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Numerical simulation of the fiber optic parametric oscillator aiming the goal to produce picosecond narrowband pulses for CARS has been performed in an extremely wide range of parameters, such as a pump pulse duration, parametric frequency shift, spectral bandwidth of the pump and the parametric pulses. It required extremely large calculation window both in time (3.5 ns) and spectral (from 433 nm to 3100 nm) domains. We managed to speed up simulation in fifty times by graphic processor unit, which allowed to define the areas of stability for different lengths of standard passive and photonic-crystal fibers used in the external linear cavity of oscillator. The cavity length reached a value of 100 meters that was resulted in parametric pulses with the energy up to 40 nJ and peak power up to 1 kW at a wavelength about 800 nm.

We have studied the harmonic generation regimes in a pulsed SRS laser with synchronous pumping. The use of phosphosilicate polarisation-maintaining fibre made it possible to obtain stable generation of Raman dissipative solitons (RDS’s) with a wavelength of 1.3 μm, and a fold increase in the length of the external cavity relative to that of the pump laser cavity by a dispersion-shifted fibre allowed us to raise the threshold for the transition to noise-like pulses and to increase the RDS energy up to 2.5 nJ with an estimated duration of a transform-limited pulse of about 200 fs.

Graded-index multimode optical fibers have recently attracted a renewed attention, thanks to the discovery of new nonlinear effects, such as Kerr beam self-cleaning. In essence, Kerr self-cleaning involves a flow of the propagating beam energy into the fundamental mode of the fiber, accompanied by a redistribution of the remaining energy among high-order modes. Increasing the fundamental mode energy leads to a significant improvement of the output beam quality. A standard method to determine beam quality is to measure the M2 parameter. However, since self-cleaning involves the nonlinear redistribution of energy among a large number of fiber modes, measuring a single beam quality parameter is not sufficient to characterize the effect. A properly informative approach requires performing the mode decomposition of the output beam. Mode decomposition permits to evaluate the energy distribution among all of the excited fiber modes, which enables investigations of nonlinear mode coupling processes at a qualitatively new level. In this work, we demonstrate an efficiency mode decomposition method based on holography, which is suitable for analyzing the self-cleaning effect. In a theoretical study, we describe the solution of the mode decomposition problem for the modes of the gradedindex multimode fiber. In an experimental investigation, we demonstrate the decomposition of both low-power (speckled) and self-cleaned beams, involving more than 80 modes.

All-fiber Raman lasers have demonstrated their potential for the efficient conversion of highly multimode pump beams into a high-quality Stokes beams. However, the modal content of these beams has not been investigated yet. Here we apply, for the first time, a mode decomposition technique for revealing intermodal interactions in different operational regimes of CW multimode Raman lasers. Our approach allows for analyzing the output laser radiation in terms of the amplitude and phase distributions of a huge number of excited modes for both the pump and the Stokes beams, which enables a new insight into nonlinear mode coupling processes. The measured contribution of the first three modes of the residual pump beam after overcoming the SRS threshold decreased on average by 25%, whereas the signal beam mainly consists of fundamental mode (40%) and the modes of the first group (20%).

Beam self-imaging of ultrashort pulses in nonlinear graded-index (GRIN) multimode optical fibers is of interest for many applications, including spatiotemporal mode-locking in fiber lasers. We obtained a new analytical description for the nonlinear evolution of a laser beam of arbitrary transverse shape propagating in a GRIN fiber. The longitudinal beam evolution could be directly visualized by means of femtosecond laser pulses, propagating in the anomalous or in the normal dispersion regime, leading to light scattering out of the fiber core via the emission of blue photo-luminescence. As the critical power for self-focusing is approached and even surpassed, a host of previously undisclosed nonlinear effects is revealed, including strong multiphoton absorption by oxygen-deficiency center defects and Germanium inclusions, splitting and shifting of the self-imaging period, filamentation, and conical emission of the guided light bullets. We discovered that nonlinear loss has a profound influence on the process of high-order spatiotemporal soliton fission. The beam energy carried by the fiber is clamped to a fixed value, and nonlinear bullet attractors with suppressed Raman frequency shift and fixed temporal duration are generated, leading to highly efficient frequency conversion of the input near-infrared femtosecond pulses into mid-infrared multimode solitons.

