Mirror Reflectometer Of Very High Reflectivity

This research paper is aimed at the measurement of high reflectivity of laser mirrors for Ring Laser Gyroscope. At present, the most advanced technique for this field is to measure the total loss of a passive resonant cavity in which the mirror to be measured is consisted. If the loss of all the other mirrors is known, the loss (including transmittance, absorption and scatter) of the special mirror can be deduced. Three methods for loss measurement of a passive resonant cavity are involved in this paper.In chapter one some simple remarks are made of the development of the technique for high reflectivity measurement of optical coatings. As the highly reflecting mirrors have been developed rapidly, the traditional methods of reflectivity measurement cannot fill the satisfactions more and more. The so-called optical cavity decay time method has proved to be the most suitable technique for measuring the very low losses of optical coatings now available. It is an abnormal method for loss measurement, the lower the loss of the cavity, the easier the measurement. Another method is specially mentioned there, which is capable of making crosscheck with the optical cavity decay time measurement.In chapter two the frequency response function of a passive resonant cavity to an Amplitude-Modulated laser is deduced while the laser is frequency-stabilized to the longitudinal mode of the cavity by dithering method. Under this condition, the amplitude ratio of the output AM intensity of the cavity to that of the incident light, or the phase difference between them is measured with the Lock-in Amplifier. The data are fitted to a theoretical function by Least-Square method, then the half width of half maximum the cavity is obtained as a parameter in the theoretical function, and the total loss of the cavity is deduced. By this method, the total loss of (977.0?.0) ppm of the cavity is obtained.In chapter three the optical cavity decay time method with cw laser is realized. At first, some very special attention is paid to the EMC design to eliminate the disturbance of the optical switch to other elements, especially to the intensity receiver, in the system. This is the key step to proceed. After that, the experimental setup is constructed to measure the total loss of a passive resonant cavity. Lots of measurements are made and it is realized in the end that it is the temporal characteristic of the power source of the optical switch to confine the precision and reliability of the experimental results.In the end of the paper, a third method is employed to verify the correction of the result of the AM method, with which the longitudinal mode frequencies of the cavity are pushed forth and back, the transmittance spectral form is read out from the oscillator scope, and the half width of half maximum of the cavity is obtained. But this is true only when the pushing frequency is very slow.