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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.

The main application area for GaSe crystals is nonlinear optics, but there are problems with machining and antireflection coatings due to the layered structure of GaSe. Large GaSe crystals with dominant ε-modification were grown. Using various harmonics of a femtosecond laser, antireflection microstructures (ARM) were fabricated by laser ablation on the plate surface and modes providing increased transmission up to 90% in the mid-IR were selected. Using SEM and optical microscopy, as well as EDX and optical spectroscopy, defects on the GaSe surface and their effect on the spectral region and the degree of transmission increase were studied.

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.

Formation of highly ordered nanostructures on a crystalline silicon surface is highly demanded for novel optoelectronic and nanophotonic designs pushing toward development of inexpensive and high-performing nanostructuring technologies. Here, we demonstrate that laser-induced periodic surface structuring of c-Si protected by a thin Hf over-layer allows one to fabricate extremely uniform high-aspect-ratio gratings with a characteristic periodicity of ≈900–950 and 450 nm. Corresponding ordering originates from interference of incident IR femtosecond laser pulses with surface plasmons as well as doubling of the grating period via interference of counter-propagating plasmons. A high-melting-point Hf over-layer regulates the c-Si ablation in the plasmon-mediated interference maxima and prevents its excessive oxidation upon multi-pulse exposure in ambient environment. Considering unique high-aspect ratio morphology (a depth-to-period ratio of up to 1.24 and a depth-to-width ratio of up to 8) of the reported nanogratings, their outstanding uniformity, and rather fast printing rate of ≈0.2 mm2/s as well as possibility for its further upscaling, we envision high practical applicability of this technology in novel optoelectronic devices, visible and near-IR optics, all-dielectric metasurfaces, and sensors.

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.

Multimode 940-nm diode radiation is shown to be converted in graded-index fibers into high-quality (M2≤2) 976-nm Raman beam due to parabolic pump beam, its homogeneous depletion above threshold, mode-selective feedback by FBG and nonlinear effects.

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.

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.

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.

Ultrafast fiber lasers with a high pulse energy at 1550 nm wavelength are important for a range of applications: from CARS [1], few-cycle pulse generation [2] and frequency metrology to THz-wave generation. These applications require not only the high pulse energy, but also short duration, and high generation stability. Highly chirped (chirp parameter > 10) dissipative soliton (HCDS) generation technique (HCDS) driven by the nonlinear polarization evolution (NPE) effect meets all the above requirements. Substantial HCDS energy increase has been obtained previously in Yb-fiber all-fiber NPE mode-locked cavity containing polarization maintaining (PM) and non-PM parts [3]. We have implemented this approach for 1.5 microns wavelength area for the first time in [4] demonstrating 165 fs dechirped duration and 0.93 nJ energy HCDS. We also observed a multi-pulse generation regime caused by NPE overdriving. In this work we extend our results and increase the single HCDS energy by significantly lengthening the all-fiber cavity.

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.

Raman fiber lasers (RFLs) represent an important class of fiber lasers with all benefits of fiber design, in which a passive fiber with pump-induced Raman gain is used as an active medium. Due to the broad Raman gain spectrum and efficient cascaded generation of higher Stokes orders, they may operate at almost any wavelength in transmission window (1–2 μm) of conventional silica fibers with an appropriate pumping [1]. Most of RFL schemes utilize single-mode fibers pumped into the core by another single-mode fiber laser such as Yb-doped fiber laser (YDFL) operating near 1 μm, which in its turn is cladding-pumped by multimode laser diodes (LDs) at 915 or 976 nm. Direct LD pumping is now one of the main challenges for RFL development. Two key opportunities have been explored recently: widely used telecom multimode graded-index fiber (GIF) or specially designed double-clad passive fibers which exhibit relatively good characteristics at weakly-multimode fiber laser pumping but degrade in efficiency at highly-multimode LD pumping, see [2] and citation therein.

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.

