Design and Realisation of InP and InAsP QDs Passively monolithic Mode-Locked Lasers

Abstract

This thesis examines the feasibility of using Indium Phosphide and Indium Arsenide Phosphide Quantum Dots for passive mode-locking through modal gain and absorption under forward and reverse bias, a novel semi-empirical approach and experimental work on fabricated passively monolithic mode-locked lasers (MLLs). There are two device configurations used in this work. The first device is a non-lasing segmented edge-emitting laser. The device’s contact width is 50 m, and its section length is 300 m. This device is used to characterise the modal gain and modal absorption of two studied materials. The second set of devices are fabricated passively monolithic MLLs with a shallow ridge of 2 m and total cavity length of 3 mm, gain- section length of 2400 m and saturable absorber-section length of 600 m. These second set of devices are used to measure and characterise the MLLs and to evaluate the reliability of the semi-empirical approach. Both devices are broad-area lasers with cleaved facets. The characterisation of the reverse-biased saturable absorber-section using the segmented contact method demonstrated red-shift with the application of reverse bias between 0– 6 V with an absorption tuning wavelength range of 728 nm–735 nm and 773 nm–781.8 nm for Indium Phosphide and Indium Arsenide Phosphide Quantum Dots, respectively. A novel semi-empirical approach has been established to simulate the conditions of mode-locking regimes; and accordingly, design the best-performing Indium Phosphide and Indium Arsenide Phosphide mode-locked lasers in terms of absorber-to-gain length ratio. The approach used the combination of modal gain under forward bias and modal absorption under reverse bias, with particular attention paid to absorber-to-gain length ratio. The ratios investigated are 1:4, 3:17, 1:9 and 1:19. This approach enabled the prediction that both materials with a total cavity length of 3 mm will not provide mode-locking regime or lasing when the ratio of the saturable absorber length exceeds 20%. However, InP QDs are predicted to produce ML regimes only for device designs of 1:4, 3:17 and 1:9, while InAsP QDs are predicted to produce ML for device designs of 1:4, 3:17 ,1:9 and 1:19. Mode-locking pulses were obtained from a fabricated Indium Phosphide Quantum Dot passively monolithic MLLs with absorber-to-gain length ratio of 1:4. Mode-locking pulse obtained with pulse width as short of 6 ps and repetition frequency of 12.5 GHz. This work provides a valuable study of the performance of InP and InAsP self- assembled QDs passively monolithic MLLs operating in the 690 nm to 780 nm wavelength range, considering significant device performance parameters such as gain current density, saturable absorber reverse bias, absorber-to-gain length ratio and total cavity length.