Alan Kost

Frontiers in OpticsTucson, AZ

October 20, 2005

Monolithically Integrated Semiconductor Components for Coarse Wavelength

Division Multiplexing

OUTLINE

• The need for photonic integrated circuits

• Coarse wavelength division multiplexing- Arrayed waveguide gratings on InP

• Quantum Well Intermixing- GaAsSb Quantum Wells

Low cost is the driver

ELECTRONIC ICs

A WDM Data Link

LASEROPTICAL

MODULATOR

MULTIPLEXER DE-MULTIPLEXER

1

2

n

1, … n ADD

FIBER

POWERAMP DROP

i

j

PRE-AMP

IN-LINEAMP

1

2

m

VARIABLEATTENUATOR

PHOTODIODE

Components are numerous and expensive

OURPROGRAM

AWG/SOA Concept

1 cm

ArrayedWaveguide

Grating (AWG)

1 to 9Coupler

9 to 4Coupler

4Semiconductor

Optical Amplifiers(SOAs)

4

InP Substrate

= 20nm

De-multiplexing, amplification, and equalization on one, InP chip

1 cm

Coarse Wavelength Division Multiplexing

DWDM

• Very high throughput (80 channels over C-Band from 1530 to 1565 nm)

• For use in long haul networks

• Stabilized lasers and narrow-band filters required

32 nm

50 GHz (~0.4 nm at 1.55 m)

CWDM

• Smaller number of channels and correspondingly smaller throughput• For use in short to medium haul networks

• Compatible with less expensive, un-stabilized lasers and broadband filters

340 nm (1270 – 1610 nm)

= 20 nm

InP

ARRAYED WAVEGUIDE GRATINGS:“HORSESHOE” TYPE

SYMMETRY LINE

Tra

nsm

issi

on

1 2 3 4 1 2 3 4

OutputWaveguide #

Free Spectral Range OPTICAL PATHLENGTH = L

L + L

L + 2L

L + 3LL + 4L

L + 5L

L = m

1

Small for CWDM

FSRL

L

~

An “S-SHAPED”Arrayed Waveguide Grating

This kind of AWG has not been previously fabricated in semiconductor

The optical path difference between waveguides in the array can be made arbitrarily small reducing the angle subtended by the arc.

Arc

Star Coupler

Star Coupler

OutputWaveguides

WaveguideArray

Input

Shallow Ridge Waveguides

0

2

4

6

8

10

0 1000 2000 3000 4000 5000 6000 7000 8000

Bend radius range for AWGsB

end

Lo

ss (

db

/cm

)

Bend Radius (m)

FundamentalMode

First HigherOrder Mode

2.5 or 3.5 m

1.45 m

0.11 m 0.30 m

InP

InGaAsP

InP 1.45 m

0.30 m

InP

InGaAsP

InP

Loss = 4.5 dB/cm

Patterned AWGs

Star Coupler

“S”

1-micron

AWG Response

-35

-30

-25

-20

-15

-10

1460

1470

1480

1490

1500

1510

1520

1530

1540

1550

1560

1570

1580

Wavelength (nm)

Tra

nsm

itte

d P

ow

er (

dB

)

Channel Width ~ 7 nm (FWHM)Cross talk ~ - 15 dB or less

Channel3

4 5 6 7 8 1

1473 15091527

154615621490

Proper AWG design should include chromatic dispersion

Yurt, Rausch, Kost, Peyghambarian, Opt. Express 13, 5535 (2005)

L small Insensitivityto dimensional

error

AWG/SOA Concept

1 cm

AWG A passive device

4SOAs

Active devices

4

InP Substrate

Conventional Approach: Epitaxial Re-Growth

Epi-Layers for AWG

SUBSTRATE

Epi-Layers for AWG

SUBSTRATE

Epi-Layers for AWG

SUBSTRATE

Epi-Layers for SOAs

1st growth of epi-layers for AWG

Selective area etch to substrate

Re-growth of epi-layers forSOAs

Technical Problems:• Poor morphology for re-grown layers• Vertical misalignment of AWG and SOA

layers• Rough AWG/SOA interface

LowYield

Ion-Induced Band Gap Modification

HEAT

ION MASK

BARRIER

QUANTUM WELL

BARRIER

DEFECT

HIGH ENERGY IONS

DIFFUSION

AB

SO

RP

TIO

N C

OE

FF

ICIE

NT

PHOTON ENERGY

ABSORPTIONEDGE

BLUE SHIFT

Advantage-No re-growth

Disadvantage- Constraints on layers

AMPLIFIER BANDWIDTH

Useful amplification range forSemiconductor Optical Amplifier

(or Erbium-Doped Fiber Amplifier)

SOA bandwidth is insufficient to cover all CWDM wavelengths

340 nm (1270 – 1610 nm)

= 20 nm

QUANTUM WELL INTERMIXINGTO ADJUST AMPLIFICATION RANGE

4

4

AB

SO

RP

TIO

N C

OE

FF

ICIE

NT ABSORPTION

EDGE

BLUE SHIFT

4321

Candidates Materials

• InGaAsP - conventional material, limited tuning range• GaInNAs• GaAsSb

GaSb MATERIALS FOR 1.5 MICRON DEVICES

GaAs

LATTICE CONSTANT IN ANGSTROMS

0.5

1.0

1.5

2.0

5.6 5.7 5.8 5.9 6.0 6.1 6.2

BA

ND

GA

P W

AV

EL

EN

GT

H(M

ICR

ON

S)

2.5

3.0

3.5

Substrate

AlSb

GaSb

AlGaSb

GaAsSb

Candidates

AlGaSb(nearly indirect band gap)

GaSb Quantum Wells(indirect gap)

GaAsSb Quantum Wells

GaASSb/AlSb Quantum Wells

G.Griffiths, K.Mohanned, S.Subbana, H.Kroemer and J.L.Merz, Appl. Phys. Lett. 43, 1059 (1983)

 

 

L

Г

X

L

Г

X

AddingQuantum

Confinement

  

 

Indirect band gap

GaSb

Adding As + Quantum

Confinement

GaAsSb

GaAsSb Quantum Wells

Photoluminescence increases dramatically with As content

1200 1300 1400 1500 1600 1700 1800

0.00

0% AsSb1690

9.1% As

GaAsSb/AlSbQuantum Wells

15.1% As

18.8% As

31% AsSb1704

Sb1707

Sb1720

Sb1682

Ph

oto

lum

ines

cen

ce (

arb

. un

its)

Wavelength (nm)

x 10

GaSb Cap 

AlSb

GaAsxSb1-x

 

AlSb

 

GaSb Substrate

60X

Kost, Sun, Peyghambarian, Eradat, Selvig, Fimland, and Chow, Appl. Phys. Lett. 85, 5631 (2004).

GaAsSb Quantum Wells

The shift is the largest for any quantum wells (in the telecom band)

= 140 nm, E = 86 meV = 195 nm, E = 123 meV

BORON ION IMPLANTATION (~300 keV, 3x1013 cm-2)

Sun, Peyghambarian, Kost, Eradat, Appl. Phys. Lett. 86, (2005)

Summary

• AWG for CWDM

→ Demonstrated first semiconductor AWGs for CWDM using an flexible “S-shape”

• Band Gap Modification for Heterogeneous Integration

→ GaAsSb/AlSb quantum wells show promise

(enabling technology for PICs)

(lower cost devices)

Future Directions

• Application for CWDM AWG- Combined wavelength and time-division

multiplexing

• New materials for intermixing- GaInNAs- Sb quantum wells on InP

© 1998 - 2005 Christian L. Deichert