2 -1 -0.85
6
7
8
9
10
11
12
13
Control Voltage Vctr (V)R
epet
ition
Rat
e (M
Hz)
(a) (b)
-2.5 -2 -1.5180
200
220
240
260
280
300
320
Control Voltage Vvar2 (V)
Pul
se W
idth
FW
HM
(ps)
-2.6 -2.4 -2.2 -2 -1.8200
205
210
215
220
225
230
235
Control Voltage Vb (V)
Pul
se W
idth
FW
HM
(ps)
(a) (b)
43
3.5.4 Analysis
There are certain differences between measurement and simulation in this design. First of
all, the supply voltage (VSS_33) in the measurement setup is higher than in the simulation.
Reasons for this difference could be that the laser diode equivalent circuit model is not accurate
enough or circuit connections in the measurement suffer the power transmission/reflection loss.
Although the supply voltage (3.65 V) is higher than the thick oxide transistor’s breakdown voltage
(3.6 V), it does not cause breakdown in the transistor due to the laser diode load circuit that is
connected with the transistor in series. The laser diode load circuit acts like a voltage dividing
circuit and also limits the in-rush current to the transistor when being powered on.
Secondly, the measured laser pulse width is very close to the simulation current pulse width.
Theoretically, the measured laser pulse width should be smaller than the simulated current pulse
width due to the gain-switching mechanism. However, as the parasitic capacitance from the die
package, the PCB and the laser diode causes a delay on the pulse rise time and fall time, the
switching current pulse width gets larger than the simulated one. A combination of these two
effects makes the measured pulse width close to the simulation current pulse width.
44
3.6 Summary
This work presents a compact picosecond pulsed laser source by gain-switching a DFB laser
diode with a CMOS integrated circuit. The CMOS circuit has a 0.3 mm2 die area. A commercial
DFB laser diode is assembled with the CMOS driver circuit and measurements on the laser light
output have been conducted. A performance summary of this laser source is given in Table 3.1.
This compact picosecond pulsed laser source can be used in SCG as the seed laser source.
Table 3.1: Performance summary of the designed pulsed laser diode
Parameter Value Parameter Value
Pulse Width 200 ps (Minimum) Pulse Energy 1.25 pJ
Repetition Rate 5.6 MHz (Minimum) Duty cycle 1:893
Average Power 7.0 µW Center Wavelength 1548.1 nm
Peak Power 5.9 mW Size (L · W) 7 cm · 4 cm
45
Chapter 4: Supercontinuum Generation in a Highly Nonlinear Fiber Using CMOS Laser Diode Drivers
4.1 Introduction and Objectives
In general, s supercontinuum generation (SCG) system consist of two parts: a laser and a
nonlinear medium. The method of SCG in a highly nonlinear fiber (HNLF) employed in this
research is shown in Figure 4.1. It consists of a gain-switched laser diode, a fiber amplifier and an
HNLF. The combination of a low power pulsed laser diode and a fiber amplifier is considered as
a simple, efficient and cost-effective solution to establish a high power pulsed laser for SCG.
CMOS drivers designed in Chapters 2 and 3 can be used to reduce the system complexity,
size and cost. The picosecond pulsed distributed feedback (DFB) laser diode using the CMOS
driver designed in Chapter 3 can be applied as a seed pulsed laser in SCG. As the peak power of
optical pulses from the laser diode is not high enough to efficiently induce the nonlinear effects in
the HNLF, an erbium-doped fiber amplifier (EDFA) is employed to boost the peak power. The
pump laser diode with the high current continuous-wave (CW) CMOS driver described in
Chapter 2 can be utilized to pump an erbium-doped fiber in the EDFA. The amplified optical
pulses will be launched into the HNLF for SCG.
The objectives of the research described in this chapter are to design a SCG system using
CMOS laser diode drivers and to compare the performance with a reference SCG system using a
commercially-available nanosecond seed laser module.
Figure 4.1: Supercontinuum generation system design block diagram
HNLF
Gain-switched Laser Diode Fiber Amplifier
Supercontinuum Output
46
4.2 Erbium-Doped Fiber Amplifier
EDFAs are widely used in the optical fiber communications systems, as they can efficiently
amplify light in the 1.55 µm wavelength region (the conventional band), which has the lowest fiber
attenuation. EDFAs have been employed in the design of SCG for pulse power amplification [17].
The erbium-doped fiber is the core technology of EDFAs. The mechanism of light
amplification in EDFAs is illustrated based on an energy-level diagram of erbium ions, as shown
in Figure 4.2 [46]. The 980 nm pump laser light excites the erbium ions to a short-lifetime excited
state 4I11/2 where they make a fast and non-radiative decay to a long-lifetime metastable state 4I13/2.
From there erbium ions undergo spontaneous emission and stimulated emission to the lower-
energy 4I15/2 state. The input light with a wavelength of 1550 nm is amplified by the stimulated
emission, while noise light with wavelengths spanning range from 1520 nm to 1570 nm is
amplified by spontaneous emission.
