Research On Optical Poling And Frequency Doubling Effect Of Germanosilicate Glass

Germanosilicate glass is one of the functional glass materials with high performance.Because of its stable chemical and mechanical properties,as well as its photosensitive characteristic which means the refractive index can be changed by ultra-violet writing,the germanosilicate glass has been widely applied in photonic devices.Including the germanosilicate glass,the isotropy in many kinds of functional glass materials can be broken by poling process.By this way,second order nonlinearity(SON)can be obtained in these glass,and make them suitable for applications of SON devices such as electro-optic switch and frequency converter.Among all the poling manners,optical poling only needs the irradiation of a relatively stronger mixed laser beam consisting of fundamental(1?)and frequency-doubled(2?)lights.The realization of optical poling is easy and most importantly,there is an intrinsic structure in the optically poled germanosilicate(OPGS)glass which has the spatial period for quasi-phase-matching(QPM).Thus there is no need for extra technical process for frequency-doubling.On one side,accompanied by the existing silicate fibers,the OPGS glass is convenient for the realization of all-fiber short-wavelength laser system;on the other side,the OPGS glass can be applied in optical information technology as optical modulator,optical storage and other photonic devices.Therefore,the OPGS glass is a significant functional glass material with high potential application value.During the development of researches on SON in poled glass,the thermal poling manner has become more and more popular for its larger nonlinear coefficient and significant nonlinear effects.But one deficiency of the thermal poling manner is that it doesn't fit the QPM condition,so the spatial QPM period must be built by mask electrode or ultra-violet writing after uniformly poling.In addition,thermal poling requires more on experimental settings.With the development of intense laser and optical fiber,although the nonlinear coefficient induced by optical poling is smaller,it can be made up by increasing the light propagation distance.Thus,the OPGS glass is still demanded and valuable to study.This thesis studies the frequency-doubling and optical poling characteristics of OPGS glass in both theory and experiments.Based on the optical poling platform and frequency-doubling testing platform,many characteristics are studied,including the angular and polarization characteristic in frequency-doubling process,and the influences of ultra-violet irradiation,poling time,poling intensity and poling polarization state in optical poling process.The main research contents for this thesis include:1.Based on the classic model of optical poling and photosensitivity of germanosilicate glass,the forming process of inner electric field in germanosilicate glass,as well as the nonlinear coefficient and QPM process in OPGS glass are investigated.In addition,influences of other physical mechanisms on optical poling are discussed,including the regular orientation of poles and optical beating phenomenon.When the germanosilicate glass is irradiated by l? and 2? lights in poling process,third order photochemical reaction occurs in the material and free electrons are generated.Since the frequencies of the lights are counteracted,these electrons turn tobe direct current and then inner electric field appears.Because of this inner electric field,accompanied by the third order nonlinearity in glass,SON can be observed in the material.Based on the analysis on symmetry of the SON coefficient,it is obtained that the OPGS glass has the SON characteristic of medium with point group of C?v.Based on derivation of efficient nonlinear coefficient,the expression of the relationship between optical electric field of poling beam and inner electric field is built.Based on the analysis on QPM process,it is demonstrated that the SON coefficient periodically turns over in a form of sine function along the propagation direction.Besides that,based on the derivation of regular orientation of poles,the contribution of this physical mechanism on SON is obtained,as well as the process that it makes the inner electric field deviate from the classic model predictions.In addition,the contribution of the optical beating phenomenon on regular orientation of poles is discussed as well.2.Optical poling platform and frequency-doubling testing platform are built in this thesis,and optical poling and frequency-doubling effect are realized.By analyzing the test data,it is demonstrated that the poling conditions and testing manners are effective.The samples adopted in the experiments are slices from fiber preforms.By testing their transmission spectrum,it is proved that the samples possess high transmittance and have no absorption peak in tested spectrum.The frequency-doubled and ultra-violet lights in the experiments are generated by corresponding nonlinear crystals,and their irradiation intensity are monitored.The frequency-doubled output are measured by an imaging system and CCD.By measuring the beam profile,spectrum,relationship between the output and input intensity,it is demonstrated that the optical poling and frequency-doubling effect are realized.It is also given in the thesis that the relative value of SON is expressed in I2/I12.3.The germanosilicate glass is optically poled with 1? and 2? lights polarized parallel and equaling in intensity.By testing the frequency-doubling properties of OPGS glass,it can be obtained that when the deviation between 1? light direction and poled region as well as the variation of QPM period are ignored,the rotating angle about the symmetry axis has no effect on the frequency-doubled output of OPGS glass.However,the rotating angle about direction perpendicular to the symmetry axis has significant effect on the output of OPGS glass.The relationship between output 2?intensity and input 1? polarization state in OPGS glass is in a form of trigonometric function.When the input 1? polarization direction is parallel to the poling 1? and 2?lights,the frequency-doubling efficiency is the highest,but when the polarization direction is perpendicular,the frequency-doubling efficiency is the lowest but not zero.By theoretically analyzing,consistency between experimental results and theoretical predications is obtained,which proves again that the OPGS glass is rotational symmetric.4.By changing the poling conditions,the optical poling characteristics of germanosilicate glass are investigated.The results show that the laser induced SON gets stronger as the poling time lasts,but finally it will saturates.Under the experimental conditions,the saturating time is about 12 hours.While the poling effectiveness is strengthened during the poling time,the new symmetry induced by poling process doesn't change.Given that the doping concentration of Ge is 30%and the poling intensity used in the experiments,the optical poling process doesn't need the presence of UV light.And no significant enhancement is observed with UV irradiation.The SON in OPGS glass is reversible.There is a threshold for poling intensity in poling process,beyond which the poling level rises fast with the poling intensity.By the study on the optical poling process,it is known that the OPGS glass has potential application values in optical information technology because its SON is reversible.With its photosensitivity,the OPGS glass can be used as recording material for laser intensity.5.In this thesis,the polarization dependence of the germanosilicate glass in optical poling is studied individually.By introducing beam-splitting-and-combining(BSC)light path,polarization controlling optical poling platform has been built,which can accurately adjust the coaxiality and synchronicity of 1? and 2co lights.Poling level and poling direction as functions of polarization angle between 1? and 2? lights in poling beam are experimentally tested.The results show that the poling level is the highestwhen the 1? and 2? lights in poling beam are polarized parallel.As the polarization angle changes,two valleys appear in the poling level curve.When the 1? and 2co lights are polarized perpendicular,the poling level is the lowest.The poling direction as function of polarization angle is nonlinear,and odd.By comparing the experimental results and theoretical expectations,they are qualitatively consistent,but different in quantity.Thus the results indicate the correctness of classic model,but on the other hand as described above,the optical poling process is complicated because there are influences of other mechanisms such as regular orientation of poles.By analyzing and studying the frequency-doubling process and poling process of OPGS glass,it is proved that the OPGS glass is a SON material with point group of C?v like uniaxial crystals.The SON coefficient in medium is distributed in form of sine function with spatial period of QPM.Therefore,the OPGS glass is naturally suitable for QPM frequency-doubling.By discussing its reversible SON,frequency-doubling characteristics,poling characteristics and polarization dependence,the potential application values of OPGS glass in photonic device is proved.