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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.

An all-fiber 4-core Yb-doped laser with a cavity formed by fiber Bragg gratings directly inscribed in each core with femtosecond laser pulses and 4% Fresnel reflection from the output fiber end face is demonstrated. It has been shown that the diameter of the active fiber winding significantly affects the power distribution between the cores, since it affects both the pump power distribution and the cross-coupling between the cores. In particular, with an active fiber winding diameter of 21 cm, the cores behave independently, and the power is distributed almost evenly over all cores. With a winding diameter of 6.5 cm, the lasing is achieved almost exclusively from one core, and a mechanism of that radiation concentration based on bending induced stress in an active multicore fiber is proposed which explains the experimental data. By analyzing the optical and radio-frequency spectra of the output laser radiation, additional details of the 4-core fiber lasing are revealed. In particular, a narrowband (several longitudinal modes) lasing with periodic linear sweeping of central wavelength in time is observed and characterized in the multicore fiber laser, for the first time to our knowledge. It is shown that crosstalk of longitudinal modes arising from different cores is greatly enhanced in the case of a strongly bent fiber.

This paper reports results of a study aimed at producing a Raman fibre laser based on a multicore fibre with high-reflectivity fibre Bragg gratings inscribed in its peripheral cores using femtosecond laser pulses. We present spectral and power characteristics of a Raman laser whose output power at a wavelength of 1090 nm is 2.5 W. The laser emission linewidth has been shown to decrease as a result of the reduction in nonlinear effects owing to the larger effective mode area in the multicore fibre in comparison with a standard single-mode fibre Raman laser.

Here, we applied direct laser-induced periodic surface structuring to drive the phase transition of amorphous silicon (a-Si) into nanocrystalline (nc) Si imprinted as regular arrangement of Si nanopillars passivated with a SiO2 layer. By varying the laser beam scanning speed at a fixed pulse energy, we successfully tailored the resulting unique surface morphology of the formed LIPSSs that change from ordered arrangement of conical protrusions to highly uniform surface gratings, where sub-wavelength scale ripples decorate the valleys between near-wavelength scale ridges. Along with the surface morphology, the nc-Si/SiO2 volume ratio can also be controlled via laser processing parameters allowing the tailoring of the optical properties of the produced textured surfaces to achieve anti-reflection performance or partial transmission in the visible spectral range. Diverse hierarchical LIPSSs can be fabricated and replicated over large-scale areas opening a pathway for various applications including optical sensors, nanoscale temperature management, and solar light harvesting. By taking advantage of good wettability, enlarged surface area and remarkable light-trapping characteristics of the produced hierarchical morphologies, we demonstrated the first LIPSS-based surface enhanced fluorescent sensor that allowed the identification of metal cations providing a sub-nM detection limit unachievable by conventional fluorescence measurements in solutions.

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

In this paper we present various configurations of erbium DBR lasers based on seven-core fiber. By using femtosecond writing technique, it is possible to fabricate fiber Bragg gratings (FBGs) in selected cores of multicore fibers (MCF) at a certain transverse and longitudinal locations in a fiber, thus allowing for fabrication of advanced reflecting elements. Different schemes of tunable lasers are presented based on both uniform and chirped FBGs.

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.

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 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.

Femtosecond point-by-point inscription of pi-shifted fiber Bragg grating in Er3+-heavily-doped phosphosilicate fiber with ultrashort length of 5.3 mm enables stable linearly-polarized single-frequency generation of 0.5 mW power and 3.5-kHz linewidth.

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 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.

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.

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

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 have studied second-harmonic generation (SHG) of a cw single-frequency ytterbium-doped fibre laser, using a periodically poled silica fibre as a nonlinear medium for frequency conversion. All-fibre external cavity SHG has been investigated for the first time. A twofold increase in second-harmonic power in a fibre ring cavity has been demonstrated and possibilities of further optimising the fibre scheme have been analysed.

Narrowband Raman lasing in a polarization-maintaining two-core fiber (TCF) is demonstrated. Femtosecond point-by-point inscription of fiber Bragg gratings (FBGs) in individual cores produces a half-open cavity with random distributed feedback. The laser linewidth in the cavity with a single FBG inscribed in one core of the TCF reduced by similar to 2 times with respect to the cavity with a fiber loop mirror. It is shown that the inscription of two FBGs in different cores leads to the formation of a Michelson-type interferometer, leading to the modulation of generation spectra near threshold. This technique offers new possibilities for spectral filtering or multi-wavelength generation. (C) 2019 Optical Society of America

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 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 paper presents the results of the thermochemical laser-induced periodic surface structures formation on Cr film at femtosecond irradiation with elliptical spot shape at different powers, repetition rate and scanning speeds.

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.

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.

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.

We report on long-period grating (LPG) writing in a standard telecom fibre, SMF-28e+, via refractive index modification by femtosecond pulses. A method is proposed for grating writing with a slit-apertured beam, which enables one to produce LPGs with reduced background losses and a resonance peak markedly stronger than that in the case of grating writing with a Gaussian beam. The method can be used to fabricate LPGs for use as spectral filters of fibre lasers and sensing elements of sensor systems.

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).

In this paper, we present the results of femtosecond point-by-point inscription of fiber Bragg gratings in different cores of multicore fibers for applications in 3D shaping sensors, Raman and random fiber lasers based on dual-core fiber.

Application of femtosecond point-by-point inscription technique make it possible to form fiber Bragg gratings (FBGs) in different cores of multicore fibers as mirrors of laser cavity. In this paper, we present various configurations of Raman laser based on dual-core fiber. Due to the use of the inscribed FBG in the cavity of dual-core fiber, lasing threshold is reduced and the output power stability is increased, comparing to existing schemes.

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].

The paper presents the results on inscription of FBG with phase shifts in passive and active rare-earth doped optical fibers by the new versatile method based on femtosecond laser direct point-by-point writing. The technique allows to create phase-shifted FBGs with different values and position of phase shift along the fiber.

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.

In this Letter, we present the results on fabrication and investigation of a holmium (Ho) distributed fiber laser based on a π-phase-shifted FBG inscribed in a heavily Ho-doped fiber by IR femtosecond laser pulses. Single-polarization and single-transverse mode operation regimes were observed with a linewidth of the order of 10 kHz and output power up to 53 mW at 2.07 μm. Lasing regimes are compared for room and cryogenic (77 K) temperatures of the laser cavity. To the best of our knowledge, this is the first realization of such type of fiber laser based on holmium active medium.

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.

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.

Experimental investigation of thermochemical laser-induced periodic surface structures (TLIPSS) formation on metal films (Ti, Cr, Ni, NiCr) at different processing conditions is presented. The hypothesis that the TLIPSS formation depends significantly on parabolic rate constant for oxide thin film growth is discussed. Evidently, low value of this parameter for Ni is the reason of TLIPSS absence on Ni and NiCr film with low Cr content. The effect of simultaneous ablative (with period ≈λ) and thermochemical (with period ≈λ) LIPSS formation was observed. The formation of structures after TLIPSS selective etching was demonstrated.

All-fiber Raman laser based on a graded-index fiber directly-pumped by multimode laser diodes is demonstrated for the first time. High-quality narrowband output of 50 W at 954 nm is generated with slope efficiency of 67%.

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.

The paper presents the results on inscription of fiber Bragg and long-period fiber gratings by femtosecond laser pulses and point-by-point technique in various types of fibers (single-mode, multimode, polarization-maintaining) for applications in fiber lasers.