Sensitivity of interferometers under conditions of intense illumination, field mismatch, and light scattering in the mirrors

The problem of ultimate sensitivity of the Fabry-Perot reflection interferometer (RI), which is used for measuring ultrasmall displacements, is considered. The sensitivity depends on the shot noise and is restricted, with the available light power margin, by the power that can be released inside the RI due to the light absorption in the mirrors. In addition, the sensitivity is affected by a mismatch between the RI and light source fields (leading to an excess shot noise) and by the light scattering in the mirrors (a favorable factor reducing the RI heating for specified total losses). An expression for the sensitivity is derived that takes into account the above factors and involves the power P _int released inside the interferometer. This makes it possible to optimize the RI parameters by minimizing P _int and maximizing the sensitivity.