Mid-Infrared Quantum Cascade Lasers: Dual-Wavelength DFBs, Dispersion Engineering and RF-Injection Control of Frequency Combs

Abstract

The mid-infrared range of the electromagnetic spectrum is of particular interest, as it allows access the fundamental ro-vibrational transitions of many key pollutant and greenhouse gas molecules. Quantum cascade lasers (QCLs) are unipolar semiconductor devices, based on intersubband transitions in coupled quantum wells and barrier systems, which have become the dominant light source in this spectral region, as well as the THz. These devices are robust, monolithic and electrically pumped and they deliver considerable output power in continuous-wave operation. They offer the possibility of precise mode control via distributed-feedback (DFB) gratings or other methods, in order to target a speci c wavelength. Quantum cascade lasers are also characterized by their broadband emission: they naturally operate as frequency combs, in both the mid-infrared and THz making them prized devices for coherent spectroscopy in those spectral regions. The characteristics of a QCL device operating as a frequency comb are a broad and phase-locked optical spectrum of equidistant modes, together with a strong narrow radio frequency (RF) beatnote generated by the beating of these optical modes. The presented work is separated into four main parts. The rst part involves the characterization of strain-compensated, high-wallplug e ciency QCL active regions, and the evaluation of MBE growth conditions. The optimal growth methods are then identi ed and used later on for the fabrication of devices used in the rest of the projects. The second part is dedicated to the design, simulation, fabrication and characterization of mid-infrared, dual-wavelength DFB QCLs for the speci c targeting of greenhouse and pollutant gases. Three di erent designs are proposed and fabricated, and successfully implemented in an absorption spectroscopy setup, allowing the detection of trace gases with a high precision down to parts per billion. The third part covers the design and fabrication of group velocity dispersion (GVD) compensation schemes, monolithically integrated with the fabricated device, in both the short (4-5 m) and long (7-10 m) midinfrared wavelengths. For the longer wavelengths, the coupling of the fundamental laser mode to a plasmon propagating in the top, heavily doped layer of the device cladding is used. This coupling is carefully tuned by changing the thickness and doping of the cladding, keeping the optical losses in an acceptable level, and the amount of compensated GVD enough to allow stable comb operation. The results are stable frequency combs, with high output powers and broad emission spectra, which are then used in multiple dual-comb spectroscopy schemes with excellent performance. For the shorter wavelengths, since the aforementioned method cannot be used, an alternative method was implemented. A passive InGaAs waveguide is integrated with the buried heterostructure device, allowing the comb operation with a broad dynamical range, also in this spectral region, which traditionally is quite challenging to produce frequency combs due to the high values of GVD naturally occuring in this range. The fourth part involves the design and fabrication of a mid-infrared QCL frequency comb with a geometry that enhances its high frequency response and subsequently its comb characteristics. The design is basically a microstrip-like geometry, with a buried heterostructure. As a result, the device can be injection locked over an extended range ( up to 1.5 MHz for shorter devices) and with low injection powers, which outperforms normal buried heterostructure schemes by more than an order of magnitude. Moreover, under RF injection at powers higher than 20 dBm, the lasing spectrum is attened and signi cantly broadened, from 24 cm 1 to 65 cm 1 in bandwidth, while at the same time, the coherence of the comb is maintained and veri ed by a coherent beatnote spectroscopy technique, SWIFTS. The dynamics of the comb are extensively studied under the in uence of varying the injection frequency and power, and analysed.