Mid-IR Doppler-Free Saturated Absorption Spectroscopy

In Doppler-free saturated absorption spectroscopy, the output from a laser is split into two counter-propagating beams: a strong "pump" beam and a weak "probe" beam. Since these two laser beams have the same frequency, only those molecules that travel perpendicular to the laser beams and, hence, have zero (first-order) Doppler shift can "feel" both laser beams simultaneously. The pump beam is modulated by an acousto-optical modulator (AOM) or a mechanical .chopper driven by a lock-in amplifier. The absorption of the probe beam by only those zero-Doppler-shift molecules is lock-in amplified. Consequently, the Doppler broadening (on the order of 100 MHz) is eliminated, which significantly narrows the spectral linewidth and improves the spectral resolution.

The idler output from a continuous-wave optical parametric oscillator (CW OPO, Toptica Photonics, TOPO) is used as the light source for the Doppler-free saturated absorption measurements. In collaboration with Toptica, our group has developed an automation system for broad-range mode-hop-free wavelength scans of the idler output (λ_idler=2.2-4.0 μm). Figure 2 shows an example of the Doppler-free saturated absorption spectrum of the υ₃ (anti-symmetric CH stretch) transition of CH₄ compared to its Doppler-broadened spectrum and transitions from the HITRAN database. Also included in Fig. 2 is an etalon trace for relative calibration. Figure 3 illustrates the Q branch of this vibrational band. The Doppler-free spectroscopy apparatus will be used to record high-resolution spectra of molecules with astrochemical and atmospheric significance.

Combining the Doppler-free saturated absorption spectroscopy and the CW CRD setups, our group is developing a novel cavity-enhanced two-photon spectroscopy technique.