Novel Integrable Semiconductor Laser Diodes

Semiconductor Laser Laboratory

Novel IntegrableSemiconductor Laser Diodes

J.J. ColemanUniversity of Illinois

1998-1999 Distinguished Lecturer SeriesIEEE Lasers and Electro-Optics Society


Definition of the Problem

1. Epitaxial structure optimizationLasers and other optical devices generally have very different optimum layer structures

2. Cleaved facet resonatorsDifficult (impossible) processingPoor optical coupling to other elements

Why aren’t conventional semiconductor diode lasers particularly suitable for integration?


Outline

• Engineering in the optical path - selective area epitaxy

• Integrable laser resonators• Examples of lasers integrated with other

optical devices• Summary


Approaches to Wafer Engineering

• Universal substrate– Compromise epitaxial layer design

• Regrowth/overgrowth/multiple growth– Coupling and plane-of-propagation

issues

• Selective area epitaxy– Multiple regrowths

T.L. Koch and U. Koren, OFC’92 Tutorial


Single Stripe Pattern

• SiO2 field• single open stripe• stripe width 25-150 µm

SiO2


Selective Epitaxy Boundary Conditions

boundary layerN = constant

= g(x)∂N∂y= 0∂N

∂y

= 0∂N∂x

= 0∂N∂x

= 0∂N∂y


Simulation Results

isoconcentration profiles thickness profile


Modeled and Experimental Data

• two stripe widths (50 and 125 µm)

• modeled (dashed)• experimental (solid)

0

500

1000

1500

2000

2500

3000

-20 0 20 40 60 80 100 120 140Distance (µm)


Wide Stripe Impracticalities

• Growth rate enhancement is too large

– Poorer quality materials

– Less control over thickness, especially for thin layers

• Deep bowing

– Makes subsequent processing difficult

– Yields non-uniform quantum well thicknesses

But, the basic parameters determined from these structures can be used to model more complex structures


Dual Stripe Pattern

• open field• dual stripe• stripe separation 2-5 µm• stripe width 2-25 µm• dimensions small with

respect to a diffusion length


Dual Stripe Growth Enhancement

0

1

2

3

-60 -40 -20 0 20 40 60

Rel

ativ

e T

hick

ness

Distance (µm)

25 µm6 µm

n

n

nn

nnn

n

0

1

2

3

0 5 10 15 20 25E

nhan

cem

ent

Fac

tor

Oxide Stripe Width (µm)

w w


Buried Heterostructure Growth Process

a) buffer layer and lower confining layer

b) selectively grown active layer

c) upper confining layer and cap layer


Buried Heterostructure SEM Cross Section


Buried Heterostructure L-I Curves

0

20

40

60

80

100

120

140

160

180

0 200 400 600 800 1000Current (mA)

length = 760 µm

0

1

2

3

0 5 10 15I (mA)

Ith = 2.65 mA


980 nm Wavelength Control

nnn

n

n n

n

n

9400

9600

9800

10000

10200

10400

10600

10800

0 5 10 15 20 25Dual oxide stripe width (µm)

n

nnnn n

nnnnn n

9400

9500

9600

9700

9800

0 2 4 6 8 10 12Element Number

S = 5.5-10.5 µm


1.55 µm Wavelength Control

• 110 nm wavelength range• uniform PL intensity• uniform PL half-width

Masahiro Aoki et al. IEEE J. Quantum Electron. 29, 2088 (1993)


Basic Buried Heterostructure Building Block

• Engineered transition energy• Automatic lateral optical waveguide• Many relatively lossless coupling schemes are possible


Integrable Resonator Geometries

• Etched Fabry-Perot facets– Scattering losses, verticality, flatness,

coupling

• Corner reflectors and ring geometries

– Mode selection

• Distributed feedback (DFB) resonators

– Processing, sensitivity to reflections

• Distributed Bragg reflectors (DBR)

– Processing, coupling


Tunable DBR Reflectors

• Asymmetric (thin p-layer) cladding structure• Conventional ridge waveguide or selective-area epitaxy buried

heterostructure• Deep surface-etched DBR grating (1st, 2nd, or 3rd order)• Separate grating electrode for tuning purposes

