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Simulations of Distributed Bragg Reflector Multi-Mode Interference

Bistable Laser Diodes (DBR-MMI-BLDs)

Maura RaburnMitsuru TakenakaYoshiaki Nakano

2/13

Reset

Set

Output

Output

Cleaved-Facet MMI Bistable Laser Diode

Active MMI coupler Saturable Absorber

MMI features:–tolerance, compactness–large bandwidth–Low polarization sensivity

M. Takenaka, et al., IEEE PTL Vol. 15, No. 8, pp. 1035-1037, 2003

Set

Cross-Coupled Modes

Reset

High Speed:Speeds above relaxation resonance frequency: Overlap of lasing modes

3/13

Need for Cascadable Flip-Flops• Single flip-flop not very useful• All-optical network switching:

– Allow label-reading for routing including storage/retrieval from optical memory

– Important for bit-length, TDM/WDM conversion

• Smaller, cheaper, more powerful circuits: on-chip integration necessary

Burst/ Packet

Label

100……

1Payload

To set optical switch

… … …

1

0

0

FIFO All-optical stack

Optical gates

1

1

1

1 11

1

0 0

0

0

Flip-flop

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Techniques for Cascading Lasers with Waveguide Devices On-Chip

• Flip-chip bonding

• Etched facets

HR-CoatLaser Waveguide

Output Light

Laser Active Waveguide

Output Light

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Distributed Bragg Reflectors for Cascadability

• Lasing with on-chip integration before and after flip-flop—no cleaved facets necessary

• Directional output• Single-moded output• Control of output wavelength

– Different wavelengths for output and output possible• Wavelength conversion• Lmmi≈600µm,Wmmi=12µm all simulations

Saturable AbsorberλDBR1min refl..

Lmmi

Wmmi

LDBRoutLSA

SET

RESET OUTPUTDBRin1

DBRin2

DBRout2

DBRout1

λDBR2min refl.

λDBR2

λDBR1

OUTPUT

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DBR-MMI-BLD structure

• Integration of active gain/SA region with passive DBRs and input/output guides

• Effective mirror method to account for DBRs

Gain Front DBR

n-InP

p-InP clad

1250-nm InGaAsP core

1550-nm InGaAsP MQW

Rear DBR

input light output light

SA

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SA Material Absorption:Reverse Bias• Complete & rigorous calculation requires carrier

density dependence, but difficult to obtain carrier density decay constant for reverse bias– Experimental estimate from 1-D BLDs:

– Need to dampen as laser starts lasing • Carrier density-dependent absorption has “built-in”

damping through rate equation• Damping dependent on magnitude and change of photon

density

)6.01(100.11

)8.21(3300),(

1314

11

rev

p

revprev

VVcmN

VVcmNV

−−

−−

+×+

+=α

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Side Mode Suppression Ratio (SMSR)

Gratings must not be too deep/short from SMSR, but cannot be too shallow from DBR length issues: κ≈30cm-1 for SMSR > 35dB

25

30

35

40

20 40 60 80 100 120 140 κ [cm-1]Etch depth [nm]5 １0 30 4020

SMSR

[dB

] LSA=50µm

LDBRout=50µm, 0VLDBRout=200µm, 0V

LDBRout=200µm, -1V

αm=mirror loss⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−∝

"",,

, 10

1

onth

out

m

m

IPSMSR

λ

λ

αα

Choose fraction of power emitted from output=0.8

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-40

-30

-20

-10

0

10

1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

L-I Simulations: Reverse Bias

• Threshold current and hysteresis width increase with SA loss

Inte

nsity

[dB

m]

Current density [kA/cm2]

LSA=50µm, 0V

LSA=100µm, 0V

LSA=50µm, -1VLDBRout=100µm

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“On” State Output Power• As mirror reflectivity increases, less power out of laser• Large SA absorption, small reflectivity: SA absorption

changes little “on” vs. “off” so I-Ith small

Uniform photon density:

( )thmi

mout IIP −

+∝

ααα

LSA=50µm, 0VLSA=100µm, 0VLSA=50µm, -1V

Pout [dBm]

LDBRout [µm]

Rear DBR

Gain (MMI) SA Front DBR

Photon density

z

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15

20

25

30

35

40 60 80 100 120 140 160 180 200

Extinction Ratio• Dominated by MMI splitting, practical value ~15dB

SPONTL

out

out

PePsplittingmmiP

SASA +≈ ∆− α)_(

LSA=50µm, 0VLSA=100µm, 0VLSA=50µm, -1V

Extinction Ratio [dB]

LDBRout [µm]

Choose LDBRout=200µm from AR coat concerns

Extinction Ratio

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-20

-15

-10

-5

0

5

-24 -20 -16 -12 -8 -4 0

• LSA=50µm, 0V: Switching achieved at –2dBm– LSA=100µm, 0V: switching ~2dBm– LSA=50µm, -1V: switching <-10dBm!

Input Power [dBm]

Output Power [dBm]

LSA=50µm, 0V J=0.562kA/cm2Larger SA loss requires larger switching power, but higher bias current provides higher gain

DBRout1 TotalDBRout2 TotalDBRout1, Lasing Mode

All-Optical Switching： LDBRout=200µm

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Conclusion• SMSR≈35dB predicted• LDBR, LSA, and reverse bias affect output

power, extinction ratio• Total prototype device length 1.2mm• DBRs offer not only cascadable, directional,

good SMSR devices but also control over output power and extinction ratio

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Bias Current• Bias current determined by device dimensions, SA

bias—not much flexibility

-30

-20

-10

0

10

1.7 1.75 1.8 1.85 1.9 1.95 2

SAbigthbias JJ α≈Intensity [dBm]

Current density [kA/cm2]

Jbias [kA/cm2]

LDBRout [µm]

κ=30cm-1

F=0.8LSA=50µm, 0V

LSA=100µm, 0VLSA=50µm, -1V

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DBR Length vs. Grating Depth• Gratings must not be too shallow• Choose splitting fraction F=0.8

DBRoutoutDBRinout

DBRoutout

PPP

F__

_

+=

200

400

600

800

1000

20 40 60 80 100 120 140 160 180 κ [cm-1]Etch depth [nm]5 20 5035

LDBRout + LDBRin [µm]

— LDBRout=200µm, F=0.8— LDBRout=50µm, F=0.8— LDBRout=50µm, F=0.95— LDBRout=50µm, F=0.99