Doppler-free Fourier transform spectroscopy

Abstract

The feasibility of sub-Doppler broadband multi-heterodyne spectroscopy with two laser frequency combs is demonstrated with two-photon excitation spectra of the 5S-5D transitions of rubidium vapor.Fourier transform spectroscopy (FTS) has been for nearly fifty years the leading technique in analytical chemistry and molecular spectroscopy, as well as an irreplaceable tool for remote sensing, industrial process control etc. Fourier transform spectrometers record on a single photo-detector, in almost any spectral region, multiplex high-resolution spectra over a broad spectral span. Interferometers with a resolution better than 30 MHz are available, even commercially; however high resolution FTS in the gas phase has been so far limited to the measurements of Doppler-broadened lines, possibly narrowed by cooling the sample e.g. in a supersonic beam or by collisions with a buffer gas.Nonlinear mechanisms for canceling the Doppler effect have been harnessed with tunable lasers or with a single frequency comb. In direct frequency comb Doppler-free two-photon spectroscopy [1,2], all comb lines may contribute and the excitation probability of any given level can be the same as with a continuous-wave (cw) laser of the same average power. Short pulses facilitate nonlinear frequency conversion to access spectral regions where cw lasers are not readily available. However, the spectrum is only retrieved modulo the comb line spacing. Therefore, direct frequency comb spectroscopy with a single frequency comb is only suitable for spectra composed of very few transitions.Here we propose and demonstrate Doppler-free Fourier transform spectroscopy by combining dual-comb spectroscopy and two-photon excitation in a standing-wave field. Dual-comb spectroscopy is a recent technique of Fourier spectroscopy without moving

Frequency comb spectroscopy

“…the modulation of the intensity of the fluorescence of the sample, providing an insightful illustration of Fourier encoding. The first proof--of--concept 94 of broadband Doppler--free two--photon spectroscopy showcases experimental spectra spanning 10 THz, which exhibit atomic line profiles of a width of 6 MHz. A strength of dual--comb spectroscopy (and other techniques of Fourier transform spectroscopy), that has been recently explored, is its ability to efficiently combine with other sampling techniques or instrumentations.…”

Dual-comb spectroscopy with spectral acquisition rate surpassing Nyquist-limited trade-off constraint

“…These two signals become combs in RF domain centered at f AOM with a line-spacing of ∆f . For demodulation process, the same lines are digitally filtered to obtain the envelop expressed as e meam (t) = R| A pm (t) A * lm (t) S m (t)| (13) e ref m (t) = R| A pm (t) A * lm (t)|…”

“…Several techniques are developed to extract the spectroscopic content encoded on the comb light, such as virtually imaged phased arrays (VIPA)-based spectroscopy 6 and Fourier transform spectroscopy (FTS) 7 . Dual-comb spectroscopy (DCS) emerges from comb-based techniques and has been implemented in linear spectroscopy [8][9][10][11][12][13] , nonlinear spectroscopy 14,15 and microscopy 16,17 .…”

“…Beating two OFCs with slightly different repetition rates, DCS retrieves each frequency component in radio frequency (RF) domain, which may fully exploit the spectral resolution and measurement bandwidth determined by OFCs. The spectral resolution can be further reduced to kHz less than line-spacing by time-consuming spectral interleave techniques 13,18,19 . In recent years, novel approaches for OFC generation have been explored, which facilitate DCS performed at different wavebands with versatility 9,11,12 .…”

Quantitative modeling of complex molecular response in coherent cavity-enhanced dual-comb spectroscopy

“…Optical frequency combs (OFCs) [1][2][3][4] enable increasingly precise applications of atomic and molecular spectroscopy [5], including primary thermometry [6,7], optical radiocarbon dating [8], sub-Doppler and Doppler-free spectroscopy [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26], ultrafast and multidimensional spectroscopy [27][28][29][30], survey spectroscopy of molecular ions and cold molecules [31][32][33], and time-resolved spectroscopy for fundamental chemical kinetics [34][35][36][37]. Applied spectroscopies [38] also make novel use of optical frequency combs for actively monitoring greenhouse gas fluxes [39][40][41], methane for open-path detection [42] and unambiguous source attribution [43] and reactive atmospheric species [44], elucidating the chemical composition of combustion and open flames…”