Characterization of single mode laser and tunable laser array based on etched high order surface gratings

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Characterization of single-mode laser and

tunable laser array based on etched high

order surface gratings

Thesis

By

Azat Abdullayev

A th esis su b m itte d to th e

University o f Dublin

for th e deg ree of

Doctor o f Philosophy

School of Physics,

University of DubHn,

T rinity College Dublin.

D eclaration

This thesis has not been subm itted as an exercise for a degree in this or any

other university. The w ork described in this thesis is entirely my ow n, with

exception o f assistance recognized in the acknow ledgem ents and the

collaborative w ork noted in the publications. I agree that Trinity C ollege library

m ay lend or copy this thesis on request.

TRINITY COLLEGE

1

MAY ?0I5

JJBRARY DUBLIN

IS

Q5/3

Summary

W a v e l e n g t h t u n a b l e s i n g l e - m o d e s e m i c o n d u c t o r l a s er s play a n e s s e n t i a l r ol e in m o d e r n o pt ic al c o m m u n i c a t i o n s y s t e m s . T h e y a r e a l so r api dl y r e p l a c i n g f i x e d - w a v e l e n g t h light s o u r c e s in d e n s e w a v e l e n g t h - d i v i s i o n - m u l t i p l e x i n g ( DWDM) s y s t e m s , w h i c h will si gnificantly r e d u c e t h e s y s t e m size a n d c ost . T h e opt i c a l c o m m u n i c a t i o n i n d u s t r y d e m a n d s light s o u r c e s w i t h r el at i vel y smal l v o l u m e , l ow i n v e n t o r y c o s t a n d high m a n u f a c t u r i n g yield p r o c e s s . T h e r e f o r e , l a s e r d i o d e s a l o n g w i t h high p o w e r , s t a b l e o p e r a t i o n a n d n a r r o w l i n ew i d t h s h o u l d h a v e s i m p l e f a b r i c a t i o n p r o c e s s a n d high yield.

Up t o n o w c o n v e n t i o n a l d i s t r i b u t e d f e e d b a c k (DFB) a n d d i s t r i b u t e d Bragg r e f l e c t o r (DBR) l a se rs a c h i e v e d g r e a t s u c c e s s in t e r m s of s t a b l e o p e r a t i o n , high p o w e r a n d w i d e t u n i n g r a n g e . H o w e v e r , t h e s e l as e r s r e q u i r e c o m p l e x f a b r i c a t i o n s t e p s a n d high r e s o l u t i o n p r o c e s s i n g by usi ng e - b e a m l i t h o g r a p h y w h i c h is t i m e c o n s u m i n g a n d a n e x p e n s i v e p r o c e s s . Pr evi ous l y it w a s d e m o n s t r a t e d t h a t s i n g l e - m o d e o p e r a t i o n c a n b e a c h i e v e d in F a b r y - P e r o t l a s e r s by i n t r o d u c i n g ref l e c t i ve d e f e c t s (slots) a t p a r t i c u l a r l o c a t i o n s a l o n g t h e cavity. But in t h e s e l a s e r s it is e x t r e m e l y h a r d t o i n t e g r a t e t h e m w i t h o t h e r p h o t o n i c c o m p o n e n t s in o n e c h i p d u e t o c l e a v a g e o f b o t h f a c e t s . This w o r k p r e s e n t s c o s t e f f e ct i v e a n d i n t e g r a b l e a n e w s i n g l e - m o d e l a s e r d es i g n . T h e s e l a s e r s b a s e d o n e t c h e d high o r d e r g r a t i n g s (slots) o n o n e s i d e o f t h e l a s er cavi t y t o p r o v i d e e n o u g h r ef l e ct i o n f o r lasing t o o c cu r . T h e r e f o r e , n o r e g r o w t h is n e e d e d . This w o r k d e s c r i b e s t h e s l o t t e d s i n g l e - m o d e l a s er p l a t f o r m f r o m d e s i g n t o f a b r i c a t i o n t o e x p e r i m e n t a l c h a r a c t e r i z a t i o n .

T h e t h e s i s is o r g a n i z e d a s fol l ows:

First, a br ie f e x p l a n a t i o n o f W D M a n d c o h e r e n t c o m m u n i c a t i o n s is i n t r o d u c e d . H e r e it is e x p l a i n e d w h a t r e q u i r e m e n t s l a s e r d i o d e s s h o u l d m e e t in t h e s e c o m m u n i c a t i o n s y s t e m s . T h e t h e o r e t i c a l p a r t c o n s i s t s o f t h e ba s i c s of l a s e r c h a r a c t e r i z a t i o n , t h e t h e o r y o f t h e l a s e r l i n ew i d t h a n d d i f f e r e n t t y p e s o f w a v e l e n g t h t u n i n g m e c h a n i s m s . To real i ze s i n g l e - m o d e l a s e r a g r a t i n g s t r u c t u r e is n e e d e d . To u n d e r s t a n d t h e t h e o r y o f t h e g r a t i n g s a br i ef il l us t rat i on o f t h e s c a t t e r i n g m a t r i x m e t h o d ( SMM) a n d t h e t r a n s m i s s i o n m a t r i x m e t h o d (TMM) is d e s c r i b e d . Also t h e w o r k i n g pr i nc i pl e o f DBR l a s e r a n d DFB l a s e r is p r e s e n t e d , a n d h o w s i n g l e - m o d e o p e r a t i o n is a c h i e v e d in t h o s e l asers.

this m eth o d d iffe re n t p aram eters are extracted to characterize th e basic FP laser including a w aveguide loss.

Further, th e concept o f slotted single-m ode laser is developed. It is d em o n strated th a t th e single-m ode o p eratio n can be achieved by etching high o rd er surface gratings on top of th e ridge o f th e laser w ith o u t any reg ro w th o f additional m aterial. These gratings or slots can be can be done by standard photolithography. The in tegration o f these lasers w ith an sem iconductor am p lifier (SOA) and an electro-abso rp tio n (EA) m o d u lato r is also presented. Fabricated devices exhibit a threshold curren t of around 30 m A and slope efficiency of around 0 .12 m W /m A . The laser p erfo rm an ce is im proved by applying high reflection and a ntireflectio n coatings so th e threshold c u rren t decreased to 19 mA and th e slope efficiency increased to 0 .1 7 m W /m A . The spectral characteristics show stable single-m ode operation w ith side-m ode suppression ratio (SMSR) around 50 dB. The linew idth is m easured using th e delayed self-hetero d yn e (DS-H) m eth o d . The laser linew idth is m easured to be around 2 -4 M H z fo r short cavity lasers and th e lin ew id th follow s th e th eo retical 1 /P tren d . It is also d em o n s trated th a t longer lasers exh ib it a n a rro w e r linew idth and fo r a laser w ith an e ffective cavity length of 4 5 0 nm th e m in im um intrinsic linew idth is estim ated to be 720 kHz.

Acknowledgements

First, I w o u l d like t o t h a n k m y s u p e r v i s o r Prof. J o h n D o n e g a n f or his e x c e l l e n t s u p e r v i s i o n

a n d f or all his h e l p d u r i n g m y PhD. I w o u l d like t o e x p r e s s m y s i n c e r e a p p r e c i a t i o n a n d t h a n k s t o J o h n f or all his a d v i c e s, g u i d a n c e a n d s u p p o r t all t h e s e y e a r s . I'm v e r y g r a t e f u l for

t h e o p p o r t u n i t y t o w o r k u n d e r t hi s p r o j e c t in s e m i c o n d u c t o r p h o t o n i c s g r o u p h e r e in Trinity

College Dublin.

I w o u l d like t o e x p r e s s m y s peci al t h a n k s t o Dr. Qi aoyi n Lu f or all h e r h e l p in t h e lab a n d

in t h e o r e t i c a l m o d e l of t h i s wo rk . I a p p r e c i a t e h e r a d v i c e s a n d n e v e r e n d i n g s u p p o r t . It w o u l d b e really h a r d t o w o r k o n t hi s p r o j e c t w i t h o u t h e r hel p. T h a n k y o u .

