Research On The Method Of Wavefront Imaging Based On Liquid-crystal Microlens Array

With the development of imaging science and technology,high performance imaging means based on information visualization and high-capacity transmission,has attracted more and more attention.Lightwave is a kind of mediums utilized to output,transmit,and acquire image information needed.Generally,imaging process is a typical course,in which the intensity distribution of the lightfield compressed by used imaging lens is recorded by photo-electronic chip employed.Lightwave carries not only the intensity information based on the amplitude expression of waved lightfields,but also other mode information including:spectrum,wavefront,polarization,and wavevector,which are closely related with targets and circumstance.It is very important to acquire those information mentioned above for efficiently detecting and identifying targets in complex background.Wavefronts with respect of images can be used to reveal the disturbance characteristics of mediums,and quickly restore or optimize the target's images via wavefront correcting.Traditional imaging setups are usually used based on measuring single light intensity or amplitude,so,it is particularly urgent and important that a new generation of smart multi-mode imaging detectors.Because of excellent controllable electro-optical properties,liquid-crystal(LC)materials show special value in the development of advanced functional imaging devices.In this thesis,the research on wavefront imaging method based on electrically controlled liquid-crystal microlens array(LCMLA)has been carried out.The main contents are as follows:Firstly,according to the basic physical properties and typical performances of a type of commercial LC materials used,several simulations containing LC molecular director distribution,electric potential distribution,and phase delay distribution of the LC devices are obtained,and then the electrode layouts of the LC devices are designed according to the simulations.The reasonable structural and technological parameters are acquired through experimental trials,and the principle samples are fabricated.Both the optical and electro-optical properties and core parameters are acquired through establishing measurement architecture,so as to lay a concrete foundation for optimizing the structure and parameters of the LC propertype.Secondly,wavefront imaging technology is developed based on the LCMLA fabricated.The traditional wavefront imaging systems perform wavefront measurement and image acquisition through two independent lightpaths,so as to demonstrate some disadvantages,such as large volume,heavy weight,complex structure,and low imaging efficiency.The LCMLA without any voltage signal can only generate a certain phase retardation of incident beams,which cannot influence the next opto-electronic imaging detection of photosensitive array.When applying a driving voltage signal over LC device,it will present desired controlling-light behaviors based on arrayed microlenses,and further can be used to measure wavefront,so as to achieve simultaneous measurements of both the wavefront and image via switching on or off the LCMLA.Considering the case that the wavefront is based on single wavelength,the influence of the wavelength region width to wavefront measurement accuracy is discussed,and then the characters of composite wavefront such as white light and monochromatic wavefront such as red,green,and blue light,are compared,and finally the method of mixing several monochromatic wavefronts to shape composite wavefront is proposed.Because the wavefronts obtained by separating or mixing special wavefronts and the final target's wavefronts are highly similar,a method using white light source to implement wavefront imaging is also proposed.To the problem that the imaging system cannot be used to obtain clear images of the target outside the depth of field(DOF),directly,when turning off the LCMLA after achieving wavefront measurements,a method of extending the DOF of the LCMLA-based wavefront imaging system is proposed.Through adjusting the driving voltage signal applied over the LCMLA,targets with different objective distance can be conducted a secondary-image based on adjusting the LCMLA,and then clear images of the targets outside of the DOF can also be acquired through simple pattern process.The proposed DOF extension method can present more target details than that obtained by traditional methods.Finally,due to the problem that the wavefront is insensitive to the target's depth so as to be difficult to get depth information of targets with relatively large size,the method of measuring the depth of the objective space is carried out.Based on the property of electrically tunable focal length of the LCMLA,a relationship between the mean-root-square voltage signal and the target depth is established.By using Sobel edge detection operator to evaluate the imaging efficiency of the arrayed sub-images shaped by the LCMLA,the optima driving voltage signals for clearly imaging are obtained.The depth calibration by tuning the driving voltage signal is also carried out.The results show the effectiveness of the method for effectively measuring target's depth.