LiGaS2 crystals are prospective media for optical parametric oscillators. In such systems efficiency and maximum power output are often limited by the laser-induced damage threshold (LIDT) of nonlinear crystals. In addition, most nonlinear crystals have a high refractive index and consequently large Fresnel losses, thus encouraging the use of antireflection coatings. However, antireflection coatings are known to compromise the LIDT. This work presents results of the LIDT testing of LiGaS2 nonlinear crystals in untreated, antireflection-coated and antireflection-microstructured variations. The tests were performed using a one-on-one method with pulsed lasers operating at 1.57, 2.09 and 2.5 μm wavelengths with pulse durations of 9, 149 and 12 ns, respectively. The paper covers damage site feature investigation and LIDT comparison of antireflection coating and antireflection microstructures. The key finding of the work is that antireflection microstructures can provide an increase in transmittance for both the pump and the signal, while maintaining a high LIDT.

High-resolution optical diagnostics in the short wavelength infrared (SWIR II) region have gained significant attention in medical research, showing great potential for tissue spectroscopy and visualization due to the region’s low water absorption and scattering coefficients. However, high-beam-quality sources covering an entire spectral range are limited. This paper presents the development of a femtosecond Cr2+:ZnSe laser with a 2.2 µm center wavelength, a pulse duration of 60 fs, a spectral width of 96.5 nm, and an energy of 4.5 nJ. The resulting source is expected to enable spectroscopy and the optical coherence tomography system for diagnosing collagen-rich tissues.

LiTaO3 crystals doped with Cr3+ and Nd3+ ions are promising for developing active nonlinear laser media. In this work, the defect structure of LiTaO3 crystals, including those doped with Cr3+ and Nd3+, is examined. X-ray patterns of all six investigated LiTaO3:Cr:Nd crystals are identical and correspond to a highly perfect structure. Using optical microscopy, the presence of defects of various shapes, microinhomogeneities, and lacunae was revealed. The optical absorption and Raman scattering spectra of a series of nonlinear, optical, double-doped LiTaO3:Cr3+:Nd3+ (0.06 ≤ [Cr3+] ≤ 0.2; 0.2 ≤ [Nd3+] ≤ 0.45 wt%) crystals showed that at concentrations of doping Cr3+ ions less than 0.09 wt% and Nd3+ ions less than 0.25 wt%, the crystal structure is characterized by a low level of defects, and the optical transmission spectra characterized by narrow lines corresponding to electron transitions in Nd3+ ions. In this case, for the radiative transition in the cation sublattice, the existence of three nonequivalent neodymium centers is observed, and for the radiative transition, two nonequivalent centers are observed. IR absorption spectroscopy in the OH−-stretching vibration range revealed two main spectral regions: 3463–3465 cm−1, associated with stoichiometry changes, and 3486–3490 cm−1, linked to complex defects such as (V-Li)-OH and (Ta4+Li)-OH. A distinct low-intensity line at ~3504 cm−1 was observed only in doped crystals, attributed to (Nd2+Li)-OH defects that significantly distort the oxygen-octahedral clusters due to the larger ionic radius of Nd3+ compared to Ta5+. In contrast, Cr-related defects cause only minor distortions. The Klauer method indicated that the highest concentration of OH−-groups occurs in the LiTaO3:Cr3+ (0.09 wt%):Nd3+ (0.25 wt%) crystal, where multiple complex defects are present.

Based on the analysis of the IR transmission spectra in the region of stretching vibrations of hydrogen atoms of OH−-groups, it was established that the oxygen-octahedral МеО6 clusters (Ме-Li+, Nb5+, vacant octahedron V, impurity ion) of the structure of the compositionally homogeneous crystal LiNbO3:Er3+(3.1 wt%) and the gradient crystal LiNbO3:Er3+(congruent composition by the main components, Er gradient of 0.55 at%/cm) have a shape close to the regular one. In this case, the value of R = [Li]/[Nb] ≈ 1, and in the structure of both crystals, there are practically no point defects in NbLi responsible for the photorefraction effect. By using the IR transmission spectra and Klauer’s method, it was found that the volume concentration of OH−-groups in the gradient crystal LiNbO3:Er3+ is almost an order of magnitude lower than in the compositionally homogeneous LiNbO3:Er3+(3.1 wt%) crystal. This fact explains the lower hydrogen conductivity of the gradient crystal LiNbO3:Er3+ and the lower photorefraction effect compared to the compositionally homogeneous LiNbO3:Er3+(3.1 wt%) crystal. The results obtained are important for the development of materials for active nonlinear laser media and for the conversion of laser radiation.

