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We study Raman lasing in a 7 m polarization-maintaining fiber with an array of fiber Bragg gratings with random phases/amplitudes. After optimization of the grating number the threshold pump power decreases to 0.8 W, whereas generated Stokes power at 1092 nm grows linearly above the threshold, reaching ~3 W for both forward and backward waves. A comparison of experimental data with a developed model shows what such a random laser generates in a coherent regime. Near the threshold, single-longitudinal-mode generation with a ~50 kHz linewidth is obtained, whereas the number of generated modes grows with power exceeding ~100 at ~0.1 W. At higher power, a transition to an incoherent regime with different power increments is observed, while the linewidth grows nonlinearly reaching ~80 pm. Insertion of a 30 m fiber before gratings enables ~1.5 times higher backward Stokes power at a narrower linewidth. Additionally, tunable parametric generation around 1140 nm is observed.

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 µm. 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 CrO2 in the case of a high-rate impact and an increase in the contribution of Cr2O3 in the case of a low repetition rate of pulses.

A distributed feedback (DFB) fiber laser based on a 32-mm long pi-phase-shifted fiber Bragg grating inscribed using the femtosecond point-by-point technique in a single-mode erbium-doped optical fiber (CorActive EDF-L 1500) is demonstrated. The lasing power of the DFB laser reaches 0.7 mW at a wavelength of 1550 nm when pumped with a laser diode at a wavelength of 976 nm and power of 525 mW. The width of the lasing spectrum is 17 kHz. It is shown that the pi-phase-shifted fiber Bragg grating fs-inscribed in a non-PM fiber provides the selection of the single polarization mode of the DFB laser. DFB laser formation in a highly doped non-photosensitive optical fiber (CoreActive SCF-ER60-8/125-12) is also demonstrated.

On the base of the phase shifted fiber Bragg grating of 4 cm length written in the active non-PM Yb-doped fiber, a resonator with distributed feedback (DFB) has been formed. The single frequency single polarization unidirectional generation with the output power up to 4.5 mW has been obtained. The spectral and energy performances of the single frequency DFB fiber laser depending on the pump laser parameters and ways of pump wave coupling have been investigated. The competition between orthogonal polarization states has been also studied, and the single polarization regime of 0.5 mW power has been achieved.

Single-frequency operation of a 1.03 μm fiber laser with random distributed feedback (RDFB) is demonstrated. The laser cavity is based on a 4 cm long fiber Bragg grating (FBG) consisting of 10 homogeneous subgratings with random phase and amplitude of refractive index modulation inscribed in a polarization maintaining (PM) Yb-doped fiber. Such RDFB laser generates single longitudinal mode with output power up to 25 mW, which is 3.5 times higher than that for a DFB laser based on regular π-shifted FBG of the same length in the same fiber. The single-frequency linewidth is measured to be

Formation of laser-induced periodic surface structures (LIPSS) on the titanium surface at the presence of sharply focused fs radiation exhibits two different regimes. Conventional ablative LIPSS with low regularity are oriented orthogonally to the polarization direction of the incident beam, while thermochemical LIPSS with highly uniform periodicity are oriented along the polarization direction. These two types of LIPSS can co-exist and influence each other for arbitrary polarization and beam scanning directions. The observed regimes help to clarify mechanisms of LIPSS formation, as well as to form LIPSS of specific shapes and degrees of regularity.

We demonstrate a high-efficiency random lasing in a 850 m span of a phosphosilicate fiber. Random distributed feedback owing to the Rayleigh backscattering in the fiber enables narrowband generation with output power of up to 7.3 W at the Stokes wavelength λS = 1308 nm from 11 Wof the pump power at λP = 1115 nm. The laser demonstrates unique generation efficiency. Near the generation threshold, more than 2 W of output power is generated from only 0.5 W of pump power excess over the generation threshold. At high pump power, the quantum conversion efficiency defined as a ratio of generated and pump photons at the laser output exceeds 100%. Itis explained by the fact that every pump photon is converted into the Stokes photon far from the output fiber end, while the Stokes photons have lower attenuation than the pump photons.

The selector-free single-frequency operation of an Yb-doped fiber laser with scanning in the range of ?20?nm is demonstrated. The frequency and intensity evolution is shown to be driven by a self-induced phase grating in the active fiber defined by gain saturation in a standing-wave. A theory has been developed that describes well the main features of the experiment and provides possibilities for optimization of laser parameters. Perspectives for utilizing the self-scanned laser in fundamental studies and practical applications are discussed.