Broadband Optical Cavity Mode Measurements at Hz-Level Precision With a Comb-Based VIPA Spectrometer

Abstract

Optical frequency comb spectrometers open up new avenues of investigation into molecular structure and dynamics thanks to their accuracy, sensitivity and broadband, high-speed operation. We combine broadband direct frequency comb spectroscopy with a dispersive spectrometer providing single-spectrum acquisition time of a few tens of milliseconds and high spectral resolution. We interleave a few tens of such comb-resolved spectra to obtain profiles of 14-kHz wide cavity resonances and determine their positions with precision of a few hertz. To the best of our knowledge, these are the most precise and highest resolution spectral measurements performed with a broadband spectrometer, either comb-based or non-comb-based. This result pushes the limits of broadband comb-based spectroscopy to Hz-level regime. As a demonstration of these capabilities, we perform simultaneous cavity-enhanced measurements of molecular absorption and dispersion, deriving the gas spectra from cavity mode widths and positions. Such approach is particularly important for gas metrology and was made possible by the Hz-level resolution of the system. The presented method should be especially applicable to monitoring of chemical kinetics in, for example, plasma discharges or measurements of narrow resonances in cold atoms and molecules.

A rapid, spatially dispersive frequency comb spectrograph aimed at gas phase chemical reaction kinetics

“…Using a VIPA has additional advantages in frequency resolution, where Hz-level precision has been reported for measurement of cavity resonances, and in frequency self-calibration possibilities. [31] In contrast, dual-comb spectroscopy (DCS) has potential advantages over spatially dispersive methods, although there are also several disadvantages. DCS can be thought of as a Fourier transform spectrometer with no moving parts.…”

Infrared Comb Spectroscopy of Buffer-Gas-Cooled Molecules: Toward Absolute Frequency Metrology of Cold Acetylene

“…Starting with MLL OFCs, in the absence of active stabilization, and would be free to drift or fluctuate due to changes in the cavity length, refractive index of laser optics, and Kerr-type nonlinear effects [ 57 , 58 , 59 ]. stabilization is commonly implemented by acting on the laser cavity length via a mirror mounted on an intra-cavity piezo-electric transducer.…”

Direct-frequency-comb spectroscopy by a scanning Fabry-Pérot microcavity resonator

“…For instance, the resolutions of Fouriertransformed and dual-comb spectroscopic approaches have been tested by measuring FP resonances of ≈200-kHz and 2-MHz linewidths, respectively, over optical bandwidths wider than 15 THz [20,21]. Very recently, virtually-imagedphase-array comb spectroscopy combined with a FP cavity to implement a Vernier scheme for filtering the comb mode line spacing demonstrated the capability to measure a 14-kHznarrow FP resonance [22]. An alternative DFCS approach capable of resolving the mode structure of an OFC based on a scanning FP microcavity resonator, SMART (Scanning Micro-cAvity ResonaTor), and a tunable single-frequency cw laser calibrator demonstrated resolution better than 20 MHz in 1-THz-wide optical bandwidth and 20-ms acquisition times, limited by the microcavity finesse and the adopted sequential 2469-9926/2020/102(3)/033510 7033510-1 ©2020 American Physical Society measurement scheme [14].…”