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

Abstract

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). This leads to a breakthrough approach for wavelength-agile high-power lasers. However, current understanding of Raman beam cleanup is restricted to a small-signal gain regime, being not applicable to describing realistic laser operation. We solved this challenge by experimentally and theoretically studying pump-to-Stokes beam conversion in a graded-index fibre cavity. We show that random mode coupling, intracavity filtering and Kerr self-cleaning all play a decisive role for the spatio-spectral control of CW Stokes beams. Whereas the depleted LD pump radiation remains insensitive to them.

Optimization of random FBG array in multimode graded-index fiber for Raman lasing with improved spatio-spectral characteristics

“…At low power the linewidth is below 0.05 nm, then it grows from ~0.1 nm to ~0.2 nm in power range 5-20 W, and exceeds 0.3 nm at maximum power of about 30 W where interference pattern is almost fully washed out by non-linear spectral broadening. Nevertheless, the value is still lower than the linewidth of ~0.4 nm in MM RFL with regular FBG fs-inscribed in GRIN fiber [3,5,13], in which the interference effects are absent.…”

“…The obtained output characteristics of the Stokes beam at 976 nm are shown to depend on the array structure. Similar to regular output FBG inscribed along the axis (1D) [3,5,13], the studied random arrays have optimal integral reflection of ~4% providing maximum output power of about 30 W at 174 W pumping, obtained with 2D reflector. Comparison of low-reflection (1-2%) arrays at the same pumping demonstrates maximum power (18 W) in case of 3D array, whereas for higher reflection (~6%) maximum power (26 W) is obtained with 1D array, all having N=14 FBGs with individual length L FBG ~0.5mm.…”

“…The multimode beam may also experience nonlinear conversion to another spectral region accompanied by specific spatial beam modification. One of the interesting spatio-spectral effects is brightness enhancement at Raman conversion of multimode beam in graded-index (GRIN) fiber due to the beam cleanup at Raman amplification and mode selective feedback in the multimode Raman laser cavity based on fiber Bragg grating (FBG) with special transverse structure [3][4][5][6]. It is also possible to explore random distributed feedback based on Rayleigh backscattering on natural refractive-index fluctuations in GRIN fibers, but it is rather weak requiring of multimode pumping at kW level [7], in contrast to random lasing in singlemode fibers with singlemode laser pumping at Watt level being enough for random lasing [8].…”

Functional Fibers and Functional Fiber-Based Components for High-Power Lasers

“…Groups of modes having a field distribution beyond the central part of fiber core in a multimode fiber could be efficiently selected. Then, the spatial-spectral beam control was realized in a Kerr-cleaned Raman fiber laser, in which FBG acted as a spatial filter to generate dominant guidance for the fundamental mode [410].…”

“…From 2019 to 2021, the SPI Lasers reported the vari-MODE-Pro technology [408][409][410][411][412][413][414][415][416], enabling adjustable output beam profile from diffraction-limited Gaussian profile to complex "quasi-ring" beam with higher-order azimuthal modes, namely as petal beam, as shown in Fig. 9.…”