A polarization maintaining thulium-doped fiber laser mode-locked by single-walled carbon nanotubes has been developed. The pulse repetition frequency varied with increasing power from 84 to 504 MHz. Following pulse parameters are achieved: the pulse duration of 440 fs, center wavelength of 1912 nm, maximum average power of 600 mW, maximum pulse energy of 1.2 nJ.

Ultrashort laser pulse sources in the wavelength range of 1.8 to 2 µm have many potential applications including medicine, materials processing, and sensing. In the use of such lasers, a crucial task is to measure their pulse's temporal intensity and phase. Such measurement devices are most useful when they are simple to build and operate and also have high speed and high sensitivity. The GRENOUILLE measurement device with few components, no moving parts, sensitivity of hundreds of picojoules, and measurement speed of hundreds of milliseconds, is commonly used to solve this problem at other wavelengths. In this paper, the measurement of ultrashort pulses by a GRENOUILLE device, developed using a silicon matrix sensor, for pulses in the wavelength range of 1.8 to 2 µm has been demonstrated. It is shown that ultrashort pulses with durations of 74 to 900 fs and a maximum spectral FWHM of 85 nm can be measured with this device. The recently developed ultra-reliable RANA approach was used for pulse retrieval from the measured traces. The device's performance was validated by comparing its measurements with those obtained by the robust FROG technique.

Large GaSe crystals were grown and various antireflection microstructures (ARMs) were fabricated on their cleaved surfaces using optimized femtosecond laser ablation, which provided the antireflection effect in a wide wavelength range of 4-16 mu m. The influence of ARMs created on the GaSe surface on the change of the laser-induced damage threshold (LIDT) of the crystal at a wavelength of 5 mu m was evaluated. The 5-mu m Fe:ZnMgSe laser with the pulse duration of 135 ns was used for the LIDT test in conditions close to single pulse exposure. The measured values of LIDT of 56 +/- 6 MW/cm2 and 51 +/- 9 MW/cm2 for two GaSe substrates, respectively, were comparable with the known data of single pulse LIDT of GaSe. The average LIDT intensities of 54 +/- 6 MW/cm2 and 52 +/- 7 MW/cm2 for the ARMs at two GaSe plates, respectively, were close to LIDT intensities for the corresponding GaSe substrates. The ARMs with lower structural quality had lower LIDT (50-52 MW/cm2) in comparison with the high-quality ARMs (58-60 MW/cm2). High LIDT for high-quality ARMs can be caused by increased selenium content in the ARMs. In any case, all the tested ARMs on the GaSe plates with different surface quality are workable for development of widely tunable mid-infrared nonlinear optical converters.

In this paper, we performed a numerical calculation of arranged-microvoid volumetric reflective gratings in ZnSe crystal. These volumetric modifications are prospective to be used instead of reflective coatings in miniaturized lasers. The calculation results revealed the effectiveness of the proposed method compared to multilayer reflective coatings.

We developed a tunable in spectral range from 2 to $3 \mu \mathrm{m}$ pulsed solid state laser based on the $\mathrm{Cr}^{2+}: \mathrm{ZnSe}^{2}$ crystal with 2.15 mJ maximum output energy at $2.35 \mu \mathrm{m}, 7 \mathrm{~ns}$ pulse length and 100 Hz repetition rate. This laser could be used in theranostic system which combines a laser therapy and optoacoustic diagnoctic.

Most infrared materials used in high-power systems, such as optical parametric generators, have high values of refractive indices, which result in high Fresnel losses. The performance of conventional antireflection coatings is limited when used in high-power and ultra-broadband systems. An alternative approach is to fabricate antireflection microstructures (ARMs) that allow for a broadband increase in transmittance without reducing the damage threshold of the material. In this work, ARMs were fabricated on the surface of ZnSe crystals using the femtosecond laser ablation assisted with wet chemical etching method. This allowed to produce high aspect ratio microstructures that increase the transmittance up to 98% in the mid- and far- infrared regions.

Time-dependent polarization measurements of ultrashort pulses at $1.9 \mu \mathrm{m}$ from a thulium-doped fiber laser system adjustable with polarization controllers were demonstrated. The measurements are based on the GRENOUILLE device and the TURTLE method. By adjusting the polarization controllers of the laser system, a variety of time-polarization pulses were obtained, some of them were characterized.

