Surface Scattering Measurement Of Optical Components Based Optical Cavity Ring-Down Technique

Surface scattering of laser optical elements has become one of the important factors limiting the development of more and more optical systems.Surface scattering often represents the surface topography of optical elements.Therefore,accurate measurement of surface scattering is conducive to the selection or processing of optical elements.Cavity ring-down technique has the advantages of high measurement accuracy,low cost and not affected by light source fluctuations in measuring the surface parameters of optical elements（such as reflectivity and transmittance）.At present,optical cavity ring-down technology has been able to achieve accurate measurement of high reflectivity,high transmittance and optical loss,but scattering loss,as one of the important parameters of optical elements,is rarely mentioned in optical cavity ring-down technology.Therefore,based on the optical feedback optical cavity ring-down technique,a multi-channel optical cavity ring-down device is built in this thesis,which can meet the surface scattering measurement of high-precision optical elements.It includes the following contents:（1）The principle of optical cavity ring-down technique for measuring high reflectivity and high transmittance is introduced.Based on this,the theory of multi-channel optical cavity ring-down technology is derived by combining the idea of spectrophotometry.This theory uses the transmission signal of cavity mirror to calibrate the scattering,reducing the measurement error caused by traditional calibration methods.The relationship between surface scattering and surface topography is derived.（2）Based on the optical feedback optical cavity ring-down experimental device,the scattering measurement module is added,and the optical feedback optical cavity ring-down experimental device for measuring the surface scattering of optical elements is built,which can measure the surface scattering of high reflectivity samples（R>99%）in a certain solid angle.At the same time,through the analysis of the optical system,the influence of the cavity mirror on the measured scattering value is studied,and an improved method is proposed.From the results,the measured scattering shows good accuracy and repeatability.（3）The sensitivity of system scattering measurement is explored.In the scattering measurement,the photomultiplier tube is used for the experiment.The gain ratio between the photomultiplier tube and the photodetector is calibrated by attenuating the sample transmission signal and the scattering signal.Finally,the sensitivity of the system scattering measurement is calculated.The optical cavity ring-down technology improves the measurement sensitivity through the power accumulation characteristics of the ring-down cavity,and the influence of ambient stray light and other background noise on the scattering measurement is greatly eliminated through the time characteristics of the scattering ring-down signal.On a laser with an output power of 8m W,the scattering measurement sensitivity of1.0×10^-12is obtained experimentally.