Multi‐correlation Fourier transform spectroscopy with the resolved modes of a frequency comb laser

Abstract

An instrument achieving 100 KHz spectral precision using multiple correlation Fourier transform spectroscopy has been demonstrated. The instrument can measure the individual frequency comb modes of 100 MHz frequency comb lasers in air. The experiments show ∼400,000 resolved modes at linewidths of 85 MHz in the region of 829 nm and ∼ 182,000 resolved modes at linewidths of 28 MHz in the region of 1.5 μm, with a recording time of few minutes. The precision of the instrument, defined by the frequency positioning, attains sub-MHz even when the scan is performed in air.

Optical frequency comb Fourier transform spectroscopy with sub-nominal resolution and precision beyond the Voigt profile

“…In this case, the number of spectral elements in the spectrum after the FFT is at least twice higher than the number of comb modes. Moreover, without careful sampling of the interferogram a fit to the spectral pattern is needed to precisely find the power of each comb mode [5,8]. Therefore, the ideal case is to measure a spectrum containing one sampling point per comb mode.…”

“…In the temporal domain, a comb is a train of pulses separated by 1/frep, where frep is the repetition rate, therefore a comb-based FTS interferogram consists of a series of equidistant bursts separated by c/frep in the OPD domain, where c is the speed of light. The spectral comb structure starts to be visible when an interferogram containing several bursts is acquired [5]. However, unless the interferogram is sampled correctly, the spectral resolution is limited to the nominal resolution of the FTS and retrieving the power of comb modes requires interpolation between the FTS sampling points [6].…”

Dual-comb spectroscopy

“…However, since the comb tooth spacing is finer than the resolution of nearly all spectrometers, the comb then simply acts as a very bright, collimated light source; its associated frequency resolution and accuracy are lost. To fully capitalize on the underlying comb structure, several high-resolution dispersive spectrometer and Fourier transform spectrometer techniques have been developed for resolving individual comb teeth, including virtually imaged phase array (VIPA) spectrometers [66][67][68], comb-cavity Vernier spectrometers [69,70], and highresolution FTIR spectrometers [71][72][73][74][75]. While VIPA and FTIR techniques have seen compelling demonstrations, to date the most widely pursued form of direct comb spectroscopy is DCS, which dispenses with the dispersive spectrometer altogether.…”

The optical frequency comb fibre spectrometer

“…This technique has also been applied to probe two-photon transitions of Rb with Doppler-limited resolution 31 . A third technique, Fourier-Transform Infrared (FTIR) spectroscopy coupled to OFC sources, has become a powerful combination enabling the measurement of broadband spectra with comb-tooth level resolution 32 . As FTIR spectroscopy is based on a Michelson interferometer with a scanning delay line on one arm, reaching comb-tooth resolution requires a sufficiently long delay line (up to 10 m for comb line spacing of 100 MHz), which implies correspondingly long measurement times and extended optical layouts sensitive to vibrations.…”