LiNbO3:Cu (0,005 wt.%), LiNbO3:Cu (0,015 wt.%), LiNbO3:Cu (0,022 wt.%), LiNbO3:Cu (0,042 wt.%), LiNbO3:Cu (0,46 wt.%) crystals were studied by IR absorption spectroscopy in the region of stretching vibrations of OH--groups. These crystals were grown by the Czochralski method using the technology of direct doping of a charge of congruent composition. It has been established that the recorded absorption bands in the infrared spectrum in the frequency range 3469-3490 cm(-1) are associated with a deviation in the composition of the LiNbO3:Cu crystal from the stoichiometric one. Such changes occur due to a deficiency of Li+ cations in the crystal structure. The incorporation of Cu2+ dopant leads to disordering of the cation sublattice and noticeable deformation of oxygen octahedra. This occurs due to an increase in the O-O bond lengths. In this case, a new absorption band with a frequency of 3487 cm(-1), corresponding to the V-Li-OH complex defect, is recorded in the IR spectrum. Calculation of the volume concentration of OH-- groups showed the highest value for crystals of LiNbO3:Cu (0,005 wt.%), LiNbO3:Cu (0,015 wt.%), LiNbO3:Cu (0,022 wt.%). These changes occur due to the simultaneous formation of two types of complex defects in the crystal structure: Cu+- OH- Cu 3- and V-Li-OH. A change in the mechanism of entry of a doping impurity into the structure of a LiNbO3:Cu crystal (0,042 wt.%) leads to a decrease in the concentration of OH-- groups.

Thisstudy presents the development of a tunable femtosecond laser on a Cr 2 + : ZnSe crystal with a Lyot filter in the mid-infrared region. The system provides continuous wavelength tunability in the range of 2.15-2.4 $\mu$m with with a maximum spectrum width of 96.5 nm at a wavelength of 2.2 $\mu$m.

Broadband supercontinuum sources are of interest for various applications. The near-infrared region (1-3 μm) is specifically useful for biomedical diagnostics. One of the promising media for supercontinuum generation in the infrared region is the strongly guiding nonlinear waveguide with an arsenic trisulfide core (As2S3) and a fused silica cladding. The geometrical and chemical properties of such a waveguide allow to finely tune the dispersion landscape and nonlinearity through the core diameter variations. Here we report the generation of octave-spanning supercontinuum in As2S3-silica hybrid nanospike waveguides pumped by a thulium-doped all-fiber femtosecond laser and amplifier system at 1.9 μm wavelength. The widest supercontinuum was obtained in the wavelength range from 1.1 to 2.5 μm (full width at −10 dB) in the waveguide with a core diameter of 1.7 μm. Generation of significant dispersive waves, as well as third harmonics component, is observed. Numerical simulation shows that the generated supercontinua are coherent in the entire spectral range and can be exploited to create a self-referenced laser comb.

Studies of the optical properties of biological tissues in the infrared range have demonstrated significant potential for diagnostic tasks. One of the insufficiently explored ranges for diagnostic problems at the moment is the fourth transparency window, or short wavelength infrared region II (SWIR II). A Cr2+:ZnSe laser with tuning capability in the range from 2.1 to 2.4 µm was developed to explore the possibilities in this region. The capability of diffuse reflectance spectroscopy to analyze water and collagen content in biosamples was investigated using the optical gelatin phantoms and the cartilage tissue samples during their drying process. It was demonstrated that decomposition components of the optical density spectra correlated with the partial content of the collagen and water in the samples. The present study indicates the possibility of using this spectral range for the development of diagnostic methods, in particular, for observation of the changes in the content of cartilage tissue components in degenerative diseases such as osteoarthritis.

LiGaSe2 is a propitious material for nonlinear parametric conversion in the mid-infrared (mid-IR) range. Its refractive index of n = 2.25 in the 2-12 µm wavelength range results in significant losses due to Fresnel reflection. However, the conventional method of increasing the transmittance with antireflection coatings (ARCs) significantly reduces the damage threshold of the material. Fabrication of the antireflection microstructures (ARMs) is an alternative approach for increasing the surface transmittance. In this work, ARMs were fabricated on the surface of a LiGaSe2 crystal using a single-pulse femtosecond laser ablation method. An average transmittance of 97.2% in the 2-8 µm spectral range and the maximum transmittance of 98.6% at 4.1 µm were achieved.

We demonstrate the features of the generation of Raman solitons in germanosilicate fibers in the wavelength range up to 2.5 µm pumped by ultrashort pulses at a wavelength of 1.9 µm.

Tm-doped fibre laser with intracavity third-order dispersion compensation is developed. The numerically simulated pulses have a smooth symmetrical shape and following characteristics: time FWHM of 2 ps, spectral FWHM of 28 nm, 50 MHz repetition rate, average power of 2.5 mW, pulse energy of 48 $pJ$ .

The main application area for GaSe crystals is nonlinear optics, but there are problems with machining and antireflection coatings due to the layered structure of GaSe. Large GaSe crystals with dominant ε-modification were grown. Using various harmonics of a femtosecond laser, antireflection microstructures (ARM) were fabricated by laser ablation on the plate surface and modes providing increased transmission up to 90% in the mid-IR were selected. Using SEM and optical microscopy, as well as EDX and optical spectroscopy, defects on the GaSe surface and their effect on the spectral region and the degree of transmission increase were studied.