Integrated Platform for High Power Quantum Walk Frequency Combs

High-power, broadband frequency combs generated by semiconductor lasers have profound implications for on-chip spectroscopy [1]. Recently, a new regime of frequency comb formation, called “Quantum Walk” was discovered in fast semiconductor ring lasers [2]. Here, we present a dry-etched racetrack quantum cascade laser (QCL) that harnesses resonant RF injection to produce the same unidirectional “quantum walk” frequency comb at mid-infrared wavelengths [3]. Evanescent field coupling to a waveguide, facilitated by the dry etch platform, enables efficient outcoupling from the racetrack geometry allowing above 100 mW of continuous-wave output power at room temperature, with beam qualities typically only achievable in Fabry-Perot lasers. Additionally, waveform reconstruction measurements confirm that the combs are frequency-modulated, rather than amplitude-modulated as in classical active mode-locking. Finally, our optimized device architecture-with thick <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Si}_{3}\mathrm{N}_{4}$</tex> insulation and low parasitic capacitance-enables 3 dB RF cutoff frequencies exceeding 10 GHz, paving the way for novel high-speed comb applications, such as dual-comb spectroscopy on a single chip. Figure 1 shows a typical device as well as gives an overview about the capabilities of the platform. Particularly noteworthy is the achieved output power of 105.6mW without modulation while the highest quantum walk comb power was measured at 80.9mW.