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.

Miniaturization of erbium fiber lasers is a crucial task, which implies a use of heavily doped fibers. In dense ensemble, interaction of erbium ions changes a configuration of quantum levels of gain medium and leads to pulsed generation. As a result, heavily doped erbium lasers demonstrate two thresholds, the first one associated with an onset of lasing in the pulsed regime, and the second with a transition to CW. Operation features near these two thresholds have been established experimentally. For the first time, a power-law behavior of the system parameters - pulses frequency, duration and peak intensity - was revealed in a wide range of pump rates around both thresholds. The power exponents were associated with critical indices of phase transition. Their values were convincingly determined different from integers and half-integers. Critical indexes were shown weakly dependent on the Fabry-Perot and distributed feedback (DFB) laser cavity parameters, which made it possible to experimentally establish the universal dependence of the pulse frequency and duration on the lasing power. The results of the work are extremely useful for determining and predicting the parameters of the designed erbium lasers, due to universality of the critical indices.

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.

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 paper presents the results of developing Er-doped optical fibers for creating random single-frequency lasers in the wavelength range of 1570–1610 nm. The possibility of broadening the luminescence band of Er3+ ions in silicate glasses in the long-wavelength region of the spectrum by introducing a high concentration of P2O5, as well as by additional doping with Sb2O3, is investigated. It is found that both approaches do not improve the dynamics of luminescence decay in the L-band. In addition, Er2O3-GeO2-Al2O3-SiO2 and Er2O3-GeO2-Al2O3-P2O5-SiO2 glasses were studied as the core material for L-band optical fibers. The developed fibers exhibited high photosensitivity and a high gain of 5 and 7.2 dB/m, respectively. In these fibers, homogeneous arrays of extended weakly reflecting Bragg gratings were recorded directly during the fiber drawing process. Samples of arrays 5 m long and with a narrow reflection maximum at ~1590 nm were used as the base for laser resonators. Narrow-band random laser generation in the wavelength region of 1590 nm was recorded for the first time. At a temperature of 295 K, the laser mode was strictly continuous wave and stable in terms of output power. The maximal power exceeded 16 mW with an efficiency of 16%.

This paper presents the results of studying the process of laser formation of microstructures from silver nanoparticles in nanoporous quartz glasses. Glass samples were impregnated with organometallic molecules Ag(hfac)COD in a supercritical carbon dioxide environment. The formation of point and linear microstructures was carried out by high-frequency (70 MHz) femtosecond laser radiation with a wavelength of 525 nm and energy in the pulse up to 1 nJ. It was found that the formation of microstructures occurs due to photo- and thermal decomposition of precursor molecules with the formation of plasmonic silver nanoparticles. It is shown that the developed temperatures can exceed the melting point of glass, which leads to the appearance of microstructures with altered refractive index. A qualitative model explaining the individual stages of cluster formation in the glass volume under point laser impact is presented.

Highly ytterbium-, aluminum- and phosphorus-co-doped silica fibers with low optical losses were fabricated by the MCVD method, utilizing an all-gas-phase deposition technique. Optical and laser properties of the active fibers with a phosphosilicate and aluminophosphosilicate glass cores doped with 1.85 mol% and 1.27 mol% Yb2O3 were thoroughly investigated. With the help of hydrogen loading, it was possible to induce highly reflective Bragg grating in both fiber samples using the standard phase-mask technique and 193 nm-UV laser irradiation. The ultra-short (less than 2 cm long) Fabry–Perot laser cavities were fabricated by inscribing two fiber Bragg gratings (highly and partially reflective FBGs) directly in the core of the fiber samples. The highest pump-to-signal conversion efficiency of 47% was demonstrated in such laser configuration using phosphosilicate fiber. The reasons for the low efficiency of aluminophosphosilicate fiber are discussed.

A highly erbium- and ytterbium-co-doped photosensitive fiber with a germanophosphosilicate glass core was fabricated by the MCVD method, utilizing an all-gas-phase deposition technique developed “in-house”. Due to doping with germanium oxide (GeO2), this fiber revealed high-grade photosensitivity (without hydrogen loading) to UV laser radiation at a 193 nm wavelength. The short (28 mm) Fabry–Perot laser cavity was designed by inscribing two fiber Bragg gratings (highly and partially reflective FBGs) directly in the core of the fabricated fiber sample. The stable single-frequency operation regime of the designed laser was observed. The laser emission peak was centered at 1540 nm, with a linewidth of 50 kHz. The slope efficiency of the laser was 10%, and the maximal output power reached a level of 35 mW.

