Ultrafast thulium-doped fibre laser for healthcare applications

Abstract

The short-wavelength infrared (SWIR) window, with wavelengths ranging from ~1650 to 1900 nm, provides outstanding performance in deep tissue, non-invasive biomedical detection applications, thanks to its high optical transparency and low scattering loss in most biomedical samples. The SWIR can be utilised in multiphoton microscopy (MPM) to acquire high-resolution biomedical images with superior signal-to-background ratios. However, MPM relies on detecting nonlinear scattering signals that require a high-peak-power pump source. Therefore, reliable sources with appropriate optical properties are essential to manifest high-performance, deep-penetration detection in biomedical samples. Ultrafast SWIR pulses, with a temporal width in the hundreds of fs regime, hold high potential to function as pumps in the MPM due to their capabilities to deliver high peak powers, while average powers are kept at low levels, preventing tissue heating issues. Mode-locked (ML) thulium-doped fibre (TDF) lasers (ML-TDFLs) are promising options for generating ultrafast SWIR pulses.
In this thesis, ML-TDFLs with different cavity designs are presented. I have successfully demonstrated an all-fiberised ML-TDFL enabled by a semiconductor saturable absorber mirror (SESAM). This cavity generates a stable dissipative-soliton ML operation at a central wavelength of 1875 nm and delivers a pulse energy of 10.3 nJ with a compressed pulse duration of 548 fs. To mitigate the wavelength limitation from the SESAM, I developed another all-fiberised ML-TDFL based on carbon nanotubes (CNTs) saturable absorber (SA). This cavity offers a single-pulse ML operation at a central wavelength of 1847 nm and generates a compressed pulse duration of 381 fs. The cavity has been deployed as a seed laser in an imaging laser system, utilised an efficient pump source in biomedical imaging experiments. Moreover, I have successfully developed an all-fiberised ML-TDFL in an all-polarisation-maintaining (all-PM) configuration that enhances cavity robustness and supports a 'turnkey' laser design. Several advanced techniques have been integrated into the cavity to realise the balance of both conservative effects (between dispersion and nonlinearity) and dissipative effects (between gain and loss), to facilitate the dissipative-soliton operation. This all-PM cavity generates a self-start, stable ML operation at a central wavelength of 1876 nm with a pulse energy of 1.1 nJ and a compressed pulse duration of 391 fs. The mode-locking can be stably maintained in a normal laboratory environment for significantly extended period without requiring any active controlling mechanisms. Additionally, an all-fiberised PM chirped pulse amplification (CPA) system seeded by such all-PM ML cavity has been developed and deployed as an imaging laser prototype. This laser system operates at a central wavelength of 1840 nm and produces an output pulse energy of ~40 nJ (an average power of ~600 mW) with a compressed pulse duration of 493 fs. The laser has been delivered to our external project partner and been utilised as a reliable laser source in biomedical imaging experiments. Finally, the study of an all-PM ML-TDFL has been further explored in a cavity enabled by a nonlinear amplifying loop mirror (NALM) SA. The cavity provides stable dissipative-soliton ML operation at a central wavelength of 1860 nm with a pulse energy of 1.5 nJ and a compressed pulse duration of 539 fs. This all-fibre cavity demonstrates the possible solution to resolve any technical difficulties of using material-based SAs.

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Identifiers

ORCID for Panuwat Srisamran: orcid.org/0009-0003-2796-4168

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

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

ORCID for Yongmin Jung: orcid.org/0000-0002-9054-4372

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