Speci al t h a n k s t o m y c o l l e a g u e a n d f r i e n d M a r t a N a w r o c k a f o r h e l p in t h e lab a n d d i s cu s si o n s . I' m really t h a n k f u l t h a t w e w o r k e d o n t hi s p r o j e c t t o g e t h e r . I a l s o w o u l d like t o t h a n k Dr. Fr ank Bello f or d is c u s s i o n s in t h e o r e t i c a l m o d e l i n g of t h e l i n e wi d t h a n d f or his a d v i ce s . T h a n k s al so t o M i c ha e l f or h e l p i n g w i t h t h e m e a s u r e m e n t s in t h e lab a n d ana l yz i ng

t h e r es u l t s. T h a n k s t o Ian f or h e l p in t h e t h e r m a l m e a s u r e m e n t s . T h a n k s t o g u y s in t h e office Vas, Xia, G r a h a m , J o h n , S a m a n d t h a n k s t o t h e r e s t of t h e p h o t o n i c s g r o u p h e r e in Trinity Col l ege f o r h e l p a n d e n c o u r a g e m e n t d u r i n g m y PhD.

I w o u l d e sp e ci a l l y like t o t h a n k t h e f o r m e r m e m b e r s of p h o t o n i c s g r o u p . First o f all, t h a n k s t o Dr. W e i - H u a G u o w h o a c t u a l l y s t a r t e d w o r k i n g o n t h e c o n c e p t o f s l o t t e d l asers. T h a n k s , t o Mick Lynch a n d V i n c e n t W e l d o n . M a n y t h a n k s t o m y f r i e n d s f r o m t h e office Rob,

Ciaran, Ed, D i a r mu i d a n d Pabl o. Lads t h a n k y o u v e r y m u c h f o r y o u r h e l p a n d a d vi c e in t h e

b e g i n n i n g of m y PhD a n d t h a n k s f o r c r e a t i n g a w a r m a n d f u n a t m o s p h e r e in t h e office a n d in t h e lab.

Also, I w o u l d like t o t h a n k m y f r i e n d s f r o m K a z a k h s t a n Oral, Olzat, Askar a n d O z h e t wi t h w h o m I a r r i v e d t o I re l and f o u r y e a r s a g o a n d s t a r t e d d o i n g PhD h e r e in Trinity. T h a n k y ou

g u y s f o r y o u r e n c o u r a g e m e n t a n d a s s i s t a n c e .

Special t h a n k s t o m y d e a r f r i e n d S a k e n w h o i n t r o d u c e d us t o I r e l an d a n d e n c o u r a g e d us

t o s t u d y h e r e . T h a n k s a l s o t o Prof. Igor S h e v t s w h o c a m e t o K a z a k h s t a n a n d i n t r o d u c e d Trinity Col l ege a n d w h o a l s o i nvi t e d us h e r e . W i t h o u t b o t h o f y o u p r o b a b l y I w o u l d miss t h e

Also I w ould like to than k our go vern m en tal scholarship "Bolashak" fo r th e o p p o rtu n ity

to study abroad. Thanks fo r th e m em b ers o f JSC "CIP" fo r th e ir financial support and also

m any thanks to th e fo rm e r president o f "CIP" Sayasat N urbek fo r his help and advices.

Finally, I w ould like to express a g reat thanks to my fam ily. Thanks to my sisters Lyazzat,

Zhannat and m y b ro th e r Bolat fo r th e ir love and support. And of course m any thanks to my

parents Ergebai and Salima fo r th e ir huge love, care and em o tio n al support. Thank you fo r

List of publications and presentations

Papers

[1]

Q,. Lu, W. H. Guo, M. Nawrocka, A. Abdullaev, C. Daunt, J. O'Callaghan, M. Lynch, V.

W eldon, F. Peters, and J. F. Donegan, "Single mode lasers based on slots suitable fo r

photonic integration,"

Optics express

Vol. 19, no. 26, pp. B140-B145, 2011.

[2] W. H. Guo, Q. Lu, M. Nawrocka, A. Abdullaev, J. O'Callaghan, M. Lynch, V. Weldon,

and J. F. Donegan, "Integrable slotted single-mode lasers,"

Photonics Technology

Letters, IEEE,

Vol. 24, no. 8, pp. 634-636, 2012.

[3] Q. Lu, W. H. G uo, A. A bdullaev, M. Nawrocka, T. N. Huynh, J. O'Callaghan, M.

Lynch, V. Weldon, L. P. Barry, and J. F. Donegan, "Two-section singlemode lasers

based on slots suitable fo r photonic integration,"

Electronics Letters,

Vol. 48, no. 15,

pp. 945-946, 2012.

[4]

W. H. Guo, Q. Lu, M. Nawrocka, A. Abdullaev, J. O'Callaghan, and J. F. Donegan,

"Nine-channel wavelength tunable single mode laser array based on slots,"

Optics

express,

Vol. 21, no. 8, pp. 10215-10221, 2013.

[5] Q. Lu, A. Abdullaev, M. Nawrocka, W. H. Guo, J. O'Callaghan, and J. F. Donegan,

"Slotted single mode lasers integrated w ith a sem iconductor optical am plifier,"

IEEE

Photonics Technology Letters,

Vol. 25, no. 6, pp. 564-567, 2013.

[6]

M. Nawrocka, Q. Lu, W. H. Guo, A. Abdullaev, F. Bello, J. O'Callaghan, T. Cathcart,

and J. F. Donegan, "W idely tunable six-section sem iconductor laser based on etched

slots,"

Optics Express,

Vol. 22, no. 16, pp. 18949-18957, 2014.

[7]

F. Bello, 0 . Lu, A. Abdullaev, M. Nawrocka, and J. F. Donegan, "Linew idth and noise

characterization fo r a partially-slotted, single mode laser,"

IEEE Journal o f Quantum

Electronics,

vol. 50, no. 9, pp. 755-759, 2014.

Conference presentations:

[1] Q. Lu, W . H. Guo, M . N aw rocka, A. Abdullaev, M . Lynch, V. W e ld o n , a n d J . F.

Donegan, "Slotted single-m ode lasers fo r m onolithic photonic in teg ration ." European

Conference and Exposition on O ptical Comm unications, pp. W e -1 0 . Optical Society of

Am erica, 2011.

[2] Q. Lu, W . H. Guo, A. Abdullaev, M . Naw rocka, J. O'Callaghan, M . Lynch, V. W eld o n ,

and J. F. Donegan, "Integrable single m ode lasers based on s\ots," Sem iconductor

Laser Conference (ISLC), 2 0 1 2 2 3 rd IEEE In tern a tio n a l, pp. 1 0 2 -1 0 3 . IEEE, 2 012.

[3] Q. Lu, W . H. Guo, A. A bdullaev, M . Naw rocka, J. O'Callaghan, M . Lynch, V. W e ld o n ,

and John F. Donegan. "High-yield tw o-section single m ode lasers based on a 37*^

o rd er surface grating," CLEO: Science and Innovations, pp. C Tu3N-4. Optical Society

o f Am erica, 2012.

[4] Q. Lu, W . H. Guo, A. A bdullaev, M . N aw rocka, J. O'Callaghan, M . Lynch, V. W e ld o n ,

and J. F. Donegan, "Tunable single m ode laser array based on slots," O ptical Fiber

C om m unication Conference, pp. O T h 3l-5. Optical Society o f A m erica, 2013.

[5] F. Bello, Q. Lu, A. Abdullaev, M . N aw rocka, and J. F. Donegan, "Linew idth and

threshold calculations fo r a slotted, Fabry-Perot sem iconductor laser," N u m erical

Sim ulation o f O ptoelectronic Devices (NUSOD), 2 0 1 3 1 3th In te rn a tio n a l Conference

on, pp. 9 3 -9 4 . IEEE, 2013.

[6] A. Abdullaev, Q. Lu, M . N aw rocka, F. Bello, W . H. Guo, J. 0 ' Callaghan, and J. F.

