Advanced ultrafast thulium-doped fibre laser systems for biomedical imaging applications

This research presents the development of a high intensity ultrafast pulsed laser in the short-wave infrared (SWIR) region for deep-tissue medical imaging. The challenge is catering for the inhomogeneous biological system. To address this and uncover deep biological tissue structure, a mode-locked (ML) laser source with an operating wavelength of 1845 nm and 20.5 MHz repetition rate (RR) was used to seed a master oscillator power amplifier (MOPA) system. In addition, an acousto-optic modulator (AOM) and a pulse picker (PP) were added before the final amplifiers for peak power scaling at lower RR without increasing the average power responsible for thermal damage. The system features chirped pulse amplification (CPA), where pulses were temporally stretched, then amplified before recompression at the output using a grating pair to increase the peak powers of the pulses. As low as ∼380 fs compressed pulse duration was attained at 2 MHz of RR, resulting in peak powers up to 410 kW. Imaging experiments were carried out using this laser in-vitro on human and animal bone samples. The results revealed an increase in the imaging depth up to 400 μm as RR was reduced. To improve soft tissue imaging, a carbon nanotube (CNT) based ML laser was developed with a shorter emission wavelength of 1789 nm, which minimises water attenuation and enhances penetration in soft tissue. This was combined with a nonlinear amplification and compression achieved by provoking self-phase modulation (SPM) to broaden the pulse spectrum using a highly nonlinear fibre (HNF) before applying CPA to achieve pulse durations of 180 fs. The limit of short wavelength generation from an ML thulium (Tm) fibre system was explored next by developing a tunable source from 1900 to 1725nm and an amplifier optimised for the short wavelength with the incorporation of a bending-induced low-pass fibre filter. Finally, a gain-switched laser diode (GSLD) was used as an “off-the-shelf” alternative for generating ultrafast pulses by initially applying SPM to broaden the spectrum, a spectral filter to clean the pulse chirp and a 2-stage compression method comprising a 100 m single-mode fibre (SMF) linear compressor and a nonlinear Tm-doped fibre amplifier compressor to achieve simultaneous pulse amplification and compression from 40 ps to 560 fs. A GSLD was also used to demonstrate an adaptive approach for selective imaging of regions of interest (ROI), increasing the resolution while minimising the effects of photo-toxicity, photo-bleaching, and thermal damage to surrounding tissue. Non-uniform pulsed patterns were generated and amplified to simulate this form of imaging, and the gain saturation effects during amplification were analysed. Analytical and numerical solutions were developed to predict and shape the amplifier input pulses to compensate for uneven pulse amplification on a single 50ns burst and adapted for a 1 ms non-uniform multi-burst signal. Various advanced ultrafast Tm fibre laser technologies were developed for deep-tissue biomedical imaging, including source and amplifier design optimisation, and pulse manipulation.