Frequency Modulation Absorption Spectroscopy of Cryogenic Free Radicals

Abstract

Frequency modulation (FM) absorption spectroscopy provides a powerful way of detecting weak optical transitions with enhanced signal-to-noise ratio (SNR). In this work, we implement FM absorption spectroscopy to probe barium monofluoride (BaF) molecules produced in a cryogenic buffer gas cell. By phase-modulating a continuous-wave laser, passing it through the BaF sample, and demodulating the transmitted signal, we obtain both in-phase and in-quadrature components of the molecular absorption. This method achieves a twenty-five–fold improvement in SNR compared to direct absorption, enabling the detection of otherwise inaccessible molecular transitions. We develop a detailed theoretical model of the FM signal, implement the experimental setup for FM spectroscopy, and identify key mechanisms of residual amplitude modulation (RAM). We suppress RAM through careful polarization alignment, reduction of parasitic etalons, and active EOM temperature stabilization. This work demonstrates FM absorption spectroscopy as a powerful tool for precision optical measurements of weakly absorbing, low-density molecular samples, providing a pathway for future high-resolution studies of cryogenic free radicals. The techniques presented here are directly applicable for monitoring the density of radioactive molecules of interest in the cryogenic buffer gas cell, as well as identifying transitions required for laser cooling and optical cycling for future precision measurements.