A brightness-enhanced random Raman fiber laser (RRFL) with maximum power of 306 W at 1120 nm is demonstrated. A half-open cavity is built based on a graded-index (GRIN) passive fiber and single high-reflective fiber Bragg grating written in it directly. Due to the beam cleanup effect in the GRIN fiber enhanced in the half-open RRFL cavity, the output beam quality factor M 2 is improved from 9.15 (pump) to 1.76–2.35 (Stokes) depending on power, while the pump–Stokes brightness enhancement (BE) factor increases proportionally to output power reaching 6.1 at maximum. To the best of our knowledge, this is the highest power GRIN RRFL with BE.

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.

Recently, a technique for calibration of the Modulation Transfer Function (MTF) of a broad variety of metrology instrumentation has been established. The technique is based on test samples structured according to binary pseudorandom (BPR) one-dimensional sequences and two-dimensional arrays. The inherent power spectral density of BPR gratings and arrays, has a deterministic white-noise-like character that allows a direct determination of the MTF with a uniform sensitivity over the entire spatial frequency range and field-of-view of an instrument. As such, the BPR samples satisfy the characteristics of a test standard: Functionality, ease of specification and fabrication, reproducibility, and low sensitivity to manufacturing error. Here we discuss our recent developments working with support of the U.S. Department of Energy on industrialization of the technique. The goal is to develop affordable BPR test samples, application procedures, and data processing software, suitable for thorough characterization of optical interferometers and microscopes, as well as x-ray, electron (scanning and transmission), and atomic force microscopes. We report on the development of BPR array test samples optimized for advanced characterization (including the instrumental MTF and aberrations) and operation optimization of large aperture optical interferometers. We describe the sample fabrication process and tests to verify the compliance to desired surface topography. The data acquisition and analysis procedures for application of the technique for precise focusing of Fizeau interferometer are discussed in detail.

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).

The results on tilted fiber Bragg gratings (TFBGs) inscription using the method of transverse scanning of the fiber core by a femtosecond laser beam is reported in this paper. As an example, TFBGs consisting of unidirectional and bi-directional grating planes and having a tilt angle up to 9° are created. It is shown that different transverse mode groups of the fiber cladding can be excited with the created structures. The corresponding resonant dips reach the amplitude up to 30 dB that indicates the inscription method efficiency.

A review of recent results on new schemes and regimes of Raman fiber lasers (RFL) is presented. The key feature of RFLs is generation in new wavelength ranges, which are not covered by existing fiber lasers based on rare-earth doped active fibers. New high-brightness 9хх nm all-fiber source based on directly diode pumped passive GRIN multimode fiber with in-core fiber Bragg gratings (FBGs) enables high-efficiency Raman generation of high-quality (M2≤2.6) beam at powers up to ~100 W with further scaling capabilities. Mirrorless scheme based on Rayleigh-backscattering random distributed feedback (RDFB) in a singlemode polarization-maintaining (PM) fiber provides ultimate pump-to-Stokes conversion efficiency for the first order as well as cascaded generation of higher orders in 1-2 μm range. Single-frequency Raman lasing in a short (~10 m) PM fiber with RDFB based on random FBG array has been also demonstrated. Pulsed operation of RFL at active Q-switching and mode-locking in passive fibers is realized with pulse energies and durations of up to 30 μJ and ~50 ps, correspondingly. New regime of Raman dissipative solitons (of the first and second Stokes orders) synchronously generated in a common or external resonant cavity and their nonlinear mixing in PCF provide generation of

In contrast to semiconductor manufacturing where features are mostly lines or contact holes, the disk drive reader has a complex, nonlinear 3D geometry. Metrology of such geometries is challenging; especially with regard to repeatability of measurements. New methods were needed to keep up with production requirements for metrology regarding uncertainty of critical dimensions (CD). We report a new method developed for CD metrology of the disk drive writer pole. The method demonstrated improved uncertainties compared to the regularly used CD-SEM algorithms and also has capability for side wall angle (SWA) metrology for process control. The method utilizes multiple steps: a) extract contours from SEM images, b) identify exact locations on a curvilinear feature where CD should be measured, and c) provide CD measurements at these locations. SEM images from a variety of production wafers were used for evaluation of the developed method. Multiple series of SEM images were processed using a software utilizing advanced algorithms without using regular brightness threshold CD-SEM methodologies. It was found that CD repeatability was improved by a factor of three compared to the results of the regular threshold based CD-SEM method. TEM imaging is used to measure side wall angle; it takes a lot of efforts for sample preparation and the feedback is slow. If extraction of SWA is possible from top down SEM images it would provide instant feedback to manufacturing and reduce cost. Monte Carlo simulations were used to understand the sensitivity of SWA to the trench CD and depth. Height of features was measured using AFM. A method to extract SWA from top down images was developed. Numerous SEM images were processed; results were compared to experiment and analyzed. It was shown that the 3-sigma repeatability of SWA measurement was 0.15 degree. It was also found that the left and the right SWA were different on multiple wafers, the results were very consistent from one image series to another one, at the same time the SWA difference between the left and right walls was considerably larger than the uncertainty of SWA measurement.

