On-Chip Coherent Dual-Comb Spectroscopy

Microresonator-based frequency comb generation has been widely applied in optical communication, optical sensing, frequency metrology, arbitrary waveform generation, optical ranging, and astronomy. Through nonlinear interaction in high-quality factor (high-Q) microresonators, a broad comb spectrum can be generated with low intensity noise and high coherence. Recently, the technique has demonstrated the possibility to operate on-chip dual-comb spectroscopy with two microresonators either in serial or in parallel. For traditional spectral measurement, it requires slow dispersive spectrometer with calibration or bulky Michelson interferometer with moving components for Fourier-transform infrared spectrometer (FTIR). Dual-comb spectroscopy alleviated these requirements. By introducing slightly different repetition rate frep1 and frep2 (with different microresonator geometries), these dual combs provide beatnotes and downmixs the optical frequency to radio-frequency (nΔf=n|frep1-frep2|, n is integer) in the interferometry through a fast photodiode and real-time scope. The beatnotes are then reconstructed by fast Fourier transform and therefore the absorption of the chemical / biological sample could be determined. However, those studies required soliton state operation for dual-comb spectroscopy. With delicate pump operation, the comb is transitioned from chaos state to coherent state with multiple soliton, and eventually to a single soliton state. A stable, narrow linewidth, and fine-tuned laser is required with optimized thermal and laser tuning control. Therefore, it strongly limited the tunability for both comb repetition rate difference (Δf=|frep1-frep2|) and spectral resolution (determined by frep1 and frep2). In this project, we propose a simple, tunable on-chip dual-comb operation in waveguide normal dispersion regime. Through mode-coupling, comb could be initiated with high temporal coherence. Even without mode-locking (short pulse operation), this dual-comb offers the potential for future on-chip dual-comb spectroscopy. Two microresonators in serial are fabricated by sharing the same pump lines at a single bus waveguide. A thermal heater on the microresonator or auxiliary ring will be studied to tune the coupling wavelength and match the pump line at selective position. Similar to the soliton operation, these microresonators are designed with slight different repetition rate, providing optical beatnotes and downmixing the optical frequency to radio-frequency (RF). The goal is to achieve noise-free dual-comb spectra with controllable repetition rate from 100 GHz to 200 GHz. The repetition rate difference is designed to be 1GHz to 2GHz (different chips) which results in a spectral bandwidth up to 4THz with a >20 GHz fast photodiode / real-time scope. In addition, with this simple coherent operation, the repetition rate could be possibly tuned by a second thermal heater. We expect the tunability of repetition rate difference is up to 250MHz which provides a 1.3X tunability for spectral bandwidth.