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Ultra-highly-Yb-doped silica-based fibers are of great interest for such application as high-repetition rate mode-lock lasers (for reduction of the cavity length) and single frequency lasers (for increase pump absorption in a short, few cm in length, cavity). Another promising application is high-peak power lasers based on pedestal-supported large-mode-area (LMA) highly-Yb-doped fiber [1] . In the last case utilization of core based on aluminophophosilicate (APS) glass doped with nearly equimolar concentration of Al 2 O 3 and P 2 O 5 is highly desirable. The reason is formation of AlPO 4 join which increase solubility of rare-earth elements and have refractive index close to that of pure silica [2] . Reduction of the core refractive index in this case greatly simplify fabrication of the matched Ge-doped pedestal. At the same time maximum possible Yb 2 O 3 concentration in the core of such fiber is highly desirable to get the shortest possible fiber length.

In this paper we systematically study a limitation for maximum of Yb dopant concentration in silica based fibers. Two the most popular glass matrixes (F-Al2O3-SiO2 and Al2O3-P2O5-SiO2) were thoroughly investigated in this respect. A possibility to introduce ultra-high doping level of Yb2O3 (in excess of 2.5 mol%) with a relatively low optical losses in fibers was demonstrated. At the same time it was investigated that at ultra-high Yb concentration in the fiber core even with maintaining very low background losses (both initial and induced by photodarkening) such a fiber can nearly completely lose its active properties. Optimal glass matrixes and optimal concentration of Yb dopant in the glass core, which allow keep up lasing properties of the developed fibers high, were studied. An ultra-short length fiber amplifier (~3.5 cm) based on the developed Al2O3-P2O5-SiO2 glass core fiber doped with 1.2 mol% of Yb2O3 (Yb ions absorption was about 1000 dB/m at 920 nm) was created.

We proposed and experimentally demonstrated a technique for the suppression of unwanted modes in double-clad fibers with a high core-to-clad diameter ratio by introducing high-index absorbing inclusions into the first cladding of the fibers. These inclusions disturb the shape of undesirable modes, and a noticeable part of the power becomes localized inside the inclusion, resulting in an increase in the propagation loss of these modes. Two fiber designs were studied and realized: one with cylindrical symmetry and an absorbing high-index ring as the inclusion and another with high-index absorbing rods inserted around the fiber core. In both cases, the possibility of achieving perfect single-mode propagation was demonstrated both theoretically and experimentally.

We propose a novel saddle-shaped Yb-doped fiber design for high power generation in the spectral range near 0.976 $\mu \text{m}$ . The fiber has a central single-mode part with a core diameter of approximately $12~\mu \text{m}$ and an ultra-thin silica clad of approximately $49~\mu \text{m}$ . At both ends of the fiber, the core/clad diameters were adiabatically increased up to 20/80 $\mu \text{m}$ to be compatible with ordinary passive fibers. A monolithic 976-nm single-mode continuous wave laser based on the proposed fiber was created. The laser’s power was a record high compared with all-fiber laser schemes, with an output power of 10.6 W. Utilization of a photodarkening-free core glass matrix has allowed us to demonstrate perfect long-term stability (without any power degradation) of the developed laser over 45 h.

Er-Yb fibers are routinely used for generation of watt-level signal near 1.55 μm. Such fibers typically have core doped with a high P 2 O 5 concentration (required for efficient energy transfer from Yb3+ to Er3+ ions) which result in a high core numerical aperture (NA) of about 0.2. As a results single-mode Er-Yb fibers has a small core diameter (∼5 μm) and they are not suitable for high peak power applications. In the current communication we report on development of Er-Yb fibers with a low core NA based on highly fluorine doped P 2 O 5 -SiO 2 glass.

Ultrashort pulse fiber laser systems with both high average and peak power are promising tools for a wide range of applications. Usually pulsed fiber laser systems exhibit either high average power [1] or high peak power [2]. Only utilization of rod-photonics-crystal-fiber technology [3] allows one to achieve both high average (130 W) and peak (870 kW) power with a price of non-monolithic design (mainly bulk elements are used in [3], which lead to the laser's high cost and lose of reliability). Moreover not perfectly single-mode output (M2 = 1.30) results in limitation of maximum average power caused by mode instability. Recently sub-MW peak power in monolithic all-fiber laser scheme was demonstrated in [4] using tapered fiber approach. Scalability of average power in tapered fiber to 28 W at peak power of 292 kW was demonstrated in [5]. In the current communication we demonstrated further progress in average and peak power scaling using this approach.

