Nonlinear infrared (IR) crystals for frequency conversion are essential part of advanced parametric devices used in medical diagnostics, environmental monitoring, and sensing. However, conversion efficiency in crystals is often limited by substantial surface reflectivity coming from the high refractive index of these materials in their transparency range. Here, using a Bridgman-grown LiInS2 as a model nonlinear crystal with a wide transparency window (0.42-12 mu m), we explored two complementary UV-laser-based pathways aiming at fabrication of sub-micron-scale anti-reflective microstructures (ARMs) on the input crystal facets, increasing shortwave IR transmittance (1.2-4 mu m) from 75% to 86%. The demonstrated ARMs include: (i) regular fish-net patterns with a period down to 500 nm directly produced by tightly focused laser beam scanning, (ii) self-organized nanogratings with the periods of 200 nm formed under loose focusing conditions, and (iii) hybrid structures combining both structure types. Our results highlight the underexplored UV femtosecond-laser nanotexturing as a promising and highly precise technique for direct liquid-free nanotexturing of nonlinear crystals towards their improved near-IR pump and frequency conversion performance.

A novel femtosecond inscription technique based on spatial light modulator has been developed to enable focal spot scanning within the static focal spot that opens the way for fabricating refractive index structures with arbitrary complex cross-section geometry including line-by-line and ring-shaped broadband reflectors with random periods and thus enhanced Rayleigh backscattering (RB). The influence of inscription parameters (pulse energy, objective NA, reflector length) on refractive index modulation amplitude, RB level enhancement and reflectivity was demonstrated. Moreover, SLM-inscribed artificial Rayleigh reflectors were used to enhance random distributed feedback strength in narrow-linewidth Er3+-doped fiber laser, as well as to select higher-order transverse mode in the output beam at the low generation threshold of down to 120 W of random Raman lasing in multimode fiber, while broadband reflectivity offers laser wavelength tunability potential.

Fiber Bragg grating (FBG) sensors are widely used for distributed temperature and strain sensing. Here we demonstrate a novel approach to calibrate densely inscribed FBG sensor at central core of multi-core fiber. Long-term memory neural networks trained on sparsely inscribed FBG sensors at peripherical cores were used to predict the positions of their reflectance peaks from complex spectrum obtained from densely inscribed FBG sensor. Our studies show that deep learning algorithms is an effective tool to increase the spatial resolution of the FBG temperature and strain sensors.

Multicore fiber (MCF) is treated now as a perspective medium for high-power fiber lasers. Here we report on the point-by-point femtosecond (fs) inscription of highly-reflective fiber Bragg grating (FBG) array in all cores of 7-core Yb-doped fibers. By using such complex mirror, we achieved high-power laser generation at a wavelength of 1064 nm in two types of active MCF with different core-to-core distance. At cladding pumping by a 976-nm laser diode of 50 W power, both the MCFs with FBG cavity generate nearly the same output power of up to 33 W, but the laser spectra behave principally different since the generation of the cores is almost independent or strongly coupled in these two cases. To study the effect, we develop analytical model and perform its experimental verification identifying the role of core coupling and FBG characteristics on the generated mode and its spectrum. It is shown that the coupling of the MCF cores leads to the formation of supermodes and their hybridization when reflected from an array of highly-reflective FBGs in a resonator, therefore generation occurs with the same spectrum in all cores, despite the significant difference in the central wavelengths and the shape of the reflection spectra of individual FBGs.

