Nonlinear Optical Pulses in Media with Asymmetric Gain.

A generic novel model governing optical pulse propagation in a nonlinear dispersive amplifying medium with asymmetric (linear spectral slope) gain is introduced. We examine the properties of asymmetric optical pulses formed in such gain-skewed media, both theoretically and numerically. We derive a dissipative optical modification of the classical shallow water equations that highlights an analogy between this phenomenon and hydrodynamic wave breaking.

Spectrum collapse in a 7-core Yb-doped fiber laser with an array of fs-inscribed fiber Bragg gratings.

Femtosecond inscription of fiber Bragg gratings (FBGs) in each core of a cladding-pumped seven-core Yb-doped fiber enables efficient (≈70%) 1064-nm lasing in a robust all-fiber scheme with ≈33 W power, nearly the same for uncoupled and coupled cores. However, the output spectrum is quite different: without coupling, seven individual lines corresponding to the in-core FBG reflection spectra sum up into a broad (0.22 nm) total spectrum, whereas the multiline spectrum collapses into a single narrow line at strong coupling.

Nonlinear beam tapering and two-dimensional ring solitons.

We examine a possibility to exploit the nonlinear lens effect-the initial stage of self-focusing to localize initially broad field distribution into the small central area where wave collapse is arrested-the nonlinear beam tapering. We describe two-dimensional localized solitary waves (ring solitons) in a physical system that presents a linear medium in the central core, surrounded by the cladding with the focusing Kerr nonlinearity. The standard variational analysis demonstrates that such solitons correspond to the minimum of the Hamiltonian.

Thermalization of Orbital Angular Momentum Beams in Multimode Optical Fibers.

We report on the thermalization of light carrying orbital angular momentum in multimode optical fibers, induced by nonlinear intermodal interactions. A generalized Rayleigh-Jeans distribution of asymptotic mode composition is obtained, based on the conservation of the angular momentum. We confirm our predictions by numerical simulations and experiments based on holographic mode decomposition of multimode beams.

Statistical mechanics of beam self-cleaning in GRIN multimode optical fibers.

Since its first demonstration in graded-index multimode fibers, spatial beam self-cleaning has attracted a growing research interest. It allows for the propagation of beams with a bell-shaped spatial profile, thus enabling the use of multimode fibers for several applications, from biomedical imaging to high-power beam delivery. So far, beam self-cleaning has been experimentally studied under several different experimental conditions.

Mechanism of brightness enhancement in multimode LD-pumped graded-index fiber Raman lasers: numerical modeling.

We develop a comprehensive theory for describing the experimental beam profiles from multimode fiber Raman lasers. We take into account the presence of random linear mode coupling, Kerr beam self-cleaning and intra-cavity spatial filtering. All of these factors play a decisive role in shaping the Stokes beam, which has a predominant fundamental mode content.

Spatio-spectral beam control in multimode diode-pumped Raman fibre lasers via intracavity filtering and Kerr cleaning.

Multimode fibres provide a promising platform for boosting the capacity of fibre links and the output power of fibre lasers. The complex spatiotemporal dynamics of multimode beams may be controlled in spatial and temporal domains via the interplay of nonlinear, dispersive and dissipative effects. Raman nonlinearity induces beam cleanup in long graded-index fibres within a laser cavity, even for CW Stokes beams pumped by highly-multimode laser diodes (LDs).

Soliton based χ combs in high-Q optical microresonators.

Investigations of the frequency combs in χ microresonators have passed a critical point when the soliton based regimes are well established and realized on different platforms. For χ microresonators, where the first harmonic (FH) and second harmonic (SH) envelopes are coupled via the SH generation and optical parametric oscillation, the comb-soliton studies are just starting. Here we report on a vast accessible dual χ soliton-comb family in high-Q microresonators with the SH and FH combs centered at the pump frequency ω and its half ω/2.

Raman dissipative solitons generator near 1.3 mkm: limiting factors and further perspectives.

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.

Walk-off controlled self-starting frequency combs in χ optical microresonators.

Investigations of frequency combs in χ optical microresonators are burgeoning nowadays. Changeover to χ resonators promises further advances and brings new challenges. Here, the comb generation entails not only coupled first and second harmonics (FHs and SHs) and two dispersion coefficients but also a substantial difference in the group velocities - the temporal walk-off.

Spectral characterization technique of self-organized distributed feedback in a self-sweeping fiber laser.

We report on spectral characterization technique of self-organized dynamical distributed feedback (DDFB) in a self-sweeping Yb-doped fiber laser. The DDFB is originated from gain and refraction index gratings formed (recorded by standing waves) in the laser's active medium and dynamically changes during frequency self-sweeping operation. Dynamic nature of the feedback requires fast characterization (reading) of corresponding reflection spectrum.

