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A Mode-Locked Microchip Laser Optical Transmitter for Fiber Radio

Amarildo J. C. Vieira+, Peter R. Herczfeld*, Arye Rosen*, Michael Ermold*, Eric Funk‡, Willi am

D. Jemison§, and Keith Willi ams‡ *Center for Microwave-Lightwave Engineering

Drexel University

32nd & Chestnut Street

Philadelphia, PA 19104 USA

Phone: (215) 895-2914 Fax: (215) 895-4968 §Dept. of ECE, Lafayette College, Easton, PA 18042 USA

†SFA, Inc., Largo, MD +Motorola Broadband Communication Sector, Horsham, PA

‡SFA, Inc., Largo, MD ‡Naval Research Laboratory, Washington, DC

Abstract - This paper is concerned with the optical domain generation and transmission of

high-quality millimeter-wave signals for fiber-radio and other applications. The mode-

locked millimeter-wave optical transmitter described here is based on simple electrooptic

microchip laser technology. The transmitter can be designed to operate from a few GHz to

100 GHz and beyond. The residual phase noise of the laser is below 100 dBc/Hz at 1 kHz

off-set which makes it well suited for optically fed millimeter-wave wireless applications. A

key feature of the transmitter is its simplicity, the very small number of elements it employs

and the high level of integration of the millimeter-wave and photonic components that

results in a small, rugged and reliable package. The paper describes the design, fabrication

and experimental evaluation of the transmitter.

Index Terms- Microchip laser, laser mode-locking, millimeter-wave generation, fiber radio,

optical transmitter.

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I. INTRODUCTION

Given the increasing demand for high-speed communications, there has been growing

interest in developing techniques that can transmit microwave and/or millimeter-waves over

optical fiber. There are both commercial and military applications for this type of technology.

Commercial applications include personal communication systems (PCS) [1-3], and broadband

distribution of interactive multimedia services to the home [4-5]. Examples of military

applications include Doppler radar [6] and phased array antenna [7].

The principal advantages of the transmission of high frequency signals over fiber is low

attenuation and cost when compared to the conventional coaxial cable or radio transmission, and

the large bandwidth even when only part of the available bandwidth is exploited. For example, at

40 GHz, a 10 % bandwidth corresponds to 4 GHz, which is sufficient to allocate 4 times the

amount of bandwidth that cable TV service providers can offer. An additional advantage that

makes millimeter-wave desirable for fiber radio systems is that these frequencies are highly

attenuated by water molecules and oxygen in the atmosphere (i.e. 16 dB/Km at 60 GHz) [1].

This can be exploited to limit signal propagation within the proximity of the cell, which is

essential for wireless secure communication and for frequency reuse.

Typical fiber radio distribution system consists of a millimeter-wave optical transmitter at

the central station and numerous base stations at the picocell sites. Cost and reliabili ty issues

mandate very simple base stations with no local oscill ator or sophisticated signal processing [8].

This implies the need for an optical transmitter that can generate all the necessary optical signals

that result in the generation of high quality millimeter-waves at a photodetector output located in

the base station. Specifically, the detected signal at the base station must include the millimeter-

wave carrier with suitable modulation for wireless transmission as described in reference [8].

There are alternate methods to achieve this, including direct modulation and mode-

locking of semiconductor lasers [1, 9], external modulation [10], laser heterodyning [11], and

subharmonic optical injection locking [12]. However, these techniques usually lack in

performance or are extremely complex and costly. In this paper, a novel optical transmitter

based on a mode-locked Neodymium-doped Lithium Niobate (Nd:LiNbO3) microchip laser is

described.

The mode-locked millimeter-wave optical transmitter (M-MOT) is comprised of two

principal parts, the mode-locked microchip laser and an optical modem, as depicted in Figure 1.