High energy, ultra-short multimode soliton pulse fission is observed and numerically studied in multimode GRIN fibers, showing complex dynamics bringing to multiple fundamental solitons that do not entirely follow standard single mode soliton perturbation theory predictions.

Today, perspectives of using the picosecond and femtosecond pulses for biological tissue analysis are limited with several problems. One of them is an absence of direct sources of radiation in water transparency windows, e.g. 1.3 and 1.7 microns. There are several techniques that can produce that kind of radiation. In order to generate it we used synchronous pump and stimulated Raman scattering in a phosphosilicate fiber inside an external cavity. Our work presents the experimental and numerical modeling results for 1.3 micron Raman dissipative soliton generation in an all-fiber system. Additionally, attempts of pulse synchronous amplification are reported.

Raman dissipative solitons (RDS) have been investigated numerically. It was found that the area of stable generation is bounded in terms of pump spectral bandwidth and pulse energy. Existing optimum is strongly affected by the net cavity dispersion. The spectral bandwidth of the generated RDS linearly depends on its energy and reaches more than 50 nm in the 5-meters long cavity. Developed numerical model reproduces all the effects observed experimentally. It predicts ability to generate high-quality pulses with energy up to 6 nJ compressible down to ∼100 fs duration. The work shows that RDS generation technique can produce high-energy ultrashort pulses at wavelengths not covered by typical active mediums.

The generation of strongly chirped dissipative solitons offers great opportunities for generating high-power laser pulses of short duration. We demonstrate the possibility of amplifying such pulses in a polarisation-maintaining tapered fibre having 32 μm mode diameter at the output end. Highly chirped pulses with a repetition rate of 14.3 MHz and a duration of 10 ps at 1055 nm centre wavelength are amplified in an all-fibre circuit to 2.11 μJ energy and compressed by diffraction gratings to a duration of ~297 fs. Reducing the pulse repetition rate to 1 MHz allows a 16.5 μJ output pulse energy to be achieved without manifestations of SRS and self-phase modulation in the spectrum.

We investigate a promising approach to the energy scaling of mode-locked erbium fiber lasers. It consists in a special cavity design, comprised of a really long polarization maintaining (PM) part and a short non-PM part. Two separate fiber parts ensure the principle of space division for highly chirped dissipative soliton formation and NPE-based self-amplitude modulation effects. As a result, the pulse energy was increased up to 4 nJ with an estimated transform-limited pulse duration of 150 fs. It was also found that further energy scaling is limited by transition to a noise-like pulse generation regime. Numerical simulations revealed that this transition is associated with a contrast of the employed spectral filter and the amplifier noise level, which agrees well with the experiment.

The process of high-energy soliton fission is experimentally and numerically investigated in a graded-index multimode fiber. Fission dynamics is analyzed by comparing experimental observations and simulations. A novel nonlinear propagation regime is observed, where solitons produced by the fission have a nearly constant Raman wavelength shift and same pulse width over a wide range of soliton energies.

We experimentally demonstrate the fission of femtosecond pulses in a GRIN standard fiber, leading to high-energy multimode solitons that undergo huge Raman frequency shifts, and exhibit complex multisoliton dynamics.

Multimode optical fibers (MMF) recently attracted a renewed attention, because of their potential for spatial division multiplexing, medical imaging and high-power fiber lasers, thanks to the discovery of new nonlinear optical effects, such as Kerr beam self-cleaning, spatiotemporal mode-locking, and geometric parametric instability, to name a few. The main feature of these effects is that many transverse modes are involved in nonlinear interactions. To advance our understanding, it is necessary to analyse the modal content of beams at the output of MMFs. In this work, based on a computer digital holography method using a phase-only spatial light modulator (SLM) as a correlation filter, we experimentally demonstrate a method of mode decomposition involving a large (≃80) number of fiber modes. To obtain this, we carried out a SLM calibration, and numerically investigated the most critical parameters which affect the fidelity of the decomposition, by comparing experimental and reconstructed beam patterns in both the linear (speckled structures) and in the nonlinear (self-cleaned beams) propagation regime.

