Determination of sub-nanometer distance from reflective surface by Fabry-Perot interferometer

The method for determination of slight change of an object’s distance from a reflective surface is proposed and experimentally verified. The method is based on interference of light using Fabry-Perot configuration where the first reflection is from the optical fiber end-air interface and the second reflection is from the surface. The evaluation is based on comparison between the reference measurement and the measurement for which we determine the change in distance. The optical power difference (in dB) of two measurements is obtained and for known value of refractive index of the environment between the optical fiber end and the reflective surface, the ratio of intensities of interfering beams and the distance from reflective surface, the change of distance could be determined from comparison of measured and computed dependences. The smallest possible change of the distance to observe by the method is approximately 30 pm. The accuracy of determination of the distance is ± 20 pm.