Advanced short pulsed 2 micron fibre lasers for medical applications

Abstract

This research concerns the development of advanced pulsed fibre lasers whose emission is sourced from the 3F4→3H6 transition in thulium, thereby making them highly applicable to the field of medicine by exploiting naturally occurring absorption peaks present in biological tissues.

To that effect two such systems are reported here. The first is a 1.95μm fibre amplifier with an average power of 11W that emits pulses from the nanosecond regime, with a pulse energy and a peak pulse power of up to 55μJ and 1.25kW, respectively. Pulses are generated between repetition rates of 250kHz and 17MHz. This wavelength was chosen because within the low-loss transmission range of silica fibres, it had the highest absorption coefficient in water, and water is the most common constituent of biological tissues. This laser system was subsequently applied as a tool for biological laser ablation.

The 1.95μm system was trialled as a laser ablation tool on two contrasting cell types: onion epidermal cells and human neuroblastoma cells of the SH-SY5Y cell line. For the former, no ablation was achieved, but the laser caused observable changes at the cellular scale. For the latter, laser ablation was demonstrated on the cellular scale with the selective destruction of a small number of individual cells that were held between two glass cover slips. These individual, though not consistent, results were achieved through careful implementation of four fundamental pulse parameters: the average power, exposure time, pulse duration, and repetition rate, with parameters of Pavg = 0.25W , texp = 0.5s, τ = 7ns, and R = 1.25M Hz proving most viable. This process showed that both the total energy delivered and the manner of its delivery are important factors. The beam was focused to the cell samples using a reflective microscope objective. This objective realised a spot size of 3.6±0.2μm and, critically, had zero chromatic aberration, enabling a coincidence of focal planes for visible light and the 1.95μm signal, enabling the capture of video footage of tissue irradiation.

Furthermore, by modifying the pulse parameters and the optical coupling system, the 1.95μm system was able to cut incisions into both bacon and human cartilage. Ablation was realised for both of these samples. There was not time for sufficient investigation, nevertheless individual incisions that were either precise (40μm), deep (650μm), and exhibiting a moderate rate of tissue removal (0.2mm3/J).

The second fibre amplifier is a 1.75μm system with an average power of 3W that emits pulses from the picosecond regime, each with a pulse energy and a peak pulse power of 88.8nJ and 2.47kW, respectively. The repetition rate could be adjusted between 34MHz an 2GHz. Analogous to the first system, this wavelength was selected because of its proximity to the highest available lipid absorption coefficient present within the thulium emission band, and because of its suitability towards detecting hydrocarbons which contain a C-H bond. This system is applicable to both lipid ablation and to photoacoustic spectroscopy/imaging. This system represents the first all-fiberised multi-Watt picosec- ond thulium-doped fibre laser operating in the 1.7μm region with a controllable repetition rate. The long-wavelength gain associated with systems operating on the short-side of this thulium emission band is successfully filtered out using bending losses. It was not possible due to time to investigate this system’s capabilities as a tool for photoacoustic spectroscopy, but the potential remains.

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Identifiers

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

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

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

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