Optical phase fluctuations thermally induced in a single-mode optical fiber

Mathematical expressions have been derived on fluctuations of optical phase and frequency of a light wave which are thermally caused in a single-mode optical fiber in terms of fiber length, thermal coherence length, and fluctuation frequency. Fluctuations of optical phase and temperature were measured at a wavelength of 0.6328 Am by Mach-Zehnder interferometry, and the results are compared with theoretical predictions. A minimum detectable temperature change through optical phase shift depends on frequency stability of a source laser. It is 10⁻⁵ K with a fractional frequency fluctuation of 10⁻⁹.