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Photoacoustic signals are typically generated using Q-switched Nd:YAG pumped OPO systems, as they can provide the necessary nanosecond pulse durations with mJ pulse energies required for photoacoustic tomography. However, these sources are often bulky, require external water cooling and regular maintenance and provide low pulse repetition frequencies (PRF<100Hz) thus limiting image frame rate.
Fibre lasers can overcome these limitations and additionally offer much greater flexibility in their temporal output characteristics (e.g. pulse shaping and duration). Although fibre lasers have been used in optical-resolution photoacoustic microscopy, they have found limited application in widefield photoacoustic tomography (PAT) due to the relatively low pulse energy (<1mJ) provided by commercial systems. These low pulse energies are a consequence of small core diameter (<25m) fibres required to achieve a high beam quality. However, for widefield PAT, high beam quality is not a requirement and therefore fibre lasers with larger core diameters (>100m) can be used, enabling significantly higher pulse energies (>10mJ) to be achieved.
A novel compact fibre laser which uses a custom drawn large core diameter fibre (100m) to provide high pulse energies (15mJ) and variable PRFs (100Hz-1kHz) and pulse durations (10-400ns) has been developed and evaluated. The fibre laser was combined with a fast Fabry Perot (FP) scanner in order to evaluate its suitability for PAT of biological tissue. The high PRF (>400Hz) of the laser has allowed tomographic images of the microvasculature of the palm of a hand to be obtained in less than one second, significantly quicker than previously achieved with a FP scanner. In addition, the ability to arbitrarily vary the temporal shape of the laser pulse offers new opportunities for controlling the acoustic frequency content of the photoacoustic signal in order to optimise penetration depth and image resolution. For example, the laser pulse duration can be increased in order to shift the acoustic frequency components to lower frequencies which are less attenuated by tissue acoustic absorption and thus improve SNR. To investigate these concepts, a tissue mimicking phantom was imaged for a range of tailored excitation pulses (e.g. different pulse durations, trains of pulses) and their effect on the contrast to noise ratio (CNR) and image resolution observed.
A novel compact fibre laser, able to provide higher pulse energies (>10mJ) than previously reported and with enhanced functionality is presented. It is demonstrated that fibre lasers are a viable alternative to standard Q-switched lasers for photoacoustic tomographic applications in medicine and biology.
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