Modern photonic devices demand low-cost, scalable methods for creating periodic patterns over diverse surfaces including nonplanar and tipped ones, the examples of which can be readily found in fiber optics. Laser-induced periodic surface structures (LIPSS) offer an attractive route for fabricating such patterns in a single-step straightforward procedure, where the temporal and spatial locality of the self-interference effects ensure robustness against variations of the laser processing parameters. In this work, we show the LIPSS-assisted oxidation of thin titanium films by near-IR femtosecond laser pulses as a promising technology for the production of regular gratings consisting of rutile ridges. The self-terminating nature of the oxidation process allows the predefinition of the grating relief by the initial thickness of the titanium film, rendering the fabrication process with reproducibility and stability to variations of the laser parameters. LIPSS were found to act as gratings providing a diffraction efficiency above 7%. Such gratings were recorded over sidewalls of the optical fibers as well as their endfaces, allowing free space light coupling into the guided mode at the incidence angles up to 65 degrees exceeding fiber acceptance angles. Additionally, gratings recorded over the polished side of the D-shaped fiber were found to act as a Bragg grating manifesting itself through a narrow resonance (<1 nm) in the reflection spectrum of fiber-guided light and suggesting possible applications in sensing and light management. Finally, by exploiting the high regularity of the formed gratings, we also demonstrated resonant coupling of the incident to the surface plasmon waves on the silver-coated LIPSS. With the large resonance amplitude and the Q-factor as large as 150, LIPSS were justified as an easy-to-fabricate template for plasmonic grating production with high potential for plasmonic biosensing and nonlinear optics.

In this Letter, we proposed a new technique for point-by-point fiber Bragg grating (FBG) writing in a static fiber by using a spatial light modulator to control the position of the focal point inside the fiber core. Various types of short-length FBGs (uniform, phase-shifted, and apodized) were demonstrated by this inscription technique. Moreover, the capability to tailor the transverse dimension of a grating pitch from a single point to more complex shapes, such as a wide plane covering a whole fiber core or a transverse ring, was shown.

Diode-pumped multi-mode graded-index (GRIN) fiber Raman lasers provide prominent brightness enhancement both in linear and half-open cavities with random distributed feedback via natural Rayleigh backscattering. Femtosecond laser-inscribed random refractive index structures allow for the sufficient reduction in the Raman threshold by means of Rayleigh backscattering signal enhancement by +50 + 66 dB relative to the intrinsic fiber level. At the same time, they offer an opportunity to generate Stokes beams with a shape close to fundamental transverse mode (LP01), as well as to select higher-order modes such as LP11 with a near-1D longitudinal random structure shifted off the fiber axis. Further development of the inscription technology includes the fabrication of 3D ring-shaped random structures using a spatial light modulator (SLM) in a 100/140 μm GRIN multi-mode fiber. This allows for the generation of a multi-mode diode-pumped GRIN fiber random Raman laser at 976 nm with a ring-shaped output beam at a relatively low pumping threshold (~160 W), demonstrated for the first time to our knowledge.

Despite the fact that monocrystalline germanium (Ge) was historically the first material where formation of laser-induced periodic surface structures (LIPSSs) was observed in 1965 and then extensively studied, practically relevant thin films of amorphous Ge (a-Ge) were occasionally ignored by these studies so far. Here, highly regular LIPSSs were observed on the surface of magnetron-sputtered a-Ge films under near-IR femtosecond laser exposure. Formation of these regular structures was explained by excitation and interference of the surface waves at the interface of photoexcited Ge that was confirmed by performed calculations and full-wave simulations. At the same time, remarkable structural regularity of these LIPSSs unveiled their ablation-free formation scenario involving laser-driven oxidation of the Ge followed by ultra-clean sublimation of the oxide. Laser patterning in vacuum was found to regulate oxidation/sublimation rates of the a-Ge and its oxide improving the structure quality and opening pathways for practical applications in optoelectronics, solar light harvesting and near-IR photonics.

Laser-induced periodic surface structures (LIPSS) offer a simple, single-step technique for creating periodic patterns on solids in an ambient air. Combined with metals like gold or silver, LIPSS enhances grating-assisted refractive index (RI) sensors using surface plasmon resonance (SPR). This study analyzes an SPR sensor in the Kretschmann configuration with a periodically modulated silver layer. The modulation is created by LIPSS formed on a titanium-coated glass prism using femtosecond laser pulses, followed by a silver coating. Reflection spectra for wavelengths lambda = 600-1700 nm were calculated for various angles of incidence and RI values (1.33-1.4). Unlike a flat sensor with a single resonance, the LIPSS-based sensor shows multiple resonances (lambda = 1000-1700 nm) due to modes on inner and outer interfaces. LIPSS reduces resonance spectral width by an order of magnitude, but also reduces sensitivity similarly, weakly affecting the Figure of Merit. The LIPSS-assisted sensor was also tested experimentally.

