Mid-infrared optical solitons may be a powerful tool for applications in on-chip integrated photonics and spectroscopy, as they provide broadband, phase-locked frequency combs. Quantum cascade lasers (QCLs) embedded in a ring cavity have been found to enable self-starting optical soliton generation. In order to study these phenomena numerically, we use a model based on coupled Maxwell-density matrix equations. The introduction of backscattering in our model stabilizes self-assembled soliton field solutions, which is in very good agreement with experimental data. In this contribution, we present our model and discuss the mechanisms that lead to soliton operation in ring QCLs.

We present low threshold quantum cascade surface emitting lasers (QCSELs) emitting at wavelengths of 4.5 micrometers or 8 micrometers. To extract the light vertically from the InP-based buried heterostructure laser a second order InGaAs/InP grating is used. Both ridge facets are formed by dry-etching followed by coating a dielectric-metal film. Due to the high reflectivity of the facets, the cavity can be shortened well below 500 micrometers reducing the threshold power to several hundred milliwatts. The proposed device concept allows large-scale fabrication and wafer-level characterization. The results are an important step towards low-cost and low-power consuming quantum cascade lasers for portable MIR gas sensors.

The quantum cascade laser (QCL) based ring structures are becoming hot research areas for laser beam control [1] and mode-locking applications [2] . Recently, with the strong nonlinearity of active region, mid-infrared frequency comb based on ring QCLs have been demonstrated [3] , [4] , showing great potential of pulse generation without using active mode-locking scheme. In this work, we report the development of mid-infrared frequency comb based on the ring QCL with optimized structure. Besides the unique lasing mode dynamics of the ring QCLs, the device shows a spectrum regime which can be reasonably fitted by a sech 2 function.

Room temperature surface emission is realized on a large area (1.5 mm × 1.5 mm) photonic crystal quantum cascade laser (PhC-QCL) driven under pulsed mode, at the wavelength around 8.75 μm. By introducing in-plane asymmetry to the pillar shape and optimizing the current injection with a grid-like window contact, the maximum peak power of the PhC-QCL is up to 5 W. The surface emitting beam has a crossing shape with 10° divergence.