4I15/2 : low energy state
Fast Decay
980 nm Pump
~1550 nm Input
4I13/2 : metastable state
4I11/2 : short-lifetime state
~1550 nm Amplified output
Stimulated emission1520 nm – 1570 nmspontaneous emission
Figure 4.2: EDFA amplification mechanism based on an energy-level diagram of erbium ions
47
The EDFA employed in this SCG system is a commercial module from Opeak
(EDFA131175). A block diagram of the EDFA module setup is shown in Figure 4.3. The core is
a single mode erbium-doped optical fiber. The fiber is pumped by light from two laser diodes
through couplers. The pumping light is input in two opposite directions such that one travels in the
forward direction and the other one travels in the backward direction with respect to the input. This
bidirectional configuration has a better noise figure, compared to the unidirectional configuration
[47]. The optical isolators are incorporated at the input and the output of this EDFA to prevent the
amplification of back reflected light [48]. The pump light from laser diodes has a wavelength
around 980 nm. Laser drivers are used to control the laser diode output.
Figure 4.3: Block diagram of the EDFA module setup
Apart from the EDFA, the commercial module also contains photodiodes and an optical
splitter for monitoring the optical power. The monitor output from the optical splitter is calibrated
as 1:10000 power ratio to the EDFA output. By measuring the monitor output, the characteristics
of the EDFA output can be studied without direct measurement. Using this approach, one can
avoid damage on equipment caused by the high peak power output from the EDFA.
Pump Laser Diode
CW Laser Driver
Erbium DopedFiber
Input Output
Pump Laser Diode
CW Laser Driver
IsolatorCouplerIsolator Coupler
Photodiode
Splitter
Monitor
Photodiode
48
The pulsed laser driven by the CMOS circuit designed in Chapter 3 is connected to the EDFA
through fiber connectors. After the input laser light is amplified, the monitor output light is
measured by an optical spectrum analyzer (OSA, Yokogawa AQ6375). The output spectrum is
shown in Figure 4.4. The average power is measured to be 6.8 µW. Since this power has a 1:10000
ratio to the EDFA output power, the amplified output power from the EDFA is expected to be
68 mW. The measured average power of the input light is 7.0 µW shown in Figure 3.12, so the
amplification gain of this EDFA is 40 dB, which matches the maximum gain listed on the device
specification sheet. The spectral width gets broadened from 1 nm to 40 nm at the -60 dBm level.
This is expected due to the amplified spontaneous emission (ASE) noise [46].
Figure 4.4: Spectrum of the output light from the EDFA monitor output
1520 1530 1540 1550 1560 1570 1580-70
-65
-60
-55
-50
-45
-40
-35
-30
-25
Wavelength (nm)
Spe
ctra
l int
ensi
ty (d
Bm
)
49
4.3 Highly Nonlinear Fiber
The HNLF has a small core and high refractive index difference between the core and the
cladding. It combines a high nonlinear coefficient with small group velocity dispersion around the
desired pumping wavelength. When high peak power laser pulses propagate through this fiber,
many nonlinear effects occur and lead to the broadening of the spectrum of the laser pulse [49].
The HNLF is a suitable nonlinear medium for fiber-based SCG. Compared to standard
dispersion-shifted fibers, the HNLF exhibits both a higher nonlinearity (5 times higher) and a lower
dispersion slope that are main factors for broadband and flat SCG [50]. Besides, the HNLF has an
advantage of a low coupling loss (typical 0.1 dB) with standard single-mode fibers (SMF) and ease
of processing, compared to popular photonic crystal fibers and tapered fibers. The detailed optical
properties of the HNLF (OFS Standard HNLF) used in this SCG system are shown in Table 4.1.
Table 4.1: Optical properties of the HNLF
Parameter Value
Dispersion @ 1550 nm -0.01 ps/(nm·km)
Dispersion Slope @ 1550 nm 0.018 ps/(nm2·km)
Zero Dispersion Wavelength 1549.5 nm
Nonlinear coefficient 11.4 (W·km)-1
Fiber Attenuation @ 1550 nm 0.72 dB/km
Total Insertion Loss @ 1550 nm 0.31 dB
Splice Loss to Standard SMF Pigtail (Typical) 0.1 dB
Effective Area (Typical) 11.6 µm²
50
4.4 Nonlinear Optical Effects
SCG in HNLFs involves the interplay between nonlinear and linear effects that occur during
the propagation of picosecond optical pulses in the fiber [7]. The major linear effect that plays a
crucial role in influencing the interactions between various nonlinear effects is chromatic
dispersion. The dominant nonlinear effects behind the process of the SCG in HNLFs with
picosecond laser pulses are expected to bet stimulated Raman scattering (SRS), four-wave mixing
(FWM) and modulation instability (MI) [51]. The following sections briefly introduce these effects
in the context of SCG.
4.4.1 Chromatic Dispersion
Chromatic dispersion is the effect that causes propagating light pulses to spread in time
domain. Chromatic dispersion arises from the frequency dependence of the refractive index of the
fiber. In general, when optical light is propagating in the fiber, the response of the fiber depends
on the optical frequency of the light. This characteristic is crucial to the performance of ultra-short
pulses with a wide wavelength range propagating in fibers. Ultra-short pulses consist of a broad
spectrum of frequencies that all experience a different refractive index in a fiber. After a certain
propagation distance, ultra-short pulses get broadened in the time domain. In other words, the
temporal pulse width of ultra-short pulses is increased. The peak power of the pulse is reduced and
thus the nonlinearity is reduced. As both the pulse peak power and the nonlinearity are important
factors for SCG, it is necessary to reduce the impact of chromatic dispersion. In practical, HNLFs
with a small dispersion slope are preferred and the wavelength of ultrashort pulses launched into
HNLFs is usually chosen to be close to the zero dispersion wavelength (ZDW) of HNLFs.
