Image Reconstruction Of Coherent Field Imaging Technology

Coherent field imaging technology, as is generally called Fourier telescope, is an imaging technology aimed at long range faint object with super high resolution. The basic principle is that the interference field produced by the coherent laser beams encodes object’s spatial frequency spectrum and then the frequency spectrum information will be decoded from the backlight. The technology has a good potential in deep space object’s recognition and surveillance due to its super high resolution and active imaging feature. As a computing indirect imaging technology, the subsequent data procedure has a huge impact on the reconstructed image. To improve the reconstructed image quality, this paper carries out study on the image reconstruction of coherent field imaging technology, and the main contents are as follows:1. Starts from the mathematics principle of the interference field and spatial frequency spectrum encode, then introduces the system structure and imaging procedure, and taking the T shaped equally spaced transmitting telescope array for example, focus on the image reconstruction technology, including the spatial frequency spectrum decode, phase closure and reconstruction algorithm, and that forms the rudiment of the image reconstruction study. Moreover taking the transmitting beam position error for example, builds the error transfer function and analyses the error effects on the imaging quality.2. Based on the shortage of the traditional reconstruction method, introduce the phase retrieval algorithm which is used in the interferometic measurement technique and combines the traditional reconstruction method, then formes an iterative reconstruction method. The starting point is that the modulus has a considerably stronger anti-noise performance than the phase does, which means that the modulus distortion is obviously smaller than the phase distortion. The traditional reconstruction method ignores this feature and does the inverse Fourier transform using both the modulus and the phase. When the noise is serious the results may be bad since the phase distortion is severe. The iterative reconstruction method recovers the phase information from the small distorted modulus using the phase retrieval algorithm; furthermore, it uses the traditional reconstructed image to generate constraints for the phase retrieval procedure, which greatly increases the reconstruction speed and accuracy. Carries out theoretical analysis on the modulus and phase anti-noise performance, also the numerical simulation and experiments are done to test the effectiveness of the method.3. Based on the measured spatial frequency distributed features of the O shaped transmitting telescope array, introduces the super-resolution method which is on the basis of extrapolation and forms a super-resolution reconstruction method that suits for the O shaped transmitting telescope array as well as other similar telescope array. The O shaped transmitting telescope array is compact and can achieve large numbers of distinct spatial frequencies, but it also has disadvantages such as small equivalent aperture, nonequispaced frequencies and low frequency spectrum effectiveness. The extrapolation method based on the sampling theory can turn the advantage of large numbers of distinct sampling frequencies into the bandwidth expansion, besides it turns the nonequispaced frequencies into the uniform frequencies which means that the method can improve not only the bandwidth but also the reconstructed image quality. Numerical simulation of the O shaped transmitting telescope array is done and the application limits are discussed. In addition, numerical simulation and analysis aimed at the USA Satellite Active Imaging National Testbed (SAINT) program is conducted.4. Based on the feature and design request of multiple-beam coherent field imaging technology, designs a multiple-beam coherent field imaging ground-based test system. The system consists of a transmitter system and a receiver system. The transmitter system adopts a T shaped uniform spaced transmitting telescope array, also to satisfy the sampling requirements and increase the measured frequencies, three highly accurate slides are used to switch the transmitting beam positions. The receiver system uses a single large-aperture reflector and a second condenser to collect backlight. Designs the system parameters and opto-mechanical structure in details and fabricates both a slide module and a beam-split module, the test result shows the reasonableness of the design.5. Based on the designed multiple-beam ground-based test system, studies on the multiple-beam weak signal demodulation and image reconstruction. The detected photons during the sample period follow the Poisson distribution and the shot noise amplitude will be very serious if one time transmits a lot of beams, at which case the high spatial frequency spectrum will be lost in the noise. Analyses the effects on the high spatial frequency spectrum with different transmitting beam number and chooses the best number. To demodulate the weak signal in the case of strong noise level, adopts the cross-correlation detection which is widely applied in the weak signal detection field. Compared to the traditional filtering method, the adopted method can lead to higher spectrum accuracy. Further, consults the3beam coherent field imaging technology and formulates the phase closure procedure. Numerical simulation result shows the effectiveness of the method.