The dynamics of high-energy Raman solitons in graded-index multimode fibers is both numerically and experimentally investigated. The propagation of high-power pulses produces nonlinear losses, that quench up to 80% of the fiber transmission. In such a regime, several solitons arising from the fission of ultra-short femtosecond pulses manifest unique features: pulse width, Raman self-frequency shift and soliton order remain nearly constant over a broad range of energies.

CARS is the one of most exciting and actively developed techniques for real-time monitoring of processes occurring in biological tissues. We present a fiber optical parametric oscillator pumped by specially designed MOPA fiber laser to get time-synchronized optical signals with a frequencies difference of 2800-3000 reverse centimeters. We utilize the fourwave mixing effect in a photonic-crystal fiber to build narrowband tunable fiber optical parametric oscillator. A complex optimization of each part of the source has been performed.

Ultrafast fiber lasers with a high pulse energy at 1550 nm wavelength are important for a range of applications: from CARS [1], few-cycle pulse generation [2] and frequency metrology to THz-wave generation. These applications require not only the high pulse energy, but also short duration, and high generation stability. Highly chirped (chirp parameter > 10) dissipative soliton (HCDS) generation technique (HCDS) driven by the nonlinear polarization evolution (NPE) effect meets all the above requirements. Substantial HCDS energy increase has been obtained previously in Yb-fiber all-fiber NPE mode-locked cavity containing polarization maintaining (PM) and non-PM parts [3]. We have implemented this approach for 1.5 microns wavelength area for the first time in [4] demonstrating 165 fs dechirped duration and 0.93 nJ energy HCDS. We also observed a multi-pulse generation regime caused by NPE overdriving. In this work we extend our results and increase the single HCDS energy by significantly lengthening the all-fiber cavity.

Numerical modelling is a powerful and well-proven tool for investigation of nonlinear polarization evolution (NPE) mode-locked fiber lasers, exhibiting various regimes of signal generation. The scalar model based on nonlinear Schrodinger equation (NLSE) was used to describe evolution of dissipative solitons (DSs) in a laser cavity in the presence of stimulated Raman scattering (SRS) [1, 2] and even to predict conditions of a Raman dissipative soliton formation [3]. However, a question of validity of the model was not properly investigated. In the later work [4] only a qualitative agreement between the simulation and experiment was achieved.

We report on a detailed investigation of two approaches to the modeling of the nonlinear polarization evolution (NPE) effect, forming part of an effective artificial saturable absorber. Comparison of scalar and hybrid models was performed on an example of an all fiber laser, generating highly chirped dissipative solitons at 1.55 µm. It was shown that distributed action of this type of saturable absorber in the specially designed cavity can be reduced to the point action and described in terms of critical power, modulation depth, and phase shift of the sinusoidal transmission function. Its parabolic approximation gives a large area with less than 15% difference between the energies and durations of the pulses in the considered models. Qualitative relations between the physical cavity parameters and parameters of the transmission function have been established.

We experimentally and theoretically investigate complex temporal pulse reshaping that accompanies Kerr beam self-cleaning in multimode optical fibers. We also study the output beam shape dependence on initial conditions.

We experimentally study polarization dynamics of Kerr beam self-cleaning in a graded-index multimode optical fiber. We show that spatial beam cleaning is accompanied by nonlinear polarization rotation, and a substantial increase of the degree of linear polarization., Comment: 5 pages, 6 figures

We show that Kerr beam self-cleaning results from parametric mode mixing instabilities that generate a number of nonlinearly interacting modes with randomized phases - optical wave turbulence, followed by a direct and inverse cascade towards high mode numbers and condensation into the fundamental mode, respectively. This optical self-organization effect is an analogue to wave condensation that is well known in hydrodynamic 2D turbulence.

