Thermal Noise Limits for Optical Time Domain Reflectometry

We derive fundamental thermodynamic limits on the strain resolution of a class of fiber optic distributed acoustic sensing (DAS) techniques based on optical time domain reflectometry. The results indicate a lower limit of around 10 -14 strain/√Hz for a 2 m long (10 ns) optical pulse. These limits indicate that interrogation of unmodified fiber optic cable is unlikely to achieve sensing performance approaching that of engineered acoustic sensors.