Liquid core optical fiber temperature sensors

In this section, we derive a simple model for the evanescence-based liquid fiber sensor. The model, based on weakly-guiding theory, assumes no dependence on the length of the sensor fiber. We will see that despite the simplifying assumptions, we obtain a fairly accurate model that matches the experimental results.
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1. Introduction
Fiber optic sensors have been an area of interest for rcsc:irchcrs for more than a decade and several specific scnsors for the measurement of strain, temperature, electric current, pressure and vibration have been developed [ 11. Tlic basic idea of modulating some property of the light 1m)p;igatingthrough an optical fiber has been exploited in sevcrnl different ways; the principle of operation of a specific sensor depends on the modulation method being used. The most sensitive methods have incorporated the modulation of (he phase of light propagating through the fiber and intcrfering the sensing beam with an unaffected reference be:tm to give rise to constructive and destructive interference paiterns. While phase-modulated sensors are among the most sensitive, their implementation requires complex stabilization nicthods and very often this additional process makes such
LIQUID CORE OPTICAL FIBER TEMPERATURE SENSORS
Marten de Vries, Bemd D. Zimmermann,t Ashish M. Vengsarkar, and Richard 0. Clam Fiber & Electro-optics Research Center, Bradley Department of Electrical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0111. FIMOD Corporation, P. 0. Box 11192, Blacksburg, VA 24063. Abstract
A n intensity based, cost-effective liquid core optical fiber tcmpcrature sensor has been developed for the measurement of temperatures from 20" C to 185" C with a resolution better chnn f0.2" C. The sensor is comprised of a hollow core optical fiber filled with a liquid of a known index of refraction which is slightly higher than that of the silica tube which acts as the cladding. The refractive index fluid is chosen such that the variation of its refractive index with temperature is well defined and linear in the desired range of operation. As the temperature surrounding the liquid core fiber increases, the index of the liquid decreases in a linear fashion until i t reaches the refractive index of the cladding and the guided modes are no longer supported by the fiber. C;ilibration of the intensity changes with temperature yields a rcpeatable method to measure temperature. By using different fluids (of various indices), and different source wavclcngths, the region of sensitivity and the degree of accuracy can be varied.
sensors unattractive to a commercial user whose requirements may not be very stringent to begin with. Phasemodulated sensors are slowly being.developed for end-user application; at the same time, intensity-based fiber optic sensors, that are less sensitive than their phase-modulated counterparts, are showing promise due to the inherent advantages of fiber optic sensing methods over conventional electrical techniques, namely, immunity from electromagnetic interference, small size, geometric flexibility and the possibility of using these sensors in harsh environments. Liquid-core fibers were first investigated as possible replacements in 1972 for solid-core silica fibers which showed high transmission losses [2, 31. As the transmission losses through pure silica fibers were brought down, research in liquid-core fibers dwindled; however, in 1982, Kuribara and Takeda [4] used hollow-core fibers filled with Kerr liquids for voltage measurement and Hartog [ 5 ] followed with a distributed temperature measurement technique that used optical time domain reflectometry (OTDR) techniques to scan the backscatter form a liquid as a function of temperature. Although a distributed sensor was shown to be feasible, the high cost requirements imposed by the OTDR system makes the sensor financially impractical for low-budget applications. In this paper, we present an intensity-based temperature sensor that uses a hollow-core optical fiber filled with liquisponse. The sensor in its simplest form does not need an OTDR system and operates on the principle that as the temperature increases, the refractive index of the liquid drops; as a result, the fiber becomes weakly guiding and eventually loses all its power due to evanescence. The liquid is chosen for the temperature range in which one observes a significant drop in the intensity; the details of sensor construction and choice of liquids are given in the following sections.