We report a highly efficient, Nd-doped all-fiber laser operating based on the three-level transition at 915 nm. A key feature of the laser is a novel active fiber with cladding-embedded absorbing rods that suppress parasitic amplified spontaneous emission (ASE) at 1060 nm. The laser demonstrates 37% slope efficiency, which is, to the best of our knowledge, the highest reported value among all-fiber lasers with near-diffraction-limited beam quality (M-2<1.2 ). Strong ASE suppression with a signal-to-ASE ratio exceeding 50 dB was achieved. Further output power and efficiency scaling can be reached by optimization of fiber components used in the laser cavity.

The present research investigates the impact of drawing temperature on the refractive index of germanium-doped (9 mol

We demonstrate a compact source of few-cycle pulses at 1.56 µm wavelength on the basis of single- and double-stage erbium fiber schemes of nonlinear pulse amplification with large-mode area (LMA) solid- and hollow-core fiber compressors. The source is designed according to the concept of self-phase modulation-induced spectrum broadening in normal-dispersion erbium-doped fiber (EDF) followed by ultra-broadband pulse compression in passive LMA fibers with negative second-order dispersion and low nonlinearity. Due to low second- and third-order dispersion of highly nonlinear EDF, just ∼10cm long LMA fiber with a 39 µm wide solid core or ∼1m long LMA fiber with a 34 µm wide hollow core is sufficient for high-quality compression of positively chirped wideband pulses down to few-cycle duration. The shortest pulse width of 26.5 fs (≈5 wave cycles), as well as the highest pulse energy of 4.42 nJ, and average power of 168.4 mW are achieved in the double-stage setup, while maximum peak power of 100 kW together with improved pulse quality is obtained in the single-stage scheme. We believe that the developed source may be a prospective versatile tool for nonlinear visualization in bio-photonics and other applications requiring few-cycle wideband pulses, successfully competing with various fiber and solid-state counterparts.

A high-resolution temperature sensor using the beat frequency measurement between the modes of two DFB fiber lasers is presented. The laser cavities are formed by the femtosecond inscription technique in a highly Er/Yb co-doped phosphosilicate fiber with low optical losses and compact design. The experimental results show a sensitivity of 1 GHz/°C, leading to a temperature resolution of 0.02 °C restricted by the thermistor used in the experiment. The maximum possible resolution determined by the laser linewidth is estimated as 2 × 10−6 °C. The operation of such a sensor at high temperatures (≈750 °C) with the possibility of further temperature increase is demonstrated. The combination of high resolution and broad temperature range makes the sensor attractive for various applications, especially in high-temperature monitoring.

The paper considers specific features of the boron content analysis in silicate preforms and optical fibers using the energy dispersive X-ray spectroscopy. It is shown that boron can be determined for both two-component (xB2O3 - (1-x)SiO2) and multi-component glass core compositions containing heavy elements, including lanthanides. Based on a comparison of the calculated data from the analysis of the boron content and experimentally measured data on the refractive index profiles of optical fibers, the refractive index coefficient of boron kB2O3 =- 4.33*10-4 was obtained, where the concentration of boron oxide is expressed as a weight percentage in the glass cores of two-component optical fibers, for which quenched conditions are achieved for the core glass during rapid cooling.

The minimum refractive index of the aluminophosphosilicate (APS) core in optical fibers has been determined for a wide range of phosphorous and aluminum concentrations. It was found that the APS core refractive index became higher by ~0.0005–0.0012 as compared to that in optical fiber preform. The analysis of the measured data has shown that at least 0.2 mol.% of Al2O3 and P2O5 remain in their ordinary form near their equimolar concentrations and do not form an AlPO4 join (the effect observed for all concentrations of AlPO4 join from 5 to 25 mol.%).

Point-by-point femtosecond laser inscribed chirped Bragg gratings are good candidates for use in fiber chirped pulse amplification systems due to the high flexibility of their dispersion properties, low cost, and no length limitation. However, such gratings have two significant drawbacks: high losses and distortions of the shortwavelength part of the spectrum when operating in the normal group velocity dispersion mode. We propose a novel approach based on writing such gratings in a highly doped active fiber, which allows us to correct both shortcomings. We have also demonstrated, for the first time to our knowledge, a successful use of such gratings as stretchers in a fiber chirped pulse amplification system.

Yb-doped tapered fiber amplifier become one of the most promising tools for amplification of pulsed radiation near 1 μm to a high peak power (sub-MW directly after amplifier), while keeping superior beam quality (M2~ 1.1). Such fibers consist of a thin part with a single-mode core, tapered section with a gradually increased core and cladding diameter and a thin part with large fundamental mode field diameter, where main signal amplification take place. If signal propagates from a thin to the thick end and tapered transition is long enough almost no high order modes are excited during signal propagation and beam quality remain to be diffraction-limited. According to S2 measurements content of high order modes in output radiation is less than 0.3% [1].