51
4.4.2 Stimulated Raman Scattering
Raman scattering is an interaction between photons of optical pulses and the silica molecules
of fibers. During the interaction, photons transfer a small fraction of energy to molecules as a result
of molecular vibrational motion and low-frequency photons are scattered. These new waves
formed by generated low-frequency photons are called Stoke waves [52]. Spontaneous Raman
scattering is a weak effect. Highly efficient Raman scattering can occur under the influence of a
high intensity pump laser beam, as the Raman gain of the pump laser beam amplifies the low-
frequency photons. This effect is called SRS. The overall nonlinear consequence of SRS is that
the spectrum of optical pulses extends to long wavelength side.
4.4.3 Four-Wave Mixing
FWM is also created by the nonlinear response of the fiber medium. However, the fiber
medium only plays a passive role in FWM, which is different with SRS. When three optical waves
with three different frequencies ω1, ω2 and ω3 interact nonlinearly inside the fiber medium, each
wave sets up a refractive index modulation of the material. If any two waves or three waves are
propagating together with phase matching, the new wave will be generated with a new frequency
given by ω4 = ω1 + ω2 + ω3. The net total energy and momentum should be conserved during the
creation of new frequencies.
4.4.4 Modulation Instability
MI includes the nonlinear and dispersive effects causing a nonlinear system to exhibit an
instability, which results in the breakup of the optical pulse envelope into a series of ultra-short
pulses and the generation of new wavelength side bands [52]. When a pump wave propagates in
a fiber and experiences anomalous dispersion, the nonlinear system including the pump wave and
the nonlinear medium becomes unstable. The main requirement is the anomalous dispersion,
52
which occurs when the wavelength of the pump wave is greater than the zero dispersion
wavelength of the nonlinear medium.
The MI effect produces a symmetric gain spectrum around the center wavelength of the
pump wave. When noise within the gain spectrum is amplified, two sidebands are formed and are
symmetric about the center wavelength. The frequency shift of the first-order MI gain spectral
peak is given by Δω = (2·γ·Pp/|β2|)1/2 [52], where γ is the nonlinear coefficient, Pp is the peak power
of the pump and β2 is the group velocity dispersion parameter of the fiber at the pump wavelength.
The wavelength shift on the spectrum can be further calculated based on the frequency shift
equation. By comparing the calculated wavelength shift to the measured spectrum shape, one can
identify whether the MI is one of the nonlinear effects, which contributes to the SCG.
In the context of SCG, FWM/MI processes dominate the broadening mechanisms in the case
of input pulse width within picosecond-nanosecond range [7]. Efficient spectral broadening
requires the pumping wavelength to be at anomalous dispersion region and close to the fiber ZDW.
The anomalous dispersion induces the MI effect to occur.
4.5 Supercontinuum Generation
4.5.1 Experimental Setup
An experimental setup diagram of the SCG system is shown in Figure 4.5. The system is
composed of a CMOS driver circuit, a DFB laser diode, an EDFA, a 1-meter-long SMF, and a 10-
meter-long HNLF. Fiber connectors (FC) are utilized to connect the DFB laser diode with the
EDFA. Fusion splicing (FS) of optical fibers is also used to make fiber connections in the setup.
The 1-meter-long SMF is placed between the EDFA and the HNLF in order to reduce the fiber
coupling loss. The DFB laser diode is gain-switched by the CMOS driver. The laser output is
optical pulses with a pulse width of 200 ps, a repetition rate of 5.6 MHz, a pulse energy of 1.25 pJ
53
and a peak power of 5.9 mW. This output is amplified by the EDFA, in which an erbium-doped
fiber is pumped by two 980 nm laser diodes at the total 550 mA maximum injection current. The
EDFA monitor output spectrum is shown in Figure 4.4 and its output pulses have a 68 mW average
power. Optical characteristics of the amplified DFB laser diode pulses are listed in Table 4.2.
Table 4.2: Characteristics of the amplified laser diode pulses
The EDFA output is connected to a SMF, which is a bridge between the erbium-doped fiber
and the HNLF. The SCG occurs once the high energy laser light is launched into the HNLF. The
supercontinuum output is analyzed by the OSA with a spectral measurement wavelength range
from 1200 nm to 2400 nm, an InGaAs photodiode and an oscilloscope. The OSA is utilized to
measure the output spectrum and the output average power. The photodiode and the oscilloscope
are employed to record the pulse characteristics in the time domain.