A high-energy (0.93 nJ) all-fiber erbium femtosecond oscillator operating in the telecom spectral range is proposed and realized. The laser cavity, built of commercially available fibers and components, combines polarization maintaining (PM) and non-PM parts providing stable generation of highly chirped (chirp parameter 40) pulses compressed in an output piece of standard PM fiber to 165 fs. The results of the numerical simulation agree well with the experiment. The analyzed intracavity pulse dynamics enables the classification of the generated pulses as dissipative solitons.

An external-cavity generation of powerful ultrashort pulses near 1.3 μm in all-fiber scheme by using a new type of phosphosilicate polarization maintaining fiber is investigated. The phosphorus-related Stokes shifted Raman pulse is observed. Its optical spectra are measured as a function of energy and chirp of the pump pulses and output coupling ratio. The output energy of compressible highly-chirped pulses reaches 0.67 nJ.

It is known that there is a significant portion of the near-IR wavelength band uncovered by rare-earth-doped active fibers. This fact stimulates many research in this direction and a new approach to generate high-power ultrashort pulses by stimulated Raman scattering was demonstrated recently both in the intra- and extra-cavity configurations [1, 2]. The generated Raman pulse was proved to be a Raman dissipative soliton (RDS) [1] that can be combined coherently with the main (pump) DS in the intra-cavity design [3]. On the other side a much higher RDS energy could be achieved in the extra-cavity configurations as far as pump and Raman lasers can be optimized independently [2].

High power mode-locked (ML) fiber lasers generating femtosecond pulses in telecom window near 1.55 μm is a field of growing interest due to the apparent success of all-normal dispersion (ANDi) fiber lasers generating chirped pulses in the wavelength region around 1 μm [1, 2]. Such lasers have a wide range of practical applications, from CARS and Terahertz spectroscopy to few-cycle pulse synthesis and telecommunications. In a majority of practical applications, short duration and high pulse energy are as important as long-term stability and usability. From this perspective the all-fiber lasers look the most appropriate due to their compact size, absence of complicated adjustments, high efficiency, intrinsic stability and perfect beam quality.

The all-fiber highly-chirped dissipative soliton (HCDS) oscillator was realised at 1.5 μm wavelength. A normal net cavity dispersion was achieved by using a conventional dispersion compensating fiber (DCF). To separate effects of the amplitude self-modulation and dissipative soliton formation, we exploit in the laser cavity both standard single mode fiber and polarization maintaining single mode fiber. The properties of the generated pulses have been varied by changing spectral filter bandwidth and DCF lengths. After compression of the nJ-level ∼6ps HCDS in the external fiber compressor, we measured the output pulse duration of 165 fs (an estimated chirp parameter 40).

The results of the design and study of femtosecond fiber lasers are reviewed. Various methods of mode-locking and generation regimes are considered. Special attention is paid to the regime of dissipative solitons in an all-fiber resonator with normal dispersion. The main results and analysis of the possibilities of energy scaling of femtosecond pulses are given.

The paper presents the results on inscription of fiber Bragg and long-period fiber gratings by femtosecond laser pulses and point-by-point technique in various types of fibers (single-mode, multimode, polarization-maintaining) for applications in fiber lasers.

All-fiber cavity for synchronous generation of conventional and Raman dissipative solitons in the telecom spectral range is designed. Through extensive numerical modelling we demonstrate 2-wavelength complex with 10 nJ energy and

We experimentally demonstrate a cascaded generation of a conventional dissipative soliton (DS) at 1020 nm and Raman dissipative solitons (RDS) of the first (1065 nm) and second (1115 nm) orders inside a common fiber laser cavity. The generated high-energy pulses are shown to be linearly-chirped and compressible to 200-300 fs durations for all wavelengths. Moreover, the pulses are mutually coherent that has been confirmed by efficient coherent combining exhibiting ~75 fs and

