4. Quasi-distributed sensors for temperature and strain measurements

The fiber Bragg gratings (FBG) were first written by Hill et al. [12] who discovered the breakthrough phenomena of photosensitivity in optical fiber. As a result of this development, FBG-based strain and temperature sensors came into existence. The method of writing FBG in sensing fiber's section involves creation of periodic modulation of fiber core's refractive index. The refractive index is modulated by spatial pattern of ultraviolet (UV) light between 240 and 260 nm. The periodic structure in fiber's core can be created by phase mask method [13, 15]. A particular pattern in a particular segment of fiber will correspond to a specific Bragg reflection wavelength. The multiple gratings can be fabricated by using a specific phase mask with different initial Bragg wavelength gratings in the same fiber causing creation of several point sensors in a single sensing fiber. Such FBG-based sensors are quasidistributed temperature sensors where temperature sensing by fiber is possible only where grating was created.

According to Bragg's law, when a broad band light is injected into the optical fiber consisting of FBG sensors, a specific wavelength of light is reflected by FBG [15]. The Bragg wavelength is determined by the product of effective refractive

reported to appreciate the improvement achieved after processing of Raman

Distributed temperature profile with processed (black color) and unprocessed (red color) Raman signals:

Zone (location) Reference temperature Measured temperature

Start of fiber (location: 0 m) 24.5°C 24°C 24°C

Hot zone-1 (location: 190 m) 85°C 77.1°C 83.9 °C

Hot zone-2 (location: 192.4 m) 50°C 42.1°C 48.3°C

End of fiber (location: 205 m) 25°C 14.2 °C 23.7°C

Comparison of error in temperature measurement at various zones with unprocessed and processed Raman

Unprocessed Processed

Error 0.5°C (2.04%) 0.5°C (2.04%)

Error 7.9 °C (9.29%) 1.1 °C (1.29%)

Error 7.9°C (15.8%) 2.4°C (3.4%)

Error 10.8°C (43.2%) 1.3°C (5.2%)

(a) view for complete fiber length and (b) zoomed view for hot zones.

Applications of Optical Fibers for Sensing

Fiber Sensors Lab., Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India has developed a Raman scattering-based OFDTS [23] with the following specifications, and the developed OFDTS is capable of working in high accelerating voltage (1.5 MV), magnetic field (1.5 T), and bremsstrahlung radiation

(a) Temperature range: 25–300°C, (b) temperature resolution: 3°C, (c) spatial resolution: 1 m (over a length of 500 m); can be improved to few cm with special fiber-laying techniques, (d) distance (dynamic range for distance covered): 500 m, (e) fire alarm: audio-visual alarms can be generated, and (f) gamma field operation;

For more ruggedness and field deployability, an all-fiber ROFDTS scheme is desirable. The schematic design of one such scheme is depicted in Figure 7. Recently, a distributed sensor using a superconducting nanowire single photon detector and chalcogenide fiber has been proposed. This scheme has the potential to

signals.

62

Table 1.

signals.

Figure 6.

present in accelerator systems.

can operate up to a gamma dose of 1 MGy.

index (neff) of the grating and the grating period (Λ) (also called pitch length) as given by the following equation:

$$
\lambda\_B = 2\mathfrak{n}\_{\mathfrak{eff}}\Lambda \tag{4}
$$

multiplexed through time division multiplexing (TDM) and wavelength division multiplexing (WDM). The information from each sensor must be separated and interpreted, which requires an interrogator system to interrogate many FBGs

A series of wavelength-encoded FBGs are used for quasi-distributed sensing applications. Several groups developed quasi-distributed FBG sensors for temperature and strain monitoring [31–33]. Central Glass & Ceramic Research Institute (CGCRI), Kolkata, India recently developed specially packaged FBGs for strain/ force monitoring of electric railway engine pantographs. However, very special packaging or specialty fibers were required for advanced applications such as strain monitoring at high temperature, strain monitoring at cryogenic temperature, and very high temperature monitoring exceeding 1000°C. FBG strain sensor for health

The dynamic range of ROFDTS is restricted by coating on the optical fiber. Polyimide coatings can permit measurement up to 350°C while the gold coating may allow the measurement up to 600°C. Beyond this, distributed sensing is possible by specialized gratings made in specialized fibers. For ultra-high temperature sensing, type II-IR gratings in silica optical fiber withstand a temperature of up to 1000°C, which are usually fabricated by using a femto-second laser with power density near the damage threshold of the fiber glass. These gratings however have disadvantages as sensing elements because of asymmetric reflection spectrum and a large spectral width of more than 0.6 nm. These create problems during distributed sensing. Gratings written on a different host material, namely sapphire gratings, can be used as a temperature-sensing probe up to 1900°C. However, the material and mode mismatch with normal silica-based optical fiber and high cost of fabrication restricts its use in distributed sensing. Identification of structural changes on a molecular scale involved with the formation of a new type of FBG named

monitoring of structure at 600°C have also been developed.

5. Ultra-high temperature distributed sensors

connected in series.

Schematic diagram of the distributed FBG sensor.

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

Figure 9.

65

Figure 8 depicts the basic principle of FBG reflection spectra and interrogation technique [12–15]. The Bragg wavelength depends on grating period of FBG and the refractive property of optical fiber.

