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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

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.

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%).

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.

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.

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.

We review our recent experimental results on the cascaded Raman conversion of highly multimode laser diode (LD) pump radiation into the first- and higher-order Stokes radiation in multimode graded-index fibers. A linear cavity composed of fiber Bragg gratings (FBGs) inscribed in the fiber core is formed to provide feedback for the first Stokes order, whereas, for the second order, both a linear cavity consisting of two FBGs and a half-open cavity with one FBG and random distributed feedback (RDFB) via Rayleigh backscattering along the fiber are explored. LDs with different wavelengths (915 and 940 nm) are used for pumping enabling Raman lasing at different wavelengths of the first (950, 954 and 976 nm), second (976, 996 and 1019 nm) and third (1065 nm) Stokes orders. Output power and efficiency, spectral line shapes and widths, beam quality and shapes are compared for different configurations. It is shown that the RDFB cavity provides higher slope efficiency of the second Stokes generation (up to 70% as that for the first Stokes wave) with output power up to ~30 W, limited by the third Stokes generation. The best beam quality parameter of the second Stokes beam is close to the diffraction limit (M2~1.3) in both linear and half-open cavities, whereas the line is narrower (

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.

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.

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.

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.

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 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.

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.

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.

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.

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 semiconductor industry is moving toward a half-pitch of 7 nm. The required metrology equipment should be one order of magnitude more accurate than that. Any metrology tool is only as good as it is calibrated. The characterization of metrology systems requires test patterns that are one order of magnitude smaller than the measured features. The test sample was designed in such a way that the distribution of linewidths appears to be random at any location and any magnification. The power spectral density of such pseudo-random test pattern is inherently flat, down to the minimum size of lines. Metrology systems add a cut-off of the spectra at high frequencies; the shape of the cut-off characterizes the system in its entire dynamic range. This method is widely used in optics, and has allowed optical systems to be perfected down to their diffraction limit. There were attempts to use the spectral method to characterize nanometrology systems such as SEMs, but the absence of natural samples with known spatial frequencies was a common problem. Pseudo-random test patterns with linewidths down to 1.5 nm were fabricated. The system characterization includes the imaging of a pseudo-random test sample and image analysis by a developed software to automatically extract the power spectral density and the contrast transfer function of the nanoimaging system.

Monte Carlo-based SEM image simulation can reproduce SEM micrographs by calculating scattering events of primary electrons inside the target materials. By using the simulated SEM images, it is possible to optimize imaging conditions prior to the specimen observation, which could save time for finding suitable observation condition. However, a recent trend of miniaturized and 3-dimentional structures of semiconductor devices, and introduction of various novel materials have created a challenge for such SEM image simulation techniques; that is, more precise and accurate modeling is required. In this paper, we present a quantitatively accurate BSE simulation and a precise parameters setting in voltage contrast simulation, for both to reproduce experimental SEM images accurately. We apply these simulation techniques to optimize the accelerating voltage of SEM for sub-surface imaging, and to analyze a charge distribution on the insulating specimen under the electron irradiation. These applications promise the advancement in developing a new device by preparing inspecting condition in a timely manner.

Femtosecond-pulse modification of the refractive index in transparent materials enables the inscription of fiber Bragg gratings with new features and extended capabilities. In this study we present the results of fiber Bragg gratings inscription in Corning 62.5/125 multimode graded index fiber with IR femtosecond laser pulses. The specifics of point-by-point inscription including single and multiple Bragg grating inscription in limited fiber segment as well as different transverse modes excitation/suppression is discussed. Multimode fiber Bragg gratings inscribed with femtosecond radiation are investigated for the first time directly in the Raman fiber laser cavity.