Donegan, "Im p ro ved perform ance o f slotted single-m ode lasers," Lasers an d

Electro-Optics Europe (CLEO EUROPE/IQEC), 2 0 1 3 Conference on a n d In te rn a tio n a l Q u a n tu m

Electronics Conference, pp. 1-1. IEEE, 2013.

[7] A. Abdullaev, Q. Lu, W . H. Guo, M . N aw rocka, F. Bello, J. O'Callaghan, and J. F.

Donegan, "Linew idth characterization o f slotted single-m ode lasers," Photonics

Irelan d 2 013, Belfast, 2 01 3.

[8] Q. Lu, W . H. Guo, A. A bdullaev, M . N aw rocka, J. O'Callaghan, and J. F. Donegan, "31

nm Quasi-continuous tuning single m ode laser array based on slots," CLEO:

Table of Contents

Chapter 1 - Introduction... xi

References... 7

Chapter 2 - Semiconductor lasers... 10

2.1. Fundamentals of laser diodes... 11

2.1.1. Spontaneous and stimulated em issions... 11

2.1.2. Optical g a in ... 12

2.1.3. Heterostructures...13

2.1.4. Optical resonator...15

2.2. Laser diode characteristics... 18

2.2.1. The rate equations...18

2.2.2. The spectral characteristics... 20

2.2.3. The laser linew idth... 22

2.3. Wavelength tuning...24

2.3.1. Tuning mechanisms... 24

2.3.2. Physical effects fo r electronic wavelength tu n in g ...26

References... 28

Chapter 3 - The gratings in laser diodes... 32

3.1. Introduction... 32

3.2. Scattering matrix and transmission matrix th e o ry ... 33

3.2.1. The dielectric interface... 35

3.2.2. Transmission line w ith no discontinuities... 37

3.2.3. Dielectric segment and the Fabry-Perot etalon...38

3.3. Gratings...40

3.3.1. DBRand DFB lasers... 42

3.4. Conclusion... 47

References... 48

Chapter 4 - Fabry-Perot laser waveguide measurements...50

4.1. Fabry-Perot laser introduction...50

4.2. Fourier series expansion m e th o d ... 52

4.2.1. Theoretical description of the ASE spectrum ... 52

4.2.2. Internal loss and Quasi-Fermi level separation... 55

4.2.4. Linewidth enhancem ent fa c to r... 57

4.3. Experimental m easurem ents... 59

4.4. Conclusion...67

R eferen ces...67

C hapter 5 - S lotted single-m ode lasers... 69

5.1. Introduction to slotted single mode lasers...69

5.2. Design and fa b ric a tio n ... 71

5.2.1. N um erical sim u latio n s...71

5.2.2. Fabrication process...75

5.3. Basic c h aracterizatio n ... 78

5.3.1. P ow er-current and current-voltage characteristics...78

5.3.2. Spectral characteristics...82

5.3.3. Slot w id th c h aracterization ... 84

5.3.4. M onolithically integration of slotted laser with SOA and EA m o d u lato r... 86

5.4. Linewidth m e a s u re m e n ts ... 93

5.4.1. Free space s e tu p ...93

5.4.2. Linewidth c h aracterizatio n ... 95

5.5. Conclusion... 100

R eferen ce... 101

C hapter 6 - W a v e le n g th tu n a b le laser array based on slo tted single-m ode la s e rs ... 104

6.1. Introduction to laser a rra y ... 104

6.2. 10 channel laser a rra y ... 106

6.2.1. Basic c h aracterizatio n ... 109

6.2.2. Optical sp ectru m ...I l l 6.2.3. The lin e w id th ...115

6.3. 12-channel laser a r r a y ... 117

6.3.1. The linewidth c h aracterizatio n ... 121

6.4. Therm al crosstalk...127

6.5. Conclusion... 137

R efe re n c e s ... 138

C hapter 7 - Conclusion and o u tlo o k ... 141

7.1. Conclusion... 141

Abbreviations

ASE

Am plified spontaneous emission

AR

A nti-reflection

CW

Continuous wave

CWDM

Coarse wavelength division m ultiplexing

DS-H

Delayed self-heterodyne

DBR

Distributed Bragg reflector

DFB

Distributed feedback

DWDM

Dense wavelength division m ultiplexing

EA

Electro absorption

ECL

External cavity laser

ESA

Electrical spectral analyzer

FP

Fabry-Perot

FSE

Fourier series expansion

FSR

Free spectral range

FWHM

Full w id th at half maxim um

HR

High reflection

ICP

Inductively coupled plasma

ITU-T

International Telecom m unication Union

M M I

M u ltim ode interference

ODMUX

Optical dem ultiplexer

OMUX

Optical m ultiplexer

OSA

Optical spectrum analyzer

PSK

Phase shift keying

QAM

Quadrature am plitude m odulation

QPSK

Quadrature phase shift keying

QSCE

Quantum confined Stark effect

SG-DBR

Sampled grating distributed Bragg reflector

SMM

Scattering m atrix method

SMSR

Side mode suppression ratio

SOA

Sem iconductor optical am plifier

SSG-DBR

Super structure grating distributed Bragg reflector

TEC

Therm o-electrical cooler

TM M

Transmission m atrix method

VCSEL

Vertical-cavity surface-em itting laser

Chapter 1 - Introduction

The research and d ev elo p m en t in optical fib e r com m unication systems started in 1970s a fte r th e groundbreaking discovery in fiber-optics by Charles Kao [1] w ho is known as th e "fa th e r of fib er optic com m unications". Before this b reakthrough, glass fib e r w e re not considered as a good tra n s m itte r fo r light due to large signal loss fro m th e light scattering at defects in th e glass. Kao realized th a t, by carefully purifying th e glass, it is possible to tran s m it th e in fo rm atio n o ver long distances w ith low loss via fibers. Accidentally, light was used in com m unications in earlier days. In ancient tim es, reflected sunlight was used as a p art of com m unication n e tw o rk in th e era o f Darius th e G reat in 5 0 0 BC. In th e 19^^ and early 20'^ century, an optical teleg rap h or a heliograph was invented w h e re visual signal transm ission was used. In 188 0 A lexander Bell invented th e p h o to p h o n e which allow ed tran s m ittin g speech via a beam of light. In terest in optical com m unications has rapidly increased a fte r th e invention of th e laser in 1960s. N ow optical fib e r com m unications is th e m ain technology in tran sm ittin g th e data, voice and video in fo rm atio n in short-distance and long-haul com m unications. The dem and in th e transm ission capacity is grow ing strongly year by year.

The introduction of w avelength-division m ultiplexing (W D M ) systems in 1990s has significantly increased th e in fo rm atio n capacity, in which m u ltiple signals (channels) are tra n s m itte d through th e sam e fib er. In W D M systems light sources o f d iffe re n t w avelengths are used, and th e signal is m o d u lated by an intensity m o d u lato r. The overall system is shown in Fig. 1.1. The m o d u lated signals are m ixed into th e optical fib e r and sent to an optical m u ltip lexer (O M U X ). Then th e received signal is d em ultip lexed (O D M U X ) and separated into d iffe re n t w avelengths. The separated w avelengths are then directed to corresponding detectors.

Converter 0 /E Converter

Optical fiber

The W D M systems are divided into tw o categories: Coarse W D M and Dense W D M .

Coarse W D M (C D W M ) systems use only a fe w channels w ith w idely spaced w avelengths.

The typical w aveleng th spacing is about 20 nm and m ore. C W D M is usually used in cable

television netw orks and in m e tro p o lita n netw orks. Dense W D M (D W D M ) systems in

contrast, use a large n u m b er o f channels w ith closely spaced w avelengths. The w avelength

spacing is around 1.6 nm and below , which corresponds to 2 0 0 GHz channel spacing.

M o d e rn D W D M systems have a much closer spacing o f 50 GHz which makes th e m high level

com m unication systems, th e re fo re , th e y are used as th e basis of th e In tern e t.

