Thermal Noise-Limited Fiber-Optic Sensing at Infrasonic Frequencies

The fundamental thermal noise in fiber-optic sensors remains to be an interesting research topic. In particular, its spectral behavior in the infrasonic frequency range has yet to be observed experimentally. In this paper, we assess the feasibility of probing the thermal noise floor at infrasonic frequencies with two sensor configurations: 1) fiber Fabry-Perot strain sensors and 2) Mach-Zehnder-Fabry-Perot hybrid phase sensors. In each case, we compare the theory-predicted thermomechanical noise (the dominant thermal noise in fibers at low frequencies) with other potential system noises such as laser noises and detector noises. Our analysis indicates that the thermal-noise-limited fiber-optic sensing can be very difficult to achieve with the strain-sensing scheme, but much more feasible with the hybrid phase sensors.