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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.

A distributed temperature monitoring inside a cavity of ytterbium DFB and holmium fiber lasers has been demonstrated with a spatial resolution of 1 and 5 mm, respectively, for the first time to the best of our knowledge. For this, we use an optical backscatter reflectometer, which measures intracore temperature, and compare it with the data of an IR thermographic camera, which measures temperature from the surface of a fiber. In the case of holmium fiber laser pumped at a wavelength of 1125 nm with a power of 6 W, the maximum temperature variation along the ∼3-m active fiber reaches ∼60 °C. In the case of ytterbium DFB laser, we observe a strong inhomogeneity of the temperature along the DFB cavity, which leads to a significant decrease in the lasing efficiency. When pumped by a single-mode laser diode at a wavelength of 976 nm with a power of up to 526 mW, the maximum temperature difference reaches 37 °C for the 37-mm DBF cavity.

We report on a pulsed regime of Raman laser with all-fiber cavity scheme based on a multimode graded-index fiber directly pumped by a CW multimode laser diode. The cavity was performed with 3.7 km 62.5/125 MM graded-index fiber with two FBGs on the both sides of it. Intra-cavity acousto-optic modulator pigtailed with 10/125 fiber allowed to obtain mode locking regime. At 27.19-kHz repetition rate corresponding to the laser cavity round-trip frequency (corresponding to mode-locking regime), stable nanosecond pulses with 6.3 W peak power have been observed at the 1st (1018 nm) Stokes order. The beam quality of generated pulses is greatly improved as compared to that for pump diode (M2

Recently, highly regular thermochemical laser-induced periodic surface structures (TLIPSS) have become the subject of active studies. TLIPSS are formed in the interference maxima due to the local oxidation of the material irradiated with ultrashort laser pulses and are characterized by the elevation of the relief that forms parallel oxide protrusions. The gas surrounding is expected to affect the morphology and chemical composition of the resulting TLIPSS; however, such effects were rarely studied so far. Here we present the results of the TLIPSS fabrication on glass-supported Si-Ti bilayer films using an astigmatic Gaussian IR femtosecond beam both in air and a nitrogen-rich atmosphere. The formation of ordered TLIPSS with the period of ≈ 920 nm is observed at slow scanning speeds (∼1 μm/s) and low fluences in a nitrogen-rich atmosphere. Raman spectroscopy revealed the presence of TiO2 (rutile) peaks, as well as bands centered at 280 cm-1 and 320 cm-1, which can be related to TiN in amorphous and polycrystalline phase.

Multimode fibres provide a promising platform for boosting the capacity of fibre links and the output power of fibre lasers. The complex spatiotemporal dynamics of multimode beams may be controlled in spatial and temporal domains via the interplay of nonlinear, dispersive and dissipative effects. Raman nonlinearity induces beam cleanup in long graded-index fibres within a laser cavity, even for CW Stokes beams pumped by highly-multimode laser diodes (LDs). This leads to a breakthrough approach for wavelength-agile high-power lasers. However, current understanding of Raman beam cleanup is restricted to a small-signal gain regime, being not applicable to describing realistic laser operation. We solved this challenge by experimentally and theoretically studying pump-to-Stokes beam conversion in a graded-index fibre cavity. We show that random mode coupling, intracavity filtering and Kerr self-cleaning all play a decisive role for the spatio-spectral control of CW Stokes beams. Whereas the depleted LD pump radiation remains insensitive to them.

The power scaling on all-fiberized Raman fiber oscillator with brightness enhancement (BE) based on multimode graded-index (GRIN) fiber is demonstrated. Thanks to beam cleanup of GRIN fiber itself and single-mode selection properties of the fiber Bragg gratings inscribed in the center of GRIN fiber, the efficient BE is realized. For the laser cavity with single OC FBG, continuous-wave power of 334 W with an M2 value of 2.8 and BE value of 5.6 were obtained at a wavelength of 1120 nm with an optical-to-optical efficiency of 49.6%. Furthermore, the cavity reflectivity is increased by employing two OC FBGs to scale the output power up to 443 W, while the corresponding M2 is 3.5 with BE of 4.2. To our best knowledge, it is the highest power in Raman oscillator based on GRIN fiber.