Traditional solid-state ultrashort pulse sources for the 2 μm band, typically comprising Ti:Al2O3 oscillators and nonlinear frequency converter, are costly, bulky, and no stable enough to satisfy the requirements of realworld applications. Robust thulium-doped fibre systems offer a compelling alternative, capable to emit ultrashort pulses with wide tuneability across the 1.65-2.2 μm band. The Mamyshev oscillator concept has recently demonstrated the potential not only to omit material saturable absorber in the cavity, which is critical for operation at longer wavelength ranges, but also substantially raise the performance of fibre laser and upscale the output powers. However, a thulium-based, self-starting all-fibre Mamyshev oscillator has not yet been realised,primarily due to the strong anomalous dispersion of silica [1].

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.

A new design of a passive large mode area (LMA) optical fiber with record (for such type of fibers) stimulated Brillouin scattering (SBS) gain suppression, no less than 11dB compared to a conventional LMA fiber, is reported. The fiber core consists of two parts: the central, doped with Al 2 O 3 and GeO 2 , and the peripheral, doped with P 2 O 5 and F, which creates the Brillouin gain spectrum (BGS) spread by 1 GHz. The SBS properties were confirmed by BGS and SBS power threshold measurements.

An innovative concept to achieve a self-starting, all-fibre and all-normal dispersion Mamyshev oscillator for the 1.9 μm wavelength domain is explored. A machine-aided approach is chosen to investigate systematically the constrictions between filter offset, output coupling ratio and pump power.

We present a high-power erbium-doped all-fiber source that generates rectangular-shaped pulses with durations ranging from 200$\mu$s to 5 ms and pulse energies up to 130 mJ.The laser has a stable single-mode output, a compact size and was designed for fractional photorejuvenation procedures.

A novel Yb-doped fiber design for improved lasing near 976 nm based on spectral filtering of the amplified spontaneous emission near 1030 nm was realized and investigated. A very sharp short-pass filter was implemented by adding appropriately chosen high-index absorbing rods into the silica cladding. In this case, the resonant interaction of the core mode with the high-index rod mode could be controlled by fiber bending, which allows for the precise adjustment of the stop-band position. It was shown that the utilization of Sm-doped absorbing rods allows one to achieve very high absorption of emission at unwanted wavelengths, but it also adds background losses for the pump near 915 nm and for the signal at 976 nm. Despite this fact, the improvement of efficiency in the 976 nm fiber amplifier, after shifting the stop-band to 1000 nm, was clearly demonstrated. Based on theoretical calculations, it was shown that, after optimizing the fiber parameters, a further twofold improvement in efficiency was possible despite the excess losses at the pump and signal wavelengths.

A newly-designed Yb-doped fiber type, intended to enhance 976 nm lasing by suppressing the spectral component including 1030 nm amplified spontaneous emission (ASE), was thoroughly researched. To implement an extremely sharp short-bandpass spectral filter, several suitably selected high-index absorber rods were incorporated into the fiber cladding. Thus, the concept of such a filter allows for a relatively accurate stop-band position adjustment as a result of the highly fiber-bend-dependent state of the intermodes resonance core-rod interaction. Finally, the impact of enhancing the power conversion efficiency (PCE) of the fiber amplifier emitting at a wavelength of 976 nm, by shifting the stop-band, is justified and demonstrated in this study.

High peak and average power Yb-doped ps-pulse fiber amplifiers are of high demand for different micromachining tools. Most of commercial lasers are designed for operation near 1030 nm, which is due to high gain at this wavelength. An ultimately high peak power of ∼ MW just after the fiber amplifier and ∼3.8 GW after pulse compression [1] was achieved in lasers operated near 1030 nm by utilization of rod-type photonic crystal fibers (PCF). However, such fibers have a well-known problem: they could not be spliced with standard fibers, and also it must be kept perfectly straight. As a result, lasers based on PCF lose most of the advantages of fiber lasers – reliability, compactness and a low production cost.

We demonstrated an optimization of a picosecond fiber amplifier based on Yb-doped tapered fiber in a spectral range of 1030 nm. Nonlinear effects limiting peak power scaling (stimulated Raman scattering and four-wave mixing) were studied and factors affecting their threshold were established, such as gain, diameter profile along the length of taper, output mode field diameter, and numerical aperture of a pump. By determining the optimal amplification regime and manufacturing advanced tapered fibers, we amplified 13 ps pulses to a record-high peak power of 1 MW at a wavelength of 1029 nm directly at the output of the fiber at an average power of 13.8 W. Four-wave mixing was the limiting factor, and the total fraction of deleterious components in the output spectrum was ~2%. The quality of the output beam was close to being diffraction limited (M2 < 1.2).