Study On Stress Induced Birefringence Measurement With Cavity Ringdown Technique

Stress level is an important point that requires strictly controlled among the fabrication and application processes of optics,for instance,optical substrates,lenses,crystals,AR/HR coatings and so on,in order to have a good yield and maintain system functionality.The non-destructive stress measurement methods for optics includes XRD?X-Ray Diffraction?method,Stoney curvature method,micro-Raman spectroscopy method,and methods based on stress induced birefringence measurement,like digital photoelasticity method,and photoelastic modulator?PEM?method.The PEM method achieved stress induced birefringence measurement precision of 5×10^-5 rad,i.e.,stress measurement precision of hundreds Pascal level,which demonstrate the highest stress measurement precision currently.In this thesis,a stress birefringence measurement method that is based on cavity ringdown?CRD?technique is proposed to achieve more accurate measurement of stress birefringence with the help of a Fabry-Perot cavity.The optical losses of measured optics are obtained simultaneously as well.Due to stress induced birefringence of optical components in testing cavity,the analyzed ringdown signal deviates from single exponential decay.It is predicted as oscillated single exponential decay with oscillation frequency linearly related to birefringence retardation in cavity based on the multi-beam interference theory and interference theory of polarized light.The oscillation is results of mode beating of the two orthogonally polarized light because of refractive index anisotropy of the stressed optics in cavity.In addition,the analyzed ringdown signal evolved to superposition of multi-exponential and oscillated signals when reflectivities of the two orthogonal modes are different in cavity,as is the case of folded cavity setup.A 633nm continue-wave polarized cavity ringdown system for stress birefringence measurement of optics was constructed based on birefringent cavity theory.The input for test cavity is linearly polarized light with normal incidence on test optics placed in cavity.Cavity output is analyzed with a Wollaston beam splitter which results in two orthogonal polarized light.For fused silica substrates,measurement reproducibilities of phase retardation of 2.38×10^-6 rad and of optical path difference of 2.4×10^-4 nm was experimentally achieved,with directly nonlinear fit of decay signals to oscillated exponential decay model.Uncertainty analysis demonstrate that measurement reproducibility dominant in total measurement uncertainty.Simultaneous mapping of stress induced birefringence and optical loss of fused silica substrate and single layer optics are realized with a xy translation stage.The absolute measurement of stress birefringence of optics based on CRD technique is measurement of oscillation frequency rather than absolute intensity,which show lower noise level and higher measurement accuracy.The direction of principal stress,i.e.,direction of fast or slow axis of stress birefringence,can be determined with analyzer rotation method via fitted amplitudes of birefringent oscillation model or fitted decay time constants of single exponential model of ringdown decays acquired at various angles.The best analyzer configuration is found to be the angle bisector of the optical axes of intracavity birefringence,which leads to the maximum amplitude of birefringent modulation to decay signals.Maximum cavity length of a stable optical resonator is recommended for best performance of system reproducibility,even with theoretical disadvantage of less oscillation periods among cavity effective lifetime for birefringent frequency extraction in the consideration of optical loss of intracavity air.The measurement precision of stress birefringence improved to 1.2×10^-6 rad after optimization processes.System dynamic range are determined experimentally to across 10⁴ range via predictable and controllable reflecting phase shift of highly reflective mirrors at non-normal incidence.It is found that mapping results of stress birefringence of single layer mirrors have defects with fixed angle of plane polarized light as incidence,which could be avoided with circular polarization as cavity input.Furthermore,direct validation of absolute measurement of super-high reflectance and ultra-low loss of highly reflective mirrors is realized through crosschecking the reflectance results measured with fixed cavity length as well as variable cavity length CRDs.With an open path CRD experimental setup and method of filtering out of random particle noises,the absolute reflectance of a highly reflective mirror of laser gyroscope is determined to be 99.99956%with an uncertainty of only approximately 0.00002%.Residual stress birefringence of highly reflective mirrors is a major problem for high accuracy interferometry.With the improvements of coating processes,residual stress birefringence of highly reflective mirrors decreases to 10^-6 rad level,which imposes challenges to characterization methods.With such low intracavity birefringence,the oscillating period is far greater than the duration that the decay signal decayed to the noise level,which makes the direct fit of the ring-down signal to oscillated exponential decay model difficult.However,non-single exponential modulations to S polarization and P polarization with?phase shift makes it possible to estimate birefringence retardation with difference of fitted decay time constants with single exponential model.With mirror rotation method together with a differential loss approximation model,the birefringences of highly reflective mirrors manufactured under the same deposition parameters but structured with different numbers of high-and low-refractive-index?HL?layer pairs are measured with the cavity ring-down technique.The measured accuracy reached 1.3×10^-77 rad.Experimental results measured at center of mirrors show that birefringence retardation increases with the increasing number of HL layer pairs.Further measurements across the mirror surface indicate a non-uniform birefringence distribution.In this thesis,the measurement accuracy of stress induced birefringence of optics improved to 10^-6 rad level with polarized cavity ringdown technique,which is at least ten times higher than commercial birefringence measurement instrument.The proposed method with cavity ringdown technique has a promise future to the characterization and research of residual stress and birefringence of optical components.