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To raise the stimulated Brillouin scattering (SBS) threshold, we have fabricated an optical fibre having a core heavily doped with germanium oxide and displaced from the fibre axis. A method has been proposed for broadening the SBS gain spectrum of the fibre by winding it onto a small-diameter spool, and a broadening of the spectrum by 5.4 dB has been demonstrated, which indicates an increase in SBS threshold by 4.1 dB.

A novel method of stimulated Brillouin scattering (SBS) gain suppression in passive single-mode optical fibers is proposed and experimentally verified. The effect is achieved due to special propagation conditions created for a number of the core-bounded acoustic modes effectively involved in the SBS, such that its gain becomes approximately evenly distributed over a wide spectral range, thereby proportionally reducing its maximum. To verify and investigate this method in depth, single-mode large-mode-area (LMA) fibers with correspondingly optimized acoustic index profiles are fabricated, experimentally tested, and analyzed. Uneven complex doping of the core with two additives, P2O5 and F, forms a high-contrast gradient acoustic index profile while maintaining a quasi-uniform optical refractive index profile. With a contrast of P2O5 of more than 6 mol.%, we achieved 8 dB of SBS suppression over conventional uniformly doped LMA fibers. Moreover, it is shown that more than 10 dB of SBS gain suppression can be achieved if the optimal dopant distribution is realized.

The feasibility of making a wavelength-selective fibre filter using fibre with absorbing rods in its silica cladding has been demonstrated theoretically for the first time. The technique described in this paper is of great practical interest for designing fibre lasers and amplifiers emitting at wavelengths where lasing is impeded by the presence of more energetically favourable radiative transitions in the active medium. In addition, the approach proposed here can be used for stimulated Raman scattering suppression. The unique feature of the fundamental core mode suppression mechanism examined is that there is a (bending) controlled spectral width of a region where excess losses are produced.

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 consider a fibre laser system generating ∼10-μJ, ∼500-fs pulses with a peak power of ∼10 MW at a repetition rate of 100 kHz and emission wavelength of 1.56 μm. The system is based on a master oscillator – power amplifier configuration. The amplifier ensures chirped-pulse amplification. The pulses are then compressed by a dispersive grating compressor. The output amplifier stage is based on a specially designed tapered large mode area erbium-doped fibre for suppressing nonlinear effects. The experimental data agree with numerical simulation results for the stretcher, amplifier and compressor. The stretcher and amplifier have been simulated using a generalised nonlinear Schrödinger equation. In addition, numerical simulation results suggest that optimising the stretcher and compressor will potentially allow the peak power of the system to be scaled up to ∼30 MW.

We have developed and demonstrated a polarisation-maintaining single-mode ytterbium-doped fibre for light amplification at a wavelength near 0.976 μm in an all-fibre configuration. A distinctive feature of the proposed fibre design is low losses due to fusion splices with standard single-mode fibre having a core diameter of 10 μm, which has made it possible to produce an all-fibre small-signal amplifier with a gain threshold near 3 W and a differential pump-to-signal conversion efficiency of 9.8 % in the saturation regime. The proposed amplifier has been shown to be well-suited for small-signal amplification. In particular, at an input signal power near 1 mW and a gain of 20 dB, the ratio of the amplified signal to the integrated luminescence intensity near 0.976 μm exceeds 20 dB. A 40-dB gain has been demonstrated for an ultrasmall signal of ∼10-μW power.

The use of a heavily doped germanosilicate fibre with a W-profile refractive index and small core diameter in stretchers of ultrashort laser pulses with their subsequent amplification and compression in all-fibre laser systems is considered. The application of fibres of this type makes it possible to stretch, amplify, and then compress a laser pulse with minimum distortions of its initial shape and width. Due to the dispersion properties of these fibres, which allow the pulse duration to be increased significantly at a small fibre length and the third-order positive dispersion of the diffraction-grating-based output compressor to be compensated for, amplified pulses with an energy of 2 μJ and width of 250 fs, free of a picosecond pedestal, are obtained. Several types of fibres intended for the use in stretchers of ultrashort laser pulses are comparatively analysed from the point of view of their dispersion compatibility with a diffraction-grating-based output compressor.

