Birefringent Crystal Depolarization Wave Superposition Analysis And Optimization

Depolarizer is a type of polarization devices that can realize the transformation frompolarized light to unpolarized light. Along with the development of polarizing technology andlaser application, depolarizers have an increasingly wide utilization in optical detection andcommunication system, such as astronomical instruments, laser processing, laser medicine,optical fiber communication, etc. So it is significant to study the performance of depolarizer andits optimal design. In this paper, the depolarizers made by crystal birefringent, namely crystalbirefringent depolarizers, are selected as the major objects. Such a depolarizer can’t transformincident wave to unpolarized light in real sense, but cause the periodic variation of transmittedpolarization states, and it seems as depolarization for average effect when the cycle is smallenough. Therefore the depolarizer is a type of pseudodepolarizer. Using the birefringentcharacteristics of crystal, the crystal birefringent depolarizer realize the depolarization of spatialor frequency domain. The Lyot depolarizer which is the earliest depolarizer belongs to thefrequency depolarizer and the others belongs to the spatial depolarizer. Operation principle of thefrequency depolarizer bases on the different phase difference caused by the transmittedpolychromatic lights and the different phase differences lead to the different polarization states.The behavior of the superposition of the different polarization states is just depolarization. Thedepolarization of the spatial depolarizer based on the continuously varying spatial phasedifferences caused by one or more crystal wedge surfaces (short for crystal wedge). Though thedifferences between the above two depolarizers, their depolarization principle can be expressedas the same as the superposition of lights of different polarization states caused by the differentphase differences.For a long time, matrix is adopted as the analytical method of depolarizers, includingMueller matrix and coherency matrix. Though their scientificity is certainly reliable, they stillhave some inevitable defects, such as complicated procedure and abstract physical meaning. Toovercome the defects, considering directly the wave properties of lights, we put forward ananalytical method based on superposition of lights. The method avoids the complicatecalculation caused by the excessive matrix elements, moreover, most of the formulas andconclusions to analyze elliptically polarized light can be cited adequately by the procedure, andthe reasonable and flexible treatments of the phase in addition, the mathematical procedure ismuch simplified. The procedure has very clear physical meaning for every analysis sequence isrelated directly to the optical process of depolarization. So the new method can disclose somedepolarization mechanisms that difficult to be covered by the matrix method.This paper mainly includes the following research contents in three fields: 1. Using the superposition of lights method, four existed depolarizers were analyzed,including the improved Lyot depolarizer, the Lyot depolarizer, the HV depolarizer, and the dualBabinet compensator depolarizer. The contrasts between it with the traditional methods weregiven.2. Using the superposition of lights method, four new designed depolarizers were analyzed,including the optical axes of crystal fan-shaped-distribution depolarizer, the different propertiesof forward-reverse applications depolarizer, the quarternary optimal design depolarizer, the Lyotmonochromatic depolarizer, among which the last two are the optimal designs. Based on thestudy conclusions of the superposition of lights, combined the advantages of the dual Babinetcompensator depolarizer, the optical axes of crystal fan-shaped-distribution depolarizer and thedifferent properties of forward-reverse applications depolarizer, the quarternary optimal designdepolarizer can achieve perfect depolarization. By further discursion based on the studyconclusions of the Lyot depolarizer by the superposition of lights, the Lyot monochromaticdepolarizer was derived to realize the depolarization of monochromatic linearly polarized light,whose performance is much better than depolarizer acted by one quarter-wave plate.3. Experiment system was set up to test the performance of samples made by quartz crystal.The main test aim is the degree of depolarization (short for D), including the variation of the Dwith azimuth angle and incident angle. In addition, lots of related experiments were conducted inthe exploratory stage. For example, the study of distribution regularities of transmitted light spots,and the dependence of the D on the size of round diaphragm aperture, wedge angle, laser wavelength, incident angle in different plane, and the dependence of the degree of visibility ofsuperposed stripes on the azimuth angle and the position of optical screen.The major innovations of this paper include the following areas:1. The idea and method of the superposition of lights are put forward to analyze depolarizer.2. The new significant conclusions were derived from the analysis of the existeddepolarizers, such as the D of the improved Lyot depolarizer and the HV depolarizer for linearlypolarized light were deduced firstly, the different ideal conditions of depolarization from someearly literature were united, etc.3. Four new depolarizers were designed and analyzed, among which the quarternary optimaldesign depolarizer and the Lyot monochromatic depolarizer can be applied widely for their ownsuperiorities.4. The dependence of the D on the wedge angle was studied through experiments, and the“saturability” of the D on the wedge angle was put forward as a result, which is an importantground for the design and machining. The “edge effect” of the D which is dependent on thewedge angle and the position of optical screen were also mentioned. 5. Based on the analysis of the superposition of lights, the ideal condition of depolarizationof the crystal birefringent depolarizer was concluded, which will be a fundamental theory usedfor the design and analysis of such depolarizer.6. The dependence of the D on the incident angle was studied through experiments, and takeminor adjustment of incident angle as the way to improve the D.