Instrument For Real-time Ultrafast Phenomenon Detection Based On Pulsed Digital Holography

Pulsed digital holography, which is a new detection method, is a combination of the pulsed laser and digital holography. It has wide application in3D measurement, superresolution, ultrafast phenomenon detection. The ultrafast phenomenon, such as3D temporal and spatial distribution of THz filed, femtosecond laser induced air plasma, can be detected by this method. The former work exists the problems of unfixed detection angle, no more than3frames of real-time detection, un-adjustable time resolution, limited spatial resolution, complicated detection structure. In this paper, sub-pulses generation with adjustable pulse interval, instrumentation of the multi-frames real-time detection system, numerical reconstruction method for improving spatial resolution, ultrafast phenomenon detection based on multiplexing pulsed digital holographic system are carried out. The details are as follows:1:The multiplexing ability for pulsed digital holography system is analyzed.Pulsed digital holography systems based on single CCD camera and multiple CCD cameras are designed. The numerical reconstruction method for improving the spatial resolution and method for adjusting the detected time resolution are proposed. Method for switching the real-time detection frames and instrumentation of the pulsed digital holography system are studied.2:Collinear propagated object sub-pulse train is generated based on cascaded birefringent crystal. Sub-pulses with adjustable pulse interval are realized by the double wedge crystal. Multiple adjustment mechanisms are designed to adjust the effective thickness of the double wedge crystal. Interference experiment and autocorrelation testing manifest the effectiveness of the designed structure. Structure constituted by3pieces of wedge crystal is used to compensate the transverse shift between the ordinary and extraordinarypulses. Sub-pulses with large pulse interval are generated through the combined delay lines and pellicle beam splitter.3:Spatial angular propagated reference sub-pulses are generated. Different strategies are adopted by8and16frames of real-time tracking system. The combination of pellicle beam splitter, optical delay and elevator, and reflection mirror generates multiple reference sub-pulses for8frames of real-time tracking system. The reference sub-pulses with adjustable temporal and spatial distribution based on Wollaston prism is one of strategy for16frames of real-time tracking system. Another strategy based on Wollaston prism generates sub-pulses with arbitrary adjustable spatial angle.4:Numerical reconstruction method for improving the spatial resolution is carried out. Spatial angular multiplexing for enlarging the detected object wavefront area with assured time resolution is proposed. Numerical iteration method and nonlinear filtering method for eliminating the zero-order term are proposed. The single hologram acquisition of both methods makes them are easy to be embedded into the ultrafast holography system.The mathematical deduction, numerical simulation and experimental results manifest the effectiveness of the method.5:The femtosecond laser induced air plasma is detected by the pulsed digital holography system with8frames of real-time tracking ability. The amplitude and phase evolution of the air plasma are digitally reconstructed. The results manifest the multiplexing system can realize the ultrafast phenomenon detection with hundreds of femtosecond time resolution. The bit of the CCD on multiplexing ability is analyzed.The innovative works obtained are as follows:1:Sub-pulses with adjustable pulse interval based on double wedge crystal are generated. Invent patent "Method and corresponding mechanisms for generating multiple sub-pulses with adjustable pulse interval and pulse numbers" is granted (No.201110397464.7). To compensate the transverse shift between the sub-pulses, a triple wedge structure with continuous pulse interval adjusting ability is designed, and apply for a patent"Pulse splitter for continuous adjusting pulse interval based on birefringent crystal"(No.201410167969.8).2:To have a large adjustable pulse interval of the sub-pulses, a new implement is designed. A patent "Multiple sub-pulses generation with pulse interval adjustable in a wide span" is being applied.3:For convenienttest the splitting parameter through the auto-correlator, a method for obtaining the pulse interval, pulse width and intensity ratio is proposed.A corresponding patent is being applied (No.201410167747.6). 4:Reference sub-pulse generation based on Wollaston Prism with temporal and spatial distribution adjustable ability is proposed. A patent "Sub-pulses with adjustable temporal and spatial distribution are generated based on Wollaston Prism."(No.201410089655.0). Also a patent "Sub-pulses generation with multiple propagation angles" is being applied.5:In order to enlarge the detected object wavefront area without sacrificing the detected spatial resolution, spatial angular multiplexing technique for enlarging the detected object wavefront area is proposed. This part of the work is published in the Optics Letters and Acta Optica Sinica.6:The numerical iterative method and nonlinear filtering method are proposed to eliminate the zero-order term. Both methods require only one hologram acquisition, makes them can be specially used for ultrafast phenomenon detection. These parts of work are published in the journal of Optics Express and Optics Communications.7:Pulsed digital holography system for real-time tracking an ultrafast phenomenon is tested. The spatial angular multiplexing is used to break through the response time of the electronic device, reaches as high as hundreds of femtosecond time resolution. The experimental result shows the system can detect the ultrafast phenomenon with a time resolution of hundreds of femtoseconds. The research results are accepted by the "ICO" conference and obtained the achievement certificate of "Real-time tracking pulsed digital holography system based on spatial angular multiplexing technique" with an international advanced level.