Irradiation of a metallic film with ultrashort high-energy laser pulses in a visible and near-infrared wavelength range leads to the formation of so-called laser-induced periodic surface structures (LIPSS) due to non-linear light absorption. These structures represent arrays of parallel protrusions or valleys on the surface of the material and the mechanism of their formation is an actively discussed topic [1] , [2] . Large areas of the material surface can be covered with LIPSS by scanning a sample with a laser beam. Morphology of LIPSS (period, orientation, modulation depth, uniformity, etc.), as well as their chemical composition, depends on the properties of the chosen material, film thickness, and irradiation parameters. Metals with relatively low surface plasmon mean free path [3] and high oxidation rate [4] support the formation of highly ordered LIPSS at high scanning speeds, and metals of the titanium subgroup (Zr, Hf) are especially interesting in this regard.

Distributed stress and temperature measurements along the active fiber of operating laser is a new dimension of laser systems analysis. Thermal loading of the active fiber can lead to the switching of the laser operation mode [1] , gain spectrum changing, and modes instability appearance. To realize distributed temperature measurements one can use external thermal camera (thermograph) or optical backscatter reflectometer. The first one measures the temperature of the fiber surface, and the second one measures temperature directly in the fiber core. By using optical backscatter reflectometry spatial resolutions down to ~10 microns and temperature resolution of ~0.1 °C are achieved [2] , which allows high-performance analysis of fiber-optic schemes. In [3] distributed temperature measurement in the active medium of a high-power ytterbium-doped fiber laser is demonstrated using this approach. On the other hand, a similar or greater temperature difference can be measured in fibers doped with active ions and pumped with a large quantum defect. Holmium (Ho 3+ ) can become the most attractive ion because it can be pumped at 1.125–1.15 µm, while emitting wavelength is around 2 µm. In this work, we study the temperature distribution in Ho-doped DBR fiber lasers pumped at a wavelength of 1.125 µm. We use LUNA OBR 4600 reflectometer with down to 1 mm spatial resolution [2] and the IR-thermograph IRTIS-2000 ME to measure temperature distribution along a fiber specimen.

We report on the demonstration and characterization of Raman laser generating at the wavelength of ~1090 nm with total output power of up to 5 W based on the 7-core passive fiber with coupled cores. The Raman gain in all cores is provided by the pump laser connected to the FBG-free central core, whereas the laser cavity is formed by two sets of highly-reflective fiber Bragg gratings (FBGs) inscribed by fs pulses in all peripherical cores at the both ends of the 7-core fiber. The output FBG set has got a random shift along the axis between individual FBGs thus forming a random array of FBGs. Along with the Stokes line narrowing reasoned by the reduction of spectral broadening via nonlinear effects due to the enlargement of effective mode area in the multicore fiber with coupled cores in comparison with a standard singlemode fiber Raman laser, the additional line narrowing effect induced by the multicore random FBG array has been also revealed. It results in the generation of single peak of

As shown recently, highly multimode (M2~30) radiation of laser diodes may be converted in graded-index fibers into high-quality (M2≤2) Raman beam at Stokes-shifted wavelength. Here we experimentally study output beam profiles for the transmitted pump and generated Stokes beams and develop an analytical balance model enabling theoretical beam profiles both near the Raman threshold and well above the threshold where significant depletion of the pump beam occurs due to the Raman conversion. Comparison of the theory and experiment allows for the identification of main mechanisms of the Raman beam clean up in this system, including linear and nonlinear effects.

Fiber-optic shape sensors based on 7-core optical fibers with twisted and straight cores were integrated into a flexible medical papillotome and the accuracy of shape reconstruction was analyzed for different fiber Bragg gratings spacing.

Graded-index multimode optical fibers have recently attracted a renewed attention, thanks to the discovery of new nonlinear effects, such as Kerr beam self-cleaning. In essence, Kerr self-cleaning involves a flow of the propagating beam energy into the fundamental mode of the fiber, accompanied by a redistribution of the remaining energy among high-order modes. Increasing the fundamental mode energy leads to a significant improvement of the output beam quality. A standard method to determine beam quality is to measure the M2 parameter. However, since self-cleaning involves the nonlinear redistribution of energy among a large number of fiber modes, measuring a single beam quality parameter is not sufficient to characterize the effect. A properly informative approach requires performing the mode decomposition of the output beam. Mode decomposition permits to evaluate the energy distribution among all of the excited fiber modes, which enables investigations of nonlinear mode coupling processes at a qualitatively new level. In this work, we demonstrate an efficiency mode decomposition method based on holography, which is suitable for analyzing the self-cleaning effect. In a theoretical study, we describe the solution of the mode decomposition problem for the modes of the gradedindex multimode fiber. In an experimental investigation, we demonstrate the decomposition of both low-power (speckled) and self-cleaned beams, involving more than 80 modes.