Figure 4.5: Experimental setup diagram of the SCG system
10 mHNLF
DFB Laser Diode
CMOS Driver
EDFA
1 mSMF
FC FCFS FS
Monitor FCOutput
FS
Parameter Value Parameter Value
Pulse Width 200 ps Pulse Energy 12.1 nJ
Repetition Rate 5.6 MHz Peak Power 57.3 W
Average Power 68 mW Center Wavelength 1548.1 nm
54
4.5.2 Measured Results and Discussions
Figure 4.6 shows the optical spectrum of the SCG system output recorded by the OSA with
0.5 nm scan resolution and 20000 sampling points. As the spectrum noise floor of the OSA is
around -66 dBm, the broadband spectrum of supercontinuum covers the whole spectral range of
the OSA when measured from the noise level. The spectral bandwidth of the supercontinuum light
is 806 nm, spanning from 1298 nm to 2104 nm at the -40 dB level. It has a flat spectrum of 10 dB
across the wavelength range from 1298 nm to 1517 nm and 1576 nm to 2104 nm. The spectral
peak around 1548 nm is due to the seed laser pumping. The average power of this supercontinuum
light is 62 mW, achieving a 91.2% power conversion efficiency from the injection pump power to
the recorded supercontinuum power. The power loss is due to fiber connection loss and fiber
attenuation.
Figure 4.6: Spectrum of supercontinuum output
1200 1400 1600 1800 2000 2200 2400-60
-50
-40
-30
-20
-10
0
10
20
Wavelength (nm)
Spe
ctra
l int
ensi
ty (d
Bm
)
10 dB
BW=806 nm
55
For picosecond pulses, and when pumping close to the zero dispersion wavelength region of
the HNLF, the SCG is initiated by the MI and the FWM [51], [7], [21]. The MI is observed in the
spectrum with the generation of modulation sidebands located around the pump wavelength. To
confirm that the supercontinuum is initiated by the MI, the spectrum broadening as a function of
the pump power is measured as shown in Figure 4.7. Spectral sidebands around the pump
wavelength are observed during the early evolution of the supercontinuum at low pump power.
When increasing the pump power, the peak power is correspondingly increased so that the
supercontinuum spectrum expands to both sides around the pump wavelength. The red shift is
caused by the SRS of solitons, which are created by the MI. The blue shift is due to a FWM
nonlinear process between solitons and dispersive waves that are also created by the MI [53]. The
red shift side expands faster than the blue shift side.
Figure 4.7: Supercontinuum output evolution at different pump power level
1200 1400 1600 1800 2000 2200 2400-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
Wavelength (nm)
Spe
ctra
l int
ensi
ty (d
Bm
)
4.8 mW Supercontinuum11.6 mW Supercontinuum18.2 mW Supercontinuum31.0 mW Supercontinuum42.9 mW Supercontinuum53.9 mW Supercontinuum62.3 mW Supercontinuum
Pump Peak1st order Sideband
1st orderSideband
2nd orderSideband
2nd orderSideband
56
The attenuated (~40 dB attenuation) supercontinuum pulse is detected by a 5 GHz InGaAs
photodiode and recorded by an 8 GHz oscilloscope. The repetition rate of supercontinuum pulses
remains the same (5.6 MHz) as the pump pulses. A temporal pulse profile of the supercontinuum
pulses is shown in Figure 4.8 with a seed laser pulse and an EDFA monitor pulse. The pulse profile
corresponds to the full bandwidth of the supercontinuum spectrum incident on the photodiode.
Compared to the seed laser pulse and the EDFA monitor pulse, the pulse width of the
supercontinuum pulse is increased around 10 ps due to the dispersion in the HNLF. In general, the
wavelength shifting does not increase the input pulse duration substantially. In other words, the
supercontinuum generation process does not affect the temporal pulse shape.
Figure 4.8: Temporal profile of the seed laser pulse, the EDFA monitor pulse and the supercontinuum pulse
0 200 400 600 800 1000 1200 1400
0
0.2
0.4
0.6
0.8
1
Time (ps)
Nor
mal
ized
Vol
tage
(V)
Seed Laser PulseEDFA Monitor PulseSupercontinuum Pulse
57
4.5.3 System Performance Comparison
For comparison, a reference SCG system using a commercially available nanosecond seed
laser module produced by Opeak (DFB131101) is tested for SCG. The setup of the reference SCG
system is the same as with Figure 4.5, except that the reference SCG system uses a seed laser
module to replace the DFB laser diode and the CMOS driver circuit. The seed laser module
contains a 1550.8 nm DFB laser diode and an electronic control circuit. The module is able to
output optical pulses with 10 ns pulse width (shortest available) and 10 kHz repetition rate under
a 5 V power supply. The temporal waveform characteristics are shown in Figure 4.9. The spectrum
characteristics of the output pulses are shown in Figure 4.10 (a). The average output power of this
seed laser module is 1.1 µW. The performance is summarized in Table 4.3.
Table 4.3: Performance summary of the seed laser module
Parameter Value Parameter Value
Pulse Width 10 ns Pulse Energy 110 pJ
Repetition Rate 10 kHz Duty Cycle 1:10000
Average Power 1.1 µW Center Wavelength 1550.8 nm
Peak Power 10.3 mW Size (L · W) 14 cm · 9 cm
58
Figure 4.9: Output optical pulses from the commercial seed laser module: the left figure shows the pulse waveform; the right figure shows a single pulse shape.
