Comb-locked frequency-swept synthesizer for high precision broadband spectroscopy

Abstract

frequency combs have made optical metrology accessible to hundreds of laboratories worldwide and they have set new benchmarks in multi-species trace gas sensing for environmental, industrial and medical applications. However, current comb spectrometers privilege either frequency precision and sensitivity through interposition of a cw probe laser with limited tuning range, or spectral coverage and measurement time using the comb itself as an ultra-broadband probe. We overcome this restriction by introducing a comb-locked frequency-swept optical synthesizer that allows a continuous-wave laser to be swept in seconds over spectral ranges of several terahertz while remaining phase locked to an underlying frequency comb. This offers a unique degree of versatility, as the synthesizer can be either repeatedly scanned over a single absorption line to achieve ultimate precision and sensitivity, or swept in seconds over an entire rovibrational band to capture multiple species. the spectrometer enables us to determine line center frequencies with an absolute uncertainty of 30 kHz and at the same time to collect absorption spectra over more than 3 THz with state-of-the-art sensitivity of a few 10 −10 cm −1. Beyond precision broadband spectroscopy, the proposed synthesizer is an extremely promising tool to force a breakthrough in terahertz metrology and coherent laser ranging.

High-Resolution Quantum Cascade Laser Dual-Comb Spectroscopy in the Mid-Infrared with Absolute Frequency Referencing

“…This method is compatible with other integrated comb technologies featuring multi-GHz repetition frequencies such as micro-resonator combs [4], and could also be applied to comb-calibrated spectroscopy [11]. Thus, our agile spectrometer provides unique performances in terms of acquisition speed and frequency accuracy compared to other broadband spectrometers with small (< 10 MHz) spectral point spacing [11,19,23,25,26]. Our spectrometer is a promising candidate towards an on-chip gas sensor that is highly sensitive and selective to many relevant molecules, allowing real-time monitoring of complex gas mixtures at Hz-level refresh rates.…”

Absolute Frequency Referencing in the Long-Wave Infrared using a Quantum Cascade Laser Frequency Comb

“…The large free spectral range of 10 GHz of the QCL-OFC is practical for closed-loop combassisted spectroscopy over hundreds of MHz, as the problem of the heterodyne beat reaching near zero frequency and folding is avoided [21,34]. However, the available detectors can provide sufficient SNR for phase-locking only up to 1.5-GHz offsets, such that accessing the full optical spectrum requires changing the transition frequency of the locked laser.…”

“…Moreover, they are polyvalent in their application to spectroscopy [11], e.g. in dual comb arrangements [12][13][14][15][16], combined with an FTIR [17,18], or in open or closed loop comb-assisted schemes [19][20][21].…”

Ultra-stable tunable THz generation based on frequency comb technology

“…Continuously tunable light is achieved by external shifting of the frequency comb spectrum. 2 The shifter is based on an electro-optical phase modulator that imprints a time-dependent phase-shift of the reference comb mode. This unique technique enables both high-precision and fast tuning of the CTL laser source with tuning rates of up to 1000 GHz/s and tuning ranges of over 10 THz at optical frequencies.…”