Key Technology Of Liquid Crystal Phase Control Array And Its Application

With the development of optoelectronic technology of laser in recent years, the control, modulation, deflection and detection of laser beam has been one of the most important essential keys in effecting the application of laser technology into optoelectronic technology. Due to the advantage of low operating voltage, small volume, little consumption and ease of controlling, liquid crystal possess great application potential in many laser spatial light modulation fields and afford an effective tool to be applied in many scientific or engineering research fields. As one class of liquid crystal spatial light modulator, liquid crystal optical phased array has attracted many research interests in the nonmechanical bean steering of laser beam because of its high efficiency, wide angle, high precision, good reliability and fast response beam steering compared to that of mechanical homologues. Although Raytheon company has fabricated prototype of liquid crystal optical phased array, research of many key issues should be implemented to further develop the liquid crystal optical pahsed array and enhance its optical performance. In the mean while, the domestic development of LCOPA was still on the way. This theis was focused on the key issues that influence the optical performance of LCOPA and discuss the potential application of related LC devices.The objective of this thesis was to solve key issues that limit the optical performance of LCOPA, including the device physics, structure optimization, new type of liquid crystal for LCOPA and control method of phase profile in LCOPA for beam steering. The fabrication of a prototype was the basis of this thesis and was tightly associated with the fabrication techniques, which was not mature enough for the domestic fabrication factory. In fabrication process, cascaded IC driver and time dependant addressing based on TFT was proposed to increase the aperture. Casvaded IC driver was experimentally demonstrated to double the effective optical aperture of liquid crystal optical phased array.Deivice phsyics and structure optimization was studied in this thesis. We analyzed the deflection angle and diffraction efficiency by numerical method which was based on the Eular-Lagrange equation and scalar integration theory. Key issues such as material parameters and structure parameters were discussed to optimize the LCOPA device to obtain maximized deflection angle and diffraction efficiency simultaneously. It was found that the optical performance should be enhanced by reducing dielectric anisotropy, increasing optical birefringence or reducing the cell thickness. It was indicated that the nonlinear correlation effect between adjacent pixies plays an important role in effecting the optical performance of LCOPA and induced a tradeoff between the deflection angle and diffraction efficiency. The nonlinear correlation induced flyback region on the phase reset point was also discussed. Threshold pixel pitch was proposed to indicate that the optical performance does not neccesorily increase with reducing the pixel pitch of LCOPA. In another aspect, both transmissive and reflective LCOPA was compared to elucidate their advantage and disadvantages.Two categories of new type liquid crystal materials wsa also focused. The first type was high birefringence liquid crystal, whose birefringence could be as up as0.42. Based on this material, new LCOPA device was fabricated to weaken the nonlinear correlation effect. The second type was fast response polymer network liquid crystal (PNLC), which could achieve submillisecond response speed and was mostly focused. The key issues that limit the phase modulation application of PNLC include light scattering and dynamic response characteristics. The polymerization effect on the light scattering and dynamic response of PNLC phase modulator was studied. The polymer network morphology was found to take charge on the optical performance of PNLC phase modulator, which was mainly divided into cross-linked fibrils and cross linked fibril bundles. Cross linked fibrils induced smaller light scattering intensity but can not endure high applied voltage. In contrast, cross linked fibril bundles could endure higher applied voltage but usually induced more light scattering intensity. There may be a tradeoff between the light scattering and high voltage endurance. It was believed that fast response PNLC optical phase array and other spatial light modulator would be achieved by controlling the morphology of PNLC based on optimizing the molecular structure of monomer, liquid crystal solvent and most important polymerization conditions.The accurate control of phase profile in LCOPA was essential to enhance the diffraction efficiency and deflection accuracy. In this chapter, we developed two methods to control the phase profile accurately. The first was based on the measurement of phase profile. The actual could be investigated based on our proposed experimental setup. The deviated pahse profile between the measured phase profile and the ideal phase profile was selected as the evaluation function to achieve maximum diffraction efficiency. The objective of this method was to minimize the difference between the actual phase profile and ideal phase profile, which correspondingly induced maximum diffraction efficiency. The second method was based on optimization control. The electro-phase delay characteristic was divided by the sum of crossed polynomials. The coeeficients of the polynomials was controlled by the SPGD optimization algorism to generate maximum diffraction efficiency and updated electro-phase delay characteristic. The diffraction of the blazed order was selected as the evaluation function. In the near future, the above two methods will be combined to obtain fast control of phase profile in LCOPA for beam steering. As shown in chapter2, the occurrence of disclination lines in LC optical phased array will deteriorate the optical performance greatly.In this chapter, a new structural configuration was proposed to eliminate the occurrence of disclination lines. Due to the complicated and expensive fabrication procedure, this proposal was verified by a similar liquid crystal microlens array (LCMLA) in which the occurrence of disclination lines limited its focal performance. Based on this new configuration, liquid crystal microlens array with improved optical performance and good focus tunability was demonstrated. Furthermore, the improved liquid crystal microlens array was employed to test the wavefront aberration dynamically, which verified the feasibility of the LCMLA to be applied in the dynamic Hartmann like wavefront aberration sensor.Based on the study of fast response polymer network liquid crystal, we attempted to fabricate fast response phase modulator and corresponding optically addressed spatial light modulators, by which beam steering was demonstrated. Some questions were also discussed to improve optically addressed liquid crystal spatial light modulators, such as to optimize the writing beam intensity, wavelength and AC voltage frequency.