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The mode-locked microchip laser has three components: the microchip laser which is housed in a

millimeter-wave cavity, the semiconductor diode array that ‘pumps’ the laser, and a millimeter-

wave source that locks the phases of the longitudinal laser modes. The output of the mode-locked

microchip laser is a train of pulses, the sum of propagating optical modes that can be represented

by the following relation:

( )ii tjitotal eII

φω −−∑=

where I i is the intensity of the i-th mode, and ωi and φi are its optical frequency and phase. The

frequency difference between the adjacent modes, ∆ω=ωi-ωi-1=ωmm is determined by the free

spectral range, which in turn is dependent on the effective length of the laser cavity. At the

output of the detector the beating of the different modes yields a millimeter-wave signal with the

frequency ωmm. The millimeter-wave locking signal assures that the modes are in phase, φj-φj-

1=0, in order to produce a very low noise millimeter-wave carrier at the detector.

Diodepump

Microchip laser in a millimeter cavity

Millimeter wavesource

Diodepump

Microchip laser in a millimeter cavity

Millimeter-wavesource

Diodepump

Microchip laser in a millimeter cavity

Millimeter wavesource

Diodepump

Microchip laser in a millimeter cavity

Millimeter-wavesource

Mode-locked microchip laser Optical Modem

Information and control signals

Mach-Zehndermodulator

Information and control signals

Mach-Zehndermodulator

Information and control signals

Mach-Zehndermodulator

Information and control signals

Mach-Zehndermodulator

TransmitterOutput

Figure 1. Block diagram of the mode-locked millimeter-wave optical transmitter (M-MOT).

The optical modem has several functions. The various information signals, namely voice,

video, data and others are organized and superimposed on the optical carrier using a Mach-

Zehnder or other suitable device. This approach promotes significant agili ty of frequency and

modulation format. Some applications require optical domain filtering which is also carried out

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at this level. In general, the optical modem would also include a detector for the signals

originating at the distributed base stations; that is not considered here.

The approach taken in the design of the microchip laser is significantly different from

past practices in two main aspects:

i. The host material for the solid-state microchip laser is an electro-optic crystal, LiNbO3,

which provides for efficient interaction between the optical and electric fields. This is

exploited for efficient mode-locking.

ii. The laser and the microwave components are fully integrated in a compact, rugged and low

cost package, which reduces parasitic effects and therefore provides for better performance.

Section II of this paper describes the design and performance of the microchip laser. This

discussion includes a general description of the laser as well as the issues involved in optimizing

the optical and microwave interaction to achieve efficient mode-locking. Several generations of

hardware development are described, each showing a successively higher level of integration.

Section III presents both optical domain and millimeter-wave domain measurement results,

which is followed by a brief section of conclusions.

II. THE MODE-LOCKED MICROCHIP LASER

The core of the mode-locked millimeter-wave optical transmitter is the solid-state

microchip laser shown in Figure 2. Microchip lasers are monolithic flat-flat optical cavities

formed by a short length of gain material with dielectric mirrors deposited directly on their

surfaces [13-15]. Since the host material, Lithium Niobate, is electro-optic, the laser chip can be

placed in a millimeter-wave cavity to facili tate effective interaction of the optical and millimeter-

waves resulting in superior mode-locking performance. Therefore, the motivation for using

microchip lasers is driven by three main factors:

i. Solid-state lasers are inherently more stable, have lower noise, and in general have a higher

output power than semiconductor lasers.

ii. The microchip configuration lends itself to a low cost, compact, short length cavity, which is

very suitable for mode-locking at high millimeter-wave frequencies.

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ii i. Solid-state lasers lend themselves to integration with the microwave subsystems, yielding a

low cost, rugged, integrated package that can be manufactured in large quantity.

DielectricMirrors

Doped ElectroopticCrystal

Mode-LockedOutput

Lc

Diode PumpBeam

Mode-LockingSignal

Figure 2. Concept of the mode-locked microchip laser.

With regard to the design of the mode-locked microchip laser emphasis was on simplicity and

minimizing the number of component employed. The following specific goals were established:

i. Efficient optical coupling of the semiconductor diode pump beam into the microchip laser

ii. Efficient coupling of the laser output into a single-mode fiber,

ii i. Optimal interaction between the millimeter-wave and optical fields

iv. Providing a simple, low noise and low cost millimeter-wave input signal for the mode

locking, and further reduction of the phase noise as required for some applications.

