Research On Liquid Crystal On Silicon Spatial Light Modulator And Liquid Crystal Filter

With the development of optical communication technology,tunable all-optical network is becoming one research focus,in which spatial light modulator(SLM)and optical filter with tunable central wavelength and bandwidth are the key optoelectronic devices.As the core device in filtering system,the liquid crystal on silicon(LCoS)device has the advantage of high diffraction efficiency,high resolution,high refresh rate,high integration level,and so on.Based on the wide use in optical communication system of liquid crystal,several types of liquid crystal are used in filters,such as cholesteric liquid crystal,blue phase liquid crystal,ferroelectric liquid crystal,and so on.Filters based on liquid crystal show the advantage of simple fabrication process,high tunable range,high flexibility,and so on.Among the parameters of LCoS,diffraction efficicncy is one of the most important parameters,which is related to the aperature rate,phase depth,flyback region,and so on.The low aperature rate,low phase depth and large flyback region will result in the low diffraction efficiency,which will limit the wide application of LCoS in communication system.In addition,polarization-independent LCoS is most desirable and several methods to realize polarization-independence have been developed,such as utilizing two orthogonal polymer network liquid crystal layers,stting external reflective polarizer,using polarization-independent liquid crystal material,and so on.However,these methods are still inevitable to lead to some other issues,including high driving voltage,complicated fabrication process,and so on.In the aspect of liquid crystal tunable filter,cholesteric liquid crystal is the dominant liquid crystal because of the large helical pitch length but with the problems of low tuable speed and high driving voltage.Among the methods for broadening the bandwidth of cholesteric liquid crystal(CLC)filter,the electrically induced and photoinduced bandwidth broadening will result in distortion of the Bragg reflection band,and the multi-layer stacking and spatially tuning need a complicated fabrication process.Therefore,the realization of polarization-independent high-performance communication band LCoS spatial light modulator and communication band tunable liquid crystal filter with low driving voltage and high filtering characteristics need to be further studied and explored.Based on the existing problems of LCoS,this paper proposes a high phase depth modulation method.The influence of the high phase depth modulation method on the performance of LCoS SLM is analyzed on the basis of simulation and experiment,and the performance of the high phase depth modulation method is compared with that of the conventional phase depth modulation method.The diffraction efficiency of 6? modulo modulation is 34% higher than that of 2? modulo modulation at the deflection angle of 4.5°.The diffraction efficiency of 4? modulo modulation is 28% higher than that of 2? modulo modulation at the deflection angle of 4.5°.To improve the polarization independence characteristic of the LCoS SLM,a new polarization-independent low driving voltage bluephase liquid crystal on silicon SLM structure is proposed.This structure multiplies the optical path length of the light beam in the liquid crystal layer and greatly increases the phase depth.The phase depth and polarization independence characteristic are measured and analyzed in simulation and experiment.With the structure,the operation voltage can be lowered to 5.5 V in simulation and 5.9 V in experiment for 2? phase modulation at 1550 nm.The value of the polarization dependence is only 3.09%.In order to solve the problems of low central wavelength tunable speed and high driving voltage of tunable liquid crystal filters in communication band,a tunable filter in communication band based on sphere phase liquid crystal is proposed.The central wavelength of Bragg reflection can be thermally tuned from 1580 nm to 1324 nm with a temperature-gradient of 42.7 nm/K.Meanwhile,the central wavelength can be electrically tuned over 76 nm within a low operating electric field of 0.3 V/?m.Among the methods for broadening the bandwidth of cholesteric liquid crystal filter in communication band,the electrically induced and photoinduced bandwidth broadening will result in distortion of the Bragg reflection band,and the multi-layer stacking and spatially tuning need a complicated fabrication process.In this paper,a polymerization process with standing treatment is proposed,which makes a distortion-free filter available.And the realization process of the filter with sectionalized polymerization process is simple.The reflection bandwidth of the CLC filter can be broadened from 120 nm to 220 nm,and the central wavelength of polymer-stabilized CLC filter can be thermally tuned from 1614 nm to 1460 nm with a stable wide bandwidth.In summary,this paper studies the technology of high performance LCoS SLM in communication band and liquid crystal filter with tunable central wavelength and bandwidth,including high phase depth LCoS SLM,polarization-independent blue-phase LCoS SLM,large temperature gradient and low driving voltage central wavelength tunable filter based on sphere phase liquid crystal in communication band and bandwidth tunable CLC filter based on sectionalized polymerization process.The performances of these devices are measured and analyzed both in simulation and experiment,and the performances are improved by using these methods.The main research results of this paper are as follows:1.To solve the existing problems of SLM,this paper proposes a high phase depth modulation method to improve the diffraction efficiency,analyzes its effect on the SLM and compares the modulation method with the conventional phase depth modulation method.By comparing the diffraction efficiency of the high phase depth modulation method with that of the conventional phase depth modulation method,the beneficial effects of the high phase depth modulation method on the diffraction efficiency of LCoS SLM are summarized.2.In order to make the macro-isotropic material blue phase liquid crystal available for the low-voltage driven liquid crystal on silicon spatial light modulator,a novel low-voltage polarization-independent blue phase liquid crystal on silicon spatial light modulator structure is proposed.By setting two light guide Aluminum reflectors outside the spatial light modulator,the total reflection of the light beam in the liquid crystal layer can be realized,and the optical path length of the light beam can be multiplied.By forming an inclined electric field on the pixel electrode,the electric field direction is parallel to the propagation direction of the light beam,so that the polarization-independent characteristic can be realized.And finally the polarization-independent blue phase liquid crystal on silicon spatial light modulator can be implemented.3.In order to solve the problems of the existing tunable liquid crystal filter in communication band,such as low central wavelength tunable speed and high driving voltage,this paper proposes a tunable liquid crystal filter based on sphere phase liquid crystal.The center wavelength of the device can be controlled by temperature and electric field,and a sphere phase liquid crystal filter with large temperature gradient and low driving voltage is achieved.4.Some problems exist in methods for liquid crystal filter bandwidth broadening,such as band distortion and fabricated process.A sectional polymerization process with standing treatment is proposed in this paper,and a tunable polymer stabilized cholesteric liquid crystal filter based on the polymerization process is obtained.The bandwidth of the polymer-stabilized cholesteric liquid crystal filter can be broadened by the holding treatment without distortion.The central wavelength of polymer-stabilized CLC filter can be thermally tuned with a stable wide bandwidth.The filter can realize distortion-free filtering,and the fabrication process is simple.The main innovations of this paper are as follows:1.A high phase depth modulation method to improve the diffraction efficiency is proposed,and liquid crystal material with large birefringence and large dielectric constant is developed.2.A polarization-independent blue phase liquid crystal on silicon device with Al reflection films is investigated,and an inclined electric field made by periodical gradient voltage is applied to match the light propagation direction.And then a polarizationindependent blue phase liquid crystal on silicon with low operation voltage can be obtained.3.A polymer stabilized cholesteric liquid crystal filter with sectionalized polymerization process is proposed.The reflection bandwidth of the filter can be broadened without distortion,and the polymer stabilized cholesteric liquid crystal filter can be thermally tuned with a stable wide bandwidth.