High-power ring laser frequency-modulated combs

Abstract

High-power, broadband frequency combs generated by semiconductor lasers have profound implications for on-chip spectroscopy. Here, we present a dry-etched racetrack quantum cascade laser that uses resonant radio-frequency injection to produce a unidirectional “quantum walk” frequency comb at mid-infrared wavelengths. Efficient light outcoupling from the racetrack resonator provides more than 100 mW of continuous-wave output power at room temperature, with beam quality on par with that of Fabry–Perot lasers. Experimental waveform reconstruction confirms that the combs are frequency-modulated, rather than amplitude-modulated (as in active mode-locking). We show excellent agreement between the experimental waveforms and numerical simulations, which are based on a reduced Maxwell–Bloch model of the laser and include fast gain dynamics, finite group velocity dispersion, and large Kerr nonlinearity. Our optimized device architecture—featuring thick Si 3 N 4 passivation and reduced parasitic capacitance—enables modulation bandwidths exceeding 10 GHz. Moreover, by asymmetrically biasing the split bus waveguide contacts, we achieve digital control over emission directionality within the racetrack without compromising comb performance. The unidirectional laser also exhibits inherent immunity to extreme levels of delayed optical feedback, underscoring the platform’s robustness for precision spectroscopic applications. Combined with high output powers and the potential for monolithic integration of multiple ring lasers and waveguide couplers on a chip, these devices serve as ideal sources for fully integrated dual-comb spectrometers.