An improved design of an ytterbium-doped fiber featuring cladding-embedded absorbing rods has been proposed, which enables spectral suppression of amplified spontaneous emission (ASE) around 1030 nm and enhances the efficiency of 976 nm signal generation. A series of theoretical investigations using COMSOL and MATLAB was conducted to analyze the impact of key fiber parameters on ASE suppression effect and 976 nm signal efficiency in an all-fiber amplifier setup, leading to optimization of the fiber configuration. Numerical studies demonstrate ~2 times increase in efficiency compared to a standard fiber design.

Ultrashort pulse formation from noise represents a fundamental self-organisation process in nonlinear dissipative systems and remains central to ultrafast photonics. Mamyshev oscillators offer a particularly valuable platform for investigating this phenomenon because they do not rely on conventional saturable absorbers. Instead, pulse formation is governed by self-phase modulation in normal-dispersion fibres combined with periodic offset spectral filtering. Achieving self-starting pulse formation in these systems is challenging, particularly at longer wavelengths, due to limited normal-dispersion components and the complex gain dynamics. This work reports, to the best of current knowledge, the first self-starting, all-fibre, all-normal-dispersion Thulium-doped Mamyshev oscillator operating near 1.9um. The cavity employs a compact Fabry-Perot design incorporating a dispersion-engineered Thulium-doped gain fibre, a highly nonlinear passive normal-dispersion fibre, and a pair of chirp-free broadband fibre Bragg gratings. The oscillator self-starts without external seeding or active modulation and stabilises noise-like pulse generation regime at a fundamental cavity repetition rate. Real-time measurements and numerical simulations reveal the build-up pathway from noise through transient multi-pulsing and pulse competition to a stationary noise-like envelope. Our results show that both tailored laser components and gain-medium-specific dynamics are essential for enabling self-starting ultrashort pulse generation in Mamyshev oscillators and for extending these laser concepts beyond the near-infrared, thereby facilitating the development of compact and robust shortwave infrared (SWIR) sources and new regimes of ultrafast self-organisation.

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.%).

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].

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.

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.

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).

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.

This work focuses on the methods of creating in-fiber devices, such as sensors, filters, and scatterers, using the fiber fuse effect. The effect allows for the creation of structures in a fiber core. However, it is necessary to know exactly how this process works, when the plasma spark occurs, what size it reaches, and how it depends on external parameters such as power and wavelength of radiation. Thus, this present study aims to create the possibility of predicting the consequences of optical breakdown. This paper describes a mathematical model of the optical breakdown initiation in a fiber core based on the thermal conductivity equation. The breakdown generates a plasma spark, which subsequently moves along the fiber. The problem is solved in the axisymmetric formulation. The computational domain consists of four elements with different thermophysical properties at the boundaries of which conjugation conditions are fulfilled. The term describing the heat source in the model is determined by the wavelength of radiation and the refractive indices of the core and the shell and also includes the radiation absorption on the released electrons during the thermal ionization of the quartz glass. The temperature field distributions in the optical fiber are obtained. Based on the calculations, it is possible to estimate the occurrence times of various phase states inside the fiber, in particular, the plasma spark occurrence time.

In this article, we studied high-energy single frequency transform-limited Er-doped amplifiers core pumped by a specially developed Yb-doped fiber laser at 980 nm and a Raman laser at 1480 nm. It was demonstrated that pumping at 1480 nm allows achieving slightly higher maximum pulse energy for long pulses in the absence of nonlinear effects. At the same time, amplifiers with a pump at 980 nm have a higher threshold of stimulated Brillouin scattering (SBS), and higher peak power of shorter pulses can be achieved in such a scheme. Lasers with more than 730 μJ pulse energy and with higher than 3.5 kW peak power were demonstrated.

All-fibre ultrafast lasers are renowned for their maintenance-free, cost-effective operation. Nevertheless, a range of applications beyond standard wavelengths (1-1.5 μm) are not feasible since reduced efficiency of saturable absorbers. Bright examples are Thulium-doped fibre lasers, which are in great demand for biomedical treatments or free-space transmissions. One solution can be an easily adaptable Mamyshev oscillator (MO), which mitigates restrictions of conventional modulators and has brought on record performances for fibre lasers [1]. Though, a major challenge of MOs and a prerequisite for use in real-world applications is a reliable self-starting. Current works have introduced so far only a pulse-seeded, free-space MO in the 2 μm band [2].

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 significant change in the refractive index profiles for the large mode area phosphoroaluminosilicate (PAS) core optical fibers was observed in comparison to that in preforms. This study shows that the refractive index of the PAS core can vary from negative (in preform) to positive (in fiber), and the difference in the refractive index between the core and preform can exceed a few thousand. By measuring a large set of fibers with different concentrations of P2O5 and Al2O3, we define the refractivity of each dopant (P2O5, Al2O3 and AlPO4 joint) after drawing fiber from the preform and discuss the possible origin of the observed refractive index variation.

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.

We report, for the first time to the best of our knowledge, the switchable generation of versatile femtosecond and picosecond spatiotemporal mode-locked (STML) pulses in an all-few-mode fiber figure-8 laser at 1.55 mu m. Both in the femtosecond and picosecond pulse regimes, the laser has pulse-pattern-tunable function, namely, the pulse patterns can be tuned from single-pulse, multiple pulses to soliton molecules. Particularly, the femtosecond soliton molecules operate at the loosely bound states while the picosecond soliton molecules can operate both at the loosely and tightly bound states. The theoretical analyses are carried out to give the further insight in the switchable generation of the femtosecond and picosecond pulses. It is found that the filtering effect of the nonlinear amplifying loop mirror (NALM) plays the major role in the generation of the femtosecond pulses while the multimode interference filtering effect induced by the active few-mode fiber-passive few-mode fiber structure plays the role in the generation of the picosecond pulses. The obtained results contribute to further exploring the characteristics of the STML fiber laser and such a laser with tunable output of versatile femtosecond and picosecond pulses has potential applications in optical communications, laser processing and measuring.

In the present work we have developed and realized a high core-to-cladding diameters ratio active optical fibers operated in a single-mode regime due to specially designed structure containing boron-doped and fluorine-doped rods. The beam quality at the output of the realized fibers was studied by three independent methods: near field investigation, M-2 technique and S-2 technique. According to the obtained results, the developed approach allows efficient suppression high-order-modes (first of all-LP11 mode) in the core and achieves a diffraction-limited beam at the output of the appropriately bent fiber (suppression of unwanted modes was better than 30 dB). Additionally, it is shown that application of such approach allows for increased bent-resistance of the fundamental mode and realizes polarization-sensitive amplification.

A tapered Er-doped fiber amplifier for high peak power pulses amplification has been developed and tested. The core diameter changed from 15.8 µm (mode field diameter (MFD) 14.5 µm) to 93 µm (MFD 40 µm) along 3.7 m maintaining single-mode performance at 1555 nm (according to the S2-method, the part of the power of high-order modes does not exceed 1.5%). The amplification of 0.9 ns pulses with spectral width below 0.04 nm up to a peak power above 200 kW (limited by self-phase modulation) with a slope pump-to-signal conversion efficiency of 15.6% was demonstrated.

The unique properties of optical fibers enable the realization of the state-of-the-art fiber lasers, which surpass other laser sources in many characteristics and act themselves as an exceptional platform for harnessing emerging technologies [...]