We review our recent results on fiber lasers with distributed feedback based on π-shifted or random fiber Bragg gratings, and random index structures inscribed by femtosecond pulses in singlemode or multicore/multimode fibers.

All-fiber Raman lasers have demonstrated their potential for the efficient conversion of highly multimode pump beams into a high-quality Stokes beams. However, the modal content of these beams has not been investigated yet. Here we apply, for the first time, a mode decomposition technique for revealing intermodal interactions in different operational regimes of CW multimode Raman lasers. Our approach allows for analyzing the output laser radiation in terms of the amplitude and phase distributions of a huge number of excited modes for both the pump and the Stokes beams, which enables a new insight into nonlinear mode coupling processes. The measured contribution of the first three modes of the residual pump beam after overcoming the SRS threshold decreased on average by 25%, whereas the signal beam mainly consists of fundamental mode (40%) and the modes of the first group (20%).

A novel ultrahigh-resolution x-ray microscope achieves 8-nm spatial resolution and accurately maps chemistry in nanomaterials., The analysis of chemical states and morphology in nanomaterials is central to many areas of science. We address this need with an ultrahigh-resolution scanning transmission soft x-ray microscope. Our instrument provides multiple analysis tools in a compact assembly and can achieve few-nanometer spatial resolution and high chemical sensitivity via x-ray ptychography and conventional scanning microscopy. A novel scanning mechanism, coupled to advanced x-ray detectors, a high-brightness x-ray source, and high-performance computing for analysis provide a revolutionary step forward in terms of imaging speed and resolution. We present x-ray microscopy with 8-nm full-period spatial resolution and use this capability in conjunction with operando sample environments and cryogenic imaging, which are now routinely available. Our multimodal approach will find wide use across many fields of science and facilitate correlative analysis of materials with other types of probes.

Specially designed composite heavily Er3+-doped fiber in combination with unique point-by-point inscription technology by femtosecond pulses at 1,026 nm enables formation of distributed-feedback (DFB) laser with ultra-short cavity length of 5.3 mm whose parameters are comparable and even better than those for conventional Er3+-doped fiber DFB lasers having much longer cavity. The composite fiber was fabricated by melting rare-earth doped phosphate glass in silica tube. The ultra-short DFB laser generates single-polarization single-frequency radiation at 1,550 nm with narrow linewidth (3.5 kHz) and 0.5 mW output power at 600 mW 980-nm pumping. The same fiber with conventional CW UV (244 nm) inscription technology using phase mask enables fabrication of 40-mm long DFB laser with > 18 mW output power at 3.3% pump conversion, which is a record efficiency for Er3+-doped fiber DFB lasers. The developed technologies form an advanced platform for Er3+-doped fiber DFB lasers operating around 1.55 µm with excellent output characteristics and unique practical features, in particular, the ultra-short DFB lasers are attractive for sensing applications.

We present experimental, analytical, and numerical methods developed for reconstruction (deconvolution) of one-dimensional (1D) surface slope profiles over the spatial frequency range where the raw data are significantly perturbed due to the limited resolution of the measurement instrument. We characterize the spatial resolution properties of a profiler with the instrument's transfer function (ITF). To precisely measure the ITF, we apply a recently developed method utilizing test surfaces with 1D linear chirped height profiles of constant slope amplitude. Based on the results of the ITF calibration, we determine parameters of an analytical model for the ITF that is used in the original reconstruction software. Here, we treat surface slope metrology data obtained with the Optical Surface Measuring System (OSMS), using as a sensor an electronic autocollimator (AC) ELCOMAT-3000. The spatial resolution of the OSMS is limited by the size of the AC light-beam-collimating aperture. For the purposes of this investigation, the OSMS is equipped with a circular aperture with a diameter of 2.5 mm. This is a typical arrangement of most AC-based slope profilers developed for surface slope metrology of state-of-the-art x-ray mirrors. Using the example of surface slope metrology of two state-of-the-art elliptically shaped x-ray focusing mirrors, we demonstrate that the developed data reconstruction procedure allows us to significantly improve the accuracy of surface slope metrology with the OSMS over the spatial wavelength range from ∼1.6 mm to 7 mm. Thus, the amplitude of the quasi-periodic error characteristic of the deterministic polishing process used appears to be higher by a factor of ∼2 than is apparent from the rough metrology data. Underestimation of the surface slope errors in this spatial wavelength range can lead to serious errors in the expected performance of x-ray mirrors in synchrotron beamlines, especially at modern light sources utilizing coherent x rays, where the perturbations can lead to increased speckle-like intensity variation.

