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

Sergey A Babin,Alexey G Kuznetsov,Alexey A Wolf,Mikhail P Fedoruk,Evgeniy V Podivilov,Stefan Wabnitz,Oleg S Sidelnikov,Ilya N Nemov,Sergey I Kablukov

doi:10.1038/s41598-021-01491-0

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.

Highlights

It is well-known that stimulated Raman scattering on molecular vibrations shifts the wavelength of laser radiation, but may improve laser beam quality
The pump beam from an individual laser diodes (LDs) almost homogeneously fills the core of the LD pigtail made of step-index fibres (SIFs) so that the transverse intensity profile of the pump beam becomes to be similar to the SIF index profile
The pump beam quality slightly degrades ( M2 ≈ 34) after the mixing of three beams coming from individual LDs in the spliced fibre combiner and the propagation of the combined beam in ~ 2 m output port of the combiner made of graded-index fibres (GIFs)

Summary

Introduction

It is well-known that stimulated Raman scattering on molecular vibrations shifts the wavelength of laser radiation, but may improve laser beam quality. Driven by applications such as laser fusion[1], Raman beam cleanup (RBC) has been extensively studied since its first demonstration in 1 9792, as it provided an effective solution for improving quality and brightness of high-power Nd:glass or excimer laser beams via Raman conversion in g aseous[2,3,4,5,6] or solid-state[7,8,9] media. This idea was further applied to fibre lasers, which became superior to other types of lasers in the 2000s due to their ultimate efficiency, beam quality and thermally mitigating a rchitecture[10]. This permits to unveil the conditions for efficient conversion of highly-multimode LD pump beams into a nearly-singlemode Stokes beam

Methods

Results

Conclusion