We have demonstrated a modified chemical vapor deposition (MCVD) process for the fabrication of multilayer photonic bandgap fiber based on high-purity silica glass. Sequential growth of layers differing in melting point has been shown to lead to distortion of the layers in the resulting photonic bandgap structure and a sharp rise in optical loss as a result of deviations from Bragg’s reflection conditions. By optimizing the chemical composition of the layers in the photonic bandgap structure, we were able to suppress excessive optical losses and reach the optical loss limit, which is only determined by the guidance properties of the photonic bandgap structure itself.

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.

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.

In this paper, we demonstrate the spectrally selective fundamental core mode suppression in single-mode fiber by mode anti-crossing technique. A unique feature of the method proposed in the current work is that the fundamental core mode can have rectangular-like excess loss spectrum with bandwidth controllable by fiber bending. This property made the proposed fiber design to be promising for suppression of lasing at unwanted wavelengths in different fiber lasers and amplifiers.

The technical process of the synthesis of a fluorophosphosilicate (FPS) glass core was thoroughly investigated for the first time utilizing a modified chemical vapor deposition (MCVD) all-gas-phase fabrication method. It was discovered that the limiting doping level of the silica glass simultaneously co-doped with phosphorus (P) and fluorine (F) was found to be confined by the formation of POF3 gas. The dopants content was achieved as high as 4.7 at% of P and 1.1 at% of F in a glass core, respectively. A developed “in-house” manufacturing method makes it possible to fabricate a large mode area (LMA) purely single-mode Er–Yb co-doped optical fibers with a core diameter of 20 μm and with a lasing efficiency comparable to commercially available LMA Er–Yb optical fibers.

We consider a novel approach for suppression of signal propagation at undesirable wavelengths. It is based on distortion and absorption of the operating fundamental mode in highly absorbing rods, inserted into the fiber cladding. As an example, we designed fiber with a high absorption at wavelength of above 1000 nm and low losses near 976 nm – structure, which can be promising for design of Yb-doped fiber amplifiers operated near 976 nm.

All-fiber ultrafast amplifier at the wavelength of 1.9 µm based on normal dispersion thulium-doped germanosilicate fiber and large mode area silica fiber compressor is developed. Pulses at the repetition rate of 23.9 MHz with the duration of 63 fs and the pulse energy of 25 nJ are obtained. We assume that the pulse peak power is 250 kW.

We present a new design of single-mode Yb-doped fiber for efficient 976 nm lasers. The fiber has 14/45 micron core/cladding diameters. Exploiting of this fiber with a specially designed pump/signal combiner allowed us to realize an all-fiber amplifier with the pump-conversion slope efficiency of 31 % and the output power of 10 W limited only by the available pump power.

a novel method of higher order modes suppression in a large mode area double clad Er-doped fiber is proposed and demonstrated. For this aim absorbing inclusions (Tm-doped rods) were inserted into the first cladding of the Er- doped fiber to achieve a resonant coupling between high order modes and modes located in the inclusions.

In this paper, we have developed Yb-doped fiber suitable for creation of all-fiber seed laser schemes operating near 977 nm. The fiber was based on a ring-doping design (cladding was partially doped with Yb-ions), which allowed us to fabricate a relatively small core and provide mode field diameter (MFD) of the active fiber comparable with standard fibers (to achieve small splicing losses with commercially available optical fibers) and, simultaneously, increase absorption from the cladding to keep a reasonably high lasing efficiency. So MFDx of the fiber was 12 μm, MFDy was 14 μm. Outer silica cladding of the active fiber was decreased to diameter of 80 μm and a special pump and signal combiner was used to inject pump and signal into the active fiber. Based on the developed Yb-doped fiber an all-fiber polarization maintaining mode-locked laser with central wavelength around 977 nm was demonstrated for the first time. SESAM was used as a saturable absorber. The laser was self-starting for pump powers above 4.6 W, with the output power of 3 mW. The autocorrelation was the best fitted with sech2 profile and pulse duration was estimated to be as long as 9.5 ps. The fundamental cavity frequency corresponded to the pulse repetition rate of 33.532 MHz. Signal-to-noise ratio measured in the radio frequency range was more than 50 dB, the line width was below 1 kHz, which indicate ultimate stability of the fabricated mode-lock laser. acceptedVersion