The influence of β-radiation exposure (the total dose up to 41.1 MGy and dose rate of 2.5 kGy/s) on the spectral characteristics of high and low reflective FBGs inscribed using femtosecond laser radiation in Ge-doped and pure-silica core fibers with polyimide and metal coating is presented. The largest Bragg wavelength induced shift (BWS) of +55 pm is observed in the case of weak reflective FBG (type I) inscribed in Ge-doped fiber. A comparable red shift in wavelength of + 50 pm is observed in the case of high reflective FBG (type II), which is explained by an increase in the concentration of GeE’-centers and, accordingly, an increase in the effective refractive index. Moreover, a significantly smaller BWS of -10 pm was obtained in the case of high reflective FBGs inscribed in pure-silica core fibers.

We investigate possibilities of using a multicore fiber containing fiber Bragg grating (FBG) in sensing applications. We use the advantages of the femtosecond point-by-point technique to inscribe FBGs in the selected cores of polyimidecoated 7-core fiber. Besides the results on 3D shape sensing we present new approaches for acoustic wave detection and environment refractive index sensing. In particular, we show that spatial division multiplexing with a multicore fiber allows one to create multipoint acoustic sensor based on a single laser source. In addition, to detect the change in environment refractive index by using point-by-point FBG, there is no need in fiber coating removal. Thus, durability of the fiber along with the FBG inscription approach is of great potential for fabrication of next generation multiple parameter sensors.

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.

Raman fiber lasers (RFLs) are usually based on single-mode fibers core-pumped by high-power rare-earth-doped fiber lasers with single transverse mode output that leads to a rather complicated design of RFLs. One of interesting possibilities to simplify the RFL design is its direct pumping by cheap and reliable high-power multimode laser diodes (LDs). It is attractive to use standard graded-index multimode passive fibers characterized by very high quality and low cost due to their wide use in telecom. In this case, one can directly couple the multimode radiation of high-power LDs with moderate brightness into the core of multimode graded-index fiber with much higher efficiency than into the core of single-mode fiber. Using commercially available multimode LDs with operating wavelengths of 915–940 nm, it is possible to obtain high-power high-beam-quality Raman lasing in wavelength range of 950–1000 nm, which is problematic for rare-earth-doped fiber lasers. Here, we review the results obtained in this direction and report on the demonstration of all-fiber LD-pumped CW Raman laser based on the graded-index fiber. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of fiber core, results in high-efficiency conversion of multimode (M2∼26) pump at 915 nm into a high-quality output beam at 954 nm. Fibers with core diameter of 62.5, 85, and 100 μ m are compared. With core enlargement, the conversion efficiency increases sufficiently (from 47% to 84%) at the expense of slight beam-quality parameter reduction (M2 = 1.3–3). The generated spectrum remains to be rather narrow ( 60 W.

In this work, long high-quality fiber Bragg gratings with phase shifts in the structure are inscribed directly in the optical fiber by point-by-point technique using femtosecond laser pulses. Phase shifts are introduced during the inscription process with a piezoelectric actuator, which rapidly shifts the fiber along the direction of its movement in a chosen point of the grating with a chosen shift value. As examples, single and double π phase shifts are introduced in fiber Bragg gratings with a length up to 34 mm in passive fibers, which provide corresponding transmission peaks with bandwidth less than 1 pm. It is shown that 37 mm π -phase-shifted grating inscribed in an active Er-doped fiber forms high-quality DFB laser cavity generating single-frequency radiation at 1550 nm with bandwidth of 20 kHz and signal-to-noise ratio of >70 dB. The inscription technique has a high degree of performance and flexibility and can be easily implemented in fibers of various types.