IIa. Microchip Laser Crystal Design

The microchip laser employs a y-cut Lithium Niobate (LiNbO3) crystal as a host material,

doped with 1-atm % of Neodymium (Nd) to provide for optical gain. In addition, 5-atm % of

Magnesium Oxide (MgO) was also added to the melt before pulli ng to reduce the effects of

photorefractive damage [16]. Lithium Niobate was selected for the laser because of its excellent

optical and electro-optical properties, principally its large electro-optic coefficients. Dielectric

mirrors were directly deposited on the crystal surfaces forming an optical cavity. The length of

the laser cavity was designed to be 3.48 mm, which corresponds to a roundtrip time of 50 ps, or

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an axial mode spacing of 20 GHz, matching the desired millimeter-wave subcarrier for this

particular experiment.

IIb. Millimeter-wave Cavity Design

In order to achieve low-noise millimeter-wave generation, the microchip laser must be

mode-locked to an external millimeter-wave signal. In our work, this millimeter-wave mode-

locking signal is applied to the Nd:LiNbO3 microchip laser through a reentrant microwave cavity

as indicated in Figure 3.

Pump Beam

Output Beam

Coarse Tuning

Cavity Tuning Cylinder

Microchip laserin cavity gap

Center Cylinder

Figure 3. Reentrant microwave cavity.

The reentrant coaxial cavity was designed to be resonant at 20 GHz. The basic design

used a transmission line analysis where the cavity is modeled as a shorted length of loss-less

coaxial transmission line that is terminated by the capacitance of the gap between the center

conductor and the bottom of the cavity [17]. The cavity length is selected such that the input

impedance of the shorted transmission line provides an inductance that cancels the gap

capacitance at the desired resonant frequency of 20 GHz. Both the fringing capacitance of the

gap [18] and the capacitance of the solid-state laser chip are included in this analysis. Thus, a

first order approximation of the required cavity length is obtained.

The mechanical design allows both course and fine frequency tuning by providing cavity

length adjustment about the nominal value and by a tuning screw in the side wall of the cavity as

shown in Figure 3. In addition, the mechanical design also incorporates an alignment groove in

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the bottom of the cavity to facili tate the alignment of the laser chip and input and output optics.

The 20 GHz signal is coupled into the cavity by a small loop probe located on the cavity

sidewall. In addition, a temperature sensor and a thermoelectric cooler were incorporated into

the microwave cavity in order to keep the cavity and the laser at a fixed temperature.

IIc. Microchip Laser Mode Locking

In order to facili tate efficient mode-locking, it is desired to optimize the interaction

between the optical and millimeter-wave fields within the solid state laser crystal. Since the

single Nd:LiNbO3 laser chip provides both the gain medium for lasing and electro-optic

properties for mode-locking, the laser chip is placed in the center of the gap between the center

conductor and the bottom of the cavity where the electric field is highest. This location will

provide the strongest electro-optic interaction and should provide the best mode-locking

performance. However, it should also be noted that mode-locking theory is not developed for this

case where both the gain (for lasing) and electro-optic (for mode-locking) properties are in the

same medium and occupy the entire laser cavity. Therefore, there is some uncertainty with

respect to the field distribution required within the crystal for optimum mode-locking.

In order to gain insight into the mode-locking, a finite element computer simulation of

the laser and cavity was initiated using a commercially available High Frequency Structure

Simulator (HFSS). The reentrant cavity, solid state laser chip, alignment groove, coupling probe,

and tuning screw are all incorporated into the model. The simulated resonance frequencies for

this model are accurate to within 1% of the measured values determined via an S11 measurement

as shown in Figure 4. The electric field distributions in the reentrant cavity and laser chip were

also computed for the dominant mode at 19.85 GHz. The z-component of the electric field for

this mode corresponding to an x-y plane cut through the center of the laser chip is shown in

Figure 5. The field distribution is stronger in the laser chip than in the open cavity as desired.

