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

1. 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

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

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

4. Wafer structure N cladding layer Active layer P cladding 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

5. Material properties 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 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

6. 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

7. 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 SOA DBR Gain SA Slot 180nm

8. 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

9. 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

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

11. 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.

12. Mode locking results ∆ʋ = 1.3 MHz Gain current 200mA, SOA current 250mA, SA -4V 26.3 ps 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.

13. Mode locking results FWHM 1.9 nm Δλ=1.14nm Gain current 220mA SOA current 250mA

14. Output peak power and TBP Gain current 220mA, VSA = -4V SOA current = 250mA, 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.

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

16. 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

17. Acknowledgements • The technical staff of JWNC at the University of Glasgow • This work is a part of EPSRCEP/E065112/1‘High Power, High Frequency Mode-locked Semiconductor Lasers’ and funded by Higher education commission of Pakistan.