Early D W D M systems used fixed -w avelen g th laser diodes, which created lots of

transceivers in th e system. This w asn 't reliable in term s of size and inventory cost. W ith th e

introduction o f tu n a b le lasers, it has been possible to reduce th e inventory cost and reduce

th e size of th e systems. The main req u irem e n ts fo r tu n a b le lasers are stable o p eratio n and

exact w aveleng th emission reco m m en d ed by ITU-T standards [2]. These standardized

w avelengths are shown in Table. 1.1.

Frequency (THz) Center wavelength (nm) Frequency (THz) Center wavelength (nm) Frequency (THz) Center wavelength (nm)

195.9 1530.33 194.5 1541.35 193.1 1552.52

195.8 1531.12 194.4 1542.14 193.0 1553.33

195.7 1531.90 194.3 1542.94 192.9 1554.13

195.6 1532.68 194.2 1543.73 192.8 1554.94

195.5 1533.47 194.1 1544.53 192.7 1555.75

195.4 1534.25 194.0 1545.32 192.6 1556.55

195.3 1535.04 193.9 1546.12 192.5 1557.36

195.2 1535.82 193.8 1546.92 192.4 1558.17

195.1 1536.61 193.7 1547.72 192.3 1558.98

195.0 1537.40 193.6 1548.51 192.2 1559.79

194.9 1538.19 193.5 1549.32 192.1 1560.61

194.8 1538.98 193.4 1550.12 192.0 1561.42

194.7 1539.77 193.3 1550.92 192.9 1562.23

194.6 1540.56 193.2 1551.72 192.8 1563.05

Table 1.1 DWDM wavelengths according to ITU-T standards [2].

The fastest data rate per channel in m odern com m unication systems is 40 G b it/s and the

next g en eratio n will be 100 G b it/s. C urrent deployed optical com m unication systems use

o n -o ff keying (OOK) d etectio n or direct detectio n techniques. The fu tu re d etection

freq u ency m odulations are used instead of intensity m o d u latio n . Thereby, these com m unication systems are called coh eren t com m unication systems.

The m ain advantages o f c oh eren t com m unications is th e high signal sensitivity, in o th e r w ords signals w ith very low intensities can be d etected . In th eo ry, only a fe w photons are req uired to d etect a one bit signal. This can be done by using h etero d yn e detectio n as shown in Fig. 1.2. The tran s m itte d signal is mixed w ith a coh eren t reference signal (local oscillator) using an optical directional coupler. Then th e m ixed signal is d etected by a p h o to d e te c to r and it will contain in fo rm atio n abo u t th e a m p litud e and th e phase o f the signal. The main req uirem en ts fo r light sources are single-m ode o p eratio n and low phase noise fluctuations.

Local oscillator

Fig. 1.2 Schem atic d iag ram o f optical h e te ro d y n e d ete c tio n [3].

Modulation format Linewidth per laser at 40Gbit/s

QPSK lOMHz

8PSK 1.6MHz

16PSK 240kHz

Star 16QAM 1.6MHz

Square 16QAM 120kHz

Square 64QAM 1.2kHz

Table 1.2 L inew idth re q u ire m e n ts fo r d iffe re n t m o d u la tio n fo rm a ts fo r 4 0 G b it/s link [4].

By taking into account th e specifications and req uirem en ts described above, th e basic req uirem en ts fo r a laser source in W D M and c oh eren t com m unications can be sum m arized

as follows:

■ High o u tp u t p o w er, so th e signal can be d etected . ■ M o d u la tio n speed should be high.

■ N arro w lin ew id th to p revent chrom atic dispersion in th e fibers and to m inim ize phase noise.

■ Stable o p era tio n w ith o u t m ode hopping. ■ Feasibility, sim ple fabrication and low cost.

The last req u irem e n ts always have been im p o rta n t ones. T eleco m m u n icatio n com panies

and system m akers seek a tu n a b le laser than can w o rk in any desirable w aveleng th range and w ith low in ven tory cost.

Table 1.3 shows th e chronological d ev elo p m en t of tu n a b le laser diodes o ver th e past 20 years. H ere, th e m ost w id ely used lasers such as d istributed Bragg re flec to r (DBR) and

distributed feed back (DFB) lasers are considered. Also, th e so-called slotted lasers

developed in our group are included in th e list.

The first w aveleng th tun ab le DBR laser was d em o n strated by Toh m o ri and Suem atsu, e t al, [5] in 1983. They d em o n s trated a m onolithic in tegration o f th e laser part w ith a phase

tuning section using B u tt-jo in t coupling. The continuous tuning o f 0 .4 nm was achieved. 10 years la te r Tohm ori and Yoshikuni, e t al. have m o d ified th e grating structure and

d em o n strated a super-structure grating (SSG) DBR laser [6], w h e re th e structure is based on

which is o ver 100 nm including m u ltim o d e regions [6], Q uasi-continuous tuning of th e

SSG-DBR laser was d em on strated by Ishii e t al, w ith the tuning range o f 34 nm [7].

In th e same year w ith th e first d em on stratio n of th e SSG-DBR laser, Jayaram an et al. from C oldren's group first proposed th e sam pled grating DBR (SG-DBR) laser w h e re conventional

gratings are divided into several in terru p ted grating sections [8]. A tuning range of 57 nm

w ith SMSR m ore than 30 dB was achieved in these lasers. Later on, th e tuning range and th e

m o d e stability w e re im proved exhibiting high SMSR m ore than 45 dB and th e linew idth

b elo w 5 M H z [9]. Typical lin ew id th o f th e SSG-DBR and SG-DBR lasers is m o re than 10 M H z.

H ow ever, th e group fro m Agilent C om m unications has d em o n s trated an SG-DBR laser

in teg rated w ith EA m o d u lato r w ith a n arro w spectral lin ew id th of 2 M H z over th e tuning

range o f 40 nm and m o d u latio n speed of 2.5 G b it/s [10]. Recently, th e group fro m

U niversity o f California, Santa Barbara (UCSB) showed a lin ew id th reduction in th e tun ab le

SG-DBR laser using a frequency lock tech niqu e [11]. The lin ew id th o f th e laser was im proved

by a fa c to r o f 27.

The w avelength tu n a b ility o f DFB lasers w e re d eveloped at th e same tim e w ith DBR

lasers. A continuous tuning of 2 nm was o b tain ed in 198 6 [12]. In 1995, a 6-channel DFB

laser array was d em on strated in [13]. Later th e group fro m NEC [14], p resented an

8-channel DFB laser array com bined w ith m u lti-m o d e in terfe ren c e ( M M I) coupler. They

successfully d em on strated th e w aveleng th coverage o ver th e e n tire S, C and L bands. Similar

w o rk was carried out by researchers fro m NTT [15], [16], [17] and Furukawa [18], These

groups d em on strated 12-channel DFB laser arrays w ith a tuning range o f m ore th a n 4 0 nm

and u ltra -n a rro w linew idth b elow 2 0 0 kHz fo r all channels [19], [20].

The lasers presented above are based on low o rd er gratings (w ith grating pitch of

a b o u t~ 2 0 0 nm ), which means th a t high-resolution techniques are needed to defin e th e m

such as electron beam lithography which is an expensive and slow process. Also, lasers such

as phase-shifted DFB lasers need an additional tw o or m o re epitaxial gro w th steps. These

req u irem e n ts will lead to high m anufacturing cost. To overcom e this pro b lem , it has been

proposed to etch high o rd er gratings (slots) on to p o f th e laser ridge so no re -g ro w th is

n eed ed . It has been shown th a t by carefully positioning these slots along th e ridge, it is

possible to obtain a tu n a b le laser [21], [22]. These types o f lasers w e re developed in our

group [21], [23]. Recently, w e have d em o n s trated a slotted tun ab le laser w ith a w id e tuning

A uthor Year ^ t u n e i (nm) P . (mW) SMSR, (dB) Av,

(M H z ) Type o f laser Institution

Tohmori [6] 1993 50-100 - >35 - SSG-DBR NTT

Jayaraman [8] 1993 57 >35 - SG-DBR UCSB

Young [13] 1995 - 3 >30 - 6 channel DFB laser

array

AT&T Bell Labs

Ishii [7] 1996 34 >10 >35 >10 SSG-DBR NTT

Mason [24] 2000 >50 2-6 >35 >10 SG-DBR Agility Com.

