Modulation Transfer Function for Distributed Temperature Measurements Using an Optical Fiber Sensor System

Optical fiber temperature sensors are useful as distributed temperature sensors in nuclear reactors and in other applications. In order to survive the nuclear reactor environment, an optical fiber temperature sensor would need to be surrounded by a capillary tube made of a tough material, such as stainless steel. The diffusion of heat energy through this capillary tube and the resulting gap between the capillary tube and fiber reduces the resolution in measurement by the fiber sensor system of a spatially varying input temperature profile. Using an analogy from optics, this paper addresses the issue of loss of spatial resolution by developing an expression for the modulation transfer function of the sensor system. The dependence of the modulation transfer function on the spatial frequency of the input temperature profile and the dimensions and properties of the components of the fiber sensor system are determined.