In the present work, we fabricated Yb-doped hybrid fiber, where unwanted mode suppression was achieved due to selective mode amplification technique. An asymptotically single-mode propagation regime of the realized hybrid fiber was observed in the amplifier scheme even for the case when significant amount of undesirable modes was excited at the fiber input. Measured dispersion of the hybrid fiber at wavelength of 1030 nm was about 70 psec/(nmkm). The realized hybrid fiber was used as nonlinear compressor in all-fiber amplifier scheme. As a seed source we used a commercially available chirped pulse laser (λ~ 1030 nm, a spectrum width was 11 nm and a pulse repetition rate was 42 MHz, pulse duration was adjusted to be of 1 psec by diffraction gratings established on the output of the seed laser). In our experiment to guarantee shorter optimal active fiber length we chose wavelength stabilized pump source at 976 nm. Thus the record high peak power (~10 kW) for all-fiber femtosecond Yb-doped laser schemes was achieved.

In this paper, we demonstrate possibility of simultaneous achievement of high peak and high average power in picosecond pulses using a monolithic amplifier based on a long Yb-doped tapered fiber. Due to a very high pump absorption (~ 25 dB/m at 976 nm) in the realized 2.4 m long tapered fiber most of the pump is absorbed near the thick tapered fiber end and a very small fraction of pump power reaches thin fiber end. As a result, signal passes through the thin part of the tapered fiber without an amplification and exhibits fast growth only near the output tapered fiber end, where a mode field diameter is large (35 μm at 1064 nm for 46 μm output core diameter), so that pulses can be amplified to a high peak power. Moreover, only a negligible fraction of pump radiation leaks at the conic part of the tapered fiber, because its most part was absorbed in the thick tapered fiber part. Thus a safe operation without polymer burning at a leakage point is possible up to a very high pump power. The developed tapered fiber was used in a final amplification stage of the all-fiber pulsed laser system, which allowed us to amplify 8.3 ps pulses with repetition rate of 18.4 MHz and central wavelength of 1064 nm to 150 W of average power and 0.92 MW peak power. The average power level was limited only by available pump power (230 W): no signs of transverse mode instability effects were observed.

We report on a fiber laser setup optimized to generate and propagate high energy solitons with megawatt peak power around 1700 nm. Picosecond pulses from a chirped-pulse amplifier system at 1560 nm trigger the formation of sub-100 fs solitons with approximately 70 nJ energy in an all-solid photonic bandgap Bragg fiber with ultra-large mode area. Upon propagation in the same fiber the soliton self-frequency shift effect allows for tuning the central wavelength up to 1680 nm in a 1.5 m long piece of fiber. This work paves the way to miniaturized endomicroscopes in the biologically relevant window around 1700 nm and, thanks to the 20-cm critical bend radius of the delivery fiber, opens the way to deep in vivo imaging of freely moving animals.

We demonstrate amplification dynamics and compression of 1.9 μm ultrashort (330 fs) pulses using a normal dispersion thulium-doped germanosilicate fiber. The maximum pulse energy is 65 nJ and minimum pulse duration is 60 fs.

We developed a highly efficient double-clad Yb-doped polarization-maintaining fiber to be implemented for small-signal amplification near 0.976 μm. The fiber was designed to have a relatively small mode field diameter compatible with standard step-index single-mode optical fibers. Another feature of the fiber was a small threshold for 0.976 μm signal amplification, which was achieved by a creation of a thin inner cladding (80 μm diameter). The unique design of the fiber allowed us to construct successfully an all-fiber picoseconds mode-locked laser at 0.98 μm for the first time to the best of our knowledge.

In this paper we consider mid-infrared Raman lasers based on gas-filled hollow-core silica fibers and provide theoretical and experimental analysis of factors that limit the efficiency and output power of these lasers. As a result, we realized an efficient ns-pulsed 4.42 {\mu}m Raman laser based on an 1H2-filled revolver silica fiber. Quantum efficiency as high as 36 % is achieved, and output average power as high as 250 mW is demonstrated. The possibilities of further improving the laser efficiency are discussed.

Mid-infrared Raman laser based on silica fiber is demonstrated. Hollow-core silica fiber filled by the mixture of 1H2 and D2 gases was used as an active medium. Pumped by nanosecond pulses of a high-power Er-doped 1.56 μ m fiber laser, the Raman laser generates radiation at 2.9, 3.3, and 3.5 μ m wavelength, which depends on gas composition and pump power. Quantum conversion efficiency as high as ∼10% is achieved. The possibilities to optimize the performance of the Raman laser are discussed.

