We observe beam degradation and transmission loss in graded-index multimode fibres under femtosecond-pulsed excitation. Transmission can be partially recovered, while the beam degradation remains permanent, pointing towards defect creation and subsequent recovery.

In this work, using a frequency degenerate two-wave-mixing (fundamental mode (FM) & higher-order mode (HOM)) formulation, we investigate the impact of inversion related gain and PD-induced loss, long-period gratings on modal interactions. We show that the higher-power dominant FM has a larger impact on the evolution of the lower-power HOM, which results in HOM content increase and output beam degradation as the PD loss increases. It should also be stressed that in contrast with the case of thermal or inversion refractive-index gratings, no relative phase shift between the interference intensity pattern and the gain/loss grating is required.

Highly birefringent (HiBi) singlemode step-index fibres (SIF), where birefringence arises from stresses induced by core ellipticity or the presence of stress rods, are widely used in applications where preserving the state of polarization of a signal is crucial, such as fibre gyroscopes, coherent communications, frequency conversion etc. It is known that in such fibres, the assumption of linearly-polarised (LP) modes is not valid [1]. Due to the non-uniform stress field, the electric field of the fundamental mode (FM) has a more complex vectorial nature and comprises two orthogonal linearly-polarised components: a major, (one-spot component) and a minor (four-spot component), with a polarisation extinction ratio (PER) not exceeding 45 dB [1].

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).

We demonstrate a highly-Yb-doped fiber with pedestal and triple-cladding stretching fiber for utilizing in chirped pulses amplification systems with high ratio of stretching/compression and low level of stimulated Raman scattering. The stretching triple-cladding fiber consisting of high-index Ge-doped core, low-index F-doped depression concentric layer and high-index Ge-doped concentric layer. By varying the fiber structure parameters (refractive indexes and geometrical size) we were able to tailor its dispersion parameter and its slope values in order to match these with a grating-pair compressor ones to obtain better pulse compression quality (shortest possible duration and pedestal-free structure). Realized fiber has dispersion parameter of -140 ps/nm/km at 1026 nm and slope of 12 um^-1. The highly-Yb-doped fiber for the final amplification stage of the CPA has aluminophosphorosilicate glass core with a germanium-doped pedestal around it; core/pedestal diameters were 14/50 um, outer cladding has square shape with edge size of 130 um. Despite of high Yb ions content (cladding absorption of 40 dB/m at 976 nm) the core NA was only 0.09 due to the creation of the pedestal. We utilized a 23 cm piece of the fiber to amplify 22ps chirped pulses with 1.432 MHz repetition rate up to 10.2W of average power, when the first stokes of Raman starts to evolve, corresponding to 320 kW of peak power. The pump-to-signal conversion efficiency was 64%. M2 factor was 1.15/1.12. Experiments on amplification of the 500 ps chirped pulses and its compression will be presented at the conference.

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.

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.

We demonstrated a simple design of a monolithic all-fiber side-coupled combiner for counter-pumped amplifiers that requires no special fiber processing systems for fabrication. The combiner based on a Yb-doped polarization-maintained tapered fiber with an output core diameter of 40 µm and a total length of 1.8 m exhibiting over 60% coupling efficiency of 976 nm 0.10 NA pump power was demonstrated and utilized to amplify 1064 nm 9.3 ps 1.84 MHz pulses up to 9.1 W of average power and 0.53 MW of peak power with near diffraction-limited beam quality. The demonstrated approach seems promising for further power scaling, retaining good output beam characteristics via design optimization.

We demonstrate an all-fiber chirped-pulse amplification system. In a stretching stage a specially designed triple-cladding fiber matching with a grating pair compressor allowed to stretch 6 ps 1026 nm pulses from master oscillator to ~500 ps was used. These pulses were amplified in a newly-developed extra-highly-Yb-doped large-mode area fiber up to ~10 W average power and then compressed in a transmission grating compressor down to 400 fs duration with resulted in a record-high 50 MW peak power.

We demonstrated an all-fiber chirped-pulse amplifier with a stretcher stage based on a triple-cladding fiber and a final amplification stage based on a newly-developed highly Yb-doped large-mode area fiber with a Ge-doped pedestal. The stretcher fiber was designed to match a diffraction grating compressor in the third-order dispersion and allowed the stretching of 6 ps 1026 nm chirped pulses up to ~500 ps. These pulses were amplified in the developed Yb-doped fiber and then compressed using a transmission grating compressor down to 670 fs duration with 3.5 W average power and $61.8~\mu \text{J}$ pulse energy, corresponding to a record-high peak power of 92 MW.

The possibility to scale-up output pulse energy in diffraction-limited Er-doped fiber amplifier has been studied. It is shown that the utilization of tapered fiber design allows one to increase the pulse energy up to 2 mJ, while keeping the diffraction-limited beam quality (M2~1.4). Factors limiting the further increase in pulse energy are revealed.

In the paper we discuss current state of the art in the field of tapered fiber development. The best results in term of high peak and high average power achieved with this type of fibers are presented together with requirements to the tapered fiber amplifier design. The report is mainly focused on tapered fiber amplifiers operated near 1 μm (Yb-doped tapered fibers), but also extension of this technique to 1.55 μm spectral range is discussed.

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.

Prospects for average power scaling of sub-MW output peak power picosecond fiber lasers by utilization of a Yb-doped tapered fiber at the final amplification stage were studied. In this paper, it was shown experimentally that a tapered fiber allows the achievement of an average power level of 150 W (limited by the available pump power) with a peak power of 0.74 MW for 22 ps pulses with no signs of transverse mode instability. Measurements of the mode content using the S 2 technique showed a negligible level of high order modes (less than 0.3%) in the output radiation even for the maximum output power level. Our reliability tests predict no thermal issues during long-term operation (10 5 hours) of the developed tapered fiber laser up to kilowatt output average power levels.

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.

Highly doped with ytterbium (up to 20 wt% of Yb2O3) Er-Yb aliminosilicate glass core fibers have been fabricated and thoroughly investigated. It has been discovered that in a particular condition the lasing properties of both Yb and Er ions can be nearly completely suppressed. Due to a very high Yb ions absorption in the 900-1000 nm range (~ 3000 dB/m at 915 nm) all together with low background losses the developed glass matrix is very promising for such applications as pump isolation in fiber lasers as well as spectral and mode filtering using highly absorbing glass dopants.

This paper presents triple-clad stretcher fibre designs in which second-, third-, and fourth-order dispersion is matched to that of a grating-pair compressor in the Treacy configuration. The use of fibre having one of the proposed designs in a chirped pulse amplifier has made it possible to produce an all-fibre very large pulse stretching system that allows one to ensure the best possible quality of stretched pulse compression to a femtosecond duration.

We have developed a cladding pumped tapered erbium-doped fibre with a record-high core diameter for erbium-doped fibres (100 mm) and a near diffraction-limited beam quality ( μ 2 ∼ 1.3). Optimisation of the tapered fibre parameters provided a high (18 %) efficiency of pump radiation conversion at a wavelength of 976 nm into signal radiation at a wavelength of 1560 nm.