Thermodynamic Limitations to the Measurement of Phase Shifts in Optical Fibers

Abstract

In discussions of the noise limitations of coherent optical sensors it is usually assumed that in absence of externally applied temperature or pressure variations, the optical path length of a single mode fiber has a perfectly well defined value. This is not strictly true. At any temperature above absolute zero, there will be statistical fluctuations of all the physical properties, including the optical path length, and this will set a lover limit to the measurable phase shift due to an external influence. These fluctuations are as unavoidable as Johnson Noise; they are significant for the fiber because of its small size. In this paper the magnitude and spectral distribution of these fluctuations are calculated. Some aspects of this problem have recently been discussed by Shelby, Levenson and Bayer (1). The present treatment approaches the problem in a slightly different way.