Optoelectronics, Instrumentation and Data Processing (v.49, #4)

Single-mode optical fibers have been widely used in optical communications, and effective fiber lasers have been designed on the basis of active fibers with linear or ring cavities. In fiber lasers, the distance between the cavity mirrors can reach 270 km (the maximum length is determined by the linear attenuation and nonlinear dephasing of the waves). In 2009, random lasing was found in a long telecommunications fiber without any cavity: the positive distributed feedback required for lasing is due to Rayleigh scattering of light, and distributed amplification is provided by stimulated Raman scattering. Such a laser can be classified into the group of so-called random lasers, actively studied recently; the fiber geometry and the weakness of Rayleigh scattering provide much better output characteristics compared to the other types of random lasers. The lasing efficiency and beam quality of this laser are comparable to those of fiber lasers with a conventional cavity. At the same time, it has a number of unique features (unlimited length, and mode-free spectrum etc.), providing new physical phenomena and new opportunities for applications in telecommunications and sensor systems. The paper presents a review of recent results of studies in this area.
Keywords: fiber laser; distributed feedback; Rayleigh scattering; stimulated Raman scattering; random laser

Tuning and doubling of the generation frequency of fiber lasers by V. A. Akulov; S. I. Kablukov (345-362).
Methods of tuning and doubling of the generation frequency of fiber lasers are discussed. It is shown that the use of a KTP crystal for second harmonic generation makes it possible to develop effective tunable sources of radiation in the visible range.
Keywords: fiber laser; second harmonic generation; Yb-doped fiber laser; Raman fiber laser; frequency tuning

Fiber optical parametric oscillators by E. A. Zlobina; S. I. Kablukov (363-382).
Recent results of experimental investigations of parametric conversion of electromagnetic radiation in fibers are presented. The main attention is paid to the description of continuous-wave (CW) and pulse fiber optical parametric oscillators (FOPOs) operating in the spectral range from 0.5 to 2 µm. Requirements to obtaining effective generation and possible problems in FOPO design are discussed. Results of the development of a CW tunable FOPO with pump by an ytterbium-doped fiber laser, which allows generation of radiation with wavelengths shorter than 1 µm, are considered in detail.
Keywords: fiber optical parametric oscillator; frequency conversion; four-wave mixing; photonics-crystal fiber; phase matching condition

This paper reviews studies of the possibility of controlling the output spectrum of Qswitched fiber lasers. Various laser configurations for producing output radiation with characteristics optimized for specific applications, such as high-power pulses for micromachining of materials, probe pulses in fiber-optic sensor systems, etc., are considered. The mechanism of broadening of the lasing line is elucidated, and methods for controlling the output spectrum in Q-switched all-fiber lasers are described. Frequency tuning in the amplification line and generation of higher harmonics in nonlinear crystals are considered.
Keywords: fiber laser; Q-switching; active fibers; self-phase modulation; frequency doubling

Generation of dissipative solitons in femtosecond fiber lasers by D. S. Kharenko; S. A. Babin (399-415).
The results of the design and study of femtosecond fiber lasers are reviewed. Various methods of mode-locking and generation regimes are considered. Special attention is paid to the regime of dissipative solitons in an all-fiber resonator with normal dispersion. The main results and analysis of the possibilities of energy scaling of femtosecond pulses are given.
Keywords: femtosecond fiber lasers; dissipative solitons; chirped pulses; scaling

Stimulated Raman scattering (SRS) of light in a single-mode optical silica fiber under excitation by a subnanosecond laser pulse with a wavelength of 532 nm is studied. The dependence of the SRS power and its spectrum on the laser pumping power is investigated. It is shown that, at relatively low pumping powers, SRS is well described by an exponential function predicted by the model, neglecting pumping depletion. At powers causing deviations from this dependence, the SRS band, whose value depends on the pumping power, is additionally broadened. Features of the broadening are similar to that of a laser line in an optical fiber. Intense anti-Stokes scattering on vibrational modes over-populated because of the SRS process is also observed.
Keywords: stimulated Raman scattering; optical fiber; tunable lasers