Figure 4.10: (a) Spectrum of optical pulses output from the seed laser module; (b) spectrum of the amplified pulses output from the monitor of EDFA
-100 0 100 200-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Time (µs)
Volta
ge (V
)
99.99 100 100.01 100.02 100.03-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Time (µs)
Volta
ge (V
)
1550 1550.5 1551 1551.5-70
-65
-60
-55
-50
-45
-40
-35
-30
Wavelength (nm)
Spec
tral in
tens
ity (d
Bm)
1520 1540 1560 1580-70
-65
-60
-55
-50
-45
-40
-35
-30
Wavelength (nm)
Spec
tral in
tens
ity (d
Bm)
59
Compared to the designed DFB laser diode with CMOS driver circuit, this seed laser module
has a lower repetition rate and much higher pulse energy. The designed DFB laser diode has the
advantage of much shorter pulse width over the commercial seed laser.
The seed laser pulses are amplified by the EDFA with the maximum pump power. The
spectrum of the monitor output from the EDFA is shown in Figure 4.10(b). As the average output
power of the monitor port is 5.9 µW, the pump pulses launched into the HNLF have an expected
average power of 59 mW, a pulse energy of 5.9 µJ, and a peak power of 554 W. Although these
pump pulses have almost the same average power as the designed DFB laser diode after
amplification, the peak power of this laser module is 9.7 times higher than the designed one. The
difference of the duty cycle between these two designs results in the peak power difference.
The spectrum at the HNLF output is measured by the OSA, shown as a red trace in
Figure 4.11. The average power of this supercontinuum is 51 mW, achieving 86.4% power
conversion efficiency. The spectrum bandwidth is 892 nm at –40 dB level, spanning from
1258 nm to 2150 nm. A 5 dB spectrum flatness covers almost the full bandwidth except the pump
wavelength region. The nonlinear mechanism leading to the SCG is similar with the picosecond
pulses pumping regime. The wavelength shifting is also initiated by the MI and enhanced by the
SRS and the FWM effects.
Compared to the reference SCG system, the SCG system using the designed DFB laser diode
has a similar supercontinuum spectrum but a high peak at the pump wavelength. The high peak
reflects that wavelength shifting is not efficient enough due to a low peak power of pump pulses.
60
Figure 4.11: SCG results from two different systems
4.6 Summary
A SCG system based on an HNLF pumped by amplified diode-laser pulses is demonstrated.
The designed DFB laser diode with the CMOS driver circuit is employed to provide diode-laser
pulses. The SCG achieves a flat supercontinuum spectrum with 806 nm bandwidth (-40 dB level)
at a low peak power and a 91.2% power conversion efficiency at 62 mW output power. For
comparison, the designed DFB laser diode is replaced by a commercially available seed laser
module in the SCG system. A comparison table between the designed SCG system and the
reference SCG system is shown in Table 4.4.
1200 1400 1600 1800 2000 2200 2400-60
-50
-40
-30
-20
-10
0
10
20
Wavelength (nm)
Spe
ctra
l int
ensi
ty (d
Bm
)
Supercontinuum with proposed DFB LaserSupercontinuum with seed laser module
51 mW 62 mW
61
This comparison shows that the designed DFB laser diode is a good candidate for the seed
laser in picosecond level SCG systems. Additionally, the designed DFB laser diode is attractive
due to reduced complexity, compact size and low cost compared to commercial seed laser modules.
Table 4.4: Comparison of the designed SCG system and the reference SCG system
Supercontinuum Parameter The Designed SCG System The Reference SCG System
Average Power 62 mW 51 mW
Spectrum Bandwidth (-40 dB) 806 nm 892 nm
Spectral Density 0.08 mW/nm 0.05 mW/nm
Pulse Duration ~200 ps ~ 10 ns
Repetition Rate 5.6 MHz 10 kHz
Pulse Energy 11.1 nJ 5.1 µJ
62
Chapter 5: Conclusions and Future Work
5.1 Conclusions
This thesis provides experimental evidence to support that CMOS laser diode drivers can be
used in supercontinuum generation (SCG) systems. SCG systems with CMOS laser diode drivers
have the advantage of compact size and low cost compared to the traditional fiber lasers and the
mode-locked lasers based SCG systems. In this thesis, two different design of CMOS laser diode
drivers are presented.
In Chapter 2, a high-current continuous-wave (CW) CMOS laser diode driver was designed
for driving pump lasers in SCG systems. Based on the design requirements, a circuit design
methodology was proposed. Circuit simulations were conducted in Cadence Design System (CDS)
to predict the driver’s performance. Experimental measurements on both the driver’s current
output and the pump laser’s optical output confirmed that the driver could provide up to 600 mA,
which corresponded to an optical output power up to 350 mW from a 975 nm laser diode. The
driver features tunable output, long-term stable operation and reduced sensitivity to power supply
voltage variations.
In Chapter 3, a picosecond pulsed laser diode driver was designed for gain-switching a
distributed feedback (DFB) laser diode. The driver design implemented CMOS analog logic
circuits to generate picosecond level current pulses. Simulations showed that the driver circuit can
provide current pulses with a pulse width of 200 ps, a repetition rate of 5.8 MHz, and a peak current
of 80 mA. Experimental measurements of the DFB laser diode output showed that the output
optical pulses had a pulse width of 200 ps at FWHM, a peak power of 5.9 mW and a repetition
rate of 5.6 MHz. The average output power of the pulsed DFB laser diode is 7.0 µW and the pulse
energy is 1.25 pJ. The driver design also features a tunable repetition rate output and a tunable
63
pulse width output. The size of the designed package, including the laser and the driver together,
is 7 cm length and 4 cm width. This compact picosecond pulsed laser diode is a suitable seed laser
for SCG systems.
