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Title: Investigation of transverse mode instability suppression in large mode area fibre
Authors: Xia, Nan
Keywords: Engineering::Electrical and electronic engineering
Issue Date: 2019
Source: Xia, N. (2019). Investigation of transverse mode instability suppression in large mode area fibre. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: In the past decades, fibre laser and amplifier systems have seen a rapid power increase due to the efficient heat dissipation and robust single-mode operation. The fibre lasers and amplifiers have become attractive sources for applications spanning from industry and defence to scientific research. In particular, the fibre lasers and amplifiers have penetrated for material processing industry including marking, cutting, welding and drilling. It had been known that the highest achievable output power in fibre laser and amplifier systems is largely limited by the nonlinear scatterings such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS). However, a new phenomenon under high power operation, the so-called transverse mode instability (TMI), was experimentally observed in a large mode area (LMA) ytterbium (Yb)-doped fibre amplifier in 2011. The TMI can occur much earlier than the SBS and SRS in average power scaling, hence becoming the first limitation in high-power fibre operations till now. The TMI refers to the output beam degradation, caused by the power transfer from the fundamental mode (FM) to the higher-order mode (HOM), for input pump power above a certain threshold. The origin of TMI is attributed to the stimulated thermal Rayleigh scattering (STRS) that provokes a phase shift, induced by the thermal diffusion time across fibre core, between the irradiance grating and thermally induced refractive index grating. The interference between two populated modes produces the irradiance grating along the fibre, which has a period equal to the beating length between two modes. Then the thermally induced refractive index grating is generated by irradiance grating through a quantum defect (QD) in fibre core. The QD created by transition from pump to signal frequency is recognized as the main heat source to modify the refractive index. Recently, it was found that photodarkening (PD) leads additional heating and contributes to the thermal grating. Hence, PD suppressed fibre becomes more important for high power scaling. As the TMI has been discovered very recently, approaches to suppressing the TMI have been very limited. HOM suppression by fibre bending has been the most popular method to suppress the TMI. TMI suppression by pump modulation was reported, but it is not a preferred method as it could lead to damage of the system. Therefore, the motivation of this thesis is to address the TMI in a fibre level through investigation of fibre designs suitable for power scaling with high TMI suppression. Firstly, I developed a four-dimensional (4D) (x,y,z,t) highly numerical model, with the fast Fourier transform (FFT) based beam propagation method (BPM), to simulate and investigate the TMI behaviours in an arbitrary fibre design. Secondly, I theoretically study the TMI in non-circular ytterbium-doped fibres to investigate the heat sink arrangement and TMI suppression. Thirdly, I investigated maximum achievable power in a PD fibre through parametric optimization. A PD fibre provides high pump absorption and high fabrication yield, which would benefit designing laser or amplifier configurations when the TMI is suppressed. Lastly, a TMI suppressed design is proposed based on annular core fibres. TMI effects are theoretically investigated in Yb-doped ring-core and multicore fibres. The annual core is found effective to reduce thermal loss, thus suppressing the TMI. The ring shaped near field beam can be converted to a Gaussian-like beam profile in a far-filed with up to 88% conversion efficiency. In the end, the ring-core fibre with outside circular diameter of 16 μm and core thickness of 2 μm was fabricated. I made the experimental lasing setup for the ring-core fibre and tested the performance of it. The thesis contributes to thorough investigation of the TMI behaviour in a fibre level, leading to new insights of fibre design aspects. The thesis concludes that power scaling in fibre lasers and amplifiers can be continuously progressive by investigating alternative fibre designs that could considerably suppress the TMI.
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