The Phasing Sensing For Optical Synthetic Aperture Imaging System

Optical synthetic aperture imaging is the main structure of high resolution optical telescope in the future.The astronomical research needs the optical telescope with higher resolution and sensitivity to study forefront science problem.At present,the diameter of single primary mirror is limited on 10 meters.Thus the existing large aperture optical telescopes are used the optical synthetic aperture imaging to break this limit.For different types of optical synthetic aperture imaging system,they all face the problem of image quality reduction due to the piston error(the optical path difference between different sub apertures to the focal plane).Only when the piston error is less than 1/10 wavelength,the piston error can be ignored.Otherwise,the angular resolution of the system may be lower than diffraction limit of a single sub aperture.The traditional adaptive optical sensor,Shack-Hartmann sensor can’t detect the piston error.This thesis focuss on the detection of piston error.Firstly,we study the dispersed fringe sensor,and realize co-phasing of the optical aperture synthesis by a dispersed Hartmann sensor.In the end,a method of simultaneous measurement of piston errors between multiple apertures is proposed and verified by a demonstration.First,different types of optical synthetic aperture imaging systems are summarized and reviewed,the characteristics of existing phasing methods are summarized.These contents reveal the characteristics of piston error and the state of phasing methods.Second,based on the above works,the thesis focuses on the research of the co-phasing with the dispersed fringe sensor.For the first time,the relationship between main factors and key parameters of the dispersed fringe sensor are analyzed by the expermental research.The result of resolution study shows that the dispersed fringe sensor has the ability to meet the requirement of the fine phasing.A method for maintaining the measurement accuracy at low signal to noise ratio(SNR)is proposed to estimate piston error.The experimental results show that the method has the advantages of strong anti-noise ability and high precision.Third,a dispersed Hartmann sensor is designed and implemented,and is composed of several dispersed fringe sensors for synchronously detecting piston errors in multi sub-apertures.A set of experimental system of optical synthetic aperture imaging is established.By analyzing the influence factors of the dispersed Hartmann sensor,the dispersion elements are matched with the experimental setup.It is the first time to realize the simultaneous white light closed-loop co-phasing of the optical synthetic aperture imaging system with a dispersed Hartmann sensor.In this experiment setup,after close-loop cophasing and the residuals of the piston errors are steady,the RMSs of the piston errors that are measured by the dispersed Hartmann sensor are less than 20nm(working waveband: 700nm~800nm).And the full width at half maximum(FWHM)of the far-field main peak for the seven mirrors is 32.1% of one single small active mirror’s FWHM,and reaches 1.07 times theoretical FWHM with a 50 mm apertureFinally,a method is proposed,called Mach-Zehnder interferometer with optical path modulation and lens array(MZIOPMLA).The scanning white-light interferometry signals between all sub apertures and reference aperture are achieved by the MZIOPMLA.With the scanning white-light interferometry,the measurement range of the MZIOPMLA is limited by modulation range,not by the features of light.This is due to that the scanning white-light interferometry does not suffer from phase ambiguities and has high accuracy.At the same time,this allows the MZIOPMLA to gain high accuracy of piston error.The componment amplitude interferometry of the MZIOPMLA makes it fit for both segmented and multi-aperture telescopes with hundreds sub apertures.Compared extracting piston error from the interferogram,the scanning white-light interferometry with the focusing of the lens improves the signal intensity.The feasibility of the MZIOPMLA is verified by experiments.The demonstration shows that measurement range of this method is larger than 1 mm,and the measurement accuracy is less than 1/10 wavelength.