Chapter 4 demonstrated an HNLF based SCG system with the designed picosecond DFB
laser diode. The supercontinuum was generated by using amplified picosecond optical pulses to
induce nonlinear effects in the HNLF. The system consists of the designed picosecond DFB laser
diode, a commercial EDFA module and the HNLF. The supercontinuum output of the designed
system has an average power of 62 mW, a spectral bandwidth of 806 nm, a pulse width of 210 ps
and a repetition rate of 5.6 MHz. For comparison, a reference SCG system using a nanosecond
pulsed commercial seed laser module was tested for supercontinuum generation. With the
commercial seed laser module, the supercontinuum output has a spectral bandwidth of 892 nm and
an average output power of 51 mW. Compared to this commercial module, the designed DFB laser
diode has the advantages of compacter size, lower cost, higher output power and shorter pulse
width. The designed SCG system has potential applications in OCT, spectroscopy and DWDM.
5.2 Future Work
Future work will be focusing on improving performance of the CMOS driver circuits and
developing a compact package for the SCG system.
In the design of the CW CMOS laser diode driver circuit, the CMOS chip size, the
potentiometer setup and the PCB assembly can be further optimized. The CMOS chip size can be
reduced by fabricating a new chip with all circuit parts integrated together. The number of
potentiometers on the PCB board can be reduced to one if the CMOS circuit is fully integrated.
The driver circuit and the laser diode can be assembled into one host PCB to make them a more
compact optoelectronic device.
64
The output of the designed picosecond pulsed DFB laser diode is sensitive to the noise from
the power supply connectors and the surroundings. A new PCB with an improved circuit grounding
and noise filtering setup can be designed to improve the circuit stability performance.
The designed SCG system uses a commercial EDFA module, which meets the requirement
for the purpose of pulse amplification. However, the module is not cost-effective and is difficult
to be integrated with other components in the system. It is necessary to develop a custom-made
EDFA with the designed pump laser. A custom-made EDFA will reduce the cost and size and also
increase the system flexibility.
The designed SCG system has not been fully packaged as a device. The largest discrete
component in this system is the 10-meter-long HNLF, which has a circular shape with an 8 cm
diameter. The device package will be built around the HNLF. The expected full package dimension
will be 10 cm length, 10 cm width and 5 cm height with a power supply input port and a
supercontinuum output port.
Silicon photonics technology has been an intense research interest recently, as it allows
optical devices to be made using standard semiconductor fabrication. This technology provides a
new platform for integration between the optical devices and CMOS circuits. It has already been
approved that the CMOS-compatible silicon wire waveguides can be used as a nonlinear medium
for SCG [18]. This report shows the opportunity for designed CMOS laser diode driver circuits.
In the future, these CMOS circuits can be integrated with silicon waveguides on the same platform
as lab-on-a-chip devices. This will be a revolutionary improvement on the design of compact and
low-cost SCG systems.
65
References
[1] R. R. Alfano and S. L. Shapiro, “Emission in the region 4000 to 7000 via four-photon
coupling in glass,” Phys. Rev. Lett., vol. 24, no. 11, pp. 584–587, 1970.
[2] R. R. Alfano, The supercontinuum laser source (Second Edition): Fundamentals with
updated references. Springer, 2006.
[3] P. B. Corkum, C. Rolland, and T. Srinivasan-Rao, “Supercontinuum generation in gases,”
Phys. Rev. Lett., vol. 57, no. 18, pp. 2268–2271, 1986.
[4] C. Lin and R. H. Stolen, “New nanosecond continuum for excited-state spectroscopy,” Appl.
Phys. Lett., vol. 28, no. 4, pp. 216–218, 1976.
[5] R. H. Stolen, C. Lee, and R. K. Jain, “Development of the stimulated Raman spectrum in
single-mode silica fibers,” J. Opt. Soc. Am. B, vol. 1, no. 4, p. 652, 1984.
[6] I. Ilev, H. Kumagai, K. Toyoda, and I. Koprinkov, “Highly efficient wideband continuum
generation in a single-mode optical fiber by powerful broadband laser pumping.,” Appl.
Opt., vol. 35, no. 15, pp. 2548–53, 1996.
[7] J. M. Dudley and J. R. Taylor, Supercontinuum generation in optical fibers. Cambridge
University Press, 2010.
[8] J. K. Ranka, R. S. Windeler, and a J. Stentz, “Visible continuum generation in air-silica
microstructure optical fibers with anomalous dispersion at 800 nm.,” Opt. Lett., vol. 25, no.
1, pp. 25–27, 2000.
[9] T. a Birks, W. J. Wadsworth, and P. S. Russell, “Supercontinuum generation in tapered
fibers.,” Opt. Lett., vol. 25, no. 19, pp. 1415–1417, 2000.
[10] N. Nishizawa and T. Goto, “Widely broadened super continuum generation using highly
nonlinear dispersion shifted fibers and femtosecond fiber laser,” Japanese J. Appl. Physics,
66
Part 2 Lett., vol. 40, no. 4 B, 2001.
[11] H. Takara, T. Ohara, K. Mori, K. Sato, E. Yamada, Y. Inoue, T. Shibata, M. Abe, T.
