A ${\mathrm{BaGa}}_4{\mathrm{Se}}_7$ nanosecond optical parametric oscillator (OPO) shows extremely wide idler tunability in the mid-IR (2.7–17 μm) under 1.064 μm pumping. The $\sim 10\mathrm{ns}$ pulses at $\sim 7.2\mathrm{\mu m}$ have an energy of 3.7 mJ at 10 Hz. The pump-to-idler conversion efficiency for this wavelength reaches 5.9% with a slope of 6.5% corresponding to a quantum conversion efficiency or pump depletion of 40%.

A AgGaSe₂ nonlinear crystal placed in a coupled cavity is intracavity pumped by the ~1.85-µm signal pulses of a 1.064-µm pumped Rb:PPKTP doubly-resonant optical parametric oscillator (OPO) operating at a repetition rate of 100 Hz. Using two samples cut for type-I and II phase-matching, the overall idler tunability of the singly-resonant AgGeSe₂ OPO covers an unprecedented spectral range from 5.8 to ~18 µm in the mid-IR.

The infrared laser photoacoustic spectroscopy (LPAS) and the pattern-recognition-based approach for noninvasive express diagnostics of pulmonary diseases on the basis of absorption spectra analysis of the patient’s exhaled air are presented. The study involved lung cancer patients ($N=9$), patients with chronic obstructive pulmonary disease ($N=12$), and a control group of healthy, nonsmoking volunteers ($N=11$). The analysis of the measured absorption spectra was based at first on reduction of the dimension of the feature space using principal component analysis; thereafter, the dichotomous classification was carried out using the support vector machine. The gas chromatography–mass spectrometry method (GC–MS) was used as the reference. The estimated mean value of the sensitivity of exhaled air sample analysis by the LPAS in dichotomous classification was not less than 90% and specificity was not less than 69%; the analogous results of analysis by GC–MS were 68% and 60%, respectively. Also, the approach to differential diagnostics based on the set of SVM classifiers usage is presented.

An overall quantum conversion efficiency of 7.8% is achieved by intracavity mixing the signal and idler of a 1.064 μm pumped Rb:PPKTP optical parametric oscillator in ${\mathrm{BaGa}}_4{\mathrm{Se}}_7$. In this way, a pulse energy of $\sim 0.71\mathrm{mJ}$ is generated at $\sim 7$ μm for a repetition rate of 100 Hz. Tuning of the mid-IR radiation is demonstrated by heating of the Rb:PPKTP crystal.

In recent decades, new nonlinear optical materials have been actively developed to create coherent tunable light sources in the mid-infrared (mid-IR) part of the spectrum used in a variety of scientific fields. In the present review, the main attention is focused on barium chalcogenide crystals, including their linear and nonlinear optical properties, laser-induced damage threshold (LIDT), and frequency down-conversion.

A human exhaled air analysis by means of infrared (IR) laser photoacoustic spectroscopy is presented. Eleven healthy nonsmoking volunteers (control group) and seven patients with chronic obstructive pulmonary disease (COPD, target group) were involved in the study. The principal component analysis method was used to select the most informative ranges of the absorption spectra of patients’ exhaled air in terms of the separation of the studied groups. It is shown that the data of the profiles of exhaled air absorption spectrum in the informative ranges allow identifying COPD patients in comparison to the control group.