This is expected since the dielectric constant of the laser chip is high (26 in the z-direction). The

field concentration is symmetric along the longitudinal axis of the laser chip with the highest

field concentration occurring at the center of the laser. It is expected that the insight gained from

these and ongoing simulations will ultimately lead to a complete mode locking theory for this

unique laser.

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Figure 4. Measured S11 of the reentrant cavity with the microchip laser. Dotted lines indicate

simulated resonant frequencies.

Cavity inside wall

Center Cylinder

Loop Antenna

Microchip laser in cavity gap

Tuning Screw

Microchip laser

(a) (b)

Figure 5. (a) Simulated z-component of electric field inside the microwave cavity. X-Y plane is

located at the center of the laser chip. (b) Simulated z-component of the electric field inside the

microchip laser. X-Y plane is located at the center of the chip

IId. System integration

During the development of the M-MOT transmitter the issues outlined earlier were

systematically addressed, leading to three different mode-locked microchip laser packaging

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configurations. Each generation used the same laser chip and basic reentrant cavity design.

However, each successive configuration employed a higher level of integration. The three

generations of laser hardware development are discussed below.

First Generation – baseline design: The main issue addressed in the first transmitter generation

was the optical alignment. The alignment of the pump beam and the collimation of the laser

output are critical to the transmitter performance. The pump for this generation was an external

semiconductor pump laser diode operating at 814 nm with an output power of 280 mW. The

pump beam is coupled to the microchip laser through a fiber imbedded in a ceramic sleeve. The

ceramic sleeve is aligned with the collimator and the laser placed in between them into an

alignment grove designed to aid in the mechanical alignment of the semiconductor laser chip.

The picture for this packaging is shown in Figure 6. The microchip laser output was coupled to a

single-mode fiber using a fiber collimator. The mode-locking signal was obtained from an

external frequency synthesizer or a Gunn oscill ator.

Output fiber

Input fiber

Coarse Tuning

Fine Tuning

Figure 6. Packaged unit with an external pumping scheme.

Second Generation – integrated mm-wave source: Millimeter-wave generation with good

stabili ty and noise performance was obtained using the baseline configuration previously

described. However, while external synthesizers provide good millimeter-wave performance,

they are bulky and costly. Therefore, the transmitter second generation emphasized the

replacement of the synthesizer with compact, low cost millimeter-wave signal sources. One of

the options that were investigated involves the use of an external Gunn oscill ator. Its noise and

power levels are compatible with the requirements of the M-MOT. Moreover, since the Gunn

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oscill ator and the reentrant microwave cavity have basically the same structure, they can both be

mounted together, leading to a more compact configuration. The RF power injected for mode

locking is +10dBm. The optical transmitter, which is comprised of these two cavities, forms a

small, rugged package. The spectra of the free-running and mode-locked signals are depicted in

Figure 7. The phase noise is adequate for many communications applications, but can be

improved by using a lower phase-noise source such as a dielectric resonator oscill ator. Efforts

are currently underway to integrate a low noise Gunn oscill ator into the microchip laser housing

that will result in further size reduction.

(a) (b)

Figure 7. Microwave spectrum of the laser output for (a) free-running and (b) mode-locked

microchip laser.

Third Generation – integrated pump: The third generation of the transmitter is already designed

and under test. It is a more compact configuration with an internal semiconductor pump. A laser

diode chip is incorporated into the reentrant microwave cavity as shown in Figure 8. With this

approach only one temperature controller is used to stabili ze the pump and reentrant cavity. Once

again, the pump beam and the collimator are aligned, and the laser chip is adjacent to the pump.

This configuration is easier to work with and allows for systematic repeatabili ty.

These efforts in system integration resulted in an integrated package housed in a rugged

cavity that is comprised of the laser chip, two active components, namely the diode pump and the

Gunn diode each driven by single DC inputs, one temperature controller, and one optical output.

