Optical Fiber Sensors Based on Nanostructured Coatings

Abstract

Optical fiber sensors have been developed from the late 1960s, when optical fiber was proposed as a practical medium for communication [1, 2]. Since then, a great effort has been dedicated for the design and development of optical fiber sensors. In fact, the use of this technology to fabricate sensors is very attractive because optical fibers make possible large sensor data capacities over long distances (kilometers). This implies that the sensing head can be very far from the electronic unit that processes the information. In addition to this, the optical fiber is made of dielectric materials which make possible to incorporate these devices in circumstances where high electromagnetic fields are applied, such as in medical magnetic resonance or in situations with high radiation doses [3]. Besides, optical fibers are made of biocompatible materials. Therefore, this technology is very suitable to develop biomedical instrumentation. Other advantages with respect to conventional sensors are that several sensors can be multiplexed in the same optical fiber or even distributed sensing along a fiber cable can be achieved [1, 2, 4]. Basically, a standard communications optical fiber is a cylindrical dielectric waveguide which consists of a core surrounded by a cladding. The fiber can transmit light along its axis due to the phenomenon of total internal reflection, which promotes the confinement of the optical signal in the core. This total internal reflection happens because the refractive index of the core is greater than that of the cladding. From an electromagnetic waveguide perspective it can be analyzed and seen that the light energy in the fiber is not completely confined to the core. Instead, a significant fraction of the energy travels in the cladding as an evanescent wave. Some optical fiber sensors are fabricated by coating the optical fiber core with a sensing cladding. This sensitive cladding is easy to excite by means of the evanescent field [5]. Thus, the interaction between light and matter is governing the sensing mechanisms of these devices.