Igain IDBR

AlGaAs:p

AlGaAs:n

InGaAs-GaAs active layer

GaAs:n


First-Order DBR Grating Lasers

• First-order gratings formed with RIE

• Six grating periods studied• Threshold currents below 10 mA

and as low as 6 mA• Slope efficiencies greater than 0.4

W/A• Minimum emission linewidth about

the same as the measurement resolution ~ 40 kHz

n

nnnnn

n

n

0

20

40

60

80

100

120

140

160

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Line

wid

th (

kHz)

20° C

Inverse Power (mW-1)

0

5

10

15

20

25

0 10 20 30 40 50 60 70 80 90 100

Pow

er (

mW

)

Current (mA)

-75

-50

-25

0

1.048 1.058 1.068In

tens

ity (

dB)

Wavelength(µm)


Spectra versus DBR Tuning Current

• T = 20C and drive current fixed at 40 mA• Single mode spectrum preserved over 100 mA tuning

current range• SMSR greater than 35 dB over an 8 nm tuning range• Current injection heating dominant tuning mechanism

llll l l

l l ll l

ll

ll

ll

l

l

1006

1008

1010

1012

1014

1016

0 20 40 60 80 100

Wav

elen

gth

(nm

)

DBR Current (mA)

-70

-60

-50

-40

-30

-20

-10

0

1000 1005 1010 1015 1020 1025

Rel

ativ

e In

tens

ity (d

B)

Wavelength (nm)

I DBR =

0 mA 50 mA 100 mA

20° CI gain = 40 mA


1.3 µm InGaAsP Ridge Waveguide DBR Lasers

• First-order gratings formed by chemically-assisted ion beam etching (CAIBE)

• Compressively-strained MQW active region

• SMSR of ~ 40dB

InP:n InGaAsP MQW active region

0.5 µm

Gain Section

First-Order DBR

Grating

0.2 µm

InGaAsP etch stop layer

InP:u/p

InP:p

0

5

10

15

20

25

30

35

40

Pow

er (m

W/fa

cet)

Current (mA)0 20 0 20 0 20 0 20 0 20 0 20 0 20 0 20 40 80 120

Ith = 26.5 ± 0.26 mA

η = 0.290 ± 0.0047 W/A

-60

-50

-40

-30

-20

-10

0

1.355 1.36 1.365 1.37 1.375

Inte

nsity

(dB

)

Wavelength (µm)


Selection of Integrated Photonic Devices

• Laser - external modulator• Laser - photodiode• Lasers for remote sensing applications• Multiple wavelength sources• An eight-channel transmitter


Example: Integrated Laser-Modulator

M. Aoki et al. IEEE J. Quantum Electron. 29, 2088 (1993)


Integrated Laser/External Modulator

• Buried heterostructure laser

grown by selective-area

epitaxy

• Tunable DBR grating as part of

the resonant cavity

• Blue-shifted electro-absorption

modulator

MQW EA Modulator

0.4 µmAl0.60Ga0.40As:p

Al0.60Ga0.40As:n

GaAs substrate

MQW DBR Laser

1 µm

DBR Gratings

SiO2

SiO2

(a)

(b)

λLD λM

150 µm


Integrated Laser/External Modulator

• Thresholds around 10 mA, single longitudinal mode operation• 18 dB extinction ratio at 1 V bias • 40 dB extinction ratio at 1.25 V bias with single mode fiber

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70

Pow

er (

mW

)

Current (mA)

-60

-40

-20

0

1.015 1.025 1.035

Pow

er (

dB)

Wavelength (µm)

-18

-15

-12

-9

-6

-3

0

3

6

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Opt

ical

Pow

er (

dBm

)

Modulator Bias (V)

AR/HR CoatedUncoated


Integrated Laser-Photodiode

• RIE etched laser facet

• PD redshifted by 150Å

• PD input facet angled by θ

• four-up contacts for flip chip bonding

LD p-contact

LD n-contact PD n-contact

PD p-contact

semi-insulating GaAs substrateGaAs:n + buffer

AlGaAs-GaAs-InGaAs selective area epitaxy

laser structure θ

d

0

2

4

6

8

10

12

14

16

18

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90 100LD Current (mA)

