23 February 2010 High repetition rate passively Q-switched fiber and microchip lasers for optical resolution photoacoustic imaging
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Optical-resolution photoacoustic microscopy is a novel imaging technology for visualizing optically-absorbing superficial structures in vivo with lateral spatial resolution determined by optical focusing rather than acoustic detection. Since scanning of the illumination spot is required, the imaging speed is limited by the scanning speed and the laser pulse repetition rate. Unfortunately, lasers with high-repetition rate and suitable pulse durations and energies are difficult to find. We are developing compact laser sources for this application. Passively Q-switched fiber and microchip lasers with pulse repetition rates up to 300 kHz are demonstrated. Using a diode-pumped microchip laser fiber-coupled to a large mode-area Yb-doped fiber amplifier we obtained 60μJ 1-ns pulses at the frequency-doubled 532-nm wavelength. The pulse-repetition rate was determined by the power of the microchip laser pump source at 808nm and may exceed 10 kHz. Additionally, a passively Q-switched fiber laser utilizing a Yb-doped double-cladding fiber and an external saturable absorber has shown to produce 250ns pulses at repetition rates of 100-300 KHz. A photoacoustic probe enabling flexible scanning of the focused output of these lasers consisted of a 45-degree glass prism in an optical index-matching fluid. Photoacoustic signals exiting the sample are deflected by the prism to an ultrasound transducer. Phantom studies with a 7.5-micron carbon fiber demonstrate the ability to image with optical rather than acoustic resolution. We believe that the high pulse-repetition rates and the potentially compact and fiber-coupled nature of these lasers will prove important for clinical imaging applications where realtime imaging performance is essential.
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Wei Shi, Ilya Utkin, Janaka Ranasinghesagara, Lei Pan, Yogesh Godwal, Shaun Kerr, Roger J. Zemp, Robert Fedosejevs, "High repetition rate passively Q-switched fiber and microchip lasers for optical resolution photoacoustic imaging", Proc. SPIE 7564, Photons Plus Ultrasound: Imaging and Sensing 2010, 75640X (23 February 2010); doi: 10.1117/12.842533; https://doi.org/10.1117/12.842533

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