High power (130 mW) 40 GHz 1.55 μm mode-locked DBR lasers with integrated optical amplifiers

J. Akbar

, L. Hou, M. Haji,

,

M. J. Strain, P. Stolarz, J. H. Marsh, A. C. Bryce and A. E. Kelly

Outline

• • • • • • • Motivation Wafer structure Material properties Device features & fabrication Device structure Device characterization Conclusions

Motivation

• • • • • • Terahertz Generation OCDMA Non-linear optical effects RZ source Optical sampling Pumping

N cladding layer

Wafer structure

P cladding layer Active layer substrate MQW decreased to 3 Farfield reduction layer AlGaInAs/InP epitaxial structure with 3- quantum well active layer. A 160nm thick Far field reduction layer (FRL) and 0.75

µm thick InP spacer layers were incorporated in the lower cladding to increase spot size while maintaining single mode operation

Material properties

Higher gain saturation energy E sat broadening in the gain section is desirable in MLLs as it reduces pulse

E sat



hvA



dg

/

dN

Increase in Esat can be achieved by:



Increasing mode cross sectional area A.



Decreasing optical confinement factor Γ



Decreasing the differential gain dg/dN Increasing A/ Γ increases saturation output power of SOAs

P sat



AI

 

FRL expands the near field towards n-cladding which results in reduced free carrier absorption.



Increase in near field pattern results in low divergence angles which improves coupling with single mode fibers

Device features

•

Optimised 3QW AlGaInAs/InP material

•

Planarisation using Hydrogen Silsesquioxane (HSQ)

•

Avoids breaks in p-contact metallization

•

Simulated results shows reduced optical losses in the DBRs

•

Surface-etched DBR :

• • •

Require only a single epitaxial growth step Simultaneously fabricated with the ridge waveguide Al-containing active layers can be used without the risk of oxidization

•

1mm long curved SOA with tilt angle of 10 degrees is fabricated

Device Structure

Cavity length = 1125 μm DBR length = 150 μm SOA length= 1000 μm SOA output tilt angle= 10˚ Gratings period ( Λ)= 734 nm Slot width = 180 nm DBR effective length = 70 μm Slot 180nm

Power measurements

Power measured from SA facet: DBR current fixed at 5mA, SOA is floating Average output power in mode locked conditions from SA side is ~ 28mW

Power measurements

Power measured from SOA end: DBR current fixed at 5mA, SOA current 250mA Average output Power in mode locked conditions from SOA end is ~ 130mW

Power measurements

Power measured from SOA end: DBR 5mA, Gain 250mA DBR current 5mA, SA reverse voltage -4V

LI & optical spectrum of SOA

SA, Gain and DBR floating, SOA biased Low amplitude of modulations in the optical spectrum indicates that effective reflectivity from the tilted facet is sufficiently reduced.

Small peaks in the optical spectrum is due to DBR stop band.

Mode locking results

Gain current 200mA, SOA current 250mA, SA -4V 26.3 ps Δt = 3.3 ps ∆

ʋ = 1.3 MHz

Minimum pulse width of 3.3ps

assuming Gaussian fit. RF peak is ~45dB above the noise floor with RF linewidth of 1.3MHz.

FWHM 1.9 nm

Mode locking results

Δλ=1.14nm

Gain current 220mA SOA current 250mA

Output peak power and TBP

SOA current = 250mA, SA = -4V Gain current 220mA, V SA = -4V With increase in SOA current, output peak power also increases whilst TBP is constant at around 0.47. This shows near transform limited pulses over wide range of SOA currents.

Farfield measurements

Devices with integrated 1mm long SOA Farfield-3D view Vertical direction Farfield-2D view Horizontal direction

Conclusions

• • • • • • • Mode-Locked DBR Laser with integrated SOA : Surface etched DBR mode locked laser Novel epitaxial structure with optimized 3 QW active region and FRL High average output power 130mW and peak power > 1W in mode locked operation Integration of SOA increases output power by a factor of ~ 5.

Minimum pulse width of 3.3 ps bandwidth of 1.14 nm with 3 dB optical spectral and TBP of 0.45 assuming Gaussian shaped pulses Reduced divergence angle Output peak power can be increased by further increasing the mode size or increasing the reflection bandwidth of DBR

Acknowledgements • • The technical staff of JWNC at the University of Glasgow This work is a part of EPSRC

EP/E065112/1

‘High Power, High Frequency Mode-locked Semiconductor Lasers’ and funded by Higher education commission of Pakistan.

HIGHER EDUCATION COMMISSION Islamabad (Pakistan)