CD-SEMs are an indispensable part of semiconductor manufacturing. Extending their functionality beyond routine work on CD metrology of a layer is highly desirable. The etch process greatly depends on CDs and the pitch of a pattern. Through pitch patterns consisting of two layers, each with different CDs and side wall angles, are widely used to develop and characterize etch processes. TEMs are widely used to measure these values for each layer. This is a time consuming and expensive process, and the feedback time to process development is long. It would be highly desirable to develop metrology for two layers, including their side wall angles, using top down SEM images in order to reduce cost and provide a fast feedback to process development and characterization. A technique was developed to measure top and bottom CDs of each layer, as well as side wall angles of the layers using SEM images. The model based myCD software was used as the basis for this development. Hundreds of SEM images were taken at multiple test pattern areas with variable pitch and dimensions. The extracted values included top and bottom CDs of the top layer, top and bottom CDs of the bottom layer, as well as side wall angles of both layers. The pitch was also extracted for verification. The results of CDs and SWA were very consistent. The verification was done in two ways: a) by comparing myCD results to results from corresponding TEM images; and b) by analyzing the noise of the measured data. It is believed that the variation of CDs and SWAs on the through pitch pattern was consistent with the designed pattern layout and their variation from the design due to process variation should be smooth. Both verifications confirmed excellent results of metrology. In particular, the repeatability of SWA measurements from top down images was found to be 0.4 degrees, a 3-sigma variation. There is no other method to our knowledge to measure SWA from top down images.

The interpretation of scanning electron microscopy (SEM) images of the latest semiconductor devices is not intuitive and requires comparison with computed images based on theoretical modeling and simulations. For quantitative image prediction and geometrical reconstruction of the specimen structure, the accuracy of the physical model is essential. In this paper, we review the current models of electron-solid interaction and discuss their accuracy. We perform the comparison of the simulated results with our experiments of SEM overlay of under-layer, grain imaging of copper interconnect, and hole bottom visualization by angular selective detectors, and show that our model well reproduces the experimental results. Remaining issues for quantitative simulation are also discussed, including the accuracy of the charge dynamics, treatment of beam skirt, and explosive increase in computing time.

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).

We report on the first demonstration of a linearly polarized cascaded Raman fiber laser based on a simple half-open cavity with a broadband composite reflector and random distributed feedback in a polarization maintaining phosphosilicate fiber operating beyond zero dispersion wavelength (~1400 nm). With increasing pump power from a Yb-doped fiber laser at 1080 nm, the random laser generates subsequently 8 W at 1262 nm and 9 W at 1515 nm with polarization extinction ratio of 27 dB. The generation linewidths amount to about 1 nm and 3 nm, respectively, being almost independent of power, in correspondence with the theory of a cascaded random lasing.

The method of femtosecond laser inscription by slit-apertured beam is applied for fabrication of long-period fiber gratings in polarization-maintaining Panda-type fiber. Polarization properties as well as ambient refractive index sensitivity are investigated for the inscribed gratings. It is shown that the method allows us to create high-quality element for the use as spectral filter of high-power fiber laser as well as high-temperature sensor.

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.

An external-cavity generation of powerful ultrashort pulses near 1.3 μm in all-fiber scheme by using a new type of phosphosilicate polarization maintaining fiber is investigated. The phosphorus-related Stokes shifted Raman pulse is observed. Its optical spectra are measured as a function of energy and chirp of the pump pulses and output coupling ratio. The output energy of compressible highly-chirped pulses reaches 0.67 nJ.

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).