S. A. Babin

The laminar-turbulent transition in a fibre laser

Studying the transition from a linearly stable coherent laminar state to a highly disordered state of turbulence is conceptually and technically challenging, and of great interest because all pipe and channel flows are of that type. In optics, understanding how a system loses coherence, as spatial size or the strength of excitation increases, is a fundamental problem of practical importance. Here, we report our studies of a fibre laser that operates in both laminar and turbulent regimes. We show that the laminar phase is analogous to a one-dimensional coherent condensate and the onset of turbulence is due to the loss of spatial coherence. Our investigations suggest that the laminar-turbulent transition in the laser is due to condensate destruction by clustering dark and grey solitons. This finding could prove valuable for the design of coherent optical devices as well as systems operating far from thermodynamic equilibrium.

Spectral broadening in Raman fiber lasers

We present an analytical theory based on wave kinetic equations that describes a Raman fiber laser (RFL) generation spectrum. It is shown both theoretically and experimentally that the quasi-degenerate four-wave mixing between different longitudinal modes is the main broadening mechanism in the one-stage RFL at high powers. The shape and power dependence of the intracavity Stokes wave spectrum are in excellent quantitative agreement with predictions of the theory.

Output spectrum of Yb-doped fiber lasers

An analytical model for self phase modulation in Yb-doped fiber laser (YDFL) describing output spectrum and its broadening with increasing power has been developed. Spectral measurements in continuous wave (CW) cladding-pumped YDFL have proved the validity of the model demonstrating hyperbolic secant shape of the spectrum and linear increase of the line width with power in 1-12 W range. At lower powers, spatial hole burning and line self-sweeping effects become important and define the lower limit for the line width.

Turbulence-induced square-root broadening of the Raman fiber laser output spectrum

The output characteristics of the conventional one-stage Raman fiber laser (RFL) are described in an optical wave turbulence formalism. Simple analytical expressions describing RFL output power and its spectral shape are presented, and square-root law for the output spectrum broadening law has been discovered. The indications of the turbulent-like spectral broadening in other types of cw fiber lasers and propagation phenomena in fibers are also discussed.

Linearly polarized random fiber laser with ultimate efficiency.

Linearly polarized pumping of a random fiber laser made of a 500-m PM fiber with PM fiber-loop mirror at one fiber end results in generation of linearly polarized radiation at 1.11 μm with the polarization extinction ratio as high as 25 dB at the output power of up to 9.4 W. The absolute optical efficiency of pump-to-Stokes wave conversion reaches 87%, which is close to the quantum limit and sets a record for Raman fiber lasers with random distributed feedback and with a linear cavity as well. Herewith, the output linewidth at high powers tends to saturation at a level of 1.8 nm.

Random fiber laser directly pumped by a high-power laser diode

A random lasing based on Rayleigh scattering (RS) in a passive fiber directly pumped by a high-power laser diode (LD) has been demonstrated. Owing to the RS-based random distributed feedback (RDFB) the low-quality LD beam (938 nm) is converted into the high-quality laser output (980 nm). Because of the relatively low excess above the threshold with the available LD, the RDFB laser output is not stationary and limited in power at the 0.5 W level. In the used gradient-index fiber, the output beam has 4.5 lower divergence as compared with the pump beam thus demonstrating a new way for development of high-power fiber lasers with high-quality output.

Generation dynamics of the narrowband Yb-doped fiber laser

We present direct real-time experimental measurements and numerical modeling of temporal and statistical properties for the Ytterbium-doped fiber laser with spectral bandwidth of ~2 GHz. The obtained results demonstrate nearly exponential probability density function for intensity fluctuations. A significant decrease below the Gaussian probability has been experimentally observed for intensity fluctuations having value more than 2.5 of average intensity that may be treated as indication of some mode correlations.

Q-switched fiber laser with spectral control for frequency doubling

A-Q switched pulsed ytterbium-doped fiber laser is created and optimized for frequency doubling and tripling. The spectral width of the laser radiation (less than 0.13 nm) is controlled using a Bragg grating. At a pulse repetition rate of 1 kHz, the peak power is greater than 13 kW and the pulse duration is 30 ns.

Tunable fiber Bragg gratings for application in tunable fiber lasers

The resonance wavelength of the fiber Bragg gratings (FBGs) is tuned using two methods. Tunable FBGs are used as the selecting elements in the cavities of tunable lasers. An ytterbium-doped fiber laser with a wavelength tuning range of 1063–1108 nm and an output power of 6 W, a Raman fiber laser with a wavelength tuning range of 1252–1303 nm and an output power of 3 W, and an erbium-doped fiber laser with a wavelength tuning range of 1530–1580 nm are realized, and their characteristics are studied.

Frequency doubling of Raman fiber lasers with random distributed feedback.

This Letter presents what we believe is the first experimental study of frequency doubling of a Raman fiber laser (RFL) with random distributed feedback (RDFB) in an MgO:PPLN crystal. We compared two laser configurations, each with a half-open cavity. The cavity contained either a broadband Sagnac mirror or a narrowband fiber Bragg grating (FBG). We found that spectral broadening in the studied configurations of the RDFB RFLs differed from that found in a conventional RFL with a linear cavity, as well as from each other. We also compared the second harmonic generation (SHG) efficiency for these three types of lasers. The highest SHG efficiency was obtained for the RDFB RFL with the FBG delivering >100  mW power at 654 nm.