Kudo [25] 2000 15.3 >6.9 >40 - 8 channel DFB laser

array NEC

Oohashi [26] 2001 46.9 >20 >40 - 16 channel DFB laser

array NTT

Hatakeyama

[14] 2003 >15 >20 >42

-8 channel DFB laser

array NEC

Coldren [9] 2004 >50 >20 >45 - 5 SG-DBR UCSB

Ishii [15] 2005 >27nm >30 >45 - 8 channel DFB laser

array NTT

Ishii [16] 2007 38 40 >45 <6 12 channel DFB laser

array NTT

Ishii [17] 2009 - 4 0 - 4 0 >50 0.58 12 channel DFB laser

array NTT

Horikawa [18] 2009 - 3 5 - 4 0 >40 2-4 12 channel DFB laser

array Furukaw/a

Byrne [21] 2009 >30 >30 >35 >10 Slotted tunable laser Trinity College Dublin Ishii [19] 2010 >40 - 4 0 >50 <0.16 12 channel DFB laser

array NTT

Yu [22] 2012 - 3 8 >10 >40 - Slotted tunable laser Zhejiang

University Kimoto [20] 2013 - 3 5 - 4 0 >40 <0.21 12 channel DFB laser

array Furukawa

Nawrocka [23] 2014 - 5 5 - >30 >10 Slotted tunable laser Trinity College Dublin

Guo [27] 2013 - 2 7 >37 >50 2-4 9 channel slotted laser array

Trinity College Dublin Lu, Abdullaev

[28] 2014 - 3 1 >35 >40 2-4

10 channel slotted laser array

Trinity College Dublin

Abdullaev 2014 - 3 6 >35 >50 - 0 .4 12 channel slotted laser array

Trinity College Dublin

In t h e past f o u r ye ars t h e c o n c e p t o f s lo tte d s in g le - m o d e laser has b e e n d e v e lo p e d and

e x p e r im e n t a ll y d e m o n s t r a t e d [29 ], [3 0] in o u r g ro u p . T h e m a in id ea is to d e v e lo p t h e lasers

w i t h s im ilar sp ecifications as c o m m e r c i a l devices, b u t w i t h a s im p le r fa b ric a tio n process and

high yield. By ta king in to a c c o u n t th e s e re q u ir e m e n t s , it w a s p ro p o s e d t o f o r m a tu n a b l e

laser a rr a y based on s lo tte d s in g le -m o d e lasers su ita b le f o r W D M and c o h e r e n t

c o m m u n ic a tio n s systems. T h e last t h r e e w o rk s in T a b le 1.3 a re co n s id e re d t o be o n e o f t h e

m a in results o f this thesis.

References

[1] K. C. Kao and G. A. Hockham, "D ielectric fib e r surface waveguide fo r optical frequency," Proceeding o f IEEE, vol. 113, no. 7, pp. 1151-1154, 1966.

[2] ITU-T, "G.692: Optical interfaces fo r m ultichannel systems w ith optical am plifiers," 10 1998. [Online]. Available: h ttp ://w w w .itu .int/rec/T-R E C -G .692-199810-l/en. [Accessed 04 02 1999].

[3] B. E. Saleh and M. C. Teich, Fundam entals o f photonics, Hoboken, New Jersey: John Wiles & Sons, Inc., 2007,

[4] M. Seimetz, "Laser line w id th lim ita tion s fo r optical systems w ith high-order m odulation em ploying feed fo rw a rd digital carrier phase e s tim a tio n ," in O ptical Fiber co m m un icatio n/N a tion al Fiber Optic Engineers Conference, San Diego, CA, 2008.

[5] Y. Tohm ori, Y. Suematsu, H. Tsushima and S. Arai, "W avelength tu n in g o f GalnAsP/lnP integrated laser w ith b u tt-jo in te d b uilt-in d istrib u te d Bragg re fle c to r," Electronics Letters, vol. 19, no. 17, pp. 656-657, 1983.

[6] Y. Tohm ori, Y. Voshikuni, H. Ishii, F. Kano, T. Tam amura, Y. Kondo and M. Yam am oto, "Broad- range w avelength-tunable superstructure grating (SSG) DBR lasers," IEEE Journal o f Quantum Electronics, vol. 29, no. 6, pp. 1817-1823, 1993.

[7] H. Ishii, H. Tanobe, F. Kano, Y. Tohm ori, Y. Kondo and Y. Yoshikuni, "Q uaslcontinuous w avelength tu nin g in super-structure-grating (SSG) DBR lasers," IEEE Journal o f Quantum Electronics, vol. 32, no. 3, pp. 433-441,1996.

[8] V. Jayaraman, A. M ath ur, L. A. Coldren and P. D. Dapkus, "Theory, design, and perform ance o f extended tu nin g range in sampled grating DBR lasers," IEEE Journal o f Q uantum Electronics, vol. 29, no. 6, pp. 1824-1834, 1993.

[9] L. A. Coldren, G. A. Fish, Y. Akulova, J. S. Barton, L. Johansson and C. W. Coldren, "Tunable sem iconductor lasers: a tu to ria l," Journal o f Lightwave T echnology, vol. 22, no. 1, pp. 193-202, 2004.

1349-1357, 2002.

[11] A. Sivananthan, H. C. Park, M. Lu, J. S. Parker, E. Bloch, L. A. Johansson, M. J. Rodwell and L. A. Coldren, "M onolithic linewidth narrowing of a tunable SG-DBR laser," in Optics Fiber Communication Conference , Anaheim, CA, 2013.

[12] Y. Yoshikuni, K. Oe, G. Motosugi and T. Matsuoka, "Broad wavelength tuning under single-mode oscillation w ith a m ultielectrode distributed feedback laser," Electronics Letters, vol. 22, no. 22, pp. 1153-1154, 1986.

[13] M. G. Young, U. Koren, B. I. Miller, M. Chien, T. L Koch, D. M. Tennant, K. Feder, K. Dreyer and G. Raybon, "Six wavelength laser array w ith integrated am plifier and m odulator," Electronics Letters, vol. 31, no. 21, pp. 1835-1836, 1995.

[14] H. Hatakeyama, K. Naniwae, K. Kudo, N. Suzuki, S. Sudo, S. Ae, Y. Muroya, K. Yashiki, K. Satoh, T. M orim oto, K. M ori and T. Sasaki, "Wavelength-selectable microarray light sources fo r S-, C-, and L-Band WDM systems," IEEE Photonics Technology Letters, vol. 15, no. 7, pp. 903-905, 2003.

[15] H. Ishii, H. Oohashi, K. Kasaya, K. Tsuzuki and Y. Tohmori, "High-power (40mW) L-band tunable DFB laser array module using current tuning," in Optic Fiber Communication Conference, Anaheim, CA, 2005.

[16] H. Ishii, K. Kasaya, H. Oohashi, Y. Shibata, H. Yasaka and K. Okamoto, "W idely wavelength tunable DFB laser array integrated w ith funnel combiner," IEEE Journal o f Selected Topics in Quantum Electronics, vol. 13, no. 5, pp. 1089-1094, 2007.

[17] H. Ishii, K. Kasaya and H. Oohashi, "Spectral linewidth reduction in widely wavelength tunable DFB laser array," IEEE Journal o f Selected Topics in Quantum Electronics, vol. 15, no. 3, pp. 514- 520, 2009.

[18] K. Horikawa, A. Yamomoto, T. Osada, H. Koshi, Y. Yafuso and T. Kurobe, "Development o f ITLA using a full-band tunable laser," Furukawa Review, vol. 35, 2009.

[19] H. Ishii, K. Kasaya and H. Oohashi, "Narrow spectral linewidth operation (<160kHz) in widely tunable distributed feedback laser array," Electronics Letters, vol. 46, no. 10, pp. 714-715, 2010.

