Phase-Coded Brillouin Optical Correlation Domain Analysis for Extreme Temperature Sensing

Abstract

Nowadays predictive maintenance is considered a key feature for industrial applications, so that unscheduled shutdowns of plants and machinery can be avoided and their lifetime extended. In this framework there is an increasing demand for sensing systems able to provide a detailed mapping of temperature/strain with high accuracy (±1°C) and high spatial resolution (in the order of a few centimeters) on relatively short lengths around 100 meters. As an example, for the monitoring of industrial boilers and steam headers, running at constant operating temperatures of 400–600 °C, no commercial sensors are able to provide distributed real-time temperature information with the desired spatial resolution. In this frame, Phase-Coded Brillouin Optical Correlation Domain Analysis (phase-BOCDA) [1] technique can prove to be a promising solution. The present work aims at demonstrating the ability of a fiber-optic phase-BOCDA sensor, described in [2], to reliably reconstruct temperature profiles up to 600°C with a 2-cm spatial resolution. One of the main advantages of phase-BOCDA is that, unlike FBG-based solutions, it does not require a-priori information about hot-spot positions, as the monitored locations along the sensing fiber can be selected on demand, providing a great flexibility. To withstand extreme operating temperatures (up to 700 °C) a special Fiber in Metal Tube (FIMT) solution, comprising one loose Single Mode Gold-Coated fiber enclosed in a protective metallic tube made of Incoloy 825, has been chosen as the sensing cable. Being a high nickel content material, Incoloy 825 can indeed guarantee good thermal conductivity as well as high mechanical strength. During preliminary high-temperature tests, substantial modifications were observed in the sensor response after successive heating cycles, thereby stating the need of performing a suitable annealing protocol [3] on the gold-coated fiber itself, before being inserted in the metal tube. Annealing ensures that any residual stress is removed from the sensing cable, thereby guaranteeing good measurement repeatability.