This study examines the local transmission of normal and pathological corneal tissues when exposed to coherent collimated near-infrared (NIR) radiation with a wavelength of 1061 nm, highlighting its potential advantages over ultraviolet (UV) radiation for antimicrobial photodynamic therapy (PDT). The proposed measurement method, which employs an original experimental setup with a collimated NIR laser source and a probe fiber for direct layer-by-layer depth profiling, allows for determining the transmission values at specific depths within the cornea. Considering that the affected areas are localized within 1/3 to 1/2 of the corneal thickness, the residual power of the NIR radiation in the posterior part of the affected zone (at a depth of 250-400 µm) does not exceed 20%. However, the residual power of the NIR radiation at the posterior epithelium of the affected cornea, after passing through its entire thickness, does not exceed 10%. Current UV-based antimicrobial corneal PDT suffers from limited depth penetration and low specificity, hindering effective treatment and requiring photoprotectors to shield the non-regenerating posterior epithelium. NIR-based PDT, using selective photosensitizers, offers a promising alternative. The results of our NIR corneal transmittance measurements are crucial for the development of more targeted, effective, and safer therapies.

A hybrid laser source with a profiled laser pulse shape is presented. It is designed with a fiber pulse shaping system and a two-stage solid-state amplifier based on two gain modules with pulsed transverse diode pumping. The ability to control the pulse shape of the hybrid laser source pulse is demonstrated, and rectangular and stepped pulse shapes with a radiation wavelength of 1064.15 nm, duration of 20 ns, repetition rate of 2 Hz, and output energy of 1 J are obtained. The resulting shapes are achieved by controlling the temporal shape of the pulses of the master oscillator to compensate for the effect of gain saturation in solid-state amplifiers. The dynamics of the pulse shape evolution as a function of the electrical pump energy in the gain modules is traced.

We have demonstrated a tunable erbium-doped ring fiber laser using an intracavity optical filter based on a tapered fiber. By adjusting the optical filter, we were able to tune the laser's central wavelength. The tapered fiber was created from a commercially available single-mode optical fiber using the Vytran GPX 3400 setup. Thin films of aerosol CVD-synthesized single-walled carbon nanotubes were used to achieve mode locking with a pulse duration of up to 1 picosecond. We compared three tapered fibers with different waist diameters and demonstrated continuous tuning of the laser wavelength over a range of 8.5 nm while maintaining stable mode locking.

The presence of different modulators within a cavity often leads to increased losses and is bound by temporal limitations. Therefore, it is reasonable to investigate alternative optical approaches to modulate and control radiation parameters. We propose an optical method for laser radiation modulation based on the use of an excited-state absorption (ESA) transition in a laser active medium under the action of resonant radiation from an external control source. The special features of this method are demonstrated by numerical simulations based on a generalized laser model with the minimum number of parameters. A comparison with the data obtained by detailed simulation of the generation dynamics of a holmium fiber laser demonstrated the matching of the key physical features. The obtained data allow us to understand the limitations of such an approach for different types of lasers. The experimental results confirm the successful application of the proposed method in a holmium fiber laser under exposure by the external pulsed control radiation at lambda(e) 1.6 mu m, corresponding to the ESA transition I-5(7)-> I-5(5) in holmium-doped fiber.

This study investigates emission characteristics of a single-mode ytterbium-doped phosphosilicate fiber laser pumped at 976 nm. The short cavity of the laser was composed of two fiber Bragg gratings directly inscribed in the fiber core by the ArF laser. The emission at 1066 nm was investigated both in continuous wave (CW) and gain-switched modes. A single-frequency CW lasing with an output power of 40 mW was demonstrated. Complex dynamics of the output pulse duration and jitter with respect to the pump pulse duration were detected in the gain-switched lasing mode.

A compact gain-switched all-fiber ytterbium laser operating at a wavelength of 1127 nm with the ability to control pulsed radiation parameters has been investigated. Gain switching was carried out by modulating the current of a semiconductor pump diode. With pumping energy ranging from 197.6 to 263.5 μJ, there was one generation pulse per each pump pulse with a duration of 0.43 to 1.1 μs, energy of 5.3 to 8.8 μJ, and a repetition rate of 100 Hz. Increasing the pumping energy above 265 μJ led to a change in the shape of generated pulses and to their energy rising up to 70 μJ.

This work presents the development and analysis of tunable spectral filters using tapered fibers. The filter was created by heating and pulling single mode optical fiber to form tapers. The optical signal propagating in the fiber within the 1.2 to $1.6 \mu \mathrm{m}$ range was filtered by bending the taper.

In this study, we explored the potential for average power scaling in a monolithic side-counter-pumped combiner based on Yb-doped tapered fibers. The optimal configuration of the pump-feeding fibers was determined through experiments with passive signal fibers. It is shown that pump coupling efficiencies higher than 83% can be achieved for fibers coated with low-index polymer with a numerical aperture (NA) around 0.45 and more than 74% for fibers with second cladding made of F-doped silica (NA ~ 0.26) for pump power up to 100 W. It was shown that the main factor significantly reducing the pump-to-signal conversion efficiency in the developed monolithic Yb-doped tapered fiber amplifiers is the pump leakage due to the decrease of the first cladding diameter along the tapered fiber and the corresponding increase of the pump NA (which becomes higher than the NA of the first cladding). A solution to this problem based on a narrowing diameter at the output end of the tapered fiber was proposed and realized. The record-high average power of 41 W, with a coupling efficiency of 77.7%, was demonstrated in a monolithic amplifier with a threshold of nonlinear effects of more than 600 kW (for ps pulses). Prospects for further power scaling in all-fiber sub-MW peak power amplifiers are discussed.

