Multiplexed Fiber Optic Temperature Monitoring Sensor Using Hard-Polymer-Clad Fiber and an Optical Time-Domain Reflectometer

Optical fiber temperature sensing systems have incomparable advantages than the traditional electric cable based monitoring systems. As of now, fiber Bragg grating (FBG) sensors are most popular because of its wavelength domain multiplexing capability. However, grating writing process is complex and takes long time and photosensitive fibers for the typical grating writing process are expensive. In addition, sensing systems for FBGs are also expensive. Therefore, this study proposes multiplexed fiber optic temperature monitoring sensor system using an economical Optical Time-Domain Reflectometer (OTDR) and Hard-Polymer-Clad Fiber (HPCF). HPCF is a specific type of optical fiber, in which a hard polymer cladding made of fluoroacrylate acts as a protective coating for an inner silica core. An OTDR is an optical loss measurement system that provides optical loss and event distance measurement in real time. Multiplexed sensor nodes were economically and quickly made by locally stripping HPCF clad through photo-thermal and photo-chemical processes using a continuous/pulse hybrid-mode laser with 10 m intervals. The core length exposed was easily controlled by adjusting the laser beam diameter, and the exposed core created a backscattering signal in the OTDR attenuation trace. The backscattering peak was sensitive to the temperature variation. Since the elaborated HPCF temperature sensor was insensitive to strain applied to the sensor node and to temperature variation in the normal HPCF line, neither strain compensation nor isolation technique are required. These characteristics are important advantages for the use as structure-integrated temperature sensors. The performance characteristics of the sensor nodes include an operating range of up to 120 C, a resolution of 1.52 C, a tensile strain resistance of 13%.