In this paper, we present the study of the thermochemical laser-induced periodic surface structures (TLIPSS) formation on a thin chromium film under femtosecond laser ( λ = 1026 nm, τ = 232 fs) irradiation focused on the astigmatic spot with an axis ratio of ≈1/10 and a width along the long side of 155 µ m. The effect of the number of laser pulses and the repetition rate (2–200 kHz) on the formation of the structures is investigated. Variation of the number of pulses allowed us to observe the dynamics of the formation of structures. It has been found that the ordering of TLIPSS structures increases with increasing repetition rate of laser pulses, which is explained by the heat accumulation effect on the oxidation process and, consequently, on the formation of TLIPSS structures in the case of a high repetition rate of pulses, which is confirmed by the results of numerical modelling. This effect leads to a different oxidation dynamics and the formation of various types of oxides: predominance of CrO ₂ in the case of a high-rate impact and an increase in the contribution of Cr ₂ O ₃ in the case of a low repetition rate of pulses.

Thermochemical laser-induced periodic surface structures (TLIPSS) are a relatively new type of periodic structures formed in the focal area of linear polarized laser radiation by the thermally stimulated reaction of oxidation. The high regularity of the structures and the possibility of forming high-ordered structures over a large area open up possibilities for the practical application for changing the optical and physical properties of materials surface. Since the mechanism of formation of these structures is based on a chemical oxidation reaction, an intriguing question involves the influence of air pressure on the quality of structure formation. This paper presents the results on the TLIPSS formation on a thin hafnium film with fs IR laser radiation at various ambient air pressures from 4 Torr to 760 Torr. Despite the decrease in the oxygen content in the ambient environment by two orders of magnitude, the formation of high-ordered TLIPSS (dispersion in the LIPSS orientation angle δθ < 5°) with a period of ≈700 nm occurs within a wide range of parameters variation (laser power, scanning speed). This behavior of TLIPSS formation is in agreement with experimental data obtained earlier on the study of the kinetics of high-temperature oxidation of hafnium at various oxygen pressures.

A technique of single-longitudinal-mode selection in a fiber laser by means of a fiber multiple-beam reflection interferometer (FRI) has been experimentally demonstrated for the first time. The laser is based on a semiconductor optical amplifier placed in a linear fiber cavity formed by a fiber Bragg grating (FBG), and the FRI generates at 1529.24 nm with output power of 1 mW in single-frequency regime with a linewidth of about 217 kHz and polarization extinction ratio of >30 dB. The FRI technique potentially enables fast tuning (within the FBG bandwidth of ~0.9 nm in our case) by varying the base length of the FRI that can be used in a number of practical applications.

A new variant of all-fiber multiple-beam interferometer capable to perform narrow-band filtration of a reflected light, with characteristics similar to those for Fabry-Perot interferometer in a transmission, is presented. The interferometer design accompanied with parameters simulation is conducted, the experimental sample is fabricated and the study of its characteristics is undertaken. Experimental results conform the calculations. This variant of reflection interferometer can be used as one of laser cavity mirrors providing frequency selection of low-powered fiber lasers and laser diodes with short linear cavities. We assume, that this device makes it possible to obtain single-frequency operation with fast continuous tuning of a laser wavelength in a wide spectral range.