0.219mW/mA

8.9 µA/mA


DBR Lasers at 915-930 nm Wavelengths

• Modified active region for shorter strained-layer emission wavelength

• Second-order gratings with three different periods

• Three emission wavelengths (915, 925, 935 nm)

AlIn

0.3 µm

1.0 µm

700 Å

20 Å

0.1 µm

0 0.2 0.4 0.60.13

80 Å

-50

-40

-30

-20

-10

0

0.91 0.915 0.92 0.925 0.93 0.935 0.94

Rel

ativ

e In

tens

ity (

dB)

Wavelength (µm)

(a) (b) (c)

0

2

4

6

8

10

12

14

16

18

20

0 10 20 30 40 50 60 70 80 90 100

Out

put P

ower

(m

W)

20C

Igain (mA)

λ = 916 nm

λ = 934 nm

λ = 924 nm


Application to Remote Sensing

• White cell filled with humid air

• Path length 85 m• Coarse tuning with

temperature, fine tuning with DBR current

• Lorentzian fit to the data within 6% of HITRAN values for halfwidth

Balanced Receiver

ADC

White Cell

TE Cooler

DBR Current Source

Gain Current Source

Mounted Laser

Variable Attenuator

center wavelength = 1009.274 nm

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

9907 9907.2 9907.4 9907.6 9907.8 9908

Abs

orpt

ion

(%)

Wavenumber (1/cm)


Dual (Redundant) Source

0

1

2

3

4

5

0 10 20 30 40 50 60 70 80 90Current (mA)

0.99 1.01 1.03 1.05 1.07Wavelength (µm)

Channel 2Channel 1

V2=-2

V1=0V2=0

V1=-2V2=0

V1=0


Multiple Wavelength Laser

A. Talneau et. al. Photon. Technol. Lett. 11, 12 (1999)

• Conventional growth for strain-compensated MQW active region

• Selectively-grown passive regions• Grating pitches adjusted for 4 nm spacing


Dual Wavelength DBR Laser

• Single ridge waveguide output aperture

• Separate grating elements • Three grating period combinations

1 µm

Λ2

Λ1

Sample Λ 1 Λ 2

1 161.1 161.9

2 161.1 162.7

3 161.1 164.2

Λ1

Λ2

Gain Section


Dual Wavelength DBR Laser

• Single mode operation (~30 db SMSR)• Dual operation over 10-12 mA range

1.042 1.047 1.052 1.057 1.062

Inte

nsity

(Lo

g sc

ale)

Wavelength (µm)

32 mA

30 mA

35 mA

37 mA.

42 mA

I = 45 mACW, 15°C

Sample ∆ λ1 4.1

2 8.4

3 16.9

1.045 1.050 1.055 1.060 1.065 1.070 1.075

Inte

nsity

(10

dB

/Div

)

Wavelength (µm)

(a)

(b)

(c)

CW, 15°C


Example: 8-Channel Transmitter

C.H. Joyner et al. IEEE Photonics Technol. Lett. 7, 1013 (1995)


Summary

• Precise control of emission wavelengths can be obtained by selective area epitaxy

• The DBR reflector is a good candidate for an integrable resonator

• High performance novel integratedphotonic devices are possible


I would like to thank….

M. Aoki (Hitachi)J.A. Dantzig (Illinois)P.D. Dapkus (USC)J.G. Eden (Illinois)C. Jagadish (ANU)A.M. Jones (Illinois)C.H. Joyner (Lucent)S.M. Kang (Illinois)

R.M. Lammert (Ortel)P.V. Mena (Illinois)M.L. Osowski (Illinois)S.D. Roh (Illinois)G.M. Smith (ATMI)T. Tanbun-Ek (Lucent)W.T. Tsang (Lucent)G.S. Walters (LEOS)

Novel Integrable Semiconductor Laser Diodes

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