A narrow-linewidth bismuth-doped fiber laser (BDFL) with the random cavity and operational wavelength at 1.67 µm was demonstrated. The laser cavity was formed by an array of weakly reflecting fiber Bragg gratings inscribed in the active fiber core directly during the fiber drawing process. Taking into account the performed analysis of optical and laser properties, the viability of this approach as applied to Bi-doped fibers with high-GeO $_{2}$ -SiO $_{2}$ glass core is shown, even despite the high sensitivity of bismuth active centers (BACs) to laser irradiation, i.e. processing does not lead to the destruction of the BACs. The maximum output power of the developed BDFL in a simple linear configuration with the use of 200 m-long bismuth-doped active fiber was $\sim$ 20 mW at room temperature when pumped by Er-Yb fiber laser at a wavelength of 1568 nm and the total power of 450 mW. The achieved width of the laser emission line was narrower than 0.02 nm. We studied the laser behavior in various configurations, and it was revealed that the laser wavelength can vary within the spectral range of 1.669–1.674 µm in dependence on the length and the ambient temperature of the active fiber. It was most likely caused by inhomogeneous distribution of the written gratings structures along the active fiber length that was induced due to the peculiarities of the used active fiber, namely, a significant core ellipticity. In addition, we demonstrated the possibility of the output power scalability of this type of lasers by using a homemade Bi-doped fiber power amplifier. As a result, the optical power of the random BDFL was increased up to 240 mW.

The Erbium "random" laser, based on the artificial Rayleigh fiber, has been comparatively studied in detail under two different pump conditions: 974.5 and 1485 nm pumping wavelengths. The artificial Rayleigh 7-m-long fiber was used as a laser cavity, it was formed by the ultraviolet (UV) inscription of the uniform array of the weakly reflective fiber Bragg grating (FBG) during the fiber drawing process. The UV photosensitivity of the Erbium-doped fiber originated from the specially developed (germanophosphosilicate) core glass composition. The emission spectrum of the fabricated "random" fiber laser had a single narrow peak at the 1548 nm wavelength. It was clearly revealed that the extension of the laser cavity by the separate wavelength-matched 90%-reflective FBG resulted in a significant laser efficiency growth. The highest laser slope efficiency of 33% and the laser output power of 80 mW were reached in the FBG-modified cavity at the 974.5-nm-wavelength pumping. The continuous-wave operation mode of this laser has been confirmed. The laser linewidth value measured by the delayed self-heterodyne technique was about 550 Hz.

A new active optical fiber with germano-phosphosilicate core, co-doped with Er and Yb, was developed by means of MCVD technology. This fiber has a high UV photosensitivity to the radiation of the 193 nm excimer laser. This feature allows us to avoid hydrogen loading of this fiber for the inscription of the Bragg gratings forming the cavity of a fiber laser. The parameters of this fiber, such as absorption and gain spectra, as well as photosensi-tivity, have been studied in detail. It was shown that high-performance single-frequency fiber lasers can be developed based on this fiber. A Fabry-Perot laser cavity with two fiber Bragg gratings was created based on a 30 mm long "pristine" (non-hydrogenated) fiber segment by means of the 193-nm radiation of the excimer laser. The slope efficiency of this laser was found to be 17% and the linewidth of about 88 kHz.

features of the phenomena of a laser nanosecond radiation In supercritical carbon dioxide are revealed. It is shown that the presence of a supercritical fluid leads to the expansion of the structures formed on the target in comparison with the air media. It has been suggested that the resulting magnification effect is due to the defocusing of the system, which causes the formation of the lens impact. Obtaining useful results is possible with the use of various technologies of laser ablation and microstructuring in supercritical fluids. Keywords: nanosecond laser radiation, supercritical fluid, metal target, fluctuations.

The lasing properties of a short-cavity laser based on the Er/Yb composite fiber with a silicophosphate core in a silica cladding were investigated in detail. A Fabry–Perot cavity was manufactured on a 25 mm long fiber segment using the ultraviolet (UV) ( λ =193nm) excimer laser radiation and phase-mask technique of Bragg grating inscription. The laser with an effective cavity length of 11 mm generated single-frequency radiation centered at 1554.45 nm. This laser demonstrated strictly continuous-wave operation under 974 nm pumping but self- Q -switched operation under 1485 nm pumping. Under 974 nm pumping, the slope efficiency was as high as 4%, and the output power reached 17 mW—a level comparable with the characteristics of commercial single-frequency laser diodes.

