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

Wafer structureN cladding layer P cladding layerActive layer

MQW decreased to 3

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

substrate

Farfield reduction layer

Material properties

dNdg

hvAEsat /

,s outsat

AIP

Increasing A/Γ increases saturation output power of SOAs

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

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

Increase in Esat can be achieved by:

Increasing mode cross sectional area A.

Decreasing optical confinement factor Γ

Decreasing the differential gain dg/dN

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 StructureCavity 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

SAGain

DBR SOA

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 measurementsPower measured from SOA end:

DBR current 5mA, SA reverse voltage -4V

DBR 5mA, Gain 250mA

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 resultsGain current 200mA, SOA current 250mA, SA -4V

∆ʋ = 1.3 MHz

Δt = 3.3 ps

Minimum pulse width of 3.3ps assuming Gaussian fit. RF peak is ~45dB above the noise floor with RF linewidth of 1.3MHz.

26.3 ps

Mode locking results

Gain current 220mA SOA current 250mA

Δλ=1.14nmFWHM 1.9 nm

Output peak power and TBP

SOA current = 250mA, SA = -4V Gain current 220mA, VSA = -4V

With increase in SOA current, output peak power also increases whilst TBPis constant at around 0.47. This shows near transform limited pulses over wide range of SOA currents.

Farfield measurementsDevices with integrated 1mm long SOA

Vertical direction Horizontal direction

Farfield-3D view Farfield-2D view

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 with 3 dB optical spectral bandwidth of 1.14 nm 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)