Fiber optic temperature sensor based on image processing of intermodal interference pattern

Abstract

Few-mode interference patterns produced by a fiber optic are composed of lobes whose intensity and displacement in the fiber cross-section are dependent on external disturbances like temperature and strain. This paper presents an image processing algorithm which computes the lobes coordinates and records their trajectory with respect to time. It is then numerically and experimentally demonstrated that the trajectory length of path is proportional to the temperature integrated all along the fiber (only thermal disturbance is considered). The heating and cooling phase discrimination issue is solved by inserting a cyclically heated fiber sample in the fiber under test and by analyzing the associated trajectory impact. A frequency-domain analysis has been used for suppressing vibration-induced disturbances. A low-cost experimental trial setup and the corresponding results are showing an accuracy of 1°C.

Fiber-Optic Surface Temperature Sensor Based on Modal Interference

“…Figure 1 a corresponds to a number of modes larger than 100. Using a few-mode configuration obtained by modal filtering, the interference pattern is only composed of one or two lobes that are easy to process (see Figure 1 b) [ 18 , 19 ]. The number of modes interfering in the pattern depicted in Figure 1 b is lower than 40.…”

“…The continuous computation of the center of mass of the pattern results in a measurement of the pattern modification. It has been shown in [ 18 ] that this indicator is linearly linked to the temperature deviation integrated along the fiber. The “deviation” is taken from an initial condition of the integrated temperature measured along the fiber.…”

“…When applied to underground power cables, the solution enabled the continuous integrated monitoring of the infrastructure temperature so that any local overheating of a cable joint between 20 °C and 60 °C could be quantified with an accuracy of 1 K [ 17 ]. In order to determine the heating and cooling phase, a cyclically-heated fiber section was used [ 18 ]. The fields of application of this solution are either linear infrastructure (1D systems) monitoring, surface (2D systems) temperature sensing as proposed in this paper and 3D monitoring, such as gas or complex volume sensing.…”

“…Compared with other fiber-optic temperature sensors, based on the principles of Raman and Brillouin, which require expensive devices, the proposed intermodal interferometric technique only requires cost effective equipment such as fixed wavelength sources and common imaging devices. The sensitivity to temperature variation measurement is proportional to the fiber length [ 15 , 16 , 17 , 18 ] and can reach the mK range for a fiber length of a few hundred meters of G.652 fiber. The major drawback of the interferometric intermodal technique is its self-referenced nature, which makes it more suitable for measuring temperature deviations from an initial condition.…”

“…Compared with previous works, the technique has been enhanced by a new cooling/heating phase discrimination. This functionality was previously made possible by means of a complex cyclically heating and cooling device [ 18 ]. This complex equipment has been replaced by proper image processing, which is applied in specific optical modes and filtering conditions.…”