All-fiber, polarization maintaining, narrow-bandwidth, Yb-doped fiber lasers with randomly distributed feedback operated near 976 nm were realized for the first time. It was shown that the laser operated in a single, longitudinal mode regime during intervals of a few seconds. At other times, the laser generated a few longitudinal modes, but its bandwidth was always below the resolution of the optical spectrum analyzer (0.02 nm). The linewidth of each single longitudinal mode of the laser was estimated to be below 20 kHz. The reasons for this observed laser behavior were discussed and methods for achieving stable, continuous wave operation in the single-longitudinal-mode regime were proposed.

A random narrow-linewidth lasing at a wavelength of 976 nm was obtained in an ytterbium-doped germanophosphosilicate fiber with an array of weakly reflecting fiber Bragg gratings (FBGs). A random laser cavity was formed by implementing the standard phase mask method of FBG inscription directly during the fiber drawing process. The UV radiation pulses of a KrF excimer laser (248 nm wavelength) synchronized with the fiber drawing speed were used to fabricate the in-fiber array of hundreds of similar FBGs. The developed laser’s slope efficiency in the backward-pumping scheme was measured as high as 33%. The stable continuous-wave operation mode of the laser was detected. The magnitude of the laser power fluctuations depends linearly on the cavity length. The random laser cavity modified with a single highlyreflected (90%) FBG demonstrates significantly better power stability and higher slope efficiency than the same one without an FBG.

Silica-based optical fibers with an ultra-high Yb concentration were systematically studied. Three the most commonly used in industry glass matrixes for active fiber core were investigated: aluminosilicate, phosphosilicate and aluminophosphosilicate. For all the glass hosts optical fibers doped with a record high concentration of Yb in a glass core were fabricated utilizing an all-gas-phase deposition based on MCVD technology. The factors limiting increase of Yb content in glasses and fibers were revealed. For the first time it was shown that highly Yb-doped fibers could nearly completely lose their active properties and the most probable reason for that is concentration quenching of luminescence.

Optical and laser properties of a new Er/Yb-doped composite optical fiber manufactured according to the "rod in-tube" technique are presented. The fabricated composite fiber with a standard outer diameter of 125 mu m was single-mode at the wavelength of 1.55 mu m. A Fabry-Perot laser cavity was manufactured on a 25 mm long fiber segment using the femtosecond technique of Bragg gratings inscription. The resulting laser with an effective cavity length of 12 mm generated strictly continuouswave single-frequency radiation at a wavelength of 1548.3 nm at room temperature (295 K). The key advantage of the studied Er/Yb-doped composite fiber laser is the excellent temporal stability of the laser output without any tendency for self-excited oscillation mode at any pump level.

Asingle-mode Yb-doped germanophosphosilicate fiber with ultra-low optical losses (less than 2 dB/km) was fabricated by means of the MCVD method utilizing an all-gas-phase deposition technique developed “in house”. The absorption and luminescent spectral properties of the fiber were thoroughly studied. The photosensitivity of the pristine (non-hydrogenated) fiber to 248 nm-laser radiation was confirmed by means of fiber Bragg grating (FBG) inscription directly during the drawing process. The random single-frequency lasing at the 1060-nm-wavelength obtained in the 21-m-long fiber with an array of weak FBG was reported. The developed laser slope efficiency in the backward-pumping scheme was measured as high as 32%.

The single-frequency linearly-polarized laser was implemented entirely in a segment of Er/Yb-codoped phosphosilicate single-mode fiber and Ge-doped polarization-maintain fiber. The slope efficiency was found to be ~9 %. The spectral linewidth of the laser did not exceed 170 kHz.

In this work we study the mechanisms of photoinduced emergence and dissociation of defects in silica glass of Er - and Al-codoped optical fiber. Defects responsible for the degradation of amplification properties of the fiber are the main focus. Photoinduced changes in UV, visible and near IR optical range of transmission spectrum of the fiber under the 193 nm UV irradiation and the subsequent 976 nm exposure are investigated. The significant role of the interstitial molecular hydrogen in the process under study is shown. All the observed processes are explained in detail by means of photochemical reactions.