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.

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.

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 ${\rm{\sim 1\ \mu J}}$ 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.

Radiation-induced attenuation (RIA) at wavelength λ=1.55 μm as well as the RIA spectra were investigated in optical fibers during and after irradiation at the IVG.1M nuclear reactor (Republic of Kazakhstan). The fibers studied were single-mode at λ=1.55 μm, had an undoped-silica-core and an-F-doped-silica cladding (”pure-silica fibers, PSFs”), and were provided by different manufacturers. In addition to five PSFs, a single-mode revolver hollow-core fiber was also studied. The PSFs had different coating materials: polyimide, copper, or aluminum. All fibers were exposed to three runs of reactor irradiation at enhanced temperature (155-355 °C) so that by the end of the 3rd run, the fast-neutron fluence amounted to Φ=4.46∙1017n/cm2 and γ-dose to D=2.91∙107 Gy. The total induced loss at λ=1.55 μm was found to be composed of 1) RIA, 2) bending loss (BL), caused by thermal expansion of the copper cylinder the PSFs were wound on, and 3) thermo-induced microbending loss (ML) in Cu-coated PSFs. At the end of the irradiation campaign, the net RIA in PSFs proved to lie in the interval of 0.12-0.16 dB/m, which appears to be admissible to intrareactor transport fibers. It was confirmed that long-wavelength RIA tail is the only RIA origin at λ=1.55 μm in PSFs in such strong radiation fields. The hollow-core fiber was found to feature zeroth RIA throughout the campaign.

Proof testing is the principal technique for revealing the largest fiber flaws that reduce fiber strength below some definite level. At the same time the proof test does not guarantee long-term (25-50 yr) reliability of optical fibers due to the growth of surface defects during proof stress unloading. In the present paper we analyze the possibility of reducing unloading effects by means of shortening the unloading time or proof testing under inert conditions. We have also estimated the reliability of hermetically coated proof-tested optical fibers and obtained simple relations that take into consideration the proof test and service conditions when evaluating the fiber reliability.

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.