High-accuracy and wide dynamic range frequency-based dispersion spectroscopy in an optical cavity.

A spectroscopic method free from systematic errors is desired for many challenging applications of gas detection. Although existing cavity-enhanced techniques exhibit very high precision, their accuracy strongly depends on propagation of the light amplitude through an optical system and its detection. Here, we demonstrate that the frequency-based molecular dispersion spectroscopy, involving sub-Hz-level precision in frequency measurements of optical cavity resonances, leads to sub-per-mille accuracy and a wide dynamic range, both previously unattainable by any other spectroscopic technique. The method offers great sensitivity of 5×10-11 cm-1, high speed, limited only by the fundamental response time of the cavity, and traceability of both axes of the spectrum to the primary frequency standard. All these features are necessary for convenient realization of comprehensive molecular spectroscopy from Doppler up to collisional regime without changing the spectroscopic method and modification of the experimental setup. Moreover, the presented approach does not require linear, high-bandwidth nor phase-sensitive detectors and can be directly implemented in existing cavity-enhanced spectrometers utilizing either continuous-wave or coherent broadband radiation. We experimentally prove the predominance of frequency-based spectroscopy over intensity-based one. Our results motivate replacement of intensity-based absorption spectroscopy with a pure frequency-based dispersion one in applications where the highest accuracy is required.

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

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.

Self-referenced, accurate and sensitive optical frequency comb spectroscopy with a virtually imaged phased array spectrometer.

We present a cavity-enhanced direct optical frequency comb spectroscopy system with a virtually imaged phased array (VIPA) spectrometer and either a dither or a Pound-Drever-Hall (PDH) locking scheme used for stable transmission of the comb through the cavity. A self-referenced scheme for frequency axis calibration is shown along with an analysis of its accuracy. A careful comparison between both locking schemes is performed based on near-IR measurements of the carbon monoxide ν=3←0 band P branch transitions in a gas sample with known composition. The noise-equivalent absorptions (NEA) for the PDH and dither schemes are 9.9×10(-10) cm(-1) and 5.3×10(-9) cm(-1), respectively.