Research On Laser Birefringent Feedback

Through optical feedback technology, the modulation of intensity andpolarization state of laser outputting is realized in the outside of resonant cavity.The laser internal nature is researched by the variation of the resonant outsidecondition. The content of laser physics will be enriched based on laser feedbackresearch. Laser anisotropic feedback, which is an important branch of laserfeedback technology, modulates the intensity and polarization states of theorthogonally polarized laser. Some parameters, such as phase retardation,displacement and so on, can be demodulated by analyzing the lasercharacteristic, based on the fact that the objective information is contained inthe feedback light. This technology can be applied to the field of precisionmeasurement. The phenomena of laser anisotropic feedback are researched inthis thesis. A new physical model which reveals the physical nature of feedbackphenomena is built. Some experimental phenomena have been instrumented andapplied to the online detection of industrial production. Some new experimantalphenomena are found and the potential applications of the phenomena areresearched. The main works of this thesis are summarized as follows.The phenomenon that the orthogonal polarization states of laser will lockinto together and the polarization flipping will not be observed when theanisotropic of optical component placed in feedback cavity is little. This is thelocking phenomenon of polarization flipping. It is an obstacle of the applicationof the measurement technique based on the optical feedback effect. A newtheoretical model which integrates the model of equivalent cavity ofFabry-Perot interferometer and Lamb’s semi-classical is built to analyze thelocking phenomenon of polarization flipping. The new physical model explainspolarization flipping and locking phenomenon perfectly. The theoretical modelis universal and the modeling idea can be applied to any feedback areas.Based on the analysis result of the new theoretical model, a scheme whichincreases the frequency difference between the orthogonal polarized laser todecrease the mode competition is given to release the locking of polarizationflipping. The experimental result shows that when a glass with very smallinternal stress is placed into the feedback cavity, the polarization flippinghappens. The system without the locking phenomenon of polarization flippingmay be used to measure the stress of optical component. This will expand theapplication field of the laser anisotropic feedback instrument. The relationship between the flipping point displacement and phaseretardation of optical component placed in feedback cavity is analyzed and themathematical relationship between them is built. The phase retardation of waveplates can be measured based on the relationship by analyzing the polarizationflipping points. A set of wave plates have been measured by the laser feedbackinstrument and frequency splitting instrument and the results of them arecompared. The results show that the phase retardation error between these twoinstruments is smaller than0.5o. This is an important application of the laserfeedback phenomenon.The optical, mechanical, electric circuit and algorithm systems of laserfeedback setup which can fulfill the industrial requirements are designed aftereliminating the polarization flipping locking and building the relationshipbetween the polarization flipping point and phase retardation. The laserfeedback setup has been instrumented. Some experts are invited to evaluate theinstrument performance and the test report is generated. The report shows thatthe measurement repeatability of the instrument is better than0.05o. The bootreplicability is better than0.019o. The measuring range is between0and360o.The application of laser anisotropic feedback in the online measurement ofwave plates manufacture is researched. Some problem are found and resolved.In wave plate manufacture, the thin slices of birefringent crystals are attached toa tray using optical cement for grinding and polishing. The phase retardation ofthe wave plate is measured while it is attached to the optical cement tray. If theresults do not meet the required phase retardation value, the wave plate can bere-ground and re-polished for modification. After meeting the required phaseretardation value, the wave plate is scraped off the optical cement tray. Theprecision of the wave plate phase retardation measured on the optical cementtray determines the wave plate quality. To manufacture high-precision waveplates, high-precision measurements are required at the manufacture stage ofwave plate. The polishing precise of wave plates polished on the same opticalcement tray is about2nm. The phase retardation deviation due to the polishingprecise is1.3°at the wavelength of632.8nm. However, the phase retardationdeviation of wave plates on the same optical cement tray is almost8o measuredby laser feedback instrument. The measurement deviation on wave platemanufacture can be subjected to the influence of optical interference effects. Anelimination structure for the optical interference effects of the wave plates hasbeen designed. The reflectivity at each of the different interfaces of the structureis tiny and the interference effects of the wave plates can be eliminated. The accuracy of wave plate manufacture can be improved greatly on the basis of thiswork. The light source of laser feedback instrument a single wavelength of632.8nm, thus, just only the phase retardation of wave plate under632.8nm ismeasured. If the phase retardation under other wavelengths needs to know, theconversion method must be used. In this process, the refractive index precisedetermines the conversion precise. In this thesis, a method for multi-wavelengthconversion is introduced. The conversion error is eliminated by the systemcalibration. The maximum error after conversion is0.26degrees. Through thismethod, the laser feedback instrument designed for a single wavelengthmeasurement can be expanded to multi-wavelength field. Temperature doesinfluence phase retardation by changing the refractive index and thickness of awave plate. In this thesis, a new nonlinear model is developed to describe thephysical mechanism underlying the temperature dependence of the phaseretardation. Whereas the refractive index is modeled using Sellmeier’s equation,wave-plate thickness is expressed as a quadratic function of temperature.Combining the two gives the temperature dependence of phase retardation in ourmodel. A set of temperature control devices is designed to modulate wave platetemperature. Based on the experimental data, the coefficients in the Sellmeier’sequation are parameter fitted. Once established, changes in phase retardation atany temperature can be calculated using the equation.The instrument for phase retardation measurement based on laser feedbackeffect is applied to industrial production. The instruments are applied to onlinemeasurement of wave plates manufacture in Castech Co., Ltd and FuzhouTongchan optoelectronic technology company. Beijing Institute of IOLmeasures phase retardation of KN crystal by this instrument.The application of laser feedback instrument in internal stress measurementis researched. Internal stress is ubiquitous in material, which is an inherentconsequence of the crystal lattice defects, processes, assembling, andtemperature gradient. Internal stress of materials and elements makes systemsusing them detracted. Therefore it is important to know the magnitude anddistribution of stresses in materials. In this thesis, the internal stress is measuredby laser feedback instrument. The vacuum tubes with optical cement stress havebeen measured and the measurement results have a reference value to airrefractive index measurement. The measurement results of stress distribution ofNd:YAG crystal are significant to improve laser quality. The measurementresults of stress of GaN semiconductor, for improving manufacturing process ofGaN semiconductor, have very important significance. Some new physical phenomena are found. These phenomena include:1.Thickness and refractive index are fundamental to the characterization ofmaterials in optical devices. A simultaneous measurement method of thickness dand refractive index n for birefringent materials is proposed in this thesis basedon laser feedback phenomenon. Results show that the precise of thickness is59nm and refractive index is0.0006. The problems are analyzed when thetechnique is used to measure the refractive index and thickness of KN crystal,liquid crystal and isotropic materials. On the basis of accurate model, thistechnique can simultaneous measure the refractive index and thickness ofuniaxial crystal, biaxial crystal and isotropic materials.2. The position ofpolarization flipping will be modulated when the wave plate which is placed inthe external cavity rotates. The position of polarization flipping is sensitive tothe rotating angle. We build a relationship between the position of polarization,or the phase retardation and the rotating angle of wave plate. The high-precisesmall-angle measurement is realized by detecting the variation of phaseretardation.3. When the optic axis direction is not consistent with the initiallaser polarization direction, the polarization states of laser output will changefrom linear polarized to elliptical polarization. The polarization states are highlysensitive to the relative position of optic axis and the initial laser polarizationdirection. Through detecting the polarization state of laser output, the optic axisazimuth can be determined. The system is low cost, fast response, compact, andpower effective.