Combined frequency comb and continuous wave cavity-enhanced optical-optical double-resonance spectrometer in the 1.7 ${\mu}$m range

TL;DR

This paper introduces a novel optical spectrometer combining frequency comb and continuous wave techniques for high-precision methane spectroscopy in the 1.7 μm range, enabling simultaneous broad coverage and high accuracy measurements.

Contribution

It presents a new hybrid spectrometer that combines broad spectral coverage with high-precision transition measurements, advancing methane spectroscopy capabilities.

Findings

Detected 43 methane transitions in the 3ν3 and 2ν3 bands.

Achieved kHz-level accuracy in measuring Lamb dips.

Demonstrated simultaneous broad and precise spectroscopic measurements.

Abstract

We present an optical-optical double-resonance (OODR) spectrometer based on a 3.3 ${\mu}$m continuous wave pump and two cavity-enhanced probes: a frequency comb tunable in the 1.64 - 1.8 ${\mu}$m range, and a comb-referenced continuous wave (CW) laser tunable in the 1.6 - 1.75 ${\mu}$m range. The comb probe provides broad spectral coverage (bandwidth up to 7 THz) for simultaneous detection of many sub-Doppler OODR transitions with sub-MHz line position accuracy, while the CW probe allows targeting individual transitions with kHz accuracy and higher signal-to-noise ratio in shorter time. Using the pump stabilized to the frequency of the R(0) transition in the ${\nu}$${_3}$ band of methane and the comb probe covering the 5550 to 6070 cm$^{-1}$ interval, we detect 37 ladder-type transitions in the 3${\nu}$${_3}$ $\leftarrow$ ${\nu}$${_3}$ band region and 6 V-type transitions in the 2${\nu}$${_3}$ band region and assign them using available theoretical predictions. Using the CW probe, we measure selected ladder- and V-type transitions with much higher precision. We also detect Lamb dips in the R(0)- R(3) transitions of the 2${\nu}$${_3}$ band and report their center frequencies with kHz level accuracy. The synergy effects of the comb- and CW-OODR open new possibilities in precision spectroscopy of levels that cannot be reached from the ground state.