3.5 Approaches to solve the problematic issues

Stoddart et al. [20] proposed to use Rayleigh instead of St from the backscattered spectrum to avoid the temperature measurement error in hydrogen-rich environments due to differential attenuation caused by the optical fiber for AS and St signal wavelengths. This resulted in better results but could not eliminate the error caused by the differential attenuation completely. The dual-ended (DE) configuration [26] (i.e. both ends of sensing fiber are connected to ROFDTS unit) and dual laser source schemes [27, 28] have also been proposed to take care of the difference in attenuation between AS and St. These schemes have resulted in improvements but add complexity and need double length of fiber, extra distributed temperature sensor (DTS) with an optical switch, and two costly lasers. A correction method to take care of the difference in attenuation for AS and St signals has been proposed with only one light source and one light detector but requires attachment of a carefully designed reflective mirror at the far fiber-end of the sensing fiber [29]. Recently, a more sophisticated correction technique [30] based on detection of AS signal alone in combination with DE configuration has been investigated. ROFDTSs based on the above schemes are important and to a certain extent become mandatory in situations where sensing fiber is exposed to the severe radiation environment or hydrogen darkening in oil wells. Requirements for less demanding situations like temperature measurement in steam pipelines of turbines, electrical cables and temperature profiling of big buildings, gas pipelines and mines etc. can be met by the technique based on digital signal processing.

In order to address the above issues satisfactorily, a discrete wavelet transform (DWT)-based dynamic self-calibration and de-noising technique is used and implemented by the authors as given in detail [23]. Briefly, wavelets are mathematical functions that can be used to segregate data into various frequency components. Each component can then be studied with a resolution matched to its scale. In DWT, a signal may be represented by its low frequency component and its high frequency component.

The DWT-based technique is simpler, more automatic, and provides a single solution to address all the above issues simultaneously. The DWT technique takes care of the difference in optical attenuation for AS and St signals by using their trend and also de-noises the AS and St signals while preserving spatial locations of peaks. Also, this technique requires just 1 m long calibration zone which is much less than the 100 m required in the previous technique. Moreover, the dynamic measurement of calibration zone's temperature eliminates the requirement of keeping the calibration zone at a constant temperature, and thus, complicated heating arrangement is avoided. Actual wavelet transform-based processed signal profile is shown in Figure 6. Table 1 presents the comparison of error in temperature measurement at various zones using Eq. (3) with unprocessed and processed Raman signals. Both absolute errors and percentage errors (in brackets) are

### Figure 6.

Distributed temperature profile with processed (black color) and unprocessed (red color) Raman signals: (a) view for complete fiber length and (b) zoomed view for hot zones.


offer sub-centimeter spatial resolution sensor, below 1°C temperature resolution

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

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

over a distance of few hundreds of meters.

Schematic diagram of an all-fiber-based ROFDTS scheme.

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

Figure 7.

where grating was created.

63

### Table 1.

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

reported to appreciate the improvement achieved after processing of Raman signals.

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 present in accelerator systems.

(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; can operate up to a gamma dose of 1 MGy.

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

Figure 7. Schematic diagram of an all-fiber-based ROFDTS scheme.

offer sub-centimeter spatial resolution sensor, below 1°C temperature resolution over a distance of few hundreds of meters.