The scheme of erbium-doped distributed feedback fiber laser is proposed, which combines pumping schemes of the single frequency oscillator and optical signal amplifier. The scheme provides maximum efficiency of pump to signal conversion while minimizing the effect of relaxation oscillations on the output radiation parameters. In this scheme single-frequency fiber laser radiation power up to 30 mW achieved at the pump power of 600 mW at 980 nm. Amplified spontaneous emission was at the level of 0.08%. The magnitude of the relative intensity noise is — 140dB/Hz at frequencies more than 10 MHz and — 100dB/Hz at the peak of relaxation oscillations. The linewidth of the laser radiation is less than 1 kHz.

We use 7-core optical fiber containing Fabry-Perot interferometers for a spatial division multiplexing of acoustic sensors. The interferometers are inscribed in different cores of the fiber using the femtosecond point-by-point inscription technique.

The paper presents a novel three-dimensional quasi-continuous shape sensor based on an FBG array inscribed by femtosecond laser pulses into a 7-core optical fiber with a polyimide protective coating. The measured bending sensitivity of individual FBGs ranges from 0.046 nm/m-1 to 0.049 nm/m-1. It is shown that the sensor allows for reconstructing 2- and 3-dimensional shapes with high accuracy. Due to the high value of the core aperture and individual calibration of each FBG we were able to measure the smallest reported bending radii down to 2.6 mm with a record accuracy of ∼1%. Moreover, we investigate the magnitude of the errors of curves reconstruction and errors associated with measurement of curvature radii in the range from 2.6 to 500 mm. The main factors affecting the accuracy of measurements are also discussed. The temperature resistance of both the inscribed FBG structures and of the protective coating, along with the high mechanical strength of the polyimide, makes it possible to use the sensor in harsh environments or in medical and composite material applications.

We report on the first demonstration of pulsed regime of Raman laser based on a multimode graded-index fiber directly pumped by a CW multimode laser diode. Proof-of-principle experiments have been performed with a 3.7-km multimode graded-index fiber with 62.5-μm core pumped by 976-nm high-power laser diode and cavity formed by bulk mirror and fiber Bragg grating with intra-cavity acousto-optic modulator providing Q-switching or mode locking. At 27.2-kHz repetition rate corresponding to the laser cavity round-trip frequency (i.e. in mode-locking regime), stable nanosecond pulses with peak power of ~300 W have been observed both at the 1st (1018 nm) and 2nd (1064 nm) Stokes orders. At that, the beam quality of generated pulses is greatly improved as compared to that for the pump diode (M2>20) reaching M2=2 for the 2nd-order Stokes wave.

We present the results of high-ordered periodic surface structure formation by femtosecond laser pulses (λ = 1026 nm, τ = 232 fs) using an astigmatic Gaussian beam with a diameter of ≈100 μm on the surface of metal films (thickness of 15 nm and 150 nm) of Hf sputtered on the glass substrate. Continuous and uniform structures with a period of ≈700 nm are formed at a scanning speed up to 2000 μm/s on the film with a thickness of 15 nm. In addition, the formation of ablative periodic substructures with a period of about 160 nm oriented perpendicularly to the main ones was observed on the oxide protrusions. In the case of 150 nm thick film, defects, cracks and phase shifts accompanied a uniform structure with a period of ≈940 nm formed at a speed up to 500 μm/s.

Output beam characteristics of a LD-pumped multimode graded-index fiber Raman laser with different cavity configurations are studied. It has been shown that specially designed 976-nm FBGs inscribed by femtosecond pulses enable selection of an individual transverse mode: fundamental LP01 mode is generated in case of FBG localized in the center of graded-index fiber core, whereas next-order LP11 mode is generated in case of FBG shifted by ~8 um from the center. Corresponding beam shapes and spectra are observed in the laser output. At that, output power at the same pumping is sufficiently higher in case of LP11 FBG. Optimization of output characteristics is also performed resulting in generation of ~50 W power at 976 nm with beam quality parameter M2≈2.

