VCSELs

1

By:

Hojjatollah Sarvari

Esmaeil Dastjerdi

Department of Physics

Shiraz University, Shiraz-Iran

March-2009

Vertical Cavity Surface Emitting Lasers

(VCSELs)

2

Outline of talk Introduction.

Why the Surface-Emitting Laser?

History of VCSEL research.

Structure:

How Does the DBR Work?

Current Confinement.

Optical Confinement.

CW Characteristics.

Rate Equations.

P–I Curve of a 1.3-um InGaAsP laser.

Applications and new structures of VCSELs

Advantages of VCSELs

Conclusion

3

Introduction

The vertical-cavity surface-emitting laser (VCSEL) is a type of

semiconductor laser diode with laser beam emission perpendicular from the

top surface.

The term VCSEL was coined in a publication of the Optical Society of

America in 1987.

The cavity is along the vertical direction, with a very short length, typically

1-3 wavelengths of the emitted light.

4


There are several advantages to producing VCSELs when compared with the production

process of edge-emitting lasers.

Edge-emitters cannot be tested until the end of the production process. If the edge-emitter

does not work, whether due to bad contacts or poor material growth quality, the production

time and the processing materials have been wasted.

VCSELs , however, can be tested at several stages throughout the process to check for

material quality and processing issues.

Additionally, because VCSELs emit the beam perpendicular to the active region of the laser

as opposed to parallel as with an edge emitter, tens of thousands of VCSELs can be processed

simultaneously on a three inch Gallium Arsenide wafer.

Some of VCSEL’s applications are as follows:

– Optical fiber data transmission .

– Analog broadband signal transmission.

– Laser printers .

– computer mouse.

5


6

History of VCSEL research

It is recognized that Mr Iga suggested VCSEL in 1977.

The first VCSEL was presented in 1979 by Soda, Iga, Kitahara and

Suematsu (Soda 1979), where the 1.3 um-wavelength GaInAsP/InP

material was used for the active region.

In 1986 a 6-mA thereshold GaAs device was made by that group.

But devices for CW operation at room temperature were not reported until

1988 (Koyama 1988).

And after that in 1989, Jack Jewell demonstrated an InGaAs SE laser

exhibiting 2-mA threshold.

Today, VCSELs have replaced edge-emitting lasers in applications for

short-range fiber optic communication such as Gigabit Ethernet and Fiber

Channel.

http://en.wikipedia.org/wiki/VCSEL#CITEREFSoda1979

http://en.wikipedia.org/wiki/VCSEL#CITEREFKoyama1988

7


8


Following figure gives plots of the lowest threshold current versus the year

of publication for 980 nm diode lasers of any type:

9


Following figure shows the improvement in the electric to optical power

conversion (or “wall-plug”) efficiency versus year:

10

Structure

The laser resonator consists of two distributed Bragg reflector (DBR) mirrors

parallel to the wafer surface with an active region consisting of one or more

quantum wells for the laser light generation in between.

The reflectivity required for low threshold currents is greater than 99.9%,

Distributed Bragg Reflectors (DBRs) are needed for this reflectivity.

11

How Does the DBR Work? The planar DBR-mirrors consist of layers with alternating high and low refractive

indices. Each layer has a thickness of a quarter of the laser wavelength in the

material, yielding intensity reflectivities above 99%.

The DBR layers also carry the current in the device, therefore, more layers increase

the resistance of the device.

The wavelength of VCSELs may be tuned, within the gain band of the active region,

by adjusting the thickness of the reflector layers.

12

Structure

In common VCSELs the upper and lower mirrors are doped as

p-type and n-type materials, forming a diode junction. In more

complex structures, the p-type and n-type regions may be

buried between the mirrors, requiring a more complex

semiconductor process to make electrical contact to the active

region, but eliminating electrical power loss in the DBR

structure.

In laboratory investigation of VCSELs using new material

systems, the active region may be pumped by an external light

source with a shorter wavelength, usually another laser. This

allows a VCSEL to be demonstrated without the additional

problem of achieving good electrical performance; however

such devices are not practical for most applications.

13

Current Confinement

The ultimate threshold current depends on how to make the active volume

small, and how well the optical field can be confined in the cavity to

maximize the overlap with the active region.

Typical models for current confinement are as follows:

a) Ring-Electrode type:

This structure can limit the current flow in the vicinity of the ring electrode.