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Laser DiodeChip

MicrochipLaser

AlignmentGrove (b)

Figure 8. (a) Shows the packaged unit with millimeter-wave input and optical output. (b) Depicts

the inside of the cavity, with the top removed. The pump diodes are permanently affixed and the

laser chip is inserted into the grove.

III. EXPERIMENTAL CHARACTERIZATION OF THE MODE-LOCKED

MICROCHIP LASER

A thorough characterization of the mode-locked optical transmitter requires

measurements in both the optical and millimeter-wave domains. This section describes the tests

performed in both of these domains.

The 20GHz mode-locking signal from a Gunn oscill ator or a synthesizer was applied to

the reentrant microwave cavity. The microwave frequency was set to correspond to the free

spectral range of the laser and the microwave cavity was adjusted to resonate at this particular

frequency. The mode-locked laser output was applied to an optical spectral analyzer, microwave

spectrum analyzer and high frequency sampling oscill oscope for the optical, microwave, and

time domain measurements, respectively.

IIIa. Optical and Time Domain Characterization

The optical spectrum of the mode-locked laser was measured using an optical spectrum

analyzer with a resolution of 0.05nm (i.e. 12.8GHz) at 1.084µm. The result, shown in Figure 9,

reveals stable modes 20GHz apart that displays a very stable and well -defined structure, with a

Lorentzian-like distribution.

For the time domain measurement, the mode-locked longitudinal modes, detected by a

high speed InGaAs Schottky photodiode (New Focus model 1014), was fed into a high

frequency sampling oscill oscope (Tektronix model CSA 803). Figure 10 depicts the waveform

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obtained when a 12.6dBm mode-locking signal was applied to the reentrant microwave cavity.

The waveform corresponds to a train of optical pulses with 18.6ps width and 20GHz repetition

rate. The shape of these pulses is limited (broadened) by the bandwidth of the measuring

instrumentation due to attenuation of higher order harmonics.

0.3

0.2

0.1

0.0

Pow

er (

mW

)

1085.51085.01084.51084.0

Wavelength (nm)

Figure 9. Optical spectrum of the mode-locked laser output. The resolution is 0.05nm.

Figure 10. Time domain measurement of the mode-locked laser output. The upper trace is the

driving signal and the lower trace corresponds to the laser pulse. The pulsewidth measured was

18.6 ps.

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IIIb. Millimeter-Wave Domain Characterization

The corresponding microwave domain signal, after the mode-locked modes are beaten in

a high-speed photodetector, is shown in Figure 11. As in the previous measurement, a very stable

signal with peak output at 20GHz was observed. The –23dBc side-bands at 350 kHz off-set

frequency are due to the laser relaxation oscill ation. We intend to suppress these by employing

feedback control to the pump laser. Peak suppression of better than 30dB has been reported in

the literature [19].

-70

-60

-50

-40

-30

-20

Po

wer

(d

Bm

)

� � � � � � �� � � � � � �� � � � � � �� � � � � � �� � � � � � �

Frequency (GHz)

� � � � � � � � � � � � �� � � � � � � � � �

� � � � � �

Figure 11. Microwave spectrum of the mode-locked laser output. The frequency span and the

resolution bandwidth for this measurement is 1MHz and 10KHz, respectively.

The phase and the amplitude noise of the generated millimeter-wave were also measured.

The single-sideband phase-noise measurement was performed using HP3048A measurement

system. The result, shown in Figure 12, indicates that the absolute phase noise is determined by

the modulating source, the Gunn oscill ator in this case. The residual phase noise was measured

as well and found to be negligible over the relevant bandwidth for most applications (< -110

dBc/Hz at 10 KHz). The absolute phase noise integrated from 10 kHz to 20 MHz is –38 dBc.

The amplitude noise of the millimeter-wave generated using the microchip laser is shown

in Figure 13. Discrete spurious signals are plotted separated from the noise power density. The -

35 dBc discrete peak at 460 kHz is due to the laser relaxation oscill ation. The exact frequency of

this oscill ation is dependent upon our alignment of the output coupler. The integrated amplitude

noise (discrete spurious interference not included) from 10 KHz to 20 MHz is –27 dBc.

