All-optical Coaxial Multiframe Coherent Imaging With Ultra-high Temporal And Spatial Resolution

Researches of ultra fast phenomena, including microscopic dynamics of materials under shock, interaction between light and materials, atomic and molecular dynamics, etc., demand very high temporal and spatial resolution. In order to breakthrough the bandwidth limitation of optoelectronic imaging devices and overcome the drawbacks of pump-probe, angular separated multiframe imaging, wavelength timed mapping photography, ultrafast holography and other high speed imaging technologies, we bring forward an all-optical coaxial multiframe coherent imaging method based on coherence shutter. We can obtain very high temporal resolution and spatial resolution near diffraction limit in a large field of view.In the thesis, we bring forward a group of coherence shutters with parallel structure, which can selectively extract complex amplitude information from arbitrary object light pulse by interference between serial object light pulses and reference light pulse with independent path and time delay. With the coherence shutters, we can realize (1) all-optical multiframe imaging to achieve very high temporal resolution and diffraction limited spatial resolution simultaneously, (2) coaxial multiframe imaging to keep identical spatial benchmark, identical temporal benchmark and identical spectral benchmark for all images, (3) non-multiplexing recording of multiframe images, i.e. completely separating interferograms in space to assign the total spatial bandwidth of a recording medium to each image and thoroughly avoid crosstalk among different images, (4) some degree of coherence gain that make it possible to use weak light as probe beam, (5) obtaining phase and amplitude information simultaneously. The proof-of-principle experiments accomplish the multi-channel gating of preset light information and verify the feasibility of the parallel coherence shutters.The coherence shutter can be sorted into two types based on mutual coherence mode or self coherence mode and expanded to several sub types according to light source. The coherence gating of light information is achieved by setting the dominant parameters of the control function for coherence shutter under certain terms. The coherence shutter based on the mutual coherence mode has good interference contrast and some gain, but it needs independent reference light path. The coherence shutter based on the self coherence mode has poor interference contrast and has no gain, but it needn’t independent light path, so it provides more flexibility and convenience. We also bring forward a special type of coherence shutter based on multi-beam interference that can obtain the highest interference contrast.We studied two operating modes of the coherence shutter, the spatial carrier mode and the phase shift mode. The spatial carrier mode is easy to be realized and is convenient to use. The phase shift mode has higher temporal and spatial resolution than the spatial carrier mode, but it is more complex. The algorithms based on both modes were studied for retrieving the object light wave of the gated interferogram. The algorithms were discussed in detail according to the properties of the object, including pure phase, pure amplitude or mixture object.An ultrafast all-optical coaxial multiframe coherent imaging system for demonstration has been constructed. The system consist of picosecond laser, serial pulses generator, optical paths of the parallel coherence shutters, imaging lenses and recording system. Multiframe interferograms of good quality are obtained by utilizing the Fourier image transferring system with optical path compensation and filtering. The system can capture four frames in a shot with interval of 34 ps, spatial resolution of 3μm, and field range of 5 mm.Two kinds of experiment including laser driving air and laser driving aluminum foil were performed for diagnostics of ultra fast physical processes. The air breakdown by intense laser radiation and evolution of the hot spot cluster were observed from the images with high temporal and spatial resolution. The maximum expanding velocity of the hot spot exceeds 1000 km/s. In the diagnostics of laser ablating aluminum foil, super fast "jet" was observed with high temporal and spatial resolution. The apparent velocity of the "jet" front exceeds 9000 km/s, and the electron density of the "jet" head is about 1019cm-3. These results confirmed that the all-optical coaxial multiframe coherent imaging method based on the parallel coherence shutters is effective for diagnostics of ultra fast processes with ultra high temporal and spatial resolution.The temporal resolution of the all-optical coaxial multiframe coherent imaging method was analyzed in frequency domain. The self coherence function and the mutual coherence function can be expressed as Fourier transform of the spectrum and the cross spectral densities, so the comprehensive influence on the temporal resolution from light pulse waveform, spectral properties, chromatic dispersion, and other properties in time and frequency domain can be studied. The spatial resolution of the imaging method was analyzed by numerical simulation of diffraction limited system. The minimum distinguishable distance between two points is acquired under mutual coherence mode and self coherence mode respectively. Moreover, the influence of recording and processing manner of interferogram on the spatial resolution was analyzed too.