Morioka, and K.-I. Sato, “More than 1000 channel optical frequency chain generation from
single supercontinuum source with 12.5 GHz channel spacing,” Electron. Lett., vol. 36, no.
25, p. 2089, 2000.
[12] T. Udem, R. Holzwarth, and T. W. Hänsch, “Optical frequency metrology,” Nature, vol.
416, no. 6877, pp. 233–237, 2002.
[13] S. Diddams, D. Jones, J. Ye, S. Cundiff, J. Hall, J. Ranka, R. Windeler, R. Holzwarth, T.
Udem, and T. Hänsch, “Direct Link between Microwave and Optical Frequencies with a
300 THz Femtosecond Laser Comb,” Phys. Rev. Lett., vol. 84, no. 22, pp. 5102–5105, 2000.
[14] I. Hartl, X. D. Li, C. Chudoba, R. K. Ghanta, T. H. Ko, J. G. Fujimoto, J. K. Ranka, and R.
S. Windeler, “Ultrahigh-resolution optical coherence tomography using continuum
generation in an air-silica microstructure optical fiber.,” Opt. Lett., vol. 26, no. 9, pp. 608–
610, 2001.
[15] K. Isobe, A. Suda, M. Tanaka, H. Hashimoto, F. Kannari, H. Kawano, H. Mizuno, A.
Miyawaki, and K. Midorikawa, “Single-pulse coherent anti-Stokes Raman scattering
microscopy employing an octave spanning pulse.,” Opt. Express, vol. 17, no. 14, pp. 11259–
11266, 2009.
[16] J. Clowes, “Next Generation Light Sources for Biomedical Applications,” Opt. Photonik,
vol. 3, no. 1, pp. 36–38, 2008.
[17] S. Moon and D. Y. Kim, “Generation of octave-spanning supercontinuum with 1550-nm
amplified diode-laser pulses and a dispersion-shifted fiber.,” Opt. Express, vol. 14, no. 1,
pp. 270–278, 2006.
67
[18] R. Halir, Y. Okawachi, J. S. Levy, M. A. Foster, M. Lipson, and A. L. Gaeta,
“Ultrabroadband supercontinuum generation in a CMOS-compatible platform.,” Opt. Lett.,
vol. 37, no. 10, pp. 1685–7, May 2012.
[19] Y. Takushima, F. Futami, and K. Kikuchi, “Generation of over 140-nm-wide super-
continuum from a normal dispersion fiber by using a mode-locked semiconductor laser
source,” IEEE Photonics Technol. Lett., vol. 10, no. 11, pp. 1560–1562, 1998.
[20] S. V. Chernikov, Y. Zhu, J. R. Taylor, and V. P. Gapontsev, “Supercontinuum self-Q-
switched ytterbium fiber laser,” Opt. Lett., vol. 22, no. 5, p. 298, 1997.
[21] C. Larsen, D. Noordegraaf, P. M. W. Skovgaard, K. P. Hansen, K. E. Mattsson, and O.
Bang, “Gain-switched CW fiber laser for improved supercontinuum generation in a PCF.,”
Opt. Express, vol. 19, no. 16, pp. 14883–91, 2011.
[22] C. Guo, S. Ruan, P. Yan, E. Pan, and H. Wei, “Flat supercontinuum generation in cascaded
fibers pumped by a continuous wave laser.,” Opt. Express, vol. 18, no. 11, pp. 11046–
11051, 2010.
[23] M. E. Fermann, “Passive mode locking by using nonlinear polarization evolution in a
polarization-maintaining erbium-doped fiber.,” Opt. Lett., vol. 18, no. 11, p. 894, 1993.
[24] T. V. Andersen, P. Pérez-Millán, S. R. Keiding, S. Agger, R. Duchowicz, and M. V. Andrés,
“All-fiber actively Q-switched Yb-doped laser,” Opt. Commun., vol. 260, no. 1, pp. 251–
256, 2006.
[25] A. S. Kurkov, “Q-switched all-fiber lasers with saturable absorbers,” Laser Physics Letters,
vol. 8, no. 5. pp. 335–342, 2011.
[26] T. Morioka, K. Mori, S. Kawanishi, and M. Saruwatari, “Multi-WDM-channel, Gbit/s pulse
generation from a single laser source utilizing LD-pumped supercontinuum in optical
68
fibers,” IEEE Photonics Technol. Lett., vol. 6, no. 3, pp. 365–368, Mar. 1994.
[27] B. Razavi, “Prospects of CMOS technology for high-speed optical communication
circuits,” IEEE J. Solid-State Circuits, vol. 37, no. 9, pp. 1135–1145, Sep. 2002.
[28] S. Galal and B. Razavi, “10-Gb/s limiting amplifier and laser/modulator driver in 0.18-
mu;m CMOS technology,” IEEE J. Solid-State Circuits, vol. 38, no. 12, pp. 2138–2146,
Dec. 2003.
[29] C. Xia, M. Kumar, M. Y. Cheng, O. P. Kulkarni, M. N. Islam, A. Galvanauskas, F. L. Terry,
M. J. Freeman, D. A. Nolan, and W. A. Wood, “Supercontinuum generation in silica fibers
by amplified nanosecond laser diode pulses,” IEEE J. Sel. Top. Quantum Electron., vol. 13,
no. 3, pp. 789–796, 2007.
