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A compact random fiber laser based on a short artificial Rayleigh reflector and heavily-doped Er fibers (custom-made and commercial as a reference) has been proposed, characterized and optimized in terms of efficiency, linewidth and noise level. A 10-cm artificial Rayleigh reflector with mean scattering level of +41.3 dB/mm relative to the natural Rayleigh scattering of the host fiber and low insertion loss level (~0.05 dB/cm at 1535 nm) was fabricated using a femtosecond direct writing technique. Its implementation as a distributed output mirror in a half-open cavity of a 980-nm diode pumped Er-doped fiber laser results in random lasing at 1535 nm in single- and few-mode regimes with power up to 100 mW, slope efficiency up to 16.5%, and signal-to-noise ratio up to 60 dB. A single-frequency regime with ~10 KHz linewidth was observed at output power up to 2.5 mW. Tunability potential of such random lasers is also demonstrated.

For the first time, radiation-induced absorption (RIA) of light is compared in two types of polarization-maintaining PANDA fibers having 1) undoped-silica core and F-doped-silica cladding, or 2) N-doped-silica core and undoped-silica cladding. RIA at λ = 1.55 μm is compared under γ-irradiation and post-irradiation recovery at room temperature (RT) and −60 °C. The post-irradiation RIA decay at RT is used to extrapolate RIA to the postulated end of the PANDA space mission (15 years) with the help of the “nth-order kinetic” model. The RIA extrapolation values for both PANDA types are found to be comparable. RIA spectra are measured in the PANDAs in the spectral range ∼0.5–4 eV in the process of both γ-irradiation and recovery; then the spectra are deconvolved into Gaussians components. The RIA spectra of both PANDA types are found to be composed of inherent and strain-assisted self-trapped holes (STHs), some undoped PANDAs demonstrating, in addition, a long-wavelength RIA tail affecting λ = 1.55 μm. No N-related color centers are revealed. Finally, RIA at λ = 1.55 μm is compared upon pulsed-X-ray irradiation. Undoped PANDAs are found to feature much smaller RIA at RT at post-pulse times larger than ∼40–1000 μs, whereas at smaller times, an N-doped PANDA proved to be superior. At −60 °C and at t > 1 ms, both PANDA types demonstrate approximately equal RIAs. Thus, the experiments showed that the two PANDA types are quite comparable, each type having its own advantages and disadvantages.

We demonstrate selective excitation and coherent amplification in the multicore fiber of the out-of-phase supermode that has equal intensities but interleaved 0/π phases in the neighboring cores. The out-of-phase supermode is beneficial due to its stability at arbitrary high powers, whereas the fundamental supermode becomes unstable at relatively high powers. We fabricated a double-clad all solid silica fiber with six highly Yb-doped coupled cores arranged in a ring structure. We obtained amplification of 50 picosecond chirped pulses up to ∼1 μJ energy with measured content of the out-of-phase mode of >80%. The experimental results were supported by 3D numerical simulation demonstrating the possibility of the out-of-phase supermode amplification in the multicore fibers with real parameters without degradation of supermode composition for powers up to damage threshold in silica glass.

The results of the numerical analysis of a new design of birefringent microstructured fibres containing three rings of identical air holes around an elliptical core with different distances between the rings of holes are presented in this study. The first ring contains two pairs of holes, which are more widely spaced than others. The study shows that the proposed design makes it possible to obtain single-mode guidance of fibres in the spectral range of 0.2 – 2.3 microns.

We experimentally demonstrate spatial beam self-cleaning in an Yb-doped graded-index multimode fiber taper, both in passive and active configurations. The input laser beam at 1064 nm was injected for propagation from the small to the large core side of the taper, with laser diode pumping in a counterdirectional configuration. The Kerr effect permits to obtain high-beam quality amplification with no accompanying frequency conversions. As a result, our nonlinear taper amplifier may provide an important building block for multimode fiber lasers and amplifiers.

Coherent propagation of supermodes in a multicore fiber is promising for power scaling of fiber laser systems, eliminating the need for the active feedback system to maintain the phases between the channels. We studied the propagation of broadband pulsed radiation at a central wavelength of 1030 nm in a multicore fiber with coupled cores arranged in a square array. We designed and fabricated a silica multicore fiber with a 5 × 5 array of cores. For controllable excitation of a desired supermode, we developed a beam-forming system based on a spatial light modulator. We experimentally measured intensity and phase distributions of the supermodes, in particular, the in-phase and out-of-phase supermodes, which matched well the numerically calculated profiles. We obtained selective excitation and coherent propagation of broadband radiation with the content of the out-of-phase supermode of up to 90% maintained without active feedback. Using three-dimensional numerical modeling with allowance for a refractive index profile similar to those of the developed fiber, we demonstrated stable propagation of the out-of-phase supermode and collapse of the in-phase supermode at a high signal power.

The copolymers of N-vinyl-2-pyrrolidone (N-VP) and condensation cardo and/or fluorinated condensation polymers (aromatic polyamide, polyimide and polyarylate) were synthesized by free radical in situ polymerization. The partial formation (14–61 wt%) of polivnylpyrrolidone (PVP) homopolymer upon the polymerization of N-VP containing dissolved condensation polymer was stated. Synthesized (co)polymers are differed in heat resistance and solubility from neat PVP. Using relevant copolymers solutions in N-methyl-2-pyrrolidone the optical fiber coatings having good adhesion properties were fabricated. Such coatings demonstrate high stability at elevated temperatures. A substantial number of defects on coatings and a slight decrease in bending strength are observed only on exposure at 200 °C for 72 h. Simultaneously holding for 1 and 24 h at the same temperature leads to hardening of the fibers: bending strength changes from ∼5 up to 6 GPa.

Highly birefringent microstructured fibers (HB MSFs) with similar mode field sizes along the two orthogonal axes have been presented. The core of the HB MSF is surrounded by two or more concentric rings having identical circular air holes. The key feature of this setup is that the first ring contains one or two pairs of holes, which are more widely spaced than others, i.e., they have one or two wider bridges. The shape of the core may be circular or elliptical. The HB MSFs were characterized using the finite element method. Simulation results show that HB MSFs with a certain relationship between the width of the wider bridges and the core ellipticity may have high birefringence (more than 2.7 × 10−3) and equal mode sizes along the two orthogonal axes. We have fabricated and investigated several such fibers, that have birefringence up to 2.9 × 10−3 and almost equal measured mode field sizes along the two orthogonal axes.