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Study on Gnenration and Control of Vector Beams Based on Liquid Crystal Spatial Light Modulators

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Tutor: GuoChengShan
School: Shandong Normal University
Course: Optics
Keywords: vector beams,liquid crystal spatial light modulators,computer-generatedholograms
CLC: TN761
Type: PhD thesis
Year:  2014
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Amplitude, phase and polarization are important parameters of optical beams which can bespatially modulated. It considers only the amplitude and phase of optical beams in classicalscalar diffraction theory without any consideration for the polarization. In recent yearspolarization property has played an important role in researching the nature of light and itsinteraction with materials. The study on the polarization of optical beams makes it important todesign optical devices and optical system with polarization-selective and polarizationcontrolling characteristic. The optical beams with inhomogeneous polarization distributiongenerally named vector beams. Vector beams’ propagation and interaction with materials haveextensively extended the application in the field of biophotonics, superresolution imaging, datastorage, optical communications and optical manipulation. Because of their interestingproperties and potential applications, there has been a rapid increase of study on vector beams.How can we transform a Gaussian beam to a wanted vector beam with spatially variantpolarization is a foundational work. The objective of this dissertation is to develop a newmethod to generate and control of vector beams in theoretical analysis and experimentalresearch. The main points of this dissertation are as follows:1. The general methods to generated vector beams have been introduced. The ways togenerate vector beams classified into active way and passive way depending on whether ornot involving amplifying media. The active methods realize the intracavity polarized modeselection with aid of the optical birefringence, Brewster optical elements, polarizinggrating mirror and so on. The passive methods rely on interferometry are mainlyintroduced include the Mach-Zehnder interferometry, the Sagnac interferometry andinterferometry based on diffractive optical elements. With the rapid development ofphotoelectronic technique, the programmed optical elements are introduced into thetraditional interferometry to realize the dynamic controlling.2. Simulating the polarization states distribution of the orthogonal linearly polarizedcomponents and their superposition basing on the radially polarized beams and azimuthalpolarized beams. The analysis is extended to the vector beams not only possess the polarization inhomogeneous distribution but also consist of hybrid polarization.3. The liquid theory and the simulation the distribution of liquid crystal directors areintroduced basing on the liquid crystal spatial light modulator applied to optical beamscontrolling. According to the requirement of the theoretical analysis, the differenceiterative method is used to calculate the distribution of liquid crystal directors, take thecode: ZLI-4792for example, from which we can approach the optimal parameters of theliquid crystal cell and its iterative algorithm.4. Numerically simulating the Jones matrix of the liquid crystal cell on ‘off’ state andanalysing the wave-guided effect of liquid crystal. Numerically simulating the relationshipbetween the Jones matrix of the liquid crystal cell on ‘on’ state and the applied voltage.Revealing the relationship between the gray levels displayed on the liquid crystal displayand the applied voltage by analyzes the modulating characteristic of liquid crystal cell. Wealso simulate the results by modulating the amplitudes, phase and polarization of theincident optical beams. We numerically calculate the modulating curve of the commercialmode, pure amplitude-modulating mode and pure phase-modulating mode which canprovide the optimal parameters for the designing of the experimental system.5. We optimize the modulating characteristic of liquid crystal spatial light modulators inorder to generate and control the vector beams. Firstly we confirm that single liquid crystalspatial light modulator can’t realize the controlling of the amplitude, phase andpolarization modulating of the orthogonal polarized components directly andsimultaneously. After that a double-pass orthogonal polarization modulating mode basedon the Mach-Zehnder interferometry is proposed and we find the optimal systemparameters for this condition. At last, we present the design of cascaded liquid crystalspatial light modulators to improve the fault of the previous interferometry and discuss theoptimal system parameters in this condition.6. We present the configuration of the vector beams generating and controlling system basedon the cascaded liquid crystal spatial light modulators. We analyse the modulatingcharacteristic of the cascaded system and the design of a double pass computer generatedhologram. The analysis indicates that we can realize the double pass and independentcontrol of the amplitude and phase of the orthogonal linearly polarized components withthe help of the double pass computer generated hologram and the cascaded liquid crystalspatial light modulators. The experiment results demonstrate that the system can carry outthe designed operation on the orthogonal linearly polarized components. We achieve thevector Bessel beams which possess the non-diffractive characteristic and the vector beams with helical phase distributions which possess polarization vortices and optical vorticessimultaneously. The most important is that we can achieve the2-D control of thepolarization singularity and the phase singularity in the beam cross section. We hope thiscascaded system can provide a convenient way to study the properties of other complexvector beams and the interaction between the optical spin angular momentum and opticalorbital angular momentum in experiments.
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