Single-mode Er-Yb fibers based on phosphorosilicate glass matrix highly doped with fluorine have been fabricated using modified all-gas phase MCVD technology. Fibers have core doped by 6.5 mol.% of P2O5, 0.9 wt.% of F and different concentration of Er and Yb. The core NA was about 0.07-0.08 relative to the pure silica level. Slope efficiency of more than 19% was achieved using amplifier scheme with co-propagating pump at 976 nm and signal at 1555 nm (input signal power was about 0.6W). Slope efficiency in the laser configuration has reached 34% relative to the input pump power.

Highly Er/Yb co-doped phosphosilicate fibers with low optical losses were fabricated by the MCVD method, utilizing an all-gas-phase deposition technique developed “in house.” Important characteristics such as photodarkening and photosensitivity of these fibers were thoroughly investigated. An almost negligible impact of photodarkening-induced losses (no more than 1 dB/m) was discovered in the near-infrared spectral region. The photosensitivity of hydrogen-loaded fiber samples was studied using excimer laser irradiation at a wavelength of 193 nm. A photoinduced refractive index value of 1 × 10 − 3 at the total UV irradiation exposure dose of 550 J / c m 2 was achieved. A single-frequency laser with a Fabry–Perot short cavity was designed by inscribing highly reflective ( R > 99.9 % ) and partially reflective ( R = 94.3 % ) fiber Bragg gratings directly in the Er/Yb co-doped phosphosilicate fiber core. The laser showed stable CW output emitting with a narrow peak centered at 1551.5 nm in the absence of any signs of Y b 3 + parasite lasing in a wavelength range from 1000 to 1100 nm.

Monolithic 976 nm laser design based on a newly developed saddle-shaped Yb-doped fiber has been proposed. The fiber has central single-mode part with core diameter of about 12 μm and ultra-thin square-shaped clad with side of about 42x42 μm. At the both ends of the saddle-shaped fiber the core and the clad sizes were adiabatically increased up to 20/(70x70) μm and the fiber could be spliced with standard (80..125 μm clad) passive fibers using commercially available equipment. Single-mode laser at 976 nm based on the developed fiber has been fabricated and photodarkening-free operation with output power of 10.6 W, which is the record high for all-fiber laser schemes, has been demonstrated.

We report a new ytterbium-doped active tapered fibre used in the output amplifier stage of a fibre laser system for the generation of megawatt peak power ultrashort pulses in the microjoule energy range. The tapered fibre is single-mode at its input end (core and cladding diameters of 10 and 80 μm) and multimode at its output end (diameters of 45 and 430 μm), but ultrashort pulses are amplified in a quasi-single-mode regime. Using a hybrid Er/Yb fibre system comprising an erbium master oscillator and amplifier at a wavelength near 1.5 μm, a nonlinear wavelength converter to the 1 μm range and a three-stage ytterbium-doped fibre amplifier, we obtained pulses of 1 μJ energy and 7 ps duration, which were then compressed by a grating-pair dispersion compressor with 60% efficiency to a 130 fs duration, approaching the transform-limited pulse duration. The present experimental data agree well with numerical simulation results for pulse amplification in the threestage amplifier.

High peak power lasers are of great importance for a wide number of applications like material processing, remote sensing and etc. The most impressive, and still record, result has been demonstrated with using of photonics crystal fibers (PCFs), where amplification to more than 1 MW peak power directly from amplifier was demonstrated [1, 2]. However such fiber are difficult in production, handling and requires use of bulk optics, that negates the benefits of truly all-fiber systems. For this reason, the development of alternative LMA design is still very actual task. One of the promising one is the tapered step-index fiber [3, 4]. Such fibers have robust single-mode (SM) input end part (to couple signal) and a very big core and outer diameters output end part. The output signal end is formally highly multi-mode, but slowly increasing diameter along the tapered fiber, ensuring of adiabatic enlargement of fundamental mode and suppress excitation of high order modes. A larger first cladding diameter at the output tapered fiber end makes possible counter-pumping using a low-brightness cheap pump diodes. Recently we have demonstrated chirped pulse amplification system, with final cascade based on Yb-doped PM fiber, generating diffraction limited (M2

Single-mode Er–Yb fibers with the core based on a phosphorosilicate glass matrix (up to 6.5 mol.% P2O5) highly doped with fluorine (up to 0.9 wt.%) were fabricated using an all-gas-phase modified chemical vapor deposition method. The core numerical aperture was in the range of 0.07–0.08 relative to the pure silica, which allowed us to increase the single-mode core diameter up to 20 μm. The slope efficiency in lasers based on the fabricated fibers reached 34% relative to the launched pump power.

