Research On The Key Techiniques For Improving The Imaging Performance Of Fourier Telescope

With the increasing of the number and importance of satellites,we need to identify their types and monitor their status.At present,most of the mainstream optoelectronic detection telescopes are passive.In order to achieve a high resolution imaging(0.5m)of medium orbit or high orbit(36,000 km)objects,the effective aperture of telescope is about 36 m(@0.5 ?m).Such a large aperture passive telescope will must adopt advanced techniques such as complex active optics and adaptive optics to overcome the effects of self-weight and atmospheric turbulence.Its technical difficulty and cost will be very challenging.Therefore,we need to explore new imaging techniques,hoping to find the ones which can obtain legible images of medium orbit and high orbit objects.Fourier telescope is a promising new imaging technique to meet the above needs.Fourier telescope has the advantages of active imaging,synthetic aperture,immunity to low-order atmospheric turbulence and low optical quality receiving mirror.The advantages mentioned above make the Fourier telescope seem to be a perfect optical imaging system for imaging the medium orbit or high orbit objects.However,after study and analysis in detail,it also has some limitations:(1)The laser coverage area is often several times larger than the imaging area.If there are other objects outside the imaging area,the laser echoes caused by other objects may cause the aliasing imaging of interested objects;(2)The intensity fluctuation and asynchronism of laser pulses and the asynchronism of received signals caused by the longitudinal depth of object may result in blurred reconstruction;(3)The imaging time of the conventional three-beam Fourier telescope is relatively long(about 6-8 hours),so it can only image the three-axis stable satellite in geostationary orbit,and can't image the unstable satellite(rapid attitude change)or the geosynchronous satellite of which the subastral point track is similar to the shape of "8";(4)For imaging of high-orbit(36,000 km)object,the transmitting power(energy)of single-beam laser is extremely high(the energy is about 40 J mentioned in GLINT program of US).At the same time,better coherence(coherence length > 1 m)is needed to form interference fringes on the object surface.This demand for high-energy,high-coherence and high-beam-quality makes the manufacture of laser very difficult.The research work of this paper focuses on the above problems to carry out a detailed analysis and demonstration and then propose feasible solutions.The main work of this paper includes the following:1)The basic principle of Fourier telescope is introduced,and the theoretical basis that Fourier telescope can break through the diffraction limit of traditional imaging is expounded by using wave optics theory;the phase closure technique is introduced,and the basic principle of eliminating atmospheric time delay and non-common path optical path difference of the transmitting system is expounded;the method of spatial spectra estimation of the object is introduced,and it is proposed that the estimation of object spectra can be obtained by using the multiplication of three product with continuous distribution after phase closure;the common baseline layout forms of Fourier telescope are introduced,and the relationship between the baseline layout and the systematic imaging indexes(resolution and field of view)are pointed out,and then the baseline form adopted in this paper is introduced;the image evaluation method is introduced,and the evaluation method used in this paper combining intuitive observation and image Strehl ratio is also introduced.2)This paper introduces the research foundation of key technologies of Fourier telescope,including indoor experiment,outfield experiment of round trip 200 m and outfield experiment of round trip 600 m.The principle and performance of key devices are also introduced,including the laser,the acoustooptic frequency shifter and the optical delay line.The final performance of these devices is demonstrated and verified by experiments.3)According to different application requirements,the layouts of launch baselines are optimized accordingly.By reducing the shortest baseline length,the imaging field of view is enlarged and the suppression performance of this method on the out-of-field echoes is studied.By changing the uniform baseline layout to the non-uniform baseline layout,the number of target spectrum acquisition is reduced to reduce the imaging time.4)This chapter contains two similar but independent research contents.Firstly,the effects of intensity fluctuation and asynchronism on the emission of pulsed laser are studied.It is pointed out that the change of laser intensity with time may cause the amplitude modulation of interference fringes,which is difficult to be distinguished from the object modulation of fringes.The reason of image quality degradation caused by intensity fluctuation and asynchrony of pulse laser is analyzed theoretically.Quantitative influence results are given by computer simulation,and possible suppression schemes are given.Then the effect of echo asynchronism caused by object depth is studied.The mechanism of echo signal asynchronism affecting imaging quality is analyzed in depth.Quantitative conclusions and possible mitigation schemes are given through simulation.5)The feasible methods to suppress noise of Fourier telescope are studied from both software and hardware aspects.The analysis results show that the signal-to-noise ratio can be significantly improved by software de-noising of the received signal before demodulation.Three software de-noising methods(median filter,Wiener filter and compressed sensing de-noising)adopted in the analysis have achieved significant improvement in the signal-to-noise ratio.The practical results of the three methods are tested by field experimental data,and it is found that compressed sensing de-noising has a better de-noising performance.Hardware methods such as ultra-narrow band filter,electronic ultra-narrow band filter and detector cryogenic refrigeration can significantly improve the detection ability of the system and suppress the noise impact.