Comb-based Fourier transform spectroscopy with sub-nominal resolution
The resolution of best traditional Fourier transform infrared spectroscopy (FTIR), based on incoherent light sources, is limited to 0.001 cm-1 by the maximum delay range of the interferometer. 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 the sub-nominal resolution sampling approach (Maslowski 2016, Rutkowski 2018), the intensities of the comb modes 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 in the 10-7 cm-1 range, limited by the width of the comb lines (Rutkowski 2017).
We use the comb-based Fourier transform spectroscopy technique in the near- and mid-infrared wavelength range for high-precision measurements of molecular bands.
References
L. Rutkowski, P. Masłowski, A. C. Johansson, A. Khodabakhsh, and A. Foltynowicz
Optical frequency comb Fourier transform spectroscopy with sub-nominal resolution and precision beyond the Voigt profile
J. Quant. Spectr. Radiat. Transf. 204, 63 (2018).
L. Rutkowski, A. C. Johansson, G. Zhao, T. Hausmaninger, A. Khodabakhsh, O. Axner, and A. Foltynowicz
Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution
Opt. Express 25, 21711 (2017).
P. Masłowski, K. F. Lee, A. C. Johansson, A. Khodabakhsh, G. Kowzan, L. Rutkowski, A. A. Mills, C. Mohr, J. Jiang, M. E. Fermann, and A. Foltynowicz
Surpassing the path-limited resolution of a Fourier transform spectrometer with frequency combs
Phys. Rev. A 93, 021802(R) (2016).
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. Following the first demonstration using a liquid-nitrogen-cooled single pass cell (Foltynowicz 2021), we implementing an enhancement cavity for the probe to increase the absorption sensitivity and improve the frequency precision (de Oliveira 2024). Most recently, we extended the spectral coverage of the comb probe and added a continuous-wave probe to the system (de Oliveira 2025).
References
V. S. de Oliveira, A. Hjältén, I. Silander, A. Rosina, M. Rey, K. K. Lehmann, and A. Foltynowicz
Combined frequency comb and continuous wave cavity-enhanced optical-optical double resonance spectrometer in the 1.7 µm range
Opt. Express 33, 38776-38802 (2025).
K. K. Lehmann, A. Hjältén, I. Silander, M. Rey, and A. Foltynowicz
Assignment of collision-induced four-level double-resonance transitions in the 3ν₃ ← ν₃ spectral region of methane
J. Chem. Phys. 163, 144304 (2025).
V. S. de Oliveira, I. Silander, L. Rutkowski, G. Soboń, O. Axner, K. K. Lehmann, and A. Foltynowicz
Sub-Doppler optical-optical double-resonance spectroscopy using a cavity-enhanced frequency comb probe
Nat. Commun. 15, 161 (2024).
A. Foltynowicz, L. Rutkowski, I. Silander, A. C. Johansson, V. S. de Oliveira, O. Axner, G. Soboń, T. Martynkien, P. Mergo, and K. K. Lehmann
Sub-Doppler double-resonance spectroscopy of methane using a frequency comb probe
Phys. Rev. Lett. 126, 063001 (2021).
