6.5.3 Novel devices, fabrication technology, and testing for high radiation dose detection

Specialty doped fibers are required to measure high dose gamma radiation. These fibers should have negligible radiation-induced attenuation in IR but should show high index changes upon irradiation. Wavelength encoded fiber gratings are attractive candidates for high level gamma dose measurements in nuclear environment. This paper explains for the first time how arc-induced long period fiber gratings can be optimally designed for gamma dose measurