Spatial beam self-cleaning in graded-index multimode fibers involves a nonlinear transfer of power among the fiber modes, which leads to robust bell-shaped output beams. The resulting output mode power distribution can be described by statistical mechanics arguments. Although the spatial coherence of the output beam was experimentally demonstrated, there is no direct study of modal phase evolution. Based on a holographic mode decomposition method, we reveal that nonlinear spatial phase-locking occurs between the fundamental and its neighboring low-order modes, in good quantitative agreement with theoretical predictions.
Linear and non-linear propagation of ultrashort pulses in a seven-core fiber was investigated experimentally and numerically in a normal dispersion regime. We observed non-uniform coupling conditions between different cores that may be the result of a random refractive index deviation. It was characterized by measurements of the power distribution and FROG traces at the output of a multicore fiber. The cores were excited by a spatial light modulator using the weighted Gerchberg-Saxton algorithm to generate phase masks. It allows us to switch-on any combination without manual alignment of the experimental setup. Finally, as the input power increased, a nonlinear coupling was observed between the selected cores, similar to a saturable absorber. So we believe that such a device could be useful for a development of high-power ultrashort fiber lasers and pulse shaping applications.
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%).
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.
We demonstrate for the first time a fibre Raman laser delivering record-setting energy (60 nJ) of double-scale picofemtosecond pulses at wavelength of 1270 nm, which is of great interest in bio-medical applications. A Stokes wave was generated in single-stage Raman conversion of amplified radiation from a mode-locked F8 Yb-doped laser passing through a synchronously pumped phosphosilicate fibre ring cavity. The converted radiation at 1270 nm amounting to 47% of the total Raman laser output was maximised by tuning the repetition rate of the pulses generated in the Yb-doped master oscillator to the fundamental inter-mode spacing frequency of the phosphosilicate fibre cavity.
The work reports for the first time on fibre-based Raman conversion with relatively large Stokes shift pumped by double-scale laser pulses having various degree of coherence. It was discovered that the degree of coherence of the pump pulses affects significantly the amount of the wavelength shift, intensity, and spectral width of frequency-downconverted radiation. At lower coherence within double-scale pulses, the magnitude of intra-pulse femtosecond field oscillation grows, leading to stronger nonlinear pulse interaction with the optical medium. This discovery suggests new approaches to nonlinear transformation of partially coherent laser pulses, typical of many mode-locked generation regimes of fibre lasers.
The work presents for the first time a comparative study of mode-locked figure-8 laser, in which two independently pumped active media are located either in the same or in different cavity loops. It is shown that the NALM2 configuration (both active media in the same cavity loop) delivers both higher average and peak radiation power. Flexibility of NALM/NALM2 technologies is further demonstrated for implementation of algorithmic electronically driven control over radiation mode-locking regimes. Also discussed are the results of experimental testing of electronic methods relying on NALM/NALM2 technologies for setting desired generation regimes.
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