Tunable mid-infrared light sources based on intersubband transitions

This thesis describes how for the first time, unidirectional operation and coupled ring tuning were realised on a quantum cascade laser material; specifically on a new strain compensated In0.7Ga0.3As/AlAs0.6Sb0.4 grown on InP substrate and operates in pulsed mode in the 3-4 micron hydrocarbon absorption region. Unidirectional ring lasers have the advantages that, in the favoured emission direction, they can have up to double the quantum efficiency of bidirectional lasers and do not suffer from spatial hole burning. In this work, this operation was realised by incorporating an "S"-crossover waveguide into the ring cavity in a manner that it introduces non reciprocal loss and gain in the counter-clockwise (CCW) and clockwise (CW) directions respectively. The measured result showed higher quantum efficiency in the CW. In fact at 1.5 times the threshold current, 90 % of the light was emitted in the favoured CW. On the other hand, the coupled ring quantum cascade laser showed nearly single mode operation, with side mode suppression ratio ~22 dB. Continuous wavelength tuning of about 13 nm was observed from one of these devices, at a tuning rate of approximately 0.4 nm/mA.EThOS - Electronic Theses Online ServiceGBUnited Kingdo