Hydrodynamic 2D Turbulence and Spatial Beam Condensation in Multimode Optical Fibers.

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.

Theoretical analysis of solutions of cubic-quintic Ginzburg-Landau equation with gain saturation.

We study highly chirped analytical solutions of the cubic-quintic Ginzburg-Landau equation with the gain saturation. Based on the analysis, we propose the analytical method of estimating the stable generation area in a long fiber laser. The results allow us to predict the stable generation in long-cavity fiber lasers without performing full mathematical modeling.

Spatial location of correlations in a random distributed feedback Raman fiber laser.

Nonlinear interactions between different components of multiwavelength radiation are one of the main processes shaping properties of quasi-CW fiber lasers. In random fiber lasers, nonlinear influence may be more complicated, as there are no distinct longitudinal modes in radiation because of the random nature of the feedback. In this Letter, we experimentally characterize internal correlations in the radiation of a multiwavelength random distributed feedback fiber laser.

Publisher Correction: Wave kinetics of random fibre lasers.

This corrects the article DOI: 10.1038/ncomms7214.

Control of mode anticrossings in whispering gallery microresonators.

Optical microresonators attract strong interest because of exciting effects and applications ranging from sensing of single atoms and molecules to quantum and nonlinear optics. For all this, control and tuning of the discrete resonances are vital. In resonators made of anisotropic materials that are beneficial for nonlinear-optical applications, anticrossings of ordinarily (o) and extraordinarily (e) polarized modes occur regularly.

Open-cavity fiber laser with distributed feedback based on externally or self-induced dynamic gratings.

Dynamic population inversion gratings induced in an active medium by counter-propagating optical fields may have a reverse effect on writing laser radiation via feedback they provide. In this Letter we report, to the best of our knowledge, on the first demonstration of an open-cavity fiber laser in which the distributed feedback is provided by a dynamic grating "written" in a Yb-doped active fiber, either by an external source or self-induced via a weak (∼0.1%) reflection from an angle-cleaved fiber end.

All-fiber highly chirped dissipative soliton generation in the telecom range.

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.

Spectral comb of highly chirped pulses generated via cascaded FWM of two frequency-shifted dissipative solitons.

Dissipative solitons generated in normal-dispersion mode-locked lasers are stable localized coherent structures with a mostly linear frequency modulation (chirp). The soliton energy in fiber lasers is limited by the Raman effect, but implementation of the intracavity feedback at the Stokes-shifted wavelength enables synchronous generation of a coherent Raman dissipative soliton. Here we demonstrate a new approach for generating chirped pulses at new wavelengths by mixing in a highly-nonlinear fiber of these two frequency-shifted dissipative solitons, as well as cascaded generation of their clones forming in the spectral domain a comb of highly chirped pulses.

Fifty-ps Raman fiber laser with hybrid active-passive mode locking.

Actively mode locked Raman lasing in a ring PM-fiber cavity pumped by a linearly polarized Yb-doped fiber laser is studied. At co-propagating pumping, a stochastic pulse with duration defined by the AOM switching time (~15 ns) is generated with the round-trip period. At counter-propagating pumping, one or several sub-ns pulses (within the AOM switching envelope) are formed.

Narrowband random lasing in a Bismuth-doped active fiber.

Random fiber lasers operating via the Rayleigh scattering (RS) feedback attract now a great deal of attention as they generate a high-quality unidirectional laser beam with the efficiency and performance comparable and even exceeding those of fiber lasers with conventional cavities. Similar to other random lasers, both amplification and random scattering are distributed here along the laser medium being usually represented by a kilometers-long passive fiber with Raman gain. However, it is hardly possible to utilize normal gain in conventional active fibers as they are usually short and RS is negligible.

High-order random Raman lasing in a PM fiber with ultimate efficiency and narrow bandwidth.

Random Raman lasers attract now a great deal of attention as they operate in non-active turbid or transparent scattering media. In the last case, single mode fibers with feedback via Rayleigh backscattering generate a high-quality unidirectional laser beam. However, such fiber lasers have rather poor spectral and polarization properties, worsening with increasing power and Stokes order.

Cascaded generation of coherent Raman dissipative solitons.

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.

Fourier synthesis with single-mode pulses from a multimode laser.

Short pulses are generated by mode-locking techniques: amplitude modulation in time domain or frequency modulation in frequency domain. Direct Fourier synthesis of radiation from several single-frequency sources offers an opportunity to generate arbitrary waveforms. Here we report on a new technique of short-pulse synthesis in the Fourier domain.