Experimental and Numerical Optimisation of a SESAM Mode-locked Holmium Fibre Laser

Abstract

The infrared region of the electromagnetic spectrum is an attractive prospect for a variety of pulsed laser applications, including biomedical surgery and imaging, defence, metrology and attosecond science, environmental detection, and range-finding. The aim of this research is to develop a robust, stable, benchtop fibre-based laser source in the mid-infrared region of the spectrum, capable of producing picosecond to femtosecond pulses. This thesis utilises a semiconductor-based absorber to create pulses in concert with various linear and nonlinear optical effects in fibre. To best optimise the laser source, a multi-step approach utilising both numerical MATLAB modelling in concert with experimental techniques. Firstly, over a half-dozen cavity designs were simulated, assembled, and tested, with each utilising alternate combinations and ordering of various fibre laser components to achieve optimal pulse-to-pulse stability, time duration and spectral quality. From these experimental results, the most optimal design was chosen and was fully characterised, in addition to a single pass pumped holmium fibre amplifier. This laser produced 2.92 ps pulses at a 20 MHz repetition rate, with a central wavelength of 2029.1 nm and a 3 dB bandwidth of 4 nm at an average power of 20.3 mW. Lastly, pulse dispersion was optimised using a length of dispersion compensating fibre (DCF). An estimate for the required dispersion compensation was calculated, numerically simulated, and then experimentally implemented via fusion splicing 13 m of DCF and cutting back in 1 m increments, characterising the laser at each length. This process was repeated using 20 cm increments. The optimal length for pulse duration was found to be 7.2 m, where the laser produced 494 fs pulses, and minimised pulse-to-pulse instability indicated by the radio frequency spectrum.