Thin Cr films 28-nm thick deposited on glass substrates were processed by scanning low-intensity femtosecond laser pulses with energy well below single-pulse damage threshold. Two types of laser-induced periodic surface structures (LIPSS) were produced, depending on the scanning velocity, (1) parallel to laser light polarization with periodicity somewhat smaller than laser wavelength and (2) perpendicular to polarization with spatial period much smaller than wavelength. All structures are formed as protrusions above the initial film surface and exhibit a high degree of oxidation. To explain formation of the LIPSS and their conversion from one to another type, a rigorous numerical approach for modeling surface electromagnetic waves in thin-film geometry has been developed, which takes into account the change of optical properties of material due to laser-induced oxidation and porosity. The approach addresses the multiplicity of electromagnetic modes allowed for thin films. It has been found that the low spatial frequency LIPSS parallel to laser polarization, which are formed at low scanning velocities, are well described by the Sipe theory for surfaces of low roughness. The SEW mode responsible for high spatial frequency LIPSS formation at high scanning velocities has been identified. The mechanisms of optical feedback and transformation between types of LIPSS with scanning velocity have been proposed.

We present the results on fabrication of 3D fiber Bragg grating (FBG) arrays in specialty 7-core optical fibers with straight or twisted (spun) cores. Femtosecond laser inscription technology allowed us to modify the fibers through the acrylate or polyimide protective coatings that significantly increases the durability of the FBG sensors as compared to conventional UV inscription approach, requiring the coating removal. Custom-made 7-core fiber with polyimide coating opens up new prospects for shape sensors operating in high-temperature environment. Twisted-core fiber makes it possible to measure not only the shape, but also the direction of fiber torsion that is essential for a free-standing sensors. A novel method enabling core-selective FBGs inscription in a 7-core spun optical fiber is presented in this work. By using the created sensors bending radii down to several millimeters can be measured with a high precision. Separation of different core FBGs by wavelength makes it possible to combine several cores during their interrogation, which allows for sensor measurements through a single optical port.

We present the results of high-ordered surface structure formation by femtosecond laser pulses $(\lambda$ =1026 nm, $\tau$ =232 fs) using an astigmatic Gaussian beam on the surface of thin metal films (thickness 30–40 nm) of Cr and Hf sputtered on the glass substrate. The difference in the structure quality and formation performance between the cases of Cr and Hf metal films was analyzed. The fabrication productivity increase by an order of magnitude was achieved in the case of Hf metal film.

We demonstrate an efficient conversion of highly-multimode (M2∼34) pump beam from fiber-combined 938-nm laser diodes into a 976-nm Stokes beam of high quality (M2=1.7-2 at output powers 10-52 W) that results in record brightness enhancement (BE=73) for Raman lasers based on graded-index fiber (GIF) and other fibers. The measured shapes of the output beams in GIF show that the near-parabolic pump beam becomes depleted above the Raman threshold so that the hole in the pump beam profile is much broader than the generated Stokes beam of near-Gaussian shape. The effect is analyzed in frames of a radially-dependent balance model with an account for the mode-selective feedback by FBG, which qualitatively describes the high-quality Stokes beam generation and the hole formation, but the hole width (and its power broadening) as well as the BE values are not consistent with experiment. Possible reasons for the difference are discussed.

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.

We experimentally and theoretically investigate complex temporal pulse reshaping that accompanies Kerr beam self-cleaning in multimode optical fibers. We also study the output beam shape dependence on initial conditions.

We present the results of fabrication and characterization of two distributed feedback fiber lasers based on 32-mm pi-phase-shifted fiber Bragg gratings, which are inscribed in non-PM CorActive EDF-L1500 and non-photosensitive CorActive SCF-ER60 fibers with a femtosecond point-by-point technique. The generation regimes, spectral and polarization characteristics of the lasers are studied depending on the fiber specifics.

The results on core-selective inscription of fiber Bragg gratings in different multicore fibers by femtosecond laser pulses are presented. Applications, including multiple parameters sensors and fiber lasers, are discussed.

