Mode control and laser beam shaping in multimode and multicore fibres

Abstract

Multimode (MM) and multicore fibres (MCFs) have been exploited in numerous important research areas, ranging from fibre lasers and telecommunications to transmission and sensing. However, designing and understanding multimodal nonlinear optical systems present significant technical and conceptual challenges. The ability to control light fields with multiple degrees of freedom is expected to become a key focus for future developments in multimode nonlinear systems. This project aims to investigate the dynamics of multimodal interactions in multimode and multicore fibres. In this thesis, mode control and laser beam shaping in homemade MCFs and commercial MMFs are explored. Firstly, the modal dynamics in multimode counterpropagating systems are investigated, including mode rejection and control when the forward signal and backward control beam (BCB) are both operating in the nonlinear regime (high peak power). Mode rejection of a specific spatial mode was successfully observed in the output forward signal when the input BCB was coupled to the same mode with comparable power, using dual-core fibres (DCFs), tri-core fibres (TCFs), and commercial MMFs with lengths of 0.4-1m, at a total peak power of 4-16 kW for counter-propagating beams with 0.5 ns pulses at a wavelength of 1040 nm. Secondly, the concept of counter-propagating nonlinear gratings is introduced when only the BCB is in the nonlinear regime. The BCB generates a multimode nonlinear grating that can be utilized for all-optical mode switching and power switching for the forward probe beam. Mode switching in 0.4 m MMFs and MCFs was observed with a BCB power of 6-12 kW in 0.5 ns pulses at a wavelength of 1040 nm. Core-to-core power switching in 0.4 m DCF and TCF was measured with a BCB power of 7-10 kW. Thirdly, wavelength and mode conversion in MCFs were investigated by exploring four-wave mixing (FWM) between supermodes in MCFs. Efficient FWM and supermode/wavelength conversion were demonstrated with a pump wavelength of 1040 nm in several homemade MCFs, including DCF, TCF, 4-core, and 7-core fibres. Finally, laser beam shaping in uncoupled MCFs was investigated. The controlled generation of ps-pulsed structured beams was achieved using a coherently combined 6-core Yb-doped MCF amplifier. This method produced linearly polarized Gaussian beams, cylindrical vector mode beams, and orbital angular momentum mode beams with peak powers of 10-14 kW and pulse durations of 92 ps at a wavelength of 1035nm. Overall, these results demonstrate mode control and beam shaping in MMFs and MCFs, highlighting the potential for all-optical light manipulations in various future photonics applications.

More information

Identifiers

ORCID for Kunhao Ji: orcid.org/0000-0002-2300-5942

ORCID for David Richardson: orcid.org/0000-0002-7751-1058

ORCID for Lin Xu: orcid.org/0000-0002-4074-3883

Catalogue record

Export record