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1. Introduction
Optical fiber sensing has been applied to a large range of measurement tasks, including voltage, strain, pressure, temperature, humidity, viscosity, and chemical species [1, 2]. Fiber optic sensors are ideally suited for remote monitoring because light can be transmitted over long distances with little attenuation. Optical fibers are relatively immune to electromagnetic interference, and sensitive personnel and equipment are removed from harsh sensing environments [3]. While the most common form of fiber sensors used today functions solely at the distal end of a fiber, an evanescent-wave sensor allows for sensing over the entire length of the fiber and/or for monitoring multiple parameters simultaneously with a single fiber.
According to Maxwell’s equations, a standing wave, known as the evanescent wave, is generated outside of the fiber core when light undergoes total internal reflection at the fiber core/fiber cladding interface [4]. The evanescent wave can optically excite sensor molecules within close proximity to, but outside of, the fiber core. Similarly, fluorescence signals emitted by sensor molecules following evanescent excitation can be captured into a guided fiber mode [5]. These evanescent fields decay exponentially from the fiber core into the fiber cladding normal to the core/cladding interface. The penetration depth of the evanescent wave depends on the refractive indices of core and cladding, on the wavelength of the light, and on the incident angle of the light on the core/cladding interface. Thus, the range of the evanescent fields varies greatly for multimode optical fibers. For practical purposes, the penetration depth is on the order of the wavelength of the light. Many such sensor regions may be created along a single fiber, thus allowing for monitoring of multiple parameters simultaneously, a single parameter redundantly, or any combination thereof.
Spatially resolved readout of individual sensors can be obtained by using a pulsed excitation light source and employing optical time-of-flight detection (OTOFD). In this technique, each sensor region has a unique optical pathlength from the source to the detector, and hence, the detected fluorescence pulses will have a unique and characteristic time delay corresponding to the respective pathlength [6–9]. Spatially resolved readout is obtained by spacing the sensors along the fiber so that the fluorescence pulses do not overlap. OTOFD can be implemented with one or more optical fibers, as...