A novel effect of long-term mode degradation in a low-average-power high-peak-power ytterbium-doped pulsed fiber lasers based on large mode area step-index fibers has been reported for the first time.

In this work we present a monolithic lidar system, based on a newly-developed double-clad large mode area (LMA) polarization-maintaining Er-doped fiber and specially designed LMA passive components. Optimization of the fiber designs resulted in as high as 100 W of SBS limited peak power. The amplifier and its passive components (circulator and collimator) were integrated in an existing lidar system. The enhanced lidar system provides three times increase of scanning range compared to one based on standard telecom-grade amplifiers.

An all-glass Yb-doped single-mode fiber laser and amplifier with output power of more than 5 W near 977 nm have been realized. Both laser and amplifier were based on a specially developed photodarking-free Yb-doped fibers with increased core-to-clad diameter ratio (up to 0.31).

A novel effect of long-term mode degradation in a low-average-power high-peak-power ytterbium-doped pulsed fiber lasers based on large mode area step-index fibers has been reported for the first time.

In this study, we present a novel monolithic ytterbium-doped fiber amplifier with more than 0.5 MW peak power output power. The amplifier is based on a 2.1 m long tapered fiber with core/cladding diameters changing from 10/80 µm (at the signal input end) to 50/430 µm (signal output, pump input). The fiber has all-glass polarization-maintaining design, that make possible utilization of conventional FC adapter and standard angle polishing to 7° for thick end. Pump absorption was measured to be 8 dB at 915 nm for the whole fiber length. Despite a very large mode area (~30 µm) the tapered fiber demonstrates low bend sensitivity (it is possible to coil tapered fiber with 9 cm radius) and a diffraction limited beam quality. In the amplifier the pump power was coupled through the thick end by means of collimating and focusing lenses. Dichroic mirror employed to separate output signal and counter-propagating pump power. We obtained 3.5 W of average output power for a 5 mW seed signal (coupled by usual fusion splicing through a thin end) corresponding to a 28 dB gain. The amplified pulses have duration of about 5 ps and energy of about 3.3 µJ that corresponding to over 0.5 MW peak power. The spectral width was 28 nm operating with center wavelength of 1057 nm.To the best of our knowledge it is the highest peak power obtained directly from the all-fiber amplifier.. Keywords: Yb-doped tapered fiber, all-fiber megawatt system, low bend sensitive tapered fiber, ultrashort pulses amplification 1. INTRODUCTION The very rapid power scaling of both pulsed and cw Yb-doped fiber lasers dictates stringent requirements on the utilized fibers, mainly in terms of nonlinear effects threshold. Conventional single-mode Yb-doped fibers exhibit a relatively low threshold for nonlinear effects since the light is confined in a small core (6-10 µm in diameter) and propagates along long distances (above 10 m in some cases). The increasing of nonlinearity threshold is the urgent problem in the case of pulsed fiber systems, which have been demonstrated to reach power levels of several hundreds of kW and even approach MW power level. The most simple and obvious way to solve this problem is the enlargement of core diameter resulting in the enlarged mode filed diameter of the fundamental mode. In the same time, this approach has so me drawbacks: the fiber becomes a multimode, leading to degradation of output beam quality, and extremely bend-sensitive, resulting in requirement to keep the fiber straight, i.e. the system becomes cumbersome. The main goal of this work was to develop a novel approach based on utilizing of so-called tapered fibers that allows one to obtain large mode field areas and maintain an acceptable bend-loss sensitivity at the same time. The fiber parameters optimization and its using in the final amplification stage of picosecond chirped-pulse amplification system have been demonstrated. The obtained output peak power was record for all-fiber amplifying systems and approached by power level of microstructured fibers based systems.