We present a review devoted to hybrid optical fibers, which combine the properties of conventional total internal reflection and antiresonant fibers and enable dispersion control at wavelengths where the material dispersion of the fiber host optical glass is normal. We discuss the main principles of mode propagation in such structures and show the prospects for their practical application.

A Raman laser with a wavelength exceeding 4 μm is designed for the first time. Using a revolver silica fibre with a hollow core filled with molecular hydrogen (1H2) as an active medium, we have obtained SRS lasing at a wavelength of 4.4 μm under pumping by a pulsed erbium fibre laser (λ = 1.56 μm, τ = 2 ns). The SRS conversion quantum efficiency reaches 15 %, and the maximum output peak power at a wavelength of 4.4 μm is 0.6 kW.

We report on a passively mode-locked oscillator based on an erbium-doped dual concentric core fiber combining high normal dispersion and large mode area. This large normal dispersion laser generates long pulses with 30 ps duration and 0.17 nm spectral width at 1530 nm wavelength. The source delivers an average power of 64 mW at a repetition rate of 16 MHz, corresponding to 4 nJ energy. This concept opens up new degrees of freedom in the design of mode-locked fiber lasers.

Single-frequency high peak power pulsed lasers operating in the 1.55 μm spectral range are required in many applications such as wind sensing LIDAR and gas column sensing. When pulse duration exceeds 10–20 ns the main limitation for peak power comes from Stimulated Brillouin Scattering (SBS). To date the highest peak power in all-fiber single-mode laser scheme was demonstrated in [1] using Er-doped (Yb-free) double clad cladding pumped large mode area fiber (EDF). The maximum peak power above 4 kW was demonstrated, however the pump to signal conversion efficiency (PCE) was only 5% with respect to launched pump power. Increase of PCE to 12% was demonstrated in the same work using longer fiber, but peak power was decreased down to 950 W. Utilization of specialty technique for SBS suppression allows one to improve these results. In particular applying of gradient stress to large mode area Er-Yb fibers (EYDF) resulted in achievement of 1 kW peak power and 16.5% PCE [2].

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.

Recently we have demonstrated perspectives of application double clad (DC) highly Er-doped (Yb-free) fibers for pulse amplification to a record-high energy and peak power [1]. Single-mode propagation regime, high pump-to-signal conversion efficiency (PCE) of 25% in L-band (near 1575 nm) and a relatively short fiber length (∼ 6 m) was achieved by optimization of fiber geometry and core compound. Core-to-cladding ratio was chosen as high as possible (core and cladding diameters were 35 μm and 125 μm correspondingly) to increase pump absorption from the cladding. Utilization of P 2 O 5- Al 2 O 3 -SiO 2 (PAS) glass matrix [2] allowed us to achieve a low core numerical aperture (NA) and, simultaneously, high PCE even for heavily Er-doped fibers.

Mid-infrared lasers operating at wavelengths above ∼3 μm are required for many applications. The lasers of a particular interest are those that can provide high average power and high quality of the output beam. A promising approach to realize such lasers is the use of gas-filled hollow-core fibers (HCF), which combine mid-infrared light guidance with all the advantages of well-established silica fiber technology. Recent progress in silica HCFs for mid-infrared spectral range has resulted in demonstration of various gas fiber lasers at wavelengths from 2.8 to 4.6 μm [1–5]. However, those lasers use bulk elements to couple the pump into a HCF, thus losing such advantages as compactness and alignment-free operation, which are expected from fiber lasers.

Yb-doped fiber lasers operating near 0.98 μm attract considerable interest in recent years. In particular, the pulsed lasers at 0.98 μm are promising for frequency doubling and quadrupling. At the same time, the Yb-doped fibers optimized for 0.98 μm generation have been designed to achieve high laser efficiency at a high average output power and hardly can be used in preamplifier stages or in scheme of master oscillator lasers. Such fibers have a large core diameter (in excess of 25 μm) and high splicing losses with standard single-mode step-index fibers (maximum diameter of those does not exceed 10 μm). The pulsed lasers based on such fibers require utilization of bulk elements [1, 2] that eliminate advantageous of all-fibers schemes (compactness, reliability, low-cost). Moreover, due to a very high lasing threshold (tens of Watts) such fibers are unsuitable for signal amplification to a middle-level output power (0.1–1 W), required for pre-amplifiers and for many of masteroscillator schemes. In opposite, core-pumped schemes are power-limited by available single-mode pump diodes at 915 nm and typically do not allow output power higher than ∼100 mW.

