Theory And Applications Of Optical Feedback Cavity Ringdown Technique

At present,the high-reflective optical components with reflectivity higher than 99.9% are widely used in laser systems,such as laser gyro,gravitational wave detection,laser fusion and so on,and its reflectivity is directly related to the performance of the whole laser system.Therefore,it is very important to accurately measure the reflectivity of high-reflective optical components for their product quality control and performance improvement.Cavity ring down technique is the only way to measure the ultra high reflectivity,which is insensitive to the fluctuation of light sources,and an absolute measurement technique with high measurement accuracy.And for the optical feedback cavity ring down technique,the system is simpler,the cost is lower,and the measurement accuracy is high enough.Therefore,this paper mainly studies the theory and application of optical feedback cavity ring down technique,including:(1)Based on the multi-beam interference theory,the output ring down signal of optical cavity was analyzed by transmission function of the linear cavity and folded cavity.The optimal analysis of high reflectivity measurement instrument was carried out form the aspects of the stability condition,the ringdown time fitting algorithm,the detector system bandwidth and so on.And the effects that the test cavity length is not consistent with the intial cavity length and the coupling mode of detector on ultra high reflectivity measurement were also analyzed.(2)Based on the Maxwell theory,polarized reflectivity of multilayer dielectric thin films was studied.The average cavity reflectivity on different incident polarization direction were simulated and experimentally measured.A novel technique simultaneously measuring polarization reflectance method of high reflective elements with out any polarizing optical elements was developed,and the two experiment systems at 633 nm and 1064 nm were set up.Compared with the traditional measurement method,the measurement error is about the ppm order.In addition,two sets respectively based on cavity ring down technique with a HeNe laser as the light source and optical feedback cavity ring down technique with semiconductor laser as the light soure were built for comparative analysis.(3)A novel method based on cavity ring dwon technique was developed to simultaneously measure the optical loss,residual reflectivity and transmittance of anti-reflective optical components with transmittance higher than 99.9%.The effect of composite cavity on measurement accuracy was also analyzed theoretically.A combined spectrophotometry and cavity ringdown technique was employed to measure the transmittance of optical components in a range extending from below 0.01% to over 99.999%.A set of antireflective samples with different transmittance were measured with thes experimental apparatus,compared with the traditional commercial instrument.The results show that the measurement uncertainty is as low as 1ppm for the transmittance higher than 99.99%,and the uncertainty of measurement is about 0.003%,which is close to 0.013%.The combined technique is capable of measuring the transmittance of any practically fabricated optical components,and has the advantages of simple structure,high measurement speed and high measurement precision.(4)A dual channel cavity ringdown technique was developed to measure or map the reflectance,transmittance,and optical loss of high-reflective or high anti-reflective optical compponents with high accuracy.The mesuremnt setup was established and acquisition frequency and scanning step were optimized.One-dimensional or two-dimensional scanning measurements of three high-reflective and two anti-reflective optical componets were performed,and the uniformity of optical componnets was also analyzed.The experimental results show that the measurement precision reached sub ppm level.For the first time,the cosine value of the phase delay of the laser in the cavity is presented as an objective evaluation index.The closer this cosine value is to 1,the better the cavity is aligned.(5)A variable lengh cavity ringdown technique was develped to not only measure the extinction coefficient of gas,but also to measure the optical loss of cavity mirror.An automatic setup was established,and the cavity lengh control,data acquisition and processing were also performed with LabVIEW virtual instrument software.The extinction coefficients of laboratory environment with different particle concentrations were measured at 635 nm.In addition,the simultaneous measurements of cavity loss at 635 nm and 1064 nm were performed,and the measurd ratio of sactttering scattering loss coefficient was consistent with the theoretical analysis results based on the Mie scattering theory.