| Optical parametric amplification is a typical nonlinear technique. Because it can get high gain, high quality, wide band power and energy amplification of simple structure, which is widely applied to many occasions and fields, including optical parametric amplification imaging. This thesis applies this technique to the field of transient optical imaging. Transient optical imaging is one of the main optical detection methods in the study of ultrafast events. It is widely used in the diagnosis of ultrafast processes, and can obtain the time and space information of the ultrafast process. Accurate acquisition of the time and space information of ultrafast process, revealing its dynamic law and controlling the use of it, have important applications in the field of physics, chemistry, biology and medicine. Is related to the national military, aerospace, scientific research, medical and industrial and other aspects of technological progress, has important scientific research, social and economic value. With the development of ultrashort pulse technology, ultrashort pulse laser has been widely used in various types of imaging for ultrafast process and promote the time resolution of imaging to picosecond and femtosecond region.This thesis proposes a transient imaging setup of femtosecond time resolution levels. The contents include the introduction of research background and the basic theory of optical parametric amplification, the theoretical analyses of optical parametric amplification imaging characteristics, a high gain and high spatial resolution optical parametric amplification imaging technology, an ultrafast frame imaging technology based on non-collinear optical parametric amplification which by using the femtosecond laser pump probe and an ultrafast real-time imaging technology non-collinear optical parametric amplifier. The main research contents can be summarized as followings.1. By theoretical analyses and numerical simulations, we analyzed the properties of the OPA gain and its spatial/temporal bandwidth for the crystals: ?-BBO, LBO and KDP. Our results show that larger gain and its spatial bandwidth is available by using ?-BBO instead of LBO and KDP crystals for the same pump and crystal thickness. We also find that smaller thickness of parametric crystal can effectively enlarge the spatial frequency bandwidth, though it will affect parametric gain. What is more, higher pump intensity is very helpful not only to raise the parametric gain, but also to broad the spatial bandwidth. Finally, special attention shall be paid to the uniform intensity of pump which can result in both the spatial non-uniformity of the gain and the non-uniform spatial bandwidths thus degrade the spatial resolution of the optical imaging. These results provide a theoretical support for our experimental study.2. We design and implement the non-collinear optical parametric amplification imaging experiment of high gain and high spatial resolution. According to the different imaging signal illuminating source, there are two kinds of experimental scheme. One is a novel scheme for non-collinear OPA imaging pumped by an intense ultrashort light pulse chain to amplify a continuous-wave illuminating laser. In this setup, the pump and the illuminating signal beams are from two independent lasers, so one can choose the illuminating wavelength with high freedom and has a simple and compact structure. The femtosecond laser pulses pump beam is helpful to obtain high gain and broad spatial resolution bandwidth for imaging. Moreover, the non-collinear angle between the signal beam and pump beam make the generated idler beam is separated from both the signal beam and the pump beam spatially, thereby can be recorded conveniently. The other is a scheme for non-collinear OPA imaging of both the signal light and the pump light are femtosecond pulses. In each scheme, two phase matching methods of type-I and type-II are designed respectively, and the high gain and high spatial resolution OPA idler images are obtained. In particular, we propose the OPA imaging scheme, which bases on the noncritical type-II phase-matching among the three interactive waves not only for optimizing angular acceptance but also for the collinear propagation between the Poynting vectors of signal and idler inside the crystal to eliminate the geometrical smearing. As a result, this design allows us to carry experimentally out idler imaging with a parametric gain up to 10~4. The spatial resolution in the horizontal direction is up to 20.16 lp/mm, and the spatial resolution in the vertical direction is up to 25.39lp/mm.3. We use the femtosecond laser pump-probe method to study the OPA ultrafast imaging device experimentally. Similarly, there are two kinds of experimental scheme. One is pumped by an intense ultrashort pulse chain to amplify a CW illuminating beam, and the other is pumped by an intense ultrashort pulse chain to amplify a weak femtosecond pulse signal beam. In experiment,we use the femtosecond laser pulses to induce plasma grating(the stripe width is about 24 m) in air for the target and successively get sequential multiple images from the idler beam. The time resolution of the OPA imaging system depends only on the accuracy of the displacement platform.4. We present a novel design for ultrafast real-time frame imaging device by use of non-collinear OPA. This device can get four pieces of sequential idler images in a single shot. The time resolution of imaging system can reach 35 fs which depends on the width of the pump pulse and the accuracy of the displacement platform. We use a CW laser or a chirp pulse to illuminate ultrafast events and an ultrashort femtosecond pulse to pump the non-collinear OPA. In our experiment, we observe the evolution of plasma grating(the stripe width is about 28 ?m) in air in 0~70ps time range. Sequential four different images of the plasma grating from four idler images are obtained by adjusting the relative delay of various pump beams. The shortest time interval is 133.3fs,the corresponding fram frequency is 7.5×10~12fps. |
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