We demonstrate the optimization of a short cavity passively mode-locked (ML) erbium-ytterbium fiber laser. The cavity consisted of a composite active fiber and a hybrid isolator-coupler-multiplexer. To realize the ML regime, aerosol-synthesized single-walled carbon nanotubes were placed in the cavity. During length optimization, the pulse repetition rate was increased from 100 to 298 MHz. At the highest repetition rate, the optical pulses had a duration of 3.6 ps and the average output power was 1.6 mW.

This paper presents a method for direct real-time estimation of the frequency modulation (chirp) of a relatively long duration laser pulse based on the use of an array of Fiber Bragg Gratings (FBGs). As an example, a 180-ps pulse generated by a mode-locked ytterbium-doped fiber laser has been studied.

Wehave investigated the generation characteristics of a distributed Bragg reflector (DBR) short cavity ytterbium fiber laser. Due to pulse pumping, the laser with an emission wavelength of 1066 nm was operated in gain switch mode with pulse durations ranging from 32 ns to 83 ns.

the dispersion parameter of composite optical fiber doped with ytterbium-erbium complex has been experimentally investigated using Kelly sidebands analysis. The dispersion values in the spectral range of 1542-1564 nm were obtained. The maximum dispersion value was $308 \mathrm{ps}^{2} / \mathrm{km}$ at the wavelength of 1542 nm.

We have demonstrated optical switching of generation in the Holmium-doped fiber laser. The generation switching from continuous-wave to pulsed regime was performed by external pulsed radiation with a wavelength of $\approx 1.6 \mu \mathrm{m}$ due to excited state absorption from ${ }^{5} I_{7}$ level.

Experimental and numerical study has been performed for three techniques of mode-locking in all-fiber Holmium laser. We have compared the fundamental repetition rate pulsed generation for mode-locking based on: nonlinear polarization evolution, polymer-free single-walled carbon nanotubes, and hybrid mode-locking. Experimental and numerical simulation results demonstrated the shortest pulse duration and maximum spectrum width for mode-locking based on the nonlinear polarization evolution: 1.3 ps, 4.2 nm and 1.3 ps, 4.1 nm, respectively. The self-starting mode in this case can vary depending on external conditions in the experiment. In Ho-doped fiber laser with polymer-free single-walled carbon nanotubes mode-locking, the small modulation depth of saturable absorption leads to a long time period of stationary single-pulse lasing development (about 10 4 cavity roundtrips in simulation, and ≈5 s in the experiment). Both experimental and numerical studies have indicated that a Ho-doped fiber laser with hybrid mode-locking provides optimal generation, enabling self-starting and a relatively fast transition to stable single-pulse lasing (less than 1.5 × 10 3 cavity roundtrips in simulation, and ≈3 s in experiment). This study presents the first employment of polymer-free single-walled carbon nanotubes for hybrid mode-locking in a Ho-doped fiber laser.

Phase transitions of GST225 thin films with a thickness of 150 nm were induced by laser pulses with durations ranging from 20 to 140 ns and energies ranging from 1 to 15 μJ. The transitions lasted from 0.4 to 0.6 μs, and the optical contrast reached up to 90%. The study also demonstrated the possibility of two-level transitions.

A hybrid fiber master oscillator solid-state power amplifier system with a Holmium fiber laser-based reference oscillator and a YSGG: $\mathrm{Cr}^{3+}: \mathbf{T m}^{3+}: \mathrm{Ho}^{3+}$ crystal-based amplifier is demonstrated. The time dynamics of the amplification of the YSGG: $\mathbf{C r}^{3+}: \mathbf{T m}^{3+}: \mathbf{H o}^{3+}$ crystal as a function of pump power and the central wavelength of the master oscillator were characterized.

The spectral and spatial output parameters of a two-channel, Yb-doped fiber laser operating in the intracavity spectral beam combining mode were investigated. We showed that by using active media with slightly different gain spectra, it is possible to implement either the spectral combining mode of the independent laser channels or the mode of collective lasing of the channels. The difference in the gain spectra of the active media was realized due to the difference in the threshold inverse population in the Yb-doped fibers.

We demonstrate experimental and numerical studies of supercontinuum generation for silica fibers with longitudinally varying diameter and dispersion. The significant difference in the spectral and temporal transformations of the pump pulse depending on the direction of propagation in the researched fiber samples is shown. Numerical simulations demonstrate the possibility of the supercontinuum spectra management by controlling the longitudinal profile of the fiber. Ways to optimize the output in terms of spectral flatness and efficient energy transfer to the desired wavelength region are presented.

The stable generation regimes of both single ultrashort pulses and pulse packages in the spectral range of 1540–1600 nm are demonstrated for two cavity designs of fiber-optic lasers: dumbbell-type and annular. In both designs the mode locking regime is provided by the nonlinear rotation of the polarization plane. It is demonstrated that using the dumbbell-type cavity design with circulator is more advantageous in erbium fiber-optic laser for obtaining the single-pulse generation compared against the classical annular design constructed on analogous elements.

Quantum key distribution (QKD) systems provide quantum-safe key exchange. Therefore, complete security analysis of implementations of QKD protocols is in the focus of interest of a worldwide information-security community. For today, a number of QKD loopholes are closed by countermeasures, which are also considered in emerging QKD security evaluation and certification [1]–[3]. However, new threats to practical QKD implementations are still found, such as the laser-damage attack, which is a powerful hacking strategy. In investigations, CW laser radiation is most often used, but in contrast to it, the interaction of pulsed laser radiation with optical materials may lead to a wide range of effects, like nonlinear effects, dielectric breakdown, etc.