The light output can be taken out from the center window.

This is easy to fabricate, but the current cannot completely be confined to a small area due to diffusion.

14

Current Confinement

b) Proton-Bombardment type:

An insulating layer was used by proton (H+) irradiation to limit the current spreading toward the area.

The process is rather simple and most commercialized devices are made by this method.

c) Buried-Heterostructure (BH) type: The mesa including the active region was buried with a wide-gap semiconductor to limit the current.

The refractive index can be small in the surrounding region.

This is one of the ideal structures in terms of current and optical confinement.

The problem is that the necessary process is rather complicated: in particular in making a tiny 3D device.

15

Current Confinementd) Selective AlAs oxidation type:

The AlAs layer was oxidized to make an insulator.

e) Oxidized DBR type:

The same method is applied to oxidize DBR consisting of AlAs and GaAs.

This is one of the volume-confinement methods and can reduce the non-radiative recombination.

16

Current Confinement

Recently the two main methods of restricting the current in a VCSEL were

characterized by two types of VCSELs: ion-implanted VCSELs and Oxide

VCSELs.

In the early 1990s, telecommunications companies tended to favor ion-

implanted VCSELs. Ions, (often hydrogen ions, H+), were implanted into

the VCSEL structure everywhere except the aperture of the VCSEL,

destroying the lattice structure around the aperture, thus inhibiting the

current.

In the mid to late 1990s, companies moved towards the technology of

oxide VCSELs. The current is confined in an oxide VCSEL by oxidizing

the material around the aperture of the VCSEL. A high content aluminium

layer that is grown within the VCSEL structure is the layer that is oxidized.

Oxide VCSELs also often employ the ion implant production step. As a

result in the oxide VCSEL, the current path is confined by the ion implant

and the oxide aperture.

17

Optical Confinement

a) Fabry-Perot type:

The optical resonant field is determined by the two reflectors.

b) Gain-Guide type:

The field simply was limited at the region where the gain exists.

18

Optical Confinement

c) Buried Heterostructure (BH):

d) Selective AlAs oxidation type:

Due to the index difference between the AlAs and the oxidized region, the optical field can

be confined as well by a kind of lens effect.

19

CW Characteristics

The operating characteristics of semiconductor lasers

are well described by a set of rate equations that govern

the interaction of photons and electrons inside the

active region.

A rigorous derivation of the rate equation generally

starts from the Maxwell’s equations.

The rate equations can also be written heuristically by

considering various physical phenomena through which

the number of photons, P , and the number of electrons,

N, change with time inside the active region.

20

Rate Equations

gm(N) = σg (N – NT) (4)

(5)

τp-1 = Vg αcav = Vg (αmir+ αint) (6)

For a single-mode laser, these equations take the form:

G = the net rate of stimulated emission

(1)

(2)

(3)

21

P–I Curve of a 1.3-um InGaAsP laser

The laser performance degrades at high temperatures.

Ith(T) = Io exp(T/To) (7)

22

CW Characteristics

In the case of CW operation at a constant current I, the time derivatives in Eqs.

(1) and (2) can be set to zero.

GP – P/ τp = 0 P ( G τp – 1) = 0 (8)

For currents such that G τp < 1, P = 0 and I/q – N/τc = 0. (9)

The threshold is reached at a current for which G τp = 1.

G τp = GN τp (N - No) = 1 Nth = No + (GN τp)-1 (10)

Ith = q Nth/ τc = q/ τc (No + 1/( GN τp)) (11)

For I > Ith, we have: -dN/dt = dP/dt.

And hence the photon number P increases linearly with I as:

P = (τp /q)(I – Ith) (12)

23

CW Characteristics

The emitted power Pe is related to P by the relation

Pe = (Vg αmir) ħωP (13)

Pe = ħω/q (ηint αmir)/(αmir+ αint) (I – Ith) (14)

dPe/dI = ħω/q ηd with ηd = ηint αmir/(αmir+ αint) (15)

ηext = (photon-emission rate)/(electron-injection rate)

ηext = (Pe/ħω)/(I/q) = q/(ħω) Pe/I (16)

ηext = ηd (1 – Ith/I) (17)

Similar to the case of LEDs, one can define the total quantum

efficiency ( or wall-plug efficiency) as ηtot = Pe/(Vo I), where

Vo is the applied voltage. It is related to ηext as:

ηtot = ħω/(qVo) ηext ~ Eg/(qVo) ηext (18)

24

VCSEL Application

• WDM Applications :

Tuning wavelength with MEMS technology

2D VCSELs Array with special wavelength for each VCSEL

• Optical Memory : VCSELs are used to read the data on compact disks and CD-ROMs ,DVD, Near field

• Optoelectronic: Printer, Laser pointer, Mobile tools, Mouses

• Optical Information Processing : Optical processors, Parallel processing

• Optical Sensing : Longer wavelength VCSELs for sensor application, Optical fiber sensing, Bar code readers, Telemetry

• Displays: Shorter wavelength VCSELs for displays, Array light sources, Multi-beam search lights, High efficiency sources

25

Material and Wavelength of VCSELs

• AlGaInP/AlGaAs for Red wavelength (650-680nm) VCSELs

• GaInAsP/AlGaAs for Near-IR wavelength (780-850nm) VCSELs

• AlGaInAs for wavelength 850nm VCSELs

• GaInAsN for Long-wavelength (1.3-1.55um) VCSELs

• Sb for Long-wavelength (1.3-1.55um) VCSELs

• III-V Nitride for Visible wavelength VCSELs

26

Single-Mode Oxide-Confined VCSEL for Printers and SensorsHiromi Otoma, Akemi Murakami, Yasuaki Kuwata, Nobuaki Ueki, Naotaka Mukoyama

2006 Electronics Systemintegration Technology Conference,Germany, 1-4244-0553-1/06/$20.00 2OO6 IEEE

• p-DBR mirror consists of 17 pairs of Al0.9Ga0.1As/Al0.12Ga0.88As

• n-DBR mirror consists of 34 pairs of Al0.9Ga0.1As/Al0.12Ga0.88As

• P-contact: Ti/Au

• N-contact: AuGe/Au

• Passivation layer (SiNx)

• Dm < Dox

• MOCVD for Fabrication

27



• Dox=3.1um & Dm=3.9um

• Lasing at λ=857nm in single mode

• P=4.7mW at I=8mA & room temperature

28



• Wavelength shifts with temperature and injection current

• Tunable VCSELs useful as light source for Atomic Clocks

• Cut off frequency is 5GHz for I=2mA

• Lifetime of device is about 30 years for I=1mA at room temperature

29



• 8*4 VCSEL Array single-mode at λ=780nm for color laser printer

• SMSR > 20dB & Ith=0.32mA

• 1.2mW at 1mA for printer with 80ppm speed & 2400DPI resolution

30

Electrically Pumped GaSb-based VCSEL

With Buried Tunnel JunctionA.Bachmann, T. Lim, K. Kashani-Shirazi, O. Dier, C. Lauer and M.-C. Amann

2008 Optical Society of America, CLEO/QELS 2008

• GaSb-based VCSELs are good light source for Mid-Infrared (2-4um)

• Active layer : Ga.63In.37As.03Sb.97 with 11nm thickness

• BTJ produce with n+InAsSb & p+GaSb

• Bottom DBR : 24 pair AlAsSb/GaSb with R=99.8%

• Top DBR : 4 pair Si/SiO2 with R=99.7%

• Ith=2.7 & 3.3mA for -10°C & 20°C respectively

31

Electrically Pumped GaSb-based VCSEL

With Buried Tunnel JunctionA.Bachmann, T. Lim, K. Kashani-Shirazi, O. Dier, C. Lauer and M.-C. Amann

2008 Optical Society of America, CLEO/QELS 2008

• Wavelength shift due to Temperature and Current make wavelength tuning as high as 10 nm

• For sensing CO gas at 2.325um

• Some feature of this VCSEL for sensor application are single-mode operation, low-cost production and tuning

32

All-Optical Flip-Flop Operation Using 1.55um Polarization

Bistable VCSELsTakeo Katayama, Toshiyuki Kitazawa, Hitoshi Kawaguchi

2008 OSA / CLEO/QELS 2008 (978-1-55752-859-9/08/$25.00 ©2008 IEEE)

• Device fabricated with low-pressure MOVPE on InP substrate

• Air gap for current confinement produced with Wet-Etching Method

• VCSEL has two lasing modes with polarization 0° & 90°

33

Optimal radii of photonic crystal holes within DBR mirrors

in long wavelength VCSELTomasz Czyszanowski, Maciej Dems, Hugo Thienpont, Krassimir Panajotov

