Broadband Diffraction Imaging Method With High Efficiency Based On Phase Encoding

Traditional optical systems are limited in many fields,such as aerospace,military and civil,because of complex structure,huge volume,strict requirements of fabrication and assembly.Phase-type diffractive lens can modulate the incident light by the surface microstructures,so as to realize the the function of imaging.However,due to the mechanism of interference and diffraction imaging,phase-type diffractive lens is seriously affected by chromatic aberration and off-axis aberration,and can only work at a single wavelength and axial field of view,which greatly limits its application.In this thesis,a broadband diffraction imaging method with high efficiency based on phase encoding is proposed.Based on this method,a phase coded diffractive lens(PCDL)with wide spectrum,large field of view and high diffraction efficiency is proposed.By changing the phase distribution of the conventional diffractive lens(CDL)and combining with image restoration algorithm,the high resolution imaging with wide spectrum,large field of view and high diffraction efficiency can be realized with a single diffractive device.The surface tolerance of PCDL is analyzed,which proves that the structure has good error tolerance,compared with CDL,it can greatly reduce the difficulty of fabrication.The main contents and results of this thesis are as follows:Firstly,the imaging principle of wavefront coding and traditional phase-type diffractive lens are introduced.Based on scalar diffraction theory,the light field distribution model of diffractive lens at focal plane is established.The wavefront coding diffractive imaging system is introduced and the wavefront coding technology is introduced into the CDL.The theory of encoding and decoding of the PCDL are discussed in detail.Secondly,based on the theory of phase encoding,a PCDL of diameter of 10 mm,a focal length of 100 mm and a phase encoding parameter of 30? is designed.The imaging characteristics of PCDL are simulated.The results show that the effective bandwidth and field of view of the PCDL are 50 nm and ±8°,respectively.Subsequently,the PCDL as mentioned above and a CDL(?=0)with the same parameters are fabricated by the technology of laser direct writing.The resolution and diffraction efficiency of the CDL and PCDL are tested by the detection optical path.The results show that PCDL can image clearly in a wide spectrum(50 nm)and a wide field of view(-8°?8°)with over 82%diffraction efficiency,which are in significant contrast over the CDL and are in good agreement with the theoretical predictions.Finally,the effects of longitudinal and transverse errors on point spread function,modulation transfer function and diffraction efficiency are analyzed.The results show that PCDL is insensitive to fabrication errors,and can achieve high-quality imaging even in a range that deviates from the theoretical value(random height error ±36%,overall height error 68%,ring width error 2.5%).Compared with CDL,the tolerance of the fabrication errors has been greatly improved,and has significant practical value.