[20] T. Kimoto, G. Kobayashi, T. Kurobe, T. Mukaihara and S. Ralph, "Narrow linewidth tunable DFB laser array fo r PDM-16QAM transmission," in OptoElectronics and Communications Conference , Kyoto, 2013.

[21] D. Byrne, J. P. Engelstaedter, W. H. Guo, Q. Lu, B. Corbett, 8. Roycroft, J. O'Callaghan, F. H. Peters and J. F. Donegan, "Discretely tunable semiconductor lasers suitable fo r photonic integration," IEEE Journal o f Selected Topics in Quantum Electronics, vol. 15, no. 3, pp. 482-487, 2009.

[22] T. Yu, L. Zou, L. Wang and J. J. He, "Single-mode and wavelength tunable lasers based on deep- submicron slots fabricated by standard UV-lithography," Optics Express, vol. 20, no. 15, pp. 16291-16298, 2012.

Express, vol. 22, no. 16, pp. 18949-18957, 2014.

[24] B. Mason, J. Barton, G. A. Fish, L. A. Coldren and S. P. Denbaars, "Design o f sampled grating DBR lasers w ith integrated sem iconudctor optical am plifiers," IEEE Photonics Technology Letters , vol. 12, no. 7, pp. 762-764, 2000.

[25] K. Kudo, K. Yashiki, T. Sasaki, Y. Yokoyama, K. Ham am oto, T. M o rim o to and M. Yamaguchi, "1.55-nm w avelength-selectable m icroarray DFB-LDs w ith m on olith ica lly integrated M M I com biner, SOA, and EA m od ula tor," IEEE Photonics Technology Letters, vol. 12, no. 3, pp. 242- 244, 2000.

[26] H. Oohashi, Y. Shibata, H. Ishii, Y. Kawaguchi, Y. Kondo, Y. Yoshikuni and Y. Tohm ori, "46.9nm W avelength-selectable arrayed DFB lasers w ith integrated M M I coupler and SOA," in In te rn atio na l Conference on Induim Phosphide and Related M aterials, Nara, Japan, 2001.

[27] W. H. Guo, Q. Lu, M. Nawrocka, A. Abdullaev, J. O'Callaghan and J. F. Donegan, "N ine-channel wavelength tunable single m ode laser array based on slots," Optics Express, vol. 21, no. 8, pp.

10215-10221, 2013.

[28] Q. Lu, W. H. Guo, A. Abdullaev, M. Nawrocka, J. O'Callaghan and J. F. Donegan, "31nm Quasi- continuous tu nin g single m ode laser array based on slots," in CLEO, San Jose, CA, 2014.

[29] Q. Lu, W. H. Guo, D. Byrne and J. F. Donegan, "Design o f slotted single-m ode lasers suitable fo r p hotonic integ ra tion ," IEEE Photonics Technology Letters, vol. 22, no. 11, pp. 787-789, 2010.

Chapter 2 - Semiconductor lasers

The first effort in t h e d e v e l o p m e n t of lasers w a s a c hie ve d by Albert Einstein in 1917 [1]

w h e n he i nt ro d u ce d t h e c o n c e p t of s t i mu l a t e d emi ssi on. However, it t o o k a b o u t 40 yea rs

b ef or e st i mu l at e d emi ssi on has b e e n d e m o n s t r a t e d a t mi cr o wa ve f r e que nc ie s. In 1958,

Schawlow a nd T o w n e s p r e s e n t e d t h e c o n c e p t of an optical m a s e r [2]. A f e w ye a rs later, t h e

first laser, t h e ruby laser, w a s d e m o n s t r a t e d by Ma i m an [3] in 1960. The first s e m i c o n d u c t o r

laser w a s realized s i mult ane ous ly and s e p a r at e l y by Hall [4], Quist [5], a n d Na th a n [6]. These

lasers w e r e homo -j un c t i on s ys te ms b a s e d on GaAs material. In t h e s a m e year, Holonyak [7]

d e m o n s t r a t e d t h e first visible laser d i od e on a GaP s ub s t r a t e . In 1960s, t h e laser wa s

c o n s i de r e d as a "solution looking for a p r obl em" . But t oda y, lasers h ave f o u n d a variety of

applications in m a n y fields including medici ne, optical fiber c o mm un i ca t i o ns , d a t a storage,

printers, sensor s, metrol ogy, at omi c clocks, optical inf or mat ion processi ng a nd m a n y others.

The significance of lasers h ave b e e n increasing since t he i r invention a n d t h e y k ee p o pe n i n g

up n e w a r e a s of applications [8]. In 2010, t h e 50**^ a nni ve r sa r y of t h e laser w a s c e l e b r a t e d

a n d t h e laser wa s a c k n o w l e d g e d as a mi le stone i nvent ion of t h e mi d- 20 ‘^ c e n t u r y in t h e

2.1. Fundamentals of laser diodes

S e m i c o n d u c t o r l a s e r s h a v e u n i q u e p r o p e r t i e s s u c h as c o m p a c t size, high efficiency, high

s p e e d d i r e c t m o d u l a t i o n , long l i fet i me, low p o w e r c o n s u m p t i o n a n d l ow loss. T h e r e f o r e ,

t h e s e d e v i c e s a r e t h e key c o m p o n e n t s in p h o t o n i c s , p a r ti c ul a rl y in opt i ca l c o m m u n i c a t i o n s .

In t hi s c h a p t e r w e will give t h e basi c c o n c e p t s o f s e m i c o n d u c t o r l a s e r o p e r a t i o n . W e will

i n t r o d u c e opt i cal gai n in s e m i c o n d u c t o r s , h e t e r o s t r u c t u r e s , s p e c t r a l c h a r a c t e r i s t i c s , t h e

l i n e wi d t h a n d t unabi l i t y.

2.1.1. Spontaneous and stimulated emissions

Fig. 2.1 s h o w s t h e basi c t y p e s of r ad i a ti v e t r a n s i t i o n . T h e solid circles a r e t h e e l e c t r o n s

a n d t h e o p e n circles a r e t h e ho l e s . T h e first t r a n s i t i o n is a s p o n t a n e o u s r e c o m b i n a t i o n

w h e r e a n e l e c t r o n - h o l e p a i r r e c o m b i n e s s p o n t a n e o u s l y t o e m i t a p h o t o n . T h e e m i t t e d

p h o t o n h a s a r a n d o m p h a s e a n d d i r ec t i o n . If t h e n u m b e r o f s u c h e m i s s i o n s will b e large,

t h e n it will r e s u l t in i n c o h e r e n t e m i s s i o n . An e x a m p l e of a d e vi c e w o r k i n g o n s p o n t a n e o u s

e m i s s i o n is t h e light e m i t t i n g d i o d e (LED) w h e r e p h o t o n f e e d b a c k is n o t p r o v i d e d . T h e

s e c o n d t r a n s i t i o n is t h e s t i m u l a t e d c a r r i e r g e n e r a t i o n . In t hi s t r a n s i t i o n t h e e l e c t r o n is

initially in t h e l o w e r e n e r g y level a n d t h e p h o t o n a b s o r p t i o n will e xc i t e t h e e l e c t r o n t o t h e

h i g h e r level in t h e c o n d u c t i o n b a n d ( Ec ) wh i l e l e a vi ng t h e h o l e in t h e v a l e n c e b a n d ( Ev ) . Th e

t h i r d t r a n s i t i o n is t h e s t i m u l a t e d e m i s s i o n . H er e a n i n c i d e n t p h o t o n e x c i t e s t h e s y s t e m t o

p r o d u c e e l e c t r o n - h o l e r e c o m b i n a t i o n a n d a t t h e s a m e t i m e a n e w p h o t o n is g e n e r a t e d . T h e

g e n e r a t e d p h o t o n h a s t h e s a m e p h a s e , f r e q u e n c y a n d d i r e c t i o n a s t h e i n c i d e n t p h o t o n .