[30] B. Razavi, Design of Integrated Circuits for Optical Communications, 1st ed. McGraw-
Hill, 2003.
[31] R. J. Baker, CMOS Circuit Design, Layout, and Simulation, 3rd ed. Wiley-IEEE Press,
2010.
[32] S. Mandal, S. Arfin, and R. Sarpeshkar, “Fast startup CMOS current references,” in IEEE
International Symposium on Circuits and Systems, 2006, pp. 1–4.
[33] Y. Kusama, Y. Tanushi, M. Yokoyama, R. Kawakami, T. Hibi, Y. Kozawa, T. Nemoto, S.
Sato, and H. Yokoyama, “7-ps optical pulse generation from a 1064-nm gain-switched laser
diode and its application for two-photon microscopy,” Opt. Express, vol. 22, no. 5, pp.
5746–5753, Mar. 2014.
[34] Y. He, Y. Li, and O. Yadid-Pecht, “A compact picosecond pulsed laser source using a fully
integrated CMOS driver circuit,” in SPIE OPTO, 2016, p. 97511B.
[35] J. Swiderski and M. Maciejewska, “Watt-level, all-fiber supercontinuum source based on
69
telecom-grade fiber components,” Appl. Phys. B, vol. 109, no. 1, pp. 177–181, Sep. 2012.
[36] P. Dupriez, A. Piper, A. Malinowski, J. K. Sahu, M. Ibsen, B. C. Thomsen, Y. Jeong, L. M.
B. Hickey, M. N. Zervas, J. Nilsson, and D. J. Richardson, “High average power, high
repetition rate, picosecond pulsed fiber master oscillator power amplifier source seeded by
a gain-switched laser diode at 1060 nm,” IEEE Photonics Technol. Lett., vol. 18, no. 9, pp.
1013–1015, May 2006.
[37] K. Y. Lau, “Gain switching of semiconductor injection lasers,” Appl. Phys. Lett., vol. 52,
no. 4, pp. 257–259, 1988.
[38] L. Abrardi and T. Feurer, “Electronic synchronization of gain-switched laser diode seeded
fiber amplifiers,” in SPIE LASE, 2012, p. 82373W.
[39] H. Liu, C. Gao, J. Tao, W. Zhao, and Y. Wang, “Compact tunable high power picosecond
source based on Yb-doped fiber amplification of gain switch laser diode.,” Opt. Express,
vol. 16, no. 11, pp. 7888–7893, 2008.
[40] N. Retdian, S. Takagi, and N. Fujii, “Voltage controlled ring oscillator with wide tuning
range and fast voltage swing,” in IEEE Asia-Pacific Conference on ASIC, 2002, pp. 201–
204.
[41] X. Zhao, R. Chebli, and M. Sawan, “A wide tuning range voltage-controlled ring oscillator
dedicated to ultrasound transmitter,” in 16th International Conference on Microelectronics,
2004, no. 1, pp. 313–316.
[42] E. F. M. Albuquerque and M. M. Silva, “Current-balanced logic for mixed-signal IC’s,” in
IEEE International Symposium on Circuits and Systems VLSI, 1999, vol. 1, pp. 274–277.
[43] S. S. Mishra, A. K. Agrawal, and R. K. Nagaria, “A comparative performance analysis of
various CMOS design techniques for XOR and XNOR circuits,” Int. J. Emerg. Technol.,
70
vol. 1, no. 1, pp. 1–10, 2010.
[44] J. Nissinen and J. Kostamovaara, “A 1 A laser driver in 0.35 microm complementary metal
oxide semiconductor technology for a pulsed time-of-flight laser rangefinder.,” Rev. Sci.
Instrum., vol. 80, no. 10, p. 104703, Oct. 2009.
[45] M. A. Leigh, “High power pulsed fiber laser sources and their use in terahertz generation,”
University of Arizona, 2008.
[46] G. Keiser, Optical fiber communications. McGraw-Hill, 2003.
[47] M. P. Shukla and A. P. K. P. Kaur, “Performance Analysis of EDFA for different Pumping
Configurations at High Data Rate,” GJRE-F Electr. Electron. Eng., vol. 13, no. 9, 2013.
[48] K.-H. Lin and J.-H. Lin, “Amplification of supercontinuum by semiconductor and Er-doped
fiber optical amplifiers,” Laser Phys. Lett., vol. 5, no. 6, pp. 449–453, 2008.
[49] G. Agrawal, Applications of nonlinear fiber optics. Academic press, 2010.
[50] A. Boucon, A. Fotiadi, P. Mégret, H. Maillotte, and T. Sylvestre, “Low-threshold all-fiber
1000nm supercontinuum source based on highly non-linear fiber,” Opt. Commun., vol. 281,
no. 15–16, pp. 4095–4098, Aug. 2008.
[51] J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal
fiber,” Rev. Mod. Phys., vol. 78, no. 4, pp. 1135–1184, Oct. 2006.
[52] G. Agrawal, Nonlinear Fiber Optics, 5th Edition. Academic Press, 2012.
[53] J. C. Travers, “Blue extension of optical fibre supercontinuum generation,” J. Opt., vol. 12,
no. 11, p. 113001, 2010.
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