The concept of random lasers exploiting multiple scattering of photons in an amplifying disordered medium in order to generate coherent light without a traditional laser resonator has attracted a great deal of attention in recent years. This research area lies at the interface of the fundamental theory of disordered systems and laser science. The idea was originally proposed in the context of astrophysics in the 1960s by V.S. Letokhov, who studied scattering with “negative absorption” of the interstellar molecular clouds. Research on random lasers has since developed into a mature experimental and theoretical field. A simple design of such lasers would be promising for potential applications. However, in traditional random lasers the properties of the output radiation are typically characterized by complex features in the spatial, spectral and time domains, making them less attractive than standard laser systems in terms of practical applications. Recently, an interesting and novel type of one-dimensional random laser that operates in a conventional telecommunication fibre without any pre-designed resonator mirrors–random distributed feedback fibre laser–was demonstrated. The positive feedback required for laser generation in random fibre lasers is provided by the Rayleigh scattering from the inhomogeneities of the refractive index that are naturally present in silica glass. In the proposed laser concept, the randomly backscattered light is amplified through the Raman effect, providing distributed gain over distances up to 100 km. Although an effective reflection due to the Rayleigh scattering is extremely small (∼0.1%), the lasing threshold may be exceeded when a sufficiently large distributed Raman gain is provided. Such a random distributed feedback fibre laser has a number of interesting and attractive features. The fibre waveguide geometry provides transverse confinement, and effectively one-dimensional random distributed feedback leads to the generation of a stationary near-Gaussian beam with a narrow spectrum. A random distributed feedback fibre laser has efficiency and performance that are comparable to and even exceed those of similar conventional fibre lasers. The key features of the generated radiation of random distributed feedback fibre lasers include: a stationary narrow-band continuous modeless spectrum that is free of mode competition, nonlinear power broadening, and an output beam with a Gaussian profile in the fundamental transverse mode (generated both in single mode and multi-mode fibres). This review presents the current status of research in the field of random fibre lasers and shows their potential and perspectives. We start with an introductory overview of conventional distributed feedback lasers and traditional random lasers to set the stage for discussion of random fibre lasers. We then present a theoretical analysis and experimental studies of various random fibre laser configurations, including widely tunable, multi-wavelength, narrow-band generation, and random fibre lasers operating in different spectral bands in the 1–1.6 μm range. Then we discuss existing and future applications of random fibre lasers, including telecommunication and distributed long reach sensor systems. A theoretical description of random lasers is very challenging and is strongly linked with the theory of disordered systems and kinetic theory. We outline two key models governing the generation of random fibre lasers: the average power balance model and the nonlinear Schrodinger equation based model. Recently invented random distributed feedback fibre lasers represent a new and exciting field of research that brings together such diverse areas of science as laser physics, the theory of disordered systems, fibre optics and nonlinear science. Stable random generation in optical fibre opens up new possibilities for research on wave transport and localization in disordered media. We hope that this review will provide background information for research in various fields and will stimulate cross-disciplinary collaborations on random fibre lasers.

Reflection properties of a fiber Bragg grating (FBG) inscribed in a multimode fiber strongly depend on the transverse profile of FBG. Point-by-point FBG fabrication by IR femtosecond pulses with highly nonlinear refraction index variation as a function of intensity within the focal spot offers an opportunity to tailor the FBG transverse profile with high spatial resolution. In this paper we review our recent results on the femtosecond inscription of FBGs in multimode fibers, study of their spectral and mode-selective properties as well as applications for mode selection in multimode and multicore fiber lasers.

We study Raman lasing in a 7 m polarization-maintaining fiber with an array of fiber Bragg gratings with random phases/amplitudes. After optimization of the grating number the threshold pump power decreases to 0.8 W, whereas generated Stokes power at 1092 nm grows linearly above the threshold, reaching ~3 W for both forward and backward waves. A comparison of experimental data with a developed model shows what such a random laser generates in a coherent regime. Near the threshold, single-longitudinal-mode generation with a ~50 kHz linewidth is obtained, whereas the number of generated modes grows with power exceeding ~100 at ~0.1 W. At higher power, a transition to an incoherent regime with different power increments is observed, while the linewidth grows nonlinearly reaching ~80 pm. Insertion of a 30 m fiber before gratings enables ~1.5 times higher backward Stokes power at a narrower linewidth. Additionally, tunable parametric generation around 1140 nm is observed.

In this paper, we present a new method of point-by-point femtosecond inscription of fiber Bragg gratings (FBG) arrays of different configurations in a 7-core spun optical fiber. The possibility of FBGs inscription with predefined periods in individual fiber cores allowed us to realize: 1) longitudinal FBG arrays with identical or variable resonant wavelengths in all side cores, 2) longitudinal FBG arrays inscribed only in the central or in the selected side core, and 3) an FBG array in a transverse cross section of a fiber consisting of an FBG inscribed in the central and three side cores. Based on the proposed method, by enabling the inscription through the acrylate protective coating of the fiber, a vector bend sensor has been created. Implementation of this sensor has shown that bending radii less than 4 mm can be measured with a high precision using a single-channel interrogation scheme.

