Comb-based Fourier transform spectroscopy with sub-nominal resolution
The resolution of traditional Fourier transform spectroscopy (FTS), based on incoherent light sources, is limited by the maximum delay range of the interferometer and the acquisition of high-resolution spectra implies long measurement times and large instrument size. This limit is overcome when the incoherent light source is replaced with an optical frequency comb and the nominal resolution is matched precisely to the comb repetition rate. Using a proper sampling approach, the intensities of the comb lines are measured precisely and one measurement yields sampling points spaced by the repetition rate of the comb. For denser sampling point spacing, the measurement is repeated with comb modes tuned to different positions, and all measurements are interleaved. The resulting spectral resolution is limited by the width of the comb lines. Using comb-based FTS allows measuring absorption lines narrower than the nominal (optical path-limited) resolution without ringing effects from the instrumental lineshape and reduces the acquisition time and interferometer length by orders of magnitude.
We use the comb-based FTS technique in the near- and mid-infrared wavelength range for high-precision measurements of entire molecular bands, e.g. the �+� band of CO2 at 1.57 𝜇m or the � band of CH3I at 3.3 𝜇m.
Sub-Doppler double-resonance spectroscopy using a comb probe
Double-resonance (DR) spectroscopy is a powerful tool for assignment of highly excited energy levels. It provides a way to use an already assigned transition to unambiguously identify the lower or upper state quantum numbers of measured spectra. In optical-optical DR spectroscopy a saturating pump laser transfers the population of a single quantum state into another state, and a weaker probe laser measures transitions from/to the selectively populated/de-populated states. When a monochromatic pump is used, only a narrow velocity group of molecules is excited, and the resulting probe transitions are free of Doppler broadening.
We use a high-power 3.3 µm continuous wave optical parametric oscillator as a pump and a 1.67 µm comb as a probe to detected sub-Doppler DR transitions in methane. The comb probe spectra are recorded using a Fourier transform spectrometer with comb-mode limited resolution. With pump tuned to 9 different transitions in the v3 fundamental band, we detected 36 ladder-type transitions to the 3v3 overtone band region, and 18 V-type transitions to the 2v3 overtone band. The accuracy of the center frequencies was ~1.7 MHz, limited by the frequency stability o the pump.
The ladder-type probe transitions allowed the first verification of the accuracy of theoretical predictions from highly vibrationally excited states, needed e.g. to model the high-temperature spectra of exoplanets. We compared the center frequencies and relative intensities of these ladder-type transitions to theoretical predictions from the TheoReTS and ExoMol line lists, demonstrating good agreement with TheoReTS.
We recently implementing an enhancement cavity for the probe to increase the increase the absorption sensitivity, improve the frequency precision, and detect a larger number of probe transitions with high signal to noise ratio.
