We report a study on the switching of the generation regimes in a high-powered thulium-doped all-fiber ring oscillator that is passively mode-locked with nonlinear polarization evolution technique with different pumping rates and cavity dispersion values. In one experimental setup, switching was observed between the noise-like pulse and the multi-soliton (in the forms of soliton bunches and soliton rain) regimes by the adjustment of the intracavity polarization controllers. We attributed this to the crucial influence of the nonlinear polarization evolution strength determined by such key parameters as saturation (over-rotation) power, linear phase bias, and nonlinear losses on the pulse evolution and stability. So the soliton collapse effect (leading to noise-like pulse generation) or the peak power clamping effect (generating a bunch of loosely-bound solitons) may determine pulse dynamics. Both the spectrum bandwidth and coherence time were studied for noise-like pulses by varying the cavity length and pump power, as well as the duration of solitons composing bunches. As a result, both noise-like pulses (with spectrum as broad as 32 nm bandwidth) and multi-soliton formations (with individual pulse-widths ranging from 748 to 1273 fs with a cavity length increase from 12 to 53 m) with up to 730 mW average power were generated at a wavelength of around 1.9 μm. The results are important for the realization of the broadband and smooth supercontinuum which can be used as a source for mid-IR vibrational spectroscopy of gas samples for breath analysis and environmental sensing.

GaSe crystals are promising as nonlinear optical converters in the mid- and far-IR ranges. However, it is challenging to increase the GaSe surface transmittance of 77% with conventional antireflection coatings because of poor surface quality, leading to coating adhesion problems. Antireflection microstructures (ARMs) offer an alternative way of increasing surface transmittance. In this work, ARMs were fabricated on the surface of a GaSe plate by single-pulse femtosecond laser ablation. An average GaSe surface transmittance of 94% in the 7–11 µm range and a maximum transmittance of 97.8% at 8.5 µm were obtained. The proposed method can be used to increase the efficiency of GaSe-based nonlinear converters.

A Yb femtosecond laser was used to fabricate antireflective microstructures on CdSSe single crystal samples. We investigated several microstructure fabrication methods, including direct single pulse ablation using 200 fs pulses, ablation with in-depth focusing, ablation in the presence of additional spherical aberration and ablation with obstruction of peripheral rays. We performed a comprehensive analysis of the implemented antireflection microstructure fabrication methods. The performances of the antireflection microstructures were measured using an infrared Fourier spectrum analyzer and the best samples demonstrated $>99\%$ transmission in the 4.5–6 µm range and average transmission near $97\%$ in range from 2.7 to 8 µm.

We report on a comprehensive characterization of Cr²⁺-doped CdSe single crystal as an efficient active material for tunable laser applications in the mid-infrared spectral region. Optical gain, thermo-optical behavior, power efficiency, scalability and wavelength tunability have been thoroughly investigated. Using an antireflection-coated crystal pumped by a Tm-fiber laser at at 1.94 μm, 1-W CW output power and 50% slope efficiency at 2.65 μm emission wavelength have been obtained in a diffraction-limited output beam. An output peak power of 2.5 W has been achieved without significant beam distortion in a quasi-CW regime. Exploiting an intra-cavity diffraction grating in a Littrow configuration, a maximum tuning range of 900 nm from 2.22 to 3.12 μm, limited by the finite bandwidth of resonator components, has been demonstrated with an uncoated crystal.

Broadband supercontinuum sources are of interest for various applications. The near-infrared region (1–3 $\mu$ m) 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 (As ₂ S ₃ ) 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 As ₂ S ₃ -silica hybrid nanospike waveguides pumped by a thulium-doped all-fiber femtosecond laser and amplifier system at 1.9 $\mu$ m wavelength. The widest supercontinuum was obtained in the wavelength range from 1.1 to 2.5 $\mu$ m (full width at -10 dB) in the waveguide with a core diameter of 1.7 $\mu$ m. 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.