Cavity-Enhanced Frequency-Comb-Based Optical-Optical Double-Resonance Spectrometer

Optical-optical double resonance spectroscopy (OODR) using a continuous wave (CW) pump and a frequency comb probe is a powerful tool for measuring and assigning hot-band transitions with high selectivity and accuracy [1]. In the first demonstration of the technique, we detected transitions in the $3v_{3}\leftarrow v_{3}$ range of methane with ~1.7 MHz accuracy, using a $3.3\ \mu\mathrm{m}$ CW pump and $1.67\ \mu\mathrm{m}$ comb probe, and a liquid-nitrogen-cooled single pass cell containing the sample [2]. Later on, we introduced a room-temperature enhancement cavity to increase the absorption sensitivity for the comb probe and reach sub-MHz accuracy on probe transition frequencies [3]. However, the transmitted comb probe coverage was limited by the dispersion of the cavity mirrors and the offset frequency ( $f_{\text{ceo}}$ ) was monitored but not actively stabilized, which complicated the data averaging process. Here we demonstrate a cavity-enhanced comb-based OODR system, illustrated in Fig. 1(a), with vastly improved performance, where the shortcomings of the first demonstration have been overcome. The pump is a 1 W narrow-linewidth (10 kHz) idler of a CW optical parametric oscillator (CW-OPO, Toptica TOPO), stabilized to the Lamb dip of a pump transition in the $v_{3}$ band of CH 4 at $\sim 3.3\mu\mathrm{m}$ . The probe is an amplified Er:fiber comb ( $f_{\text{rep}}=250\ \text{MHz}$ ) covering 6 THz of bandwidth around a center wavelength, tunable between $1.65-1.8\ \mu\mathrm{m}$ via a non-linear fiber. A sample of pure CH 4 is contained in a 60-cm-long cavity, yielding a free spectral range equal to $f_{\text{rep}}$ , with a finesse of 1000 and low dispersion (|GDD| < 100 fs ² ) in the entire comb probe range. A fraction of the comb spectrum is locked to the cavity using the Pound-Drever-Hall scheme by acting on the comb cavity length, and the $f_{\text{rep}}$ is locked to a tunable direct digital synthesizer (DDS), referenced to a GPS-disciplined Rb oscillator, by acting on the sample cavity length. Finally, $f_{\text{ceo}}$ is set to maximize the width of the transmission spectrum and then locked to another DDS. The comb probe spectra are measured using a Fourier transform spectrometer with comb-mode-limited resolution. Spectra measured at a series of $f_{\text{rep}}$ steps are interleaved to yield a sample point spacing of 2 MHz in the optical domain. At each $f_{\text{rep}}$ step we use the shutter to acquire spectra with and without the pump. We then take their ratio to remove the background originating from the Doppler-broadened transitions of the $2v_{3}$ band. Figure 1(b) shows a small window of a background-normalized and interleaved spectrum averaged 8 times (2 h acquisition), with the pump locked to the $v_{3}$ P(7,A 2 ) transition. Two sub-Doppler transitions are clearly visible with a high signal to noise ratio. The standard deviation of the noise on the baseline yields noise equivalent absorption of $4.4\times 10^{-6}$ cm ⁻¹ Hz ^−1/2 and a figure of merit of $2.8\times 10^{-9}$ cm ⁻¹ Hz ^−1/2 per spectral element. The improved setup can be used to observe and assign the ~9000 cm ⁻¹ eigenstates of a large variety of molecules that have pump and probe transitions in the respective spectral range.