Broadband Midinfrared Comb-Resolved Fourier Transform Spectroscopy

Surpassing the path-limited resolution of Fourier-transform spectrometry with frequency combs

“…In contrast to traditional FTIR spectrometers, in OFC-based FTS the time-domain interferogram consists of a series of bursts appearing at optical path differences (OPDs) equal to integer multiples of c/frep (where c is the speed of light and frep is the repetition rate of the comb) instead of a single centerburst at OPD = 0 9 . Individual comb lines can be resolved by acquiring interferograms with multiple bursts both with mechanical FTS [9][10][11] and DCS, where resolution down to the comb linewidth has been achieved using thousands of bursts 12,13 . Resolution above the nominal limit has been demonstrated recently with DCS in the THz frequency range 14 .…”

Frequency combs for cavity cascades: OPO combs and graphene-coupled cavities

“…This type of optical referencing will not work in the degenerate case as the OPO CEO does not change with cavity length, and intensity based locking is needed [11]. In addition to RF frequency measurements, we have also indirectly confirmed that the OPO produces a stable midinfrared comb using a comb-resolved FTS for absorption lines that are narrower than the comb line spacing [13,14]. Figure 4 shows a clearly resolved example spectrum of the R6 line in carbon monoxide at 11 Torr pressure in a 100m multipass cell, at about 4.6 μm wavelength.…”

“…The signal and idler wavelengths are about 3-6 μm centered at 4 μm, half the energy of the 2 μm pump. This particular OPO can operate both degenerately (indistinguishable signal and idler) [11] or nondegenerately (different signal and idler combs) which is needed to reach wavelengths far from 4 μm [13,14]. The uncoated surfaces of a thin calcium fluoride wedge acts as the output coupler, generating two output beams.…”

Midinfrared frequency comb from self-stable degenerate GaAs optical parametric oscillator

“…If the goal is simplicity, a stable, monolithic OPO could act as a passive downconversion device, with the pump repetition rate or OPO temperature used for long term corrections. If the goal is superior frequency stability, an existing option is to operate the OPO nondegenerately, and phase lock the OPO to the pump [18,19], providing frequency stability at the level of the optical reference used, but requiring beat note generation and locking. We have also phase locked the degenerate OPO, by locking the pump CEO against the OPO CEO beat, which acts as an f-to-1.5f interferometer for measuring half of the pump CEO, and locking the pump to the optical reference as usual.…”