simulations of distributed bragg reflector multi … of distributed bragg reflector multi-mode...
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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
4/13
Techniques for Cascading Lasers with Waveguide Devices On-Chip
• Flip-chip bonding
• Etched facets
HR-CoatLaser Waveguide
Output Light
Laser Active Waveguide
Output Light
5/13
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
6/13
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
7/13
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
8/13
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
10/13
“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
13/13
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
14/13
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
15/13
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