In this chapter we briefly review the basic principles of Raman fiber lasers operating via random distributed feedback, including details of feedback mechanism, various cavity designs, and corresponding power and spectral characteristics, as well as their statistical properties. We also compare performances of the random Raman fiber lasers (RRFLs) with that for conventional Raman fiber lasers (RFLs) with linear cavity based on two reflectors. The performance analysis includes polarization management, optimization of conversion efficiency, cascaded generation of higher Stokes orders, opportunities for short-wavelength generation via direct pumping by high-power laser diodes, or frequency doubling of random Raman fiber laser radiation. Pulsed operation of random Raman fiber lasers via active or passive Q-switching is also analyzed. The analysis shows that the output characteristics of Raman fiber lasers with random distributed feedback reached to the moment already outperform in many aspects those for conventional Raman fiber lasers. The unique performance of random fiber lasers opens the door to their application in advanced technologies, such as long-distance amplifier-free transmission and remote sensing, low-coherence IR, and visible sources for bio-imaging and others.

A review of recent results on synchronous generation of Raman dissipative solitons in dissipative-soliton fiber laser cavity, as well as comb of highly-chirped pulses obtained at mixing of Raman and main dissipative solitons, is given.

The formation of thermochemical laser-induced periodic surface structures (TLIPSS) on 400-nm Ti films deposited onto a glass substrate is investigated under irradiation by a femtosecond laser with a wavelength of 1026 nm, pulse duration of 232 fs, repetition rate of 200 kHz, and with different spot sizes of 4–21 μm. The optimal fluence for TLIPSS formation reduces monotonously with increasing the spot diameter in the range. It is found that the standard deviation of the TLIPSS period depends significantly on the beam size and reaches approximately 2% when the beam diameter is in the range of 10–21 μm. In addition to TLIPSS formation with the main period slightly smaller than the laser wavelength, an effect of TLIPSS spatial frequency doubling is detected. The optical properties of TLIPSS (reflection spectrum and diffraction efficiency at different incident angles and polarizations) are investigated and compared with theoretical ones to give a basis for the development of an optical inspecting method. The refractive index and absorption coefficient of oxidized ridges of the TLIPSS are theoretically estimated by simulation of the experimental reflection spectrum in the zeroth diffraction order.

We experimentally demonstrate a cascaded generation of a conventional dissipative soliton (DS) at 1020 nm and Raman dissipative solitons (RDS) of the first (1065 nm) and second (1115 nm) orders inside a common fiber laser cavity. The generated high-energy pulses are shown to be linearly-chirped and compressible to 200-300 fs durations for all wavelengths. Moreover, the pulses are mutually coherent that has been confirmed by efficient coherent combining exhibiting ~75 fs and

Random Raman lasers attract now a great deal of attention as they operate in non-active turbid or transparent scattering media. In the last case, single mode fibers with feedback via Rayleigh backscattering generate a high-quality unidirectional laser beam. However, such fiber lasers have rather poor spectral and polarization properties, worsening with increasing power and Stokes order. Here we demonstrate a linearly-polarized cascaded random Raman lasing in a polarization-maintaining fiber. The quantum efficiency of converting the pump (1.05 μm) into the output radiation is almost independent of the Stokes order, amounting to 79%, 83% and 77% for the 1st (1.11 μm), 2nd (1.17 μm) and 3rd (1.23 μm) order, respectively, at the polarization extinction ratio >22 dB for all orders. The laser bandwidth grows with increasing order, but it is almost independent of power in the 1–10 W range, amounting to ~1, ~2 and ~3 nm for orders 1–3, respectively. So, the random Raman laser exhibits no degradation of output characteristics with increasing Stokes order. A theory adequately describing the unique laser features has been developed. Thus, a full picture of the cascaded random Raman lasing in fibers is shown.

We demonstrate a linearly-polarized high efficiency random Raman lasing of the 1st-order Stokes wave and cascaded generation in 0.5- and 1-km-long PM fiber, respectively, under polarized pumping. Quantum efficiency of converting input pump radiation (1.05μm) into the 1st (1.11μm), 2nd (1.17μm) and 3rd-order (1.23μm) Stokes waves is about 80% in the cascaded generation, regardless of the order, and amounts to 92% for the 1st-order Stokes wave in the 0.5-km PM fiber. Polarization extinction ratio is >22 dB for all the waves at output powers of up to 10 W. An analytical model describing adequately the generated power for all components of the cascaded random Raman fiber laser has been developed. The laser bandwidth increases with Stokes order, amounting to ~1, ~2 and ~3 nm for the consecutive orders, respectively.

Digital planar holography enables the creation of a new generation of integrated photonic circuits with desired transfer function. We give here, for the first time, the basis for designing computer-generated planar holograms and demonstrate their application for spectroscopy-on-chip. Nanospectrometer chips are demonstrated with unmatched spectral resolution of up to 2 ·105. A specific configuration is demonstrated for easy integration of planar holograms into the full spectrometer system. The ultraminiaturization and very high performances of the devices are a breakthrough in spectroscopy and open a novel route for the digital processing of light.

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.