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Figure 12. Absolute single-sideband phase noise measurement at 20 GHz. The dotted line is the

Gunn oscill ator phase noise, the darker line is after the M-MOT and the lighter and lower line is

the residual phase noise measurement.

Figure 13. RF Amplitude noise measurement (in dBc/Hz) as a function of the carrier offset

frequency for the mode-locked laser at 20 GHz. The vertical lines indicate discrete spurs.

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IV. SUMMARY AND CONCLUSIONS

A novel millimeter-wave optical transmitter was designed, fabricated and experimentally

characterized. The heart of the transmitter is a mode-locked electrooptic microchip laser whose

most desirable attributes are simplicity, compactness, high level integration of the millimeter-

wave and photonic components and superior performance, particularly regarding phase noise.

The transmitter was conceived as part of a fiber-radio system and it meets all the requirements

for this application. Table I summarizes the important characteristics of the microchip laser

subsystem.

Table I – Summary of the microchip laser subsystem results

Parameter Value

Microchip laser host material LiNbO3

Microchip laser doping Nd (1 atm %)

Microchip laser size (for 20 GHz signal generation) 1x1x3.4 mm

Lasing wavelength 1.084 µm

Optical linewidth <25KHz

Lasing efficiency 18%

Laser output power 50 mW

Millimeter-wave frequency 20 GHz

Millimeter-wave mode locking power (threshold) 6 dBm

Modulation index 98%

linewidth of millimeter-wave signal <80 KHz

Phase noise of millimeter-wave signal - integrated -38dBc

Amplitude noise of millimeter-wave signal - integrated -27dBc

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Current efforts, with promising preliminary results, are focusing on developing a chip

laser in Er:LiNbO3. The Erbium doped laser will operate at the more conventional wavelength of

1.531 µm. A more powerful pump source and better optical coupling from the pump into the

laser is expected to increase the optical output power to several hundred milliwatts. The

millimeter-wave frequency generated by the laser can be readily extended to 100 GHz and

beyond by proper scaling of the laser chip. Subharmonic mode-locking would also allow a lower

frequency locking source to be used. Specifically, we successfully obtained a stable 40 GHz

mode-locked component at the laser output when locked to either a 20 GHz or to a 40 GHz

source.

The simplicity of the mode-locked microchip laser coupled with efficient, integrated

packaging of the millimeter wave and photonics components imply high reliabili ty and reduced

cost. Although the Mach-Zehnder interferometer is not discussed here, it should be pointed out

that since both the microchip laser and the modulator use LiNbO3 there is a distinct possibili ty

for their chip level integration. This is presently studied, as it would lead to simplified optical

coupling between these devices that translates to better performance and potentially greatly

reduced cost.

The generation and transmission of high quality millimeter-wave signals has numerous

applications. The M-MOT device discussed in this paper was specifically designed for fiber-

radio and LMDS, and it meets and exceeds the requirements for this particular application.

However by suitable tailoring of the device it can be adopted to other applications such as the

optically fed and controlled millimeter wave phased array antenna and high sampling rate

photonic Analog-Digital converter (ADC). Some other applications, like Pulsed Doppler radar at

94 GHz, have more stringent requirements particularly in terms of phase noise (-130 dBc/Hz or

less at 100 KHz off-set). Optical domain feed back can dramatically increase the Q of the

millimeter wave resonator and thus reduce noise [20]. Conventional optical feedback employs ~2

km fiber (a compact spool), a high-speed detector, millimeter-wave amplifier and filter, which

add noise and cost. The two terminal Gunn device can be directly ill uminated by the light and

eliminating the need for the detector, amplifier and filter.

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ACKNOWLEDGMENTS

We would like to thank Greg Mizell and Greg Quarles from VLOC for growing and processing

the Nd-doped Lithium Niobate crystals and Ron Esman and Paul Matthews for valuable

suggestions. This work was support by the Office of Naval Research (ONR) under the grant #

N00014-00-1-0781.

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