Generation of highly-chirped dissipative solitons (HCDS) in normal-dispersion cavity is treated now as a prospective technique to increase energy of femtosecond pulses delivered by an oscillator. Based on recently developed analytical model describing stable HCDS solutions, we have realized experimentally the regime of highly-chirped pulses in an Yb-doped fiber laser with passive mode locking via nonlinear polarization evolution (NPE) and tested the model predictions. It has been shown that the generated pulses satisfy the condition of equality for nonlinearity and dispersion induced phase shifts characterizing dissipative solitons, and their chirp parameter is growing nearly linearly with increasing cavity length. This results in nearly linear increase of the pulse energy, but the linear scaling is valid only up to some limit after which the stability of the regime is worsen significantly. At the limit length of ~ 5 m the pulses with nJ energy level and several ps duration are generated being compressed to ~ 200 fs out of the cavity. The reason of the stability break is shown to be an excessive rotation of polarization ellipse in the cavity, so it is proposed to use polarization-maintaining fibers to increase scaling range in HCDS regime obtained by means of NPE mode locking.

The frequency doubling and tripling in a Q-switched all-fiber laser are studied. It is demonstrated that the main limitations on the efficiency of the harmonic generation are related to the random polarization that is nonuniform with respect to axes, the asymmetric pulse shape with a flat trailing edge, and the significant spectral broadening of the multimode radiation of the fiber master oscillator in the fiber amplifier. The methods to increase the efficiency are proposed. For an IR pulse energy of about 0.3 mJ, duration of about 40 ns, and repetition rate of 1 kHz, the second- and third-harmonic pulse energies are greater than 60 and about 10 μJ, respectively.

An external-cavity generation of powerful ultrashort pulses in an all-fiber scheme by using a new type of phosphosilicate polarization maintaining fiber is investigated. The phosphorus-related Stokes shifted Raman pulse near 1.3 microns is observed. Optimization of Stokes output spectrum depending on pump pulse duration (chirp), energy and output coupling ratio of the cavity is performed. As result, the output energy of highly-chirped pulses compressible to 570 fs reaches 1.6 nJ.

655 nm laser radiation with power of >60 mW is generated by frequency doubling of a broadband randomly-polarized 1.31-μm phosphosilicate Raman fiber laser (RFL). The red power appears to grow linearly with increasing RFL power up to 7 W at efficiency comparable with that for single-frequency lasers. It has been shown that multiple sum-frequency mixing processes involving different RFL modes provide the main contribution to the output, which is enhanced by 2 times due to the modes stochasticity.

The all-fiber all-normal-dispersion cavity length and mode-field diameter have been increased simultaneously up to 40-m and 10-μm. By suppressing Raman effect, highly-chirped pulses with energy >50 nJ at 250 fs compressed duration are generated.

The cascaded generation of a conventional dissipative soliton (at 1020 nm) together with Raman dissipative solitons of the first (1065 nm) and second (1115 nm) orders inside a common fiber laser cavity is demonstrated experimentally and numerically. With sinusoidal (soft) spectral filtering, the generated solitons are mutually coherent at a high degree and compressible down to 300 fs. Numerical simulation shows that an even higher degree of coherence and shorter pulses could be achieved with step-like (hard) spectral filtering. The approach can be extended toward a high-order coherent Raman dissipative soliton source offering numerous applications such as frequency comb generation, pulse synthesis, biomedical imaging, and the generation of a coherent mid-infrared supercontinuum.

We demonstrate cascaded generation of first- and second-order Raman dissipative solitons in an all-fiber laser cavity. Optimization and coherent combining of dissipative soliton and second-order Raman dissipative soliton is performed.

Conventional (1015 nm) and Raman (1055 nm) dissipative solitons generated in an all-fiber Yb laser are mixed in an external photonic crystal fiber (PCF) at pulse energy of up to 4 nJ at the input. It has been found that red-shifted ~20 ps pulses with energy up to 1 nJ are generated in the parametric process. Their peak wavelength is tunable from 1084 to 1102 nm by means of the delay variation between the input pulses. At that, the parametric pulses are shown to be coherent with the input ones and compressible to ~2 ps that is useful in applications. The performed modeling explains the main features of generated pulses.