Soft 2D tactile sensors are becoming increasingly important in robotics and human-machine interaction. In this paper, we propose a new approach to develop a soft tactile sensor using fiber Bragg gratings (FBGs) and machine learning algorithms. The sensor consists of a layer of silicone elastomer with embedded 192 FBGs that can detect deformations caused by point impact. The FBG responses are then processed by machine learning algorithms to measure the position and the force of impacts with the mean absolute errors of 2.1 mm and 0.34 N, respectively.

Fiber optic sensors which use reflectometry methods to process Rayleigh backscattering signal, are susceptible to any optical losses due to the inherently low level of Rayleigh backscattering in standard telecom optical fibers. In this work, we fabricate and study the shape sensor based on a multicore optical fiber with randomly spaced discrete-point reflectors inscribed in its cores by femtosecond laser pulses. By testing the sensor on different 3D shape samples, we demonstrate the robustness of the shape reconstruction accuracy to introduced optical losses of the signal up to 20 dB with the relative reconstruction error being less than 4 %. In contrast, such level of optical losses dramatically increases the reconstruction error when the unmodified fiber cores are used. A higher signal-to-noise ratio together with low birefringence of the inscribed point reflectors enable accurate shape sensing including the forms with low curvature.

We demonstrate low-order Raman lasing in multimode diode-pumped GRIN-fiber with mode-selective thin-film central mirror with 976-nm Stokes beam quality M2 similar to 2.3 and power up to 23W. Real-time mode dynamics is studied showing instability at low powers. (c) 2024 The Author(s)

We present the experimental results on development of 3D shape and temperature fiber sensors based on random and regular reflectors inscribed by femtosecond laser radiation in 7 -core fiber. In the first case the 3D shape measurements accuracy of $\lt 5 \%$ was achieved, whereas in the second case temperature sensor utilizing a machine learning algorithm for a spectral analysis was demonstrated.

Femtosecond (fs) pulse inscription of fiber Bragg gratings (FBGs) in a multicore fiber (MCF) offers new opportunities of controlling the spatio-spectral properties of the generated beam in all-fiber scheme. With coupled cores, interference of partially reflected beams from individual FBGs in different cores becomes important. We present our recent results on the effect of narrowing/collapse of the laser spectrum generated in a cavity based on FBG array fs-inscribed in coupled cores of active (Yb-doped) MCF, which is shown to arise due to the supermodes formation and their hybridization. Output beam concentration in one core observed at Raman lasing in passive MCF with FBG arrays is potentially possible in Yb-doped MCFs. Applications and benefits of such all-fiber lasers are discussed.

In this Letter, we investigated the potential scalability of output power of a cladding-pumped laser and a power amplifier (booster) based on a multimode Bi-doped fiber (BDF) using the mode-selection approach. We fabricated the multimode double-clad graded-index (GRIN) fiber with a confined Bi-doped germanosilicate glass core with a diameter of ≈30 and ≈60 µm. Using femtosecond (fs) inscription technology with high spatial resolution, Bragg gratings of a special transverse structure allowing the selection of low-order modes were written into the core of BDFs. The operation features of the cladding-pumped multimode bismuth-doped GRIN fiber lasers with the inscribed Bragg gratings with various reflection coefficients were investigated. In addition, the behavior of the output power and the beam quality (M 2 parameter) of the optical radiation of the developed devices was studied. The CW laser and booster operating at nearly 1.45 µm with maximum output powers of ≈0.8 and ≈1 W, respectively, based on the 60-µm-core BDF under pumping by multimode laser diodes at 808 nm were developed, which are, to the best of our knowledge, the most powerful cladding-pumped BDF devices to date. Near single-mode lasing (M 2 <1.3) is demonstrated for a 30-µm-core fiber. The experimental data open new possibilities to achieve higher powers in cladding-pumped BDF sources, which are more cost-effective compared to core-pumped counterparts.