Towards optoacoustic transient shaping using a flexible fiber laser system

We aim to increase the efficiency of optoacoustic signal generation for precise, in vivo, real-time tissue temperature monitoring during thermal retinal interventions, by matching the timing of multiple laser excitation events to the acoustic response of the examined specimen. To achieve this goal, we utilized a home-built Ytterbium-based master oscillator power amplifier (MOPA) fiber laser system that provides unprecedented control over the temporal pulse structure, allowing for pulse-burst durations from picoseconds to nanoseconds and arbitrary repetition rates for investigating the influence of the excitation duration on the amplitude of the resulting optoacoustic transients. Methodologically, experiments were performed on ex vivo explants of porcine retinal pigment epithelium (RPE) consisting of the RPE, choroid, and sclera embedded in a cuvette filled with saline solution. Optoacoustic transients were detected using a piezoelectric ring transducer (fres = 1 MHz, Medical Laser Center Lübeck, Germany) integrated into a standard ophthalmic contact glass with a distance of 24 mm to the specimen. We systematically investigated the influence of pulse-burst durations between 10 and 100-ns with the total burst energy of 3 μJ matching a typical probe pulse energy. Each burst was produced with a repetition rate of 500 MHz. Results demonstrate that, at typical pulse energies of 3 μJ, shorter pulse-burst durations down to 30 ns significantly increase the amplitude of the generated acoustic transients compared to longer pulse-bursts. While higher burst energy consistently results in stronger signals, signal generation efficiency is highly dependent on the temporal burst width. With decreasing burst durations, the amplitude of the resulting transients decreases lower than that of the 30-ns burst. We hypothesize that shorter excitation bursts result in a signal consisting of higher-frequency components that are stronger attenuated in water. These findings highlight that tailoring the temporal excitation profile is essential for maximizing signal-to-noise ratio. The compact and scalable fiber-based MOPA architecture offers a versatile alternative to traditional bulk lasers, providing the necessary degrees of freedom for optimized optoacoustic tissue characterization and in future real-time monitoring.