A dual frequency comb spectrometer is realized by electro-optic modulation of a 1550 nm laser and subsequent conversion to the mid-infrared by difference-frequency generation (DFG). Using an optical parametric oscillator for the DFG the combs can be tuned from 3 μm to 4.7 μm with 440 comb modes covering 220 GHz (< 6 cm-1). Trace gas detection of nitrous oxide, carbon dioxide and methane is demonstrated with a 7.2-m-multi-pass cell while a sufficiently low noise-equivalent absorbance is reached in already 1 s. The bandwidth normalized noise-equivalent-absorption coefficient is consistently below 2.8 × 10-6 Hz-1/2 cm-1 while the precision of the determined concentrations is better 2 % Hz-1/2.
Nonlinear interferometers based on non-degenerate spontaneous parametric down-conversion (SPDC) create a link between separate spectral ranges. This allows for measurements in remote spectral regions while detecting light in easily accessible wavelengths. In our work, we use periodically poled lithium niobate to create correlated signal (visible or near-infrared) and idler (mid-infrared) photon pairs. Using a nonlinear interferometer in Michelson geometry, we obtain broadband mid-infrared spectra from light detected with a silicon avalanche photodiode. Combining the nonlinear interferometer with a measurement scheme in close analogy to classical Fourier-transform infrared spectroscopy allows for sub-wavenumber spectral resolution, which opens up possibilities for applications such as precise spectroscopic gas analysis.
Mid-infrared spectroscopy is one of the most important techniques in chemical analysis. However, the detectors for the mid-infrared range suffer from lower specific detectivities in comparison to their visible counterparts, cost more and often require cryogenic cooling. Nonlinear interferometers allow measuring mid-infrared spectra by detecting only visible light using the induced coherence effect. In our work, we realize a nonlinear interferometer designed for broadband mid-infrared spectra with high resolution, which is easily tunable, and in analogy to classical Fourier transform infrared (FTIR) spectrometers requires no additional spectral selection.
The thermo-optic coefficient of lithium niobate (LiNbO3) has been measured in the temperature range from 10 to 160 °C
using an interferometric setup. Undoped and magnesium-doped congruently melting LiNbO3 and undoped stoichiometric
LiNbO3 were studied over a wide wavelength range in the visible and near infrared (450 – 600 nm and 900 – 1130 nm)
using a frequency-doubled cw optical parametric oscillator. Experimental results for congruently grown lithium niobate
were aggregated using a Schott equation to describe the wavelength and temperature dependence of the thermo-optic
We demonstrate the continuous-wave operation of a cascade that has been successfully applied so far only for
picosecond systems: A doubly-resonant optical-parametric oscillator (OPO) based on lithium niobate generates
signal and idler waves close to degeneracy. Subsequently, these two light fields are converted to a terahertz wave
via difference frequency mixing in an orientation-patterned gallium arsenide crystal placed inside the OPO cavity.
Using this scheme, we achieved tunability from 1 to 4:5 THz frequency, a linewidth smaller than 10 MHz, and
a Gaussian beam profile. The output power is of the order of tens of μW, with a scalability into the milliwatt