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