The Calibration Methods For The Phase Modulation Non-uniformity In The Applications Of Phase-only Liquid Crystal On Silicon Spatial Light Modulator

Phase-only liquid crystal on silicon(LCOS)spatial light modulators(SLMs)which holds the advantages of high light efficiency,high fill factor and fast respond speed,has drawn widely attention around the world in recent years.There are many applications based on phase-only LCOS SLMs,such as optical tweezers,holographic projection and display,optical switch and so on.However,during the assembling process of the LCOS device,many reasons can cause the non-uniformity in the thickness distribution of the liquid crystal(LC)layer within the device,for example,the tiny curvature of the glass front plane and the silicon back plane,the thickness variation of the glass front plane,unexpected large size particles in the LC layer.Since the thickness non-uniformity distribution of the LC layer can bring in phase retardance difference and severely affect the performance of phase modulation,it is necessary to compensate the non-uniformity of phase retardance before using the LCOS SLMs.In this Ph.D.dissertation,combining both theoretical and experimental research,we focus on the compensation method for the phase retardance non-uniformity with a parallel aligned nematic LCOS SLM.The main research processes and progresses are as follows:1.Based on the relationship between white light birefringence color in the system with crossed polarizers and the thickness of liquid crystal layer,we developed a characterization method for the LC thickness distribution.This characterization method builds up the look up table of birefringence color and LC thickness,then calculates the LC thickness distribution via analyzing the white light birefringence photo of LCOS device.This method can reach 55nm accuracy and holds the advantages of high efficiency,high stability,easy calibration,low cost,etc.2.Developed an automatic analysis software which can transform the information in the birefringence photo of LCOS device into the corresponding LC thickness distribution.The outcome of the software includes the LC thickness value for each pixel on the LCOS device.The entire calculation process including image noise reduction,resolution adjustment,look up table generation,thickness distribution calculation,data selection and interpolation,etc.With algorithm of parallel computing,the whole process can be finished in just a few seconds which shows the potential in the LCOS industry.3.Using the thickness distribution of LCOS device and the phase retardance curve measured with crossed polarizers,we used the traditional crossed-polarizers compensation method to correct the phase non-uniformity,and compare the performance before and after the compensation.The birefringence photos shows that this compensation method can correct the non-uniformity of the phase retardance between the ordinary and extraordinary light,meanwhile,proves the validity of the LC thickness distribution.4.Starting from the phase modulation theory,we established a diffraction model of binary phase gratings and simulate the influence of non-uniform LC layer.Also,we evaluate the performance of the traditional crossed-polarizers compensation method The simulation results show that the uneven thickness distribution of liquid crystal layer will make the dynamic range of the first diffraction order from 45 dB down to 12-17 dB,and the crossed-polarizers compensation method can improve it to 23 dB.This illustrates the limitation of traditional crossed-polarizers compensation method.5.Based on the essence of phase retardance,we systematically studied the compensation theories of phase modulation nonuniformity.With the method of infinitesimal analysis,we explain the phenomenon in the simulation results,and propose a new compensation method,single polarizer compensation method,which based on the equivalent phase retardance.In the case of phase-only applications,this method can completely compensate the phase retardance non-uniformity due to the uneven LC thickness distribution.This compensation method can significantly improve the accuracy of the phase modulation and the device performance.To evaluate the performance of the single polarizer compensation method,we design and build the LCOS phase-only modulation optical system.The experimental results of binary phase grating diffraction show that the dynamic range of the first diffraction order can be increased from 15-21 dB up to 38 dB with the help of the compensation method presented in this thesis,and the traditional crossed polarizers compensation method can only reach the level of 23 dB.