Experimental Investigation Of All-solid-state High Repetition Frequency Sub-nanosecond Pulsed Laser And Its Application In All-optical Photoacoustic Imaging

All-solid-state high repetition frequency sub-nanosecond pulsed lasers have been widely used in laser ranging,lidar,laser microprocessing,biomedical imaging and so on,owing to their advantages such as compact structure,good beam quality,easy maintenance and long lifetime.Especially in the field of photo-acoustic imaging,in order to obtain a high resolution image of the biological sample,the pulse width of laser need to be smaller than the thermal confinement time and pressure confinement time of the tissue.Biological tissue absorbers commonly used in photoacoustic imaging are vascular tissue,the minimum diameter of capillaries in biological tissue is 4 μm,its thermal confinement time is about 0.2 ms,and the pressure confinement time is about 1.3 ns.Therefore,only when the pulse width of laser is less than 1.3 ns,it can be considered that during the laser irradiation stage,there is no thermal diffusion between the absorber and outside,and the ultrasonic waves generated at the center of the absorber do not propagate to the surface of the absorber.In addition,in order to increase the imaging speed while ensuring high resolution,a laser with high repetition frequency（≥1 k Hz）and sub-nanosecond pulse width（<1.3 ns）needs to be developed.In this paper,the experimental investigations of all-solid-state high repetition frequency sub-nanosecond pulsed lasers and its application in all-optical non-contact photoacoustic imaging are described in detail.The main research results are as follows:1.Based on electro-optical cavity dumped technology,we conducted theoretical and experimental research on all-solid-state high repetition frequency sub-nanosecond pulsed laser.Firstly,the effects of the extinction ratio and the falling edge of electro-optical Q-switch,and the cavity length on output behaviors of the pulse laser were theoretically studied.By optimizing the mechanical design of the resonant cavity and the falling edge of electro-optical Q switch,as well as based on the precise temperature control of electro-optical Q switch and laser crystal,in case that the cavity length was 70 mm,a single longitudinal mode 1064 nm pulsed laser with pulse width of 1.57 ns,repetition frequency of 1 k Hz,and single pulse energy of 0.31 m J was fabricated.The peak power fluctuation of the pulsed laser was better than ± 1.5% during 10 ms.The measured beam qualities of the laser were Mx2=1.27 along the horizontal direction and My2=1.39 along the vertical direction.2.Based on electro-optical Q-switched technology,we also conducted theoretical and experimental research on all-solid-state high repetition frequency sub-nanosecond pulsed laser.Using the rate equation of a four-level Q-switched laser,the effects of transmittance of output coupling mirror,mode matching between pump laser and oscillating laser,the driving voltage sent to the electro-optical Q-switch and the Nd³⁺ doping concentration of the Nd:YVO4 crystal on output behaviors of the pulse laser were theoretically studied.In the experiment,a reflection type volume Bragg grating was used as the input mirror of resonant cavity to achieve single longitudinal mode laser output.By optimizing transmittance of output coupling mirror,magnification ratio of telescope system,the driving voltage sent to the electro-optical Q-switch and the doping concentration of the gain medium,in case that the cavity length was 72 mm,the transmittance was 60%,the peak pump power was 25 W,when the Nd³⁺ dope concentration of Nd:YVO4 crystal was 0.7 at.%,a single longitudinal mode 1064 nm sub-nanosecond pulsed laser with repetition frequency of 1 k Hz,pulse width of 0.77 ns,single pulse energy of 1.04 m J was obtained.The measured beam qualities of the laser were Mx2=1.26 and My2=1.32.When the Nd³⁺ dope concentration of Nd:YVO4 crystal was 0.5 at.%,by tuning the driving voltage sent to the electro-optical Q-switch,the laser pulse width can be precisely and quickly adjusted to a certain length in the range from 0.78 ns to 1.3 ns.3.The developed all-solid-state high repetition frequency sub-nanosecond pulse laser was used in the experimental research of all-optical non-contact photoacoustic imaging.A novel all-optical non-contact photoacoustic imaging device was fabricated,in which a Michelson interferometer combined with an optical amplifier was used as photoacoustic sensor and a low noise balanced detector was used to extract photoacoustic signals with high signal-to-noise ratio.Experimentally,the spatial resolution of the photoacoustic imaging device was improved by optimizing the bandwidth of the balanced detector,the lateral resolution was increased from 96 μm to 27 μm,and the axial resolution was increased from 35 μm to 17 μm,and the imaging depth was 3.36 mm.The photoacoustic imaging device was used to perform two-dimensional high resolution photoacoustic imaging of cross black tape and black hair embedded in a biophantom.The innovative works of this paper are as follows:1.Based on the cavity dumping laser operation mechanism,a quasi-quantitative theoretical model was established.For the first time,the effect of the extinction ratio of the electro-optical Q-switch on output behaviors of the pulse laser was simulated.On this basis,the effects of the falling edge of electro-optical Q-switch and the cavity length on output behaviors of the pulse laser were simulated.In the experiment,by precisely controlling the temperature of electro-optical Q-switch,the problem of reducing the extinction ratio of the electro-optical Q-switch due to thermal depolarization effect under high pump intensity was solved.By optimizing circuit of the electro-optical Q-switch,the falling edge was shortened from 26.5 ns to 3.8 ns,the pulsed laser with pulse width of 1.57 ns was achieved.2.Based on the theoretical and experimental studies of electro-optical Q-switched laser,a new mechanism for narrowing the laser pulse width was proposed.Theoretically,for the first time,the effect of driving voltage sent to the electro-optical Q-switch on output behaviors of the pulse laser under the conventional cavity length was analyzed.On this basis,by analyzing the effect of the doping concentration of the gain medium on output behaviors of the pulse laser,the experimental conditions for generating a high repetition frequency sub-nanosecond pulsed laser were proposed.3.A novel all-optical non-contact photoacoustic imaging device was fabricated by using developed all-solid-state high repetition frequency sub-nanosecond pulsed lase,in which a Michelson interferometer with optical amplifier was used as photoacoustic sensor to improve the signal-to-noise ratio（SNR）of the photoacoustic imaging.P-polarized laser was used as the detection light of the interferometer.The reference light and probe light were converted into S-polarized lights after passing through the quarter-wave plate twice,and these two beams interfere at the beam splitter,by using the low-noise balanced detection technology,the SNR was further improved.By optimizing the bandwidth of the balanced detector,the lateral resolution of the photoacoustic imaging device was increased from 96 μm to 27 μm,and the axial resolution was increased from 35 μm to 17 μm.