![]()
Part I
Photonic and Optoelectronics Sensors
![]()
1 | Optical Fiber Sensors Devices and Techniques Rogério Nunes Nogueira, Lúcia Maria Botas Bilro, Nélia Jordão Alberto, Hugo Filipe Teixeira Lima, and João de Lemos Pinto |
CONTENTS
1.1 Introduction
1.2 Intensity-Based Sensors
1.2.1 Transmission and Reflection Schemes
1.2.2 Spectrally Based Sensors
1.2.3 Evanescent Wave–Based Sensors
1.2.4 Self-Reference Techniques
1.3 Phase-Based Sensors
1.3.1 Phase Detection
1.3.2 Mach–Zehnder
1.3.3 Michelson
1.3.4 Fabry–Perot
1.3.5 Sagnac
1.4 Wavelength-Based Sensors
1.4.1 Multiparameter Sensors
1.4.2 FBG Inscription Methods
1.4.3 Interrogation of FBG Sensors
References
1.1 INTRODUCTION
The research on optical fiber sensors produced and still continues to give life to a variety of measurement techniques for different applications. The technology is now in a mature state, with different applications already using commercial optical fiber sensors as a standard. This includes not only massive deployment for real-time structural health monitoring in airspace, civil, and oil industry but also more specific applications, such as environment monitoring, biochemical analyses, or gas leak monitoring in hazardous environments.
The success of this technology relies on the intrinsic flexibility, low weight, immunity to electromagnetic interference, passive operation, and high dynamic range, associated with remote monitoring and multiplexing capabilities, which allow these optical fiber sensors to succeed in difficult measurement situations where conventional sensors fail.
Optical fiber sensors operate by modifying one or more properties of the light passing through the sensor, when the parameter to be measured changes. An interrogation scheme is then used to evaluate the changes in the optical signal by converting them to a signal that can be interpreted. In this way, depending on the light property that is modified, optical fiber sensors can be divided into three main categories: intensity-, wavelength-, and phase-based sensors.
1.2 INTENSITY-BASED SENSORS
Intensity-based sensors represent one of the earliest and perhaps the simplest type of optical fiber sensors. They offer the advantages of ease of fabrication, low price–performance ratio, and simplicity of signal processing. These make them highly attractive, particularly in applications where the cost of implementation frequently excluded more expensive optical fiber systems. Grating and interferometric sensors allow high-resolution measurements, but this is not always necessary and less costly intensity-based sensing methods may offer an option in industry.
A wide number of intensity-based sensors are being presented and developed using different schemes; still they can be grouped in two major classes: intrinsic and extrinsic. In the extrinsic sensors, the optical fiber is used as a means to transport light to an external sensing system. In the intrinsic scheme, the light does not leave the optical fiber to perform the sensing function. Here, the fiber plays an active role in the sensing function, and this may involve the modification of the optical fiber structure.
1.2.1 TRANSMISSION AND REFLECTION SCHEMES
An additional classification scheme is related to the way the optical signal is collected. If the receiver and emitter are in opposite ends of the fiber(s), the sensor is of a transmission kind, otherwise it is of a reflection kind. An example of the first situation is the dependence of the power transmitted from one fiber to another on their separation [1]. With respect to reflection, most methods use reflecting surfaces to couple the light again in the fiber [2]. Other sensors are based on Fresnel reflection mechanisms [3,4] or special geometries of the fiber tip [5].
Intensity variation detection can also be performed through bending. If the bend radius is reduced below a critical value, the loss in the transmitted signal increases very rapidly, allowing the construction of a macrobending optical fiber sensor. These devices can be used to measure parameters such as deformation [6], pressure [7], and temperature [8].
Among the transmission and reflection systems reported, there are several transduction mechanisms, namely, evanescent wave–based sensors and spectrally based sensors.
1.2.2 SPECTRALLY BASED SENSORS
Spectroscopic detection has been a reliable method for the design of optical fiber sensors and is popularly used for chemical, biological, and biochemical sensing [9]. When a properly designed sensor reacts to changes in a physical quantity like refractive index (RI), absorption, or fluorescence intensity, a simple change of optical signal can be correlated to the concentration of a measurand [10].
Generally, the design of the sensors can simply comprise optical fibers with a sample cell, for direct spectroscopic measurements, or be configured as fiber optrodes, where a chemical selective layer comprising chemical reagents in suitable immobilizing matrices is deposited onto the optical fiber (Figure 1.1).
In its simplest form, the technique involves confining a sample between two fibers and the quantification of the light transmitted through the sample [11,12]. The fiber can play an active role acting as a sensing probe. The activation can be accomplished replacing the original cladding material, on a small section or end of the fiber, by a chemical agent or an environmentally sensitive material. There are a wide number of sensors that make use of this technique. Goicoechea et al. [13], using the reflection method, developed an optical fiber pH sensor based on the indicator neutral red. Regarding humidity sensing, most spectroscopic-based configurations are based on moisture-sensitive reagents (such as cobalt chloride, cobalt oxide, rhodamine B) attached to the tip of the fiber, usually with the aid of a polymeric material to form the supporting matrix [10]. Also, SiO2 nanoparticles were pointed as a po...
