IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 49, NO. 1, FEBRUARY 2000 25 An Intrinsic Fiber Optic Temperature Sensor Giovanni Betta, Associate Member, IEEE, and Antonio Pietrosanto Abstract—This paper deals with a new fiber optic temperature sensor. The relationship between temperature and refractive index of a “reference” liquid is used to obtain that the sensing fiber output follows temperature changes of an external fluid with very low re- sponse time. The fiber optic sensing element and the low-cost mea- surement hardware are described in the paper together with the first experimental results. Index Terms—Fiber optics, instrumentation, liquids, optical transducers, temperature measurement. I. INTRODUCTION T TEMPERATURE sensors cover a meaningful part of today’s world sensor market, due to the availability of many physical principles, wide range of performance, and numerous application fields [1]. Nevertheless, unsatisfied re- quirements still exist in constrained sectors, whenever specific needs in terms of costs, performance, size, or reliability, do not receive fully satisfactory answers by using the available temperature sensors. This is the case of temperature measure- ment of fluid in the presence of significant electromagnetic disturbances. In this framework, in fact, traditional solutions such as thermocouples and RTD’s do not assure satisfactory performance. As far as innovative solutions based on optical fibers are concerned, they implement a number of different measurement principles based on the temperature dependence of the following. 1) Optical characteristic of a semiconductor, located be- tween a source and a receiving fiber [2]. 2) Chromatic properties of an inorganic solution in which the sensing fiber is immersed [3]. 3) Behavior of Bragg-grating fibers, namely fibers with a suitable grating impressed on the core [4]. 4) Transmission/reflection spectrum of a multi-layer dielec- tric fiber [5]. 5) Spectrum of luminescence emission of a phosphor [6]. 6) Micro-deformation of suitable material used to modulate the light in the fiber [7]. 7) Loss effect of cladding and jacket [8]. In any case, they are all characterized by noticeable costs. In this paper, the authors propose a low-cost temperature sensor, whose sensing element is a probe made of an uninter- rupted optic fiber connecting a light source with a receiver. In the middle of this path, a small part of the fiber cladding is Manuscript received May 14, 1998; revised October 27, 1999. G. Betta is with the Department of Automation, Electromagnetism, Informa- tion Engineering, and Industrial Mathematics (DAEIMI), University of Cassino, Cassino 03043, Italy. A. Pietrosanto is with the Department of Information and Electrical Engi- neering, University of Salerno, Fisciano, Italy (e-mail: pietrosa@diiie.unisa.it). Publisher Item Identifier S 0018-9456(00)02231-2. substituted by a suitable “reference” liquid, whose refraction index versus temperature characteristic is known. This allows the temperature of the fluid, in which the probe is immersed, to be simply evaluated by a measurement of the fiber output power. The sensing element and the measurement hardware of the designed prototype are described in the following together with the first experimental results. II. DESIGNED PROTOTYPE A. TSensing Element As is well known, in an optic fiber, rays are guided or leaky depending on whether the field phase is real or not (evanes- cent). In a cylindrical coordinate system the evanes- cence of the field depends on the solutions to the following so-called eikonal equation [9]: (1) where is the refraction index and having posed Real solutions correspond to real field, while imaginary solutions give an evanescent field. Once and are fixed, solutions depend on the fiber index profile. Let us consider the step index profile shown in Fig. 1, where the term is drawn versus radius for two different couples is the core radius, and are the core and cladding refractive indices, respectively. Case a): The real field is present only in a circular crown inside the fiber core: rays are guided. Case b): The eikonal equation gives real solutions also out- side the core: rays are leaky. If influence parameters make core or cladding refractive in- dices change, guided rays may become leaky or vice versa leaky rays may become guided, depending if gets closer to or far- ther from As a consequence, once the excitation conditions have been fixed, the output power depends on the cladding and core refractive indices. The nearer the former are to the latter, the more the leaky rays and so the lower the output power, which is mainly due to the number of guided rays. Temperature is the main influencing parameter for the refrac- tive index of fluid substances [10]. In particular, the tempera- ture coefficient is almost always negative (with values typically equal to 4 10 K ) and often very much higher than the temperature coefficient of the refractive index of silica cores ( 1.28 10 K ) and of the clad. In fact, manufacturers usu- ally assure that the ratio between core and clad refractive indices is constant in a wide temperature range (e.g., 25 to 125 C for the fiber used in the experimental tests). 0018–9456/00$10.00 © 2000 IEEE