2. What is distributed sensing?

Distributed sensing is a technique whereby one sensor cable is capable to collect data (continuous/quasi-continuous profiling) that are spatially distributed over many individual measurement points. The various modes of sensing can be understood from Figure 1 [3, 4]. Briefly, a point sensor means monitoring a parameter at a discreet point; a quasi-distributed sensor system involves an arrangement of a finite number of discreet sensors as a linear array, while in fully distributed sensing mode, the measuring parameter of interest is monitored continuously along the fiber path, providing a spatial mapping of the parameter along fiber.

wavelength division multiplexing (WDM) are generally used for distributed sensing. In OTDR (Figure 2), a pulsed laser is coupled to an optical fiber through a directional coupler/splitter. The backscattered light originating from density and composition variation is monitored continuously in time. The spatial location of an event is determined from time of flight measurements, that is, the device calculates the distance of the measuring point based on the time it takes for the reflected light

For example, if the backscattered light is detected after 10 ns from the starting point, it is set to originate from 1 m distance from origin of fiber. This can be easily calculated from OTDR equation X = ct/2n, where X is the distance from origin (start of fiber taken as zero time), <sup>t</sup> is the time of event detection, <sup>c</sup> (3 <sup>10</sup><sup>8</sup> m/s) is the velocity of light in the vacuum, and n is the refractive index of fiber for wavelength of operation. If we use a sensing fiber with core refractive index (n) of 1.5 and wish to measure distance (X) traveled after t = 10 ns, then it is easy to calculate that

In wavelength multiplexing, a device such as Bragg grating is used to encode a series of resonant wavelengths in the fibers [12–15]. The wavelengths in turn are monitored by wavelength interrogator. The resonant wavelengths are affected by measuring parameters and are thus monitored in a quasi-distributed manner.

3. All fiber Raman optical fiber distributed temperature sensor with

areas of temperature measurements require a large area of coverage with high localization accuracy. Raman optical fiber-based distributed temperature sensors (ROFDTSs) are equipped with the ability of providing temperature values as a continuous function of distance along the fiber. In an ROFDTS, every bit of fiber works as a sensing element as well as data transmitting medium, to substitute the role played by several point sensors, thus allowing reduced sensor network cost. ROFDTSs have attracted the attention as a means of temperature monitoring and fire detection in power cables, long pipelines, bore holes, tunnels, and critical installations like oil wells, refineries, induction furnaces, and process control industries. The basic principle of temperature measurement using ROFDTS involves

Temperature sensors are ubiquitous devices that permeate our daily lives. Many

X = 1.0 m by putting the values in OTDR equation [21, 23].

dynamic self-calibration

to return.

57

Figure 2.

Schematic diagram of OTDR.

Distributed, Advanced Fiber Optic Sensors DOI: http://dx.doi.org/10.5772/intechopen.83622

In conventional sensing, say for temperature, an individual sensor such as a thermocouple or platinum probe is needed for each point of interest whereas distributed sensing addresses many points simultaneously along with their spatial location [5]. With proper design architecture, it can contribute to enhanced safety and security by providing early warnings of gas and coolant leakages, structural cracks, onset of fire, hot spot detection in pipelines, radiation leaks, etc. Two techniques such as optical time-domain reflectometry (OTDR) [4, 16–23] and

Figure 1.

Various modes of sensing: point, multipoint quasi-distributed, and fully distributed.

Distributed, Advanced Fiber Optic Sensors DOI: http://dx.doi.org/10.5772/intechopen.83622

availability of lasers, fibers, and low noise detectors in the mid-IR region, it has become possible to design novel distributed sensor devices for sensing hazardous volatile compounds for homeland security especially at airports, underground

A wide range of techniques such as intensity modulation, wavelength encoding, and polarization provide powerful sensing capabilities. Further, several detection techniques have been investigated for development of optical fiber-based distributed sensors. Radiation-induced absorption, scintillation, fluorescence, optically stimulated luminescence, and induced refractive index changes have been used for real-time dose measurements. Optical fiber grating [12–15]-based specialty sensors have been used for distributed strain measurements in very low temperature, very high temperature, or high radiation environment. Raman and Brillouin scatteringbased techniques are used for distributed temperature measurements for fire and hot spot detection. Mid-IR and near-IR absorption measurements coupled with hollow core fibers are used for leak detection of hazardous gases. This chapter will describe the basic principles, main components, various sensing systems for advanced applications, and future potential of distributed fiber sensors.

Distributed sensing is a technique whereby one sensor cable is capable to collect data (continuous/quasi-continuous profiling) that are spatially distributed over many individual measurement points. The various modes of sensing can be understood from Figure 1 [3, 4]. Briefly, a point sensor means monitoring a parameter at a discreet point; a quasi-distributed sensor system involves an arrangement of a finite number of discreet sensors as a linear array, while in fully distributed sensing mode, the measuring parameter of interest is monitored continuously along the

In conventional sensing, say for temperature, an individual sensor such as a thermocouple or platinum probe is needed for each point of interest whereas distributed sensing addresses many points simultaneously along with their spatial location [5]. With proper design architecture, it can contribute to enhanced safety and security by providing early warnings of gas and coolant leakages, structural cracks, onset of fire, hot spot detection in pipelines, radiation leaks, etc. Two techniques such as optical time-domain reflectometry (OTDR) [4, 16–23] and

fiber path, providing a spatial mapping of the parameter along fiber.

Various modes of sensing: point, multipoint quasi-distributed, and fully distributed.

metro stations, and big event areas.

Applications of Optical Fibers for Sensing

2. What is distributed sensing?

Figure 1.

56

wavelength division multiplexing (WDM) are generally used for distributed sensing. In OTDR (Figure 2), a pulsed laser is coupled to an optical fiber through a directional coupler/splitter. The backscattered light originating from density and composition variation is monitored continuously in time. The spatial location of an event is determined from time of flight measurements, that is, the device calculates the distance of the measuring point based on the time it takes for the reflected light to return.

For example, if the backscattered light is detected after 10 ns from the starting point, it is set to originate from 1 m distance from origin of fiber. This can be easily calculated from OTDR equation X = ct/2n, where X is the distance from origin (start of fiber taken as zero time), <sup>t</sup> is the time of event detection, <sup>c</sup> (3 <sup>10</sup><sup>8</sup> m/s) is the velocity of light in the vacuum, and n is the refractive index of fiber for wavelength of operation. If we use a sensing fiber with core refractive index (n) of 1.5 and wish to measure distance (X) traveled after t = 10 ns, then it is easy to calculate that X = 1.0 m by putting the values in OTDR equation [21, 23].

In wavelength multiplexing, a device such as Bragg grating is used to encode a series of resonant wavelengths in the fibers [12–15]. The wavelengths in turn are monitored by wavelength interrogator. The resonant wavelengths are affected by measuring parameters and are thus monitored in a quasi-distributed manner.
