Thermal noise limitations in u fiber optic seismic sensor

Abstract

Optical fibers are neither sensitive nor selective as a transduction medium. The high sensitivity of most fiber optic sensors has been a consequence of the fact that optical interferometric demodulation is capable of resolving phase changes of order 1 μrad/Hz. In a fiber sensor of 10‐m length, this is equivalent to a resolution of one part in 1014 during a 1‐s observation using light of 900‐nm vacuum wavelength. For comparison, a good quality 1‐in. condenser microphone can resolve a relative change in diaphram‐to‐backplate separation of one part in 108 during a 1‐s observation. This detection advantage of 120 dB has obscured many “sins” in several fiber optic sensor designs. This presentation will concentrate on the sensitivity limitations imposed by the fluctuation‐dissipation theorem [L. D. Landau and E. M. Lifshitz, Statistical Physics (Addison‐Wesley, Reading, MA, 1969), 2nd ed., Chap XII] at nonzero temperatures and will demonstrate that the thermal limitations are entirely analogous to fluctuations observed in “mirrored galvanometers” that also use “optical leverage” to increase sensitivity. The results will be applied to the performance of a seismic sensor with a sensitivity that is limited by its intrinsic thermal noise. [Work supported by the Office of Naval Technology and the Office of Naval Research.]