It is clear from Eq. (4) that any change in the pitch length or refractive index will induce a shift in the resonant wavelength. Consequently, temperature, strain or deformations of the fiber can be monitored by the corresponding resonant wavelength shift. The Bragg wavelength is strain- and temperature-dependent through physical elongation or thermal change of the sensor and through the change in the fiber refractive index due to photoelastic and thermo-optic effects.

There are essentially three types of gratings which vary in photosensitivity. They are known as type I, II, and IIA with the details of each type given in [15]. Type I gratings are written with moderate intensities and exhibit an index grating right across the core. Type II gratings can be written with much higher intensities within very short times, often with a single nanosecond pulse from an excimer laser (single shot damage gratings). Type IIA gratings are regenerated gratings (RGs) specifically designed for high temperature operation. In addition, there are different physical types of gratings such as long period gratings (LPGs), chirped gratings, tilted (blazed) gratings, and micro-structured FBGs. Typical temperature sensitivity of FBG is 10 pm/°C (at 1550 nm in standard silica-based single mode fiber) and strain sensitivity is 1.2 pm/micro-strain [13].

A strong point of FBGs is their capability of multiplexing in wavelength that enable multiple points or quasi-distributed sensing. The schematic diagram of the distributed FBG sensor is shown in Figure 9.

There have been significant developments in two of the areas that have constrained the progress of fiber grating technology. Firstly, the issue of temperature and strain isolation has been overcome by using various techniques reported in the literature, from simply having collocated sensors that are exposed to the same temperature fluctuations to isolate stress and strain, to more complex methods, such as using tilted or chirped gratings to distinguish between the different measurands. Secondly, with improved data processing methods, simpler interrogation techniques are being utilized such that the optical signal can easily be transposed into the electrical domain, allowing the optical networks to be interfaced seamlessly with electronic systems. In addition, the production of FBGs has improved significantly through draw tower processes and automated manufacturing. One of the main advantages of FBG sensors is their ability to be easily

Figure 8. Schematic representation of FBG sensor.

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

index (neff) of the grating and the grating period (Λ) (also called pitch length) as

Figure 8 depicts the basic principle of FBG reflection spectra and interrogation technique [12–15]. The Bragg wavelength depends on grating period of FBG and the

It is clear from Eq. (4) that any change in the pitch length or refractive index will induce a shift in the resonant wavelength. Consequently, temperature, strain or deformations of the fiber can be monitored by the corresponding resonant wavelength shift. The Bragg wavelength is strain- and temperature-dependent through physical elongation or thermal change of the sensor and through the change in the

There are essentially three types of gratings which vary in photosensitivity. They are known as type I, II, and IIA with the details of each type given in [15]. Type I gratings are written with moderate intensities and exhibit an index grating right across the core. Type II gratings can be written with much higher intensities within very short times, often with a single nanosecond pulse from an excimer laser (single shot damage gratings). Type IIA gratings are regenerated gratings (RGs) specifically designed for high temperature operation. In addition, there are different physical types of gratings such as long period gratings (LPGs), chirped gratings, tilted (blazed) gratings, and micro-structured FBGs. Typical temperature sensitivity of FBG is 10 pm/°C (at 1550 nm in standard silica-based single mode fiber) and

A strong point of FBGs is their capability of multiplexing in wavelength that enable multiple points or quasi-distributed sensing. The schematic diagram of the

measurands. Secondly, with improved data processing methods, simpler interrogation techniques are being utilized such that the optical signal can easily be transposed into the electrical domain, allowing the optical networks to be interfaced seamlessly with electronic systems. In addition, the production of FBGs has improved significantly through draw tower processes and automated manufactur-

ing. One of the main advantages of FBG sensors is their ability to be easily

There have been significant developments in two of the areas that have constrained the progress of fiber grating technology. Firstly, the issue of temperature and strain isolation has been overcome by using various techniques reported in the literature, from simply having collocated sensors that are exposed to the same temperature fluctuations to isolate stress and strain, to more complex methods, such as using tilted or chirped gratings to distinguish between the different

fiber refractive index due to photoelastic and thermo-optic effects.

λ<sup>B</sup> ¼ 2neffΛ (4)

given by the following equation:

Applications of Optical Fibers for Sensing

refractive property of optical fiber.

strain sensitivity is 1.2 pm/micro-strain [13].

distributed FBG sensor is shown in Figure 9.

Figure 8.

64

Schematic representation of FBG sensor.

Figure 9. Schematic diagram of the distributed FBG sensor.

multiplexed through time division multiplexing (TDM) and wavelength division multiplexing (WDM). The information from each sensor must be separated and interpreted, which requires an interrogator system to interrogate many FBGs connected in series.

A series of wavelength-encoded FBGs are used for quasi-distributed sensing applications. Several groups developed quasi-distributed FBG sensors for temperature and strain monitoring [31–33]. Central Glass & Ceramic Research Institute (CGCRI), Kolkata, India recently developed specially packaged FBGs for strain/ force monitoring of electric railway engine pantographs. However, very special packaging or specialty fibers were required for advanced applications such as strain monitoring at high temperature, strain monitoring at cryogenic temperature, and very high temperature monitoring exceeding 1000°C. FBG strain sensor for health monitoring of structure at 600°C have also been developed.