T h e r e f o r e , a large n u m b e r o f s u c h p r o c e s s e s will r e s u l t in c o h e r e n t e m i s s i o n . This p r o c e s s

Ec

#

Es

AAT

AAT

O

#

Fig. 2 .1 Electronic transition b e tw e e n th e conduction and valence bands. Solid circles correspond to filled

states (electrons). O pen circles correspond to unfilled states (holes), (le ft) Spontaneous rec o m b in a tio n ,

(cen ter) Stim u lated c a rrier g e n e ra tio n , (right) S tim u lated reco m b ination .

The processes described above relate to radiative transitions w h e re th e energy is released as a photon. T h ere is a n o th er typ e o f tran sitio n, n o n -ra d iativ e process. In non- radiative processes, th e elec tro n -h o le pair recom bines w ith o u t e m ittin g a p h oton. Instead of this, th e energy can be dissipated as h eat and g en e rate phonons or it can be given to electron or hole in the form of kinetic energy. The la tte r process is called Auger reco m b in ation . N o n -rad iative recom bination m ay also occur due to im p erfections and defects in th e active region o f th e laser.

2.1.2. Optical gain

Several conditions should be satisfied to realize laser diode [9]:

■ an optical gain to provide am plification ■ an optical w aveguide to confine th e photons ■ an optical reso n ato r to provide a feedback

The optical gain in sem iconductor lasers can be achieved w hen th e stim ulated emission in a strongly p um ped region is larger than th e optical losses. In sem iconductor lasers, the active region acts as a gain m edium w h e re th e optical field propagates. If w e consider a plane w ave, w e can characterize th e propagation o f this w ave in th e m edium throu g h the com plex propagation constant

w h e re /cg is th e free-space propagation, n ' and n " are th e real and im aginary parts o f the com plex refractive index. From [10], th e intensity o f th e plane w ave varies exp o n en tially

/ = — n 'l^ o P exp(2/con"z),

w h e re Eq is th e a m p litu d e o f th e e le c tric fie ld . The co rre sp o n d in g gain is a negative value o f th e loss and can be d efined as

The im p o rta n t p ro p e rty o f laser diodes is th a t th e gain is ve ry large, w h ich is o rd e r o f m agn itu d e larger th a n any o th e r types o f lasers [11]. T h e re fo re , dio d e lasers can be very small b u t have high pow er. A d d itio n a l fe a tu re o f laser diodes is th a t th e gain curve is w id e (~ 1 0 nm ), w h ich is due to o p tica l tra n s itio n b e tw e e n a p a ir o f energy bands, ra th e r th a n w e ll-d e fin e d a to m -lik e states. A n o th e r fe a tu re is th a t th e re la tio n s h ip b e tw e e n th e gain and th e ca rrie r density can be a p p ro x im a te d as a lin e a r fu n c tio n [12]

w h e re a is a d iffe re n tia l gain and Nq is a tra n sp a re n cy value o f th e c a rrie r density. The lin e a r a p p ro x im a tio n in (2.4) is invalid fo r q u a n tu m w e ll lasers. The d e s c rip tio n o f q u a n tu m w e ll lasers w ill be given in th e fo llo w in g sections.

2.1.3. Heterostructures

The basic stru c tu re o f th e s e m ic o n d u c to r lasers include an active region (u ndope d) sandw iched b e tw e e n tw o w id e -b a n d g a p m aterials, one n -doped and second p-doped fo rm in g p-n ju n c tio n . Such ty p e o f s tru c tu re is know n as a d o u b le -h e te ro s tru c tu re . The co n ce p t o f th e d o u b le -h e te ro s tru c tu re laser was in tro d u c e d by A lfe ro v [13] and K roem er [14] in 1963. The firs t co n tin u o u s-w a ve (CW) o p e ra tio n o f these lasers was d e m o n s tra te d in 1970 [15], [16]. A band diagram o f a fo rw a rd -b ia s e d d o u b le -h e te ro s tru c tu re is show n in Fig. 2.2. U nder fo rw a rd bias c o n d itio n , e le ctro n s fro m th e n-doped region and holes fro m th e p- doped region are in je cte d in to th e active region. Due to th e high-ban dgap energy fro m th e b o th sides o f th e active region, th e in je cte d e le ctro n s and holes c a n n o t escape th e active region. Therefore, th e carriers are co n fin e d in th is region and fo rc e d to re co m b in e to g e n e ra te photons. M o re o v e r, th e active region also co n fin e s th e p h o to n s because th e

(2.3)

refractive index of the active region is higher than the refractive indices of the carrier confinement layers. Thus, the active layer will serve as an optical waveguide. For efficient recombination, direct bandgap semiconductor materials should be used. Typically, these are compound materials and well known I I I - V materials, including GaAs, InP, InGaAs and InGaAsP materials.

n-type j undoped [ p-type

© — :

--- l e e e

Fig. 2 .2 Sim plified band diagram o f d o u b le -h e te ro s tru c tu re u n d er fo rw ard -b ia s condition . The b o tto m plot

shows th e guided w aveg u id e m ode.

Energy

Q W

^ Bulk

Density o f states

Fig. 2 .3 D e n s ity o f s ta te s o f q u a n tu m w e ll (solid lin e ) a n d b u lk s e m ic o n d u c to r (d a s h e d c u rv e ).

Although, th e carrier c o n fin em en t is very strong in q u an tu m wells, th e optical

c on fin em en t is w eak due to th e small thickness o f th e active region. To increase th e optical

con fin em en t, an additional optical w aveguide around th e q u an tu m w ell is needed. Such

structures are called separate c o n fin em en t h ete ro stru ctu re q u an tu m w ell (SCH-QW )

structures. If th e w aveguide consists of a graded com position, th en th e corresponding

structure will be a graded refractive index SCH-QW structure (GRINSCH-QW structure).

M o re details abo u t q u an tu m w ell structures and physics can be found in [17], [18]. Due to

th e ir unique optical and electronic properties, q u an tu m w ell structures are used in alm ost

all m odern laser diodes.

2.1.4. Optical resonator

As was m en tio n ed above, to g et th e laser to w o rk optical feed back is required. The

feedback is g en erated by placing an active m edium in an optical resonator, w h e re th e light

is reflected b etw e en tw o mirrors. Usually in laser diodes, th e feed back is provided by simply

cleaving th e facets form ing tw o parallel mirrors. In th e previous section, w e described

dou ble-h etero structures w h e re th e active region acts as a dielectric w aveguide due to

higher refractive index than th e c o n fin em en t layers. The dielectric w aveguide consists of

one or m ore guided transverse modes, which are polarized e ith e r transverse electric (TE) or

transverse m agnetic (T M ). Since th e active layer acts as a dielectric w aveguide, these modes

layer is defined by the confinement factor

F ,

d,

F

(2.5)

where \Ey\ is the amplitude of electric field (with assumption that TE mode is dominant).

The F factor is dependent on refractive index difference and the active layer thickness. The wavelength dependent modal gain, is given by this F factor and the active layer gain and w ritten as

F ig. 2 . 4 S c h e m a t i c c r o s s -s e c tio n o f a d i e l e c t r ic w a v e g u i d e w i t h t h ic k n e s s d . r i i is t h e r e f r a c t i v e in d e x o f a c t iv e l a y e r , a n d Uq is r e f r a c t i v e in d e x o f c o n f i n e m e n t la y e r s [ 1 0 ] .

The simplest type of a laser is a Fabry-Perot laser which typically consists of tw o cleaved facets with m irror reflectivity and

/?2

p

d e f f ~ ^ 9 a - (2.6)

X

no

d/ 2

0

d

n i

-d/2

no

(2.7)

9 n e t ~ d e f f (2.8)

P2 R2

Gain medium

P i

Fig. 2 .5 S c h e m a t ic v i e w o f F a b r y - P e r o t la s e r w i t h l e n g t h L a n d m i r r o r r e f l e c t i v i t ie s R j a n d /?2'

To satisfy a steady-state oscillation condition, th e ro u n d -trip gain o f th e cavity m ust be

equal to 1 and can be expressed as

r^V2e

=

1

,

w h e re r j and r2 are th e a m p litu d e reflectivities. The im aginary and real parts o f equation

(2.9) give tw o d iffe re n t conditions, one is fo r th e a m p litu d e and th e o th e r is fo r th e phase

condition. From th e phase condition, th e ro u n d -trip phase is equal to an in teg er m u ltiple of

2n.

b etw e en tw o mirrors. This will fo rm a set o f longitudinal m odes w ith w avelength

X

2rigfj^L

m

(

2.10)

and m ode spacing

A A p p —

X r

X.

w h e re n ^ ,e // is th e group refractive index.