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.

Recent years, high power fiber lasers at the spectral region near 1550 nm attract a lot of attention as promising sources for numerous civil and military applications. There are four main type of lasers allowing to generate high power radiation at this spectral range: Er/Yb-co doped fiber lasers, Yb-free Er-doped fiber lasers cladding-pumped at 980 nm, Er-doped fiber lasers core-pumped by Raman lasers at 1480 nm, and Er-doped fiber laser in-band pumped at 1532 nm. In this chapter we review current states, limiting factors and future perspectives of all these approaches.

Novel configuration of fiber amplifier based on combining Er-doped Yb-free and Er-Ybcodoped fiber in single amplifier was realized. Peak power of 3.7 kW in 150 ns single-frequencypulses with differential efficiency of 23% was demonstrated.

A quasi continuous wave 1535 nm Er-doped all-fiber laser was developed for 1.6 μm Er:crystal lasers pumping. Increase in efficiency of Er:YAG lasers as compare to pumping with commercial 1470 nm pump diodes was demonstrated.

Two Er-doped fibers with different core compositions and similar waveguiding properties were used to demonstrate an increase in SBS threshold to 1.8 kW, which is 2.5 dB higher compared to single-fiber amplifier.

We have studied the effect of heating on the performance of erbium-doped fibre based devices and determined temperaturedependent absorption and emission cross sections of the erbium ion in silica glass. The results demonstrate that heating of fibres in claddingpumped high-power (~100 W) erbium-doped fibre lasers causes no significant decrease in their efficiency. In contrast, superluminescent sources operating in the long-wavelength region (1565 – 1610 nm) are extremely sensitive to temperature changes.

Germanosilicate and phosphosilicate-based glasses are two of the most common materials used to amplify light in the near and shortwave infrared wavelengths by stimulated Raman scattering in optical fiber, but are limited in total output power and efficiency that can be achieved due to the onset of stimulated Brillouin scattering (SBS). We experimentally investigate both germanosilicate and phosphosilicate Raman gain fibers with selective transverse doping profiles in the context of altering the fiber acoustic waveguide properties for increasing the threshold at which total Raman amplified power is saturated by SBS. It is shown that selective transverse doping offers greater potential for SBS suppression in germanosilicate gain fibers in contrast to those manufactured from phosphosilicate glasses. Initial SBS suppressed germanoslicate fibers are demonstrated with a 5 to 6 dB reduction in the maximum peak Brillouin gain as compared to a standard step-index fiber such as Corning HI-1060, which allows for over 5 dB in SBS suppression and an increase in saturated Raman amplifier gain from 15 dB to greater than 22 dB.

A method for stabilising the radiation wavelength of a mode-locked fibre nanosecond laser is proposed and realised. Generation of nanosecond laser pulses with a spectral bandwidth of , which corresponds to a transform-limited pulse, is demonstrated.

— We report on the efficiency enhancement of 4400 nm hydrogen Raman laser based on revolver silica fiber. A record-high quantum efficiency of 47 % is achieved and average power as high as 260 mW at 4400 nm is demonstrated.

A novel tapered Er3+-doped fiber design for high peak power amplification has been developed and tested. The core diameter was changing along 2.5 meters from 22.5 μm (single-mode operation) to 86 μm. Amplification of 2 ns pulses has resulted in peak power of 105 kW (0.25 mJ) with nearly diffraction-limited beam quality (M2

An all-fiber pulsed erbium laser with pulse width of 2.4 ns working in a MOPA configuration has been created. Cladding pumped double clad erbium doped large mode area fiber was used in the final stage amplifier. Peculiarity of the current work is utilization of custom-made multimode diode wavelength stabilized at 981±0.5 nm – wavelength of maximum absorption by Er ions. It allowed us to shorten Er-doped fiber down to 1.7 m and keep a reasonably high pump-to signal conversion efficiency of 8.4%. The record output peak power for all-fiber amplifiers of 84 kW was achieved within 1555.9±0.15 nm spectral range.

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.