The authors report the fabrication of widely tunable monolithic quantum cascade lasers (QCLs) with coupled Fabry–Pérot (FP) cavities on indium phosphide. Quasicontinuous tuning of the single mode emission over a total spectral range of 242 nm was realized at two regions between 8.394 and 8.785 μm. An absorption experiment with ammonia shows principle feasibility of gas detection with multisegment QCL devices. Good agreement of the experimentally observed tuning behavior with the one expected from calculated FP mode-combs indicates that the change in the refractive index is mainly due to thermal heating as a result of current injection.

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Quantum cascade ridge waveguide lasers with coupled ring resonators have been fabricated. Coupling of an actively pumped microring resonator to a ridge waveguide device allows for filtering the numerous Fabry–Pérot modes emerging in the ridge waveguide. Due to the large free spectral range of the ring resonators mode selection is accomplished, resulting in stable single mode emission for an optimized design. Thus, side mode suppression ratios of up to 26 dB over a temperature range of 140 K are attained by on-chip coupling of a ridge waveguide device with a microsquare ring resonator. The tuning rate of the microring resonator laser is 0.40 nm/K. Output powers of several milliwatts are obtained.

Edge emitting quantum cascade microlasers based on InP have been fabricated and investigated. Deeply etched Bragg mirrors have been manufactured at the end of the resonators, to reduce the mirror losses of short cavity devices. In order to achieve the required high aspect ratio, an optimized plasma dry etching process has been developed, which allows monolithic device fabrication in a single dry etching step. The short resonator length results in a large Fabry-Pérot mode spacing, which leads to a single mode operation. A side mode suppression ratio of about 20dB at 80K has been observed.

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