2007 OSA,5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1301

• 4QWs AlGaInAs active region with 10nm thickness for λ=1.3um

• Top & bottom mirror with 28 & 34 pairs of AlGaAs/GaAs

• Photonic crystal produce by Etching in top DBR mirror

• Structure uses tunnel junction for better current confinement

• Advantages of using PC in DBR mirror :

Better current confinement

Increase SMSR & Single-Mode operation

34

Nano electro-mechanical optoelectronic tunable VCSELMichael C.Y. Huang, Ye Zhou, and Connie J. Chang-Hasnain

2007 Optical Society of America, 5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1222

• QWs active layer

• Bottom DBR : 34 pairs

• Top mirror : 4 pairs and HCG(High-index-Contrast sub-wavelength Grating)

• 4 pairs DBRs for inject current

• AlO2 aperture for current confinement

• HCG with 230nm thickness cause :

- R > 99.9%

- Single-Mode with SMSR > 40dB

- Tuning wavelength on 2.5nm distance

- Small thickness instead DBRs and therefore reduce mass, increase

tuning speed and reduce tuning voltage

• HCG consists of periodic strip of Al.6Ga.4As and Air as clad

35

• Laser characteristics without tuning voltage :

Ith=1.3mA and P0=.5mW at 2.3Ith

At 1.3Ith λ=861nm with SMSR=40dB



36



• Laser characteristics with tuning voltage

• For tuning λ a reverse bias voltage applied across HCG & P-DBR

37

• Variation of Air Gap with applied voltage and wavelength with Air Gap

• This tunable laser useful for WDM applications, optical filters and

detectors



38

Advantages of VCSEL

• Easier to test on wafer

• Wavelength is “tunable”

• Low current needed due to small active volume

• Circular cross-section that can be easily coupled

• The VCSEL is cheaper to manufacture in quantity

• Efficiency and speed of data transfer is improved for fiber optic communications

• Array layouts Could be treated as the 2D arrays for optical signal processor, interconnects, and image processing and to do the calculation

39

Conclusion

• The active layer is configured sometimes as a single quantum well (SQW) and usually as a multiple quantum well (MQW) to improve threshold current.

• The semiconductor layers are manufactured using epitaxial growth on a substrate that is transparent to in the emission wavelength.

• Example: a 980 nm VCSEL uses InGaAs as the active layer to provide 980 nm emission along with a GaAs crystal substrate that is transparent at 980 nm.

40

Conclusion

• The height of the cavity is small (several microns) and only permits one longitudinal mode, however there may be one or more lateral modes.

• A VCSEL is capable of delivering more power than a conventional laser diode up to a few watts with a matrix VCSEL which a matrix emitter is a broad area surface emitting laser with applications in optical interconnect and optical computing.

41

References• [1] “Vertical Cavity Surface Emitting Laser Devices” by H.Li & K.Iga,Springer

• [2] “Single-Mode Oxide-Confined VCSEL for Printers and Sensors” Hiromi Otoma, Akemi Murakami,

Yasuaki Kuwata, Nobuaki Ueki, Naotaka Mukoyama,2006 Electronics Systemintegration Technology Conference,Germany, 1-4244-0553-1/06/$20.00 2OO6 IEEE

• [3] “Electrically Pumped GaSb-based VCSEL With Buried Tunnel Junction”A.Bachmann, T. Lim, K. Kashani-Shirazi, O. Dier, C. Lauer and M.-C. Amann, 2008 Optical Society of America, CLEO/QELS 2008

• [4] “All-Optical Flip-Flop Operation Using 1.55 m Polarization Bistable VCSELs”Takeo Katayama, Toshiyuki Kitazawa, Hitoshi Kawaguchi2008 OSA / CLEO/QELS 2008 (978-1-55752-859-9/08/$25.00 ©2008 IEEE)

• [5] “Optimal radii of photonic crystal holes within DBR mirrors in long wavelength VCSEL”Tomasz Czyszanowski, Maciej Dems, Hugo Thienpont, Krassimir Panajotov2007 OSA,5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1301

• [6] “Nano electro-mechanical optoelectronic tunable VCSEL”, Michael C.Y. Huang, Ye Zhou, and Connie J. Chang-Hasnain, 2007 Optical Society of America, 5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1222

42

Thanks for your attention

VCSELs

Engineering

Transcript of VCSELs