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Numerical simulation of the fiber optic parametric oscillator aiming the goal to produce picosecond narrowband pulses for CARS has been performed in an extremely wide range of parameters, such as a pump pulse duration, parametric frequency shift, spectral bandwidth of the pump and the parametric pulses. It required extremely large calculation window both in time (3.5 ns) and spectral (from 433 nm to 3100 nm) domains. We managed to speed up simulation in fifty times by graphic processor unit, which allowed to define the areas of stability for different lengths of standard passive and photonic-crystal fibers used in the external linear cavity of oscillator. The cavity length reached a value of 100 meters that was resulted in parametric pulses with the energy up to 40 nJ and peak power up to 1 kW at a wavelength about 800 nm.

Graded-index fibers with special in-fiber Bragg gratings are shown to convert efficiently highly multimode (M2 ~30) laser diode pump radiation in all-fiber Raman laser configuration into a high-quality Stokes beam at 954 and 976 nm (M2 =1.6-2.6 in different configurations). The beam quality improvement is provided by well-known Raman beam cleanup effect defined by Raman gain profile, and additional impact of transverse mode-selective properties of cavity feedback provided by special femtosecond-pulse inscribed fiber Bragg gratings and/or random Rayleigh backscattering distributed along the fiber. Here we study frequency doubling of the diode-pumped Raman fiber laser in a simple singlepass scheme with 5-mm PPLN crystal demonstrating efficient second harmonics generation at 477 and 488 nm with high beam quality after elimination of interfering effects.

CARS is the one of most exciting and actively developed techniques for real-time monitoring of processes occurring in biological tissues. We present a fiber optical parametric oscillator pumped by specially designed MOPA fiber laser to get time-synchronized optical signals with a frequencies difference of 2800-3000 reverse centimeters. We utilize the fourwave mixing effect in a photonic-crystal fiber to build narrowband tunable fiber optical parametric oscillator. A complex optimization of each part of the source has been performed.

Random lasers utilize multiple scattering in disordered amplifying medium which provides necessary feedback even in the absence of resonant cavity. 1D random lasing via Rayleigh backscattering in single-mode fibers (SMF) with pump-induced Raman gain enabled generation of stable high-quality narrow-line output beam, which can even outperform traditional Raman fiber lasers (RFLs) with cavity made of fiber Bragg grating (FBG) pairs. Use of multicore or multimode fibers complicates transverse mode structure of random fiber lasers making them closer to traditional bulk-media random lasers, but they offer new opportunities. Here we review our latest results on investigation of random lasing in multicore and multimode fibers. First, random lasing has been explored in a PM twin-core fiber (TCF) with half-open cavity formed by Rayleigh backscattering distributed along TCF and lumped reflector at one fiber end: either a broad-band fiber-loop mirror (FLM) or femtosecond-pulse inscribed in-core FBGs (in one core or in both cores). The inscription of two FBGs in different (but coupled) cores leads to the formation of a Michelson-type interferometer resulting in the multi-wavelength generation with single peak width of $ . Second, direct pumping of multimode graded-index (GRIN) fibers by high-power LDs at 915 nm provides Raman lasing at Stokes line near 950 nm selected by multimode fs-inscribed FBGs, whereas 2-order random lasing in half-open cavity with one fs FBG appears possible in a broad wavelength range (978—996 nm). At that, the cascaded Raman conversion is accompanied by sufficient improvement of the output beam quality in comparison with that for multimode LDs. As a result, high-efficiency generation of nearly diffraction limited beam in the all-fiber LD-pumped GRIN RFL has been demonstrated.

Frequency doubling of multimode diode-pumped graded-index fiber lasers generating high-quality (M2=2–2.5) beam at 954 and 976 nm is studied. Efficient generation in PPLN crystals of second harmonics at 477 and 488 nm has been demonstrated.

Raman lasing in a graded-index fiber (GIF) attracts now great deal of attention due to the opportunity to convert high-power multimode laser diode radiation into the Stokes wave with beam quality improvement based on the Raman clean-up effect. Here we report on the cascaded Raman generation of the 2nd Stokes order in the 1.1-km long GIF with 100-μm core directly pumped by 915-nm diodes. In the studied all-fiber scheme, the 1st Stokes order is generated at 950–954 nm in a linear cavity formed at GIF ends by two fiber Bragg gratings (FBGs) securing beam quality improvement from M2 ≈ 30 to M2 ≈ 2.3 due to special transverse structure of FBGs. The 2nd Stokes wave is generated either in linear (two FBGs) or half-open (one FBG) cavity with random distributed feedback via Rayleigh backscattering. Their comparison shows that the random lasing provides better beam quality and higher slope efficiency. Nearly diffraction limited beam (M2 ≈ 1.6) with power up to 27 W at maximum gain (996 nm), and 17 W at the detuned wavelength of 978 nm has been obtained, thus demonstrating that the 2nd-order random lasing in diode-pumped GIF with FBGs provides high-efficiency high-quality beam generation in a broad wavelength range within the Raman gain spectral profile.

Frequency doubling of multimode diode-pumped GRIN-fiber Raman laser with improved beam quality (M2=1.9-2.6 depending on configuration) in a simple single-pass scheme with 5-mm PPLN crystal is studied. After scheme optimization and elimination of back reflection and crystal heating effects, an efficient conversion into blue spectral range with output power of about 0.4 W@488 nm and 0.64 W@477 nm has been demonstrated.

We demonstrate a high-power, high-efficiency Raman laser based on a 100 µm core graded-index (GRIN) fiber directly pumped by 915 nm multimode laser diode modules in all-fiber configuration. Optimization of GRIN fiber length and pumping scheme was performed. As a result, 62 W of CW power has been obtained at a wavelength of 954 nm with a slope efficiency of 85%. The joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of the GRIN fiber core results in significant beam quality enhancement for the generated Stokes beam ( 30). In addition, the exact value of the operating wavelength near the Raman gain maximum was varied. To the best of our knowledge, this is the first study of the impact of a Raman gain spectral profile on the power, spectrum and beam quality parameter M 2 of multimode Raman laser. It appears that they very much dependent on the spectral region of Raman gain at which we work, when the wavelength is selected by a fiber Bragg grating inscribed in a multimode GRIN fiber.

An external-cavity generation of powerful ultrashort pulses in an all-fiber scheme by using a new type of phosphosilicate polarization maintaining fiber is investigated. The phosphorus-related Stokes shifted Raman pulse near 1.3 microns is observed. Optimization of Stokes output spectrum depending on pump pulse duration (chirp), energy and output coupling ratio of the cavity is performed. As result, the output energy of highly-chirped pulses compressible to 570 fs reaches 1.6 nJ.