The spectrum o f a Fabry-Perot laser is m u m o d e as it is shown in Fig. 2.6. The m u

lti-m ode spectrulti-m has negative consequences in long-haul fib er-o p tic colti-m lti-m unications due to

th e dispersion in th e fibers which can lim it th e achievable data transm ission rate. For this

purpose, a single-m ode laser is required. The ways to achieve a single-m ode o p eratio n will

-70

-1520 1530 1540 1550 1560 1570 1580 W avelength (nm)

Fig. 2 .6 Optical spectrum o f a Fabry-Perot laser w ith emission peak at 1 5 5 0 nm. T he individual lines are th e

longitudinal m odes o f th e laser.

2.2. Laser diode characteristics

2.2.1. The rate equations

The simplest way to describe the behavior of laser diodes is to use the rate equations.

These equations express the balance of carriers and photons in the system. The rate

equations for the carrier density

N

Np^

can be w ritten as [10], [19]

where

Vg

R ( N )

gain and

R^p

For a steady-state case, carrier and photon densities do not change

dyv

/

d t ~ qVa

^adeffNph

R (N ).

2

11

diV

dF = '’ ’

d t

Fig. 2.7 shows a schematic view of the output power as a function of current and carrier density as a function of current. The point where the output power increases linearly is called the threshold current which can be obtained from the rate equation (2.11), assuming that the photon density is zero

Itn = qVaR(Nt n), (2.14)

where the carrier density remains constant above threshold N = and this behavior is called gain-clamping.

Pout N

s tim u la te d emission

AP

Spontaneous emission

/

Fig, 2 .7 Schem atic view o f (left) o u tp u t p o w e r vs. c u rre n t, and (right) carrier d en sity vs. cu rre n t. Below laser thresh old th e re is no p hoton e m itte d e xcep t spontaneously e m itte d photons th e re fo re , th e laser o p erates in spontaneous emission m ode. Above thresh old , th e laser o u tp u t p o w e r increases linearly w ith th e cu rre n t. The c a rrier density has a constant value above thresh old and this e ffe c t is called gain-clam ping.

The photon density above threshold will be w ritten as

I - Itn

N h = --- (2.15)

where Uf-gf is the total cavity loss which is the sum of mirrors loss (a^j^) and internal loss

(2.16)

Above t h e t h re s h o l d cur rent , t h e laser o u t p u t pov\/er i ncr e ases linearly with t h e injection

cur rent . The differential efficiency

r j a

is def ine d as

w h e r e d P / d / i s t h e slope efficiency whi ch s h o ws t h e efficiency of a laser d i od e and

m e a s u r e d in a units of mW/ mA.

The left h an d side of Fig. 2.7 is usually r e fe r r ed as a l ight-current curve

o r L — I

curve.

From this curve it is possible t o d e t e r m i n e t h e basic characteristics such as t hr es ho l d

cur rent ,/ fft , a n d sl ope efficiency, d P / d / . Mor e ove r , fr om t h e L — / curve internal laser

p a r a m e t e r s can be calculated, including t h e internal optical losses a n d t h e q u a n t u m

efficiency of a laser.

The

L — I

curve is st rongly af fe c te d by t h e t e m p e r a t u r e an d t h e t h re s h o l d c ur re nt

c h a n g e s exponentially with t h e c h a n g e of t e m p e r a t u r e

w h e r e

T

is t h e characteri stic t e m p e r a t u r e which def ine s t h e t e m p e r a t u r e stability of t he

device. F urt her exp l an at i o n a b o u t t h e characteristic t e m p e r a t u r e a n d t h e m e a s u r e m e n t

r esults will be given in C h a p t e r 5.

2.2.2. The spectral characteristics

T h e r e ar e t w o basic t y p e s of lasers, which ar e t e r m e d si ng le - mo de a n d m ul t i - m od e . We

have al re ady d e m o n s t r a t e d a Fabry-Perot laser o p e r a t i n g with a m u l t i - m o de s p e c tr u m. It

wa s m e n t i o n e d t h a t it has a n ega ti ve infl uence on d a t a t rans mi ss ion t h e r e f o r e , t h e spectral

purity of laser d io de s is crucial. In this sec t i on w e will i n t ro du ce s i ng l e- mo de o p e r at i o n .

A laser can be def ine d as a si ngl e-mode, if t h e ratio of t h e p o w e r in t h e s t r o n g e s t m o d e

d ete rm in es the single-m ode o p eratio n is called a sid e-m o d e suppression ratio (SMSR). The

SMSR can be calculated using rate equations [20] fo r steady-state condition

=

0

R,p(A^) +

=

0

RspiK^,) +

2

20

w h e re m and m - I - 1 are the d o m in an t and the second strongest m odes, respectively. The

SMSR will be th e ratio of these modes

S M S R = (2.2 1)

^PUm+i

From the exp erim ental m easurem ents, th e SMSR can be calculated as th e d ifference in

th e optical p o w er of th e main m ode and th e next strongest m ode. An exam p le of this single­

m o d e o p eratio n is d em on strated in Fig. 2.8. The optical p o w er of th e m ain m ode is -1 0 dBm

and th e optical p o w er o f th e next strongest m ode is around -56 dBm. Hence, th e SMSR of

th e given single-m ode laser is around 46 dB. It should be noted th a t th e SMSR u n it is given

in dB and not in dBm. The reason is th a t dB is used to evalu ate th e ratio b e tw e e n tw o

optical pow ers (intensities), w h ile dBm is used to q u an tify an absolute value o f th e pow er.

-10

-20

I -30

-40 0)

O -50 CL

...

-70

1550 1560 1570

1520 1530 1540

W a v e le n g th (nm )

Fig. 2.8 M ea s u re d optical spectru m o f a sing le-m od e laser.

T h ere are several ways o f achieving single-m ode o p eratio n such as m aking a laser cavity

t h u s onl y o n e m o d e will exi st n e a r t h e gain p e ak . O p e r a t i o n o n a s i n g l e - m o d e c a n b e also

a c h i e v e d by us i n g m u l t i p l e - m i r r o r r e s o n a t o r s [21]. E x a mp l e s o f a ch i e vi n g s i n g l e - m o d e

o p e r a t i o n a r e p r e s e n t e d in [11]. H o w e v e r , t h e m o s t w i d el y u s e d m e t h o d is t o u s e p e r i o di c

s t r u c t u r e s o r g r a t i n g s f o r m o d e selectivity. It s h o u l d b e m e n t i o n e d t h a t t h e l a se rs in t hi s

t h e s i s a r e b a s e d o n h i g h - o r d e r g r at i n g s . T h e d e t a i l e d d e s c r i p t i o n of g r a t i n g t h e o r y a n d t h e

e x a m p l e s o f l as er s e x p l o i t in g g r a t i n g s in t h e i r s t r u c t u r e s will b e gi ven in C h a p t e r 3.

2.2.3. The laser linewidth

S c h a w l o w - T o w n e s l i n ew i d t h

T h e s p e c t r a l l i n e wi d t h o f a l a s e r d i o d e is a n i m p o r t a n t p a r a m e t e r in c o h e r e n t

c o m m u n i c a t i o n s y s t e m s a n d in t h e s y s t e m s w i t h high o r d e r o p ti c a l m o d u l a t i o n f o r m a t s

[ di fferent i al p h a s e shi f t i ng keyi ng (DPSK) a n d d i f fe re nt i a l q u a d r a t u r e p h a s e shi ft keyi ng

(DQPSK)]. For t h e s e high s p e e d m o d u l a t i o n f o r m a t s , t h