We report on the transverse mode selection in an all-fiber CW Raman laser based on a multimode graded-index fiber directly pumped by multimode laser diodes. Selection properties of special fiber Bragg gratings inscribed by UV CW or IR femtosecond radiation in the 100-μm core of graded-index fiber are experimentally compared. It is also theoretically explained why the better fundamental mode selection occurs in the femtosecond fiber Bragg grating inscribed in the fiber with lower core diameter. Fibers with core diameter of 62.5, 85 and 100 um are compared in the experiment. With core enlargement, the output power and slope efficiency increase sufficiently (from 47% to 84%) at the expense of slight beam-quality parameter increase (M2 =1.3-3).

We report for the first time, to the best of our knowledge, an all-fiber Raman graded-index (GRIN) fiber laser pumped by a fiber laser. This configuration points to potential future power and brightness increases. Continuous-wave power of 135 W with an M2 value of 2.5 was obtained at a wavelength of 1081 nm with an optical-to-optical efficiency of 68%. A commercial GRIN core fiber acts as the Raman fiber in a power oscillator configuration that includes fiber Bragg gratings (FBGs) written onto the GRIN fiber. The efficiency and brightness demonstrated here are, to the best of our knowledge, the highest reported in any Raman GRIN fiber laser. A brightness enhancement of the pump beam by a factor of 5.6 is attained due to the transverse profiles of Raman gain and FBG reflection in the GRIN fiber.

We report on the first all-fiber CW Raman laser based on a multimode graded-index fiber directly pumped by multimode laser diodes. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of fiber core, results in high-efficiency conversion of a multimode (M2~26) pump at 915 nm into a high-quality output beam at 954 nm. Fibers with core diameter of 62.5, 85 and 100 um are compared. With core enlargement, the output power and slope efficiency increase sufficiently (from 47% to 84%) at the expense of slight beam-quality parameter reduction (M2=1.3-3).

Experimental method of temperature and strain discrimination with fiber Bragg gratings (FBGs) sensors embedded in carbon fiber-reinforced plastic is proposed. The method is based on two-fiber technique, when two FBGs inscribed in different fibers with different sensitivities to strain and/or temperature are placed close to each other and act as a single sensing element. The nonlinear polynomial approximation of Bragg wavelength shift as a function of temperature and strain is presented for this method. The FBGs were inscribed with femtosecond laser by point-by-point inscription technique through polymer cladding of the fiber. The comparison of linear and nonlinear approximation accuracies for array of embedded sensors is performed. It is shown that the use of nonlinear approximation gives 1.5–2 times better accuracy. The obtained accuracies of temperature and strain measurements are 2.6–3.8°C and 50–83 μεin temperature and strain range of 30–120°C and 0–400 με, respectively.

Fiber lasers have been actively developed in the last decade demonstrating unique efficiency and performance. It is known that the laser generation is possible not only in active fibers, but also in passive fibers owing to the effect of stimulated Raman scattering (SRS). In the SRS process, pump radiation induces amplification of the Stokes-shifted scattered light (by ~13 THz and ~40 THz for Si0 2 /Ge02 and P 2O5 , respectively), see [1] for a review. The Raman gain spectrum in silica-based fibers is rather broad (>10 THz). Therefore, Raman fiber lasers (RFLs) are treated as perspective laser sources in spectral bands, which are not available from rare-earth (RE) doped fiber lasers [2]. The RFL cavity is usually formed by a set of fiber Bragg gratings (FBGs) which reflect Stokes waves of the first and higher orders. As a result, a cascaded generation with stepwise tuning in a broad spectral range is possible, which may be combined with continuous tuning within the Raman gain spectrum of individual Stokes components. However, a relatively low Raman gain requires rather long (0.1-1 km) passive fibers with high-power pumping into the single-mode fiber core, e.g. by 1-μm single-mode Yb-doped fiber lasers (YDFLs) securing cascaded Raman generation in 1-1.5 μm range [1].

We show that Kerr beam self-cleaning results from parametric mode mixing instabilities that generate a number of nonlinearly interacting modes with randomized phases - optical wave turbulence, followed by a direct and inverse cascade towards high mode numbers and condensation into the fundamental mode, respectively. This optical self-organization effect is an analogue to wave condensation that is well known in hydrodynamic 2D turbulence.

A high-energy (0.93 nJ) all-fiber erbium femtosecond oscillator operating in the telecom spectral range is proposed and realized. The laser cavity, built of commercially available fibers and components, combines polarization maintaining (PM) and non-PM parts providing stable generation of highly chirped (chirp parameter 40) pulses compressed in an output piece of standard PM fiber to 165 fs. The results of the numerical simulation agree well with the experiment. The analyzed intracavity pulse dynamics enables the classification of the generated pulses as dissipative solitons.

Raman fiber lasers (RFLs) are treated as perspective laser sources in spectral bands which are not available from rare-earth (RE) doped fiber lasers. Raman lasing can be easily obtained from 1.1 to 2 μm in single-mode fibers that are core-pumped by RE-doped fiber lasers generating single transverse mode. However, the RFL generation below 1, Mm is difficult because of a lack of suitable high-power single-mode pump sources in the short-wavelength range, e.g. the lower limit for the Yb-doped fiber laser is ∼980 nm, while the influence of photodarkening effect increases with the wavelength shortening [1]. A perspective solution is to develop RFLs based on a multimode graded-index (GRIN) fiber directly core-pumped by cheap and reliable high-power multimode laser diodes (LDs) operating at wavelengths below 1 μm. This approach allows one to simplify the laser scheme and eliminate the problems associated with RE-doped fiber lasers, e.g. photodarkening. Such RFLs have shown to date the ability of high power generation (Pout ∼ 150 W) by shortening the GRIN fiber length to 0.2 km [2]. Effect of beam clean-up arising at the Raman conversion of pump radiation in GRIN fibers provides much better quality of the RFL output beam in comparison with that for LDs, while the RFL cavity is formed by a broadband bulk mirror and a cleaved fiber end [2, 3]. But it is not sufficient to provide single transverse mode operation. We propose to use fiber Bragg gratings for an additional mode selection [4].

Fiber Bragg gratings (FBGs) with phase shifts in the structure has a wide field of applications in sensor and laser devices: distributed-feedback and multiwavelength fiber lasers, optical sensors for simultaneous temperature/tension measurements and high-sensitivity ultrasonic detectors, optical signals processing and filtering elements. Advantages of femtosecond (fs) laser inscription technology — modification of non-photosensitive materials, FBG inscription through the fiber plastic coating, high-temperature and radiation resistance of FBGs etc — make the technology attractive to create fiber devices and components with advanced technical features [1].

It is known that there is a significant portion of the near-IR wavelength band uncovered by rare-earth-doped active fibers. This fact stimulates many research in this direction and a new approach to generate high-power ultrashort pulses by stimulated Raman scattering was demonstrated recently both in the intra- and extra-cavity configurations [1, 2]. The generated Raman pulse was proved to be a Raman dissipative soliton (RDS) [1] that can be combined coherently with the main (pump) DS in the intra-cavity design [3]. On the other side a much higher RDS energy could be achieved in the extra-cavity configurations as far as pump and Raman lasers can be optimized independently [2].

High power mode-locked (ML) fiber lasers generating femtosecond pulses in telecom window near 1.55 μm is a field of growing interest due to the apparent success of all-normal dispersion (ANDi) fiber lasers generating chirped pulses in the wavelength region around 1 μm [1, 2]. Such lasers have a wide range of practical applications, from CARS and Terahertz spectroscopy to few-cycle pulse synthesis and telecommunications. In a majority of practical applications, short duration and high pulse energy are as important as long-term stability and usability. From this perspective the all-fiber lasers look the most appropriate due to their compact size, absence of complicated adjustments, high efficiency, intrinsic stability and perfect beam quality.

The all-fiber highly-chirped dissipative soliton (HCDS) oscillator was realised at 1.5 μm wavelength. A normal net cavity dispersion was achieved by using a conventional dispersion compensating fiber (DCF). To separate effects of the amplitude self-modulation and dissipative soliton formation, we exploit in the laser cavity both standard single mode fiber and polarization maintaining single mode fiber. The properties of the generated pulses have been varied by changing spectral filter bandwidth and DCF lengths. After compression of the nJ-level ∼6ps HCDS in the external fiber compressor, we measured the output pulse duration of 165 fs (an estimated chirp parameter 40).

The paper presents the results of the formation of thermochemical laser induced periodic surface structure on Cr thin films (40–350 nm) that was sputtered on the glass substrate. The different dependence of TLIPSS formation on processing rate in case of thin (40 nm) and thick (350 nm) is observed.