A high-resolution microcavity transmission spectrometer

Spectral analysis is one of the most powerful tools for studying and understanding matter. As a key branch, absorption spectroscopy is widely used in material detection, isotope analysis, trace gas detection, and the study of atomic and molecular hyperfine structures. Traditional mode-locked optical frequency combs, which feature broad spectra and low repetition rates, have enabled high-precision absorption measurements through dual-comb techniques. These combs have found applications in trace gas detection, spectral imaging, and isotope analysis. However, their complexity, bulkiness, and large size limit their use outside laboratories. In contrast, low-noise optical frequency combs generated by optical micro-resonators offer significant potential advantages for spectroscopy due to their chip-scale size and lightweight design. We present a microcavity-based transmission spectrometer using a single silicon nitride microcavity soliton, achieving a 4 THz bandwidth with 200 kHz resolution. This system combines the stable dissipative Kerr soliton (DKS) comb from a silicon nitride micro-resonator with the dual-sideband scanning from an intensity electro-optic modulator (EOM), transferring sub-Hz RF precision to the optical domain. The resulting frequency-modulated (FM) comb inherits the high precision of the RF domain, with optical accuracy dominated by the pump laser and repetition rate stability. The DKS comb allows independent locking of the pump laser and repetition rate, facilitating ultra-precise FM comb generation. The frequency-modulated comb is then imaged onto a 2D CCD array using a VIPA in tandem with a diffraction grating, enabling the recording of a composite spectrum during scanning. It is anticipated that using an ultra-narrow linewidth laser locked to an ultra-stable cavity as the pump source could enable Hz-level precision and stability. Given the integration advantages of the key components in this approach, it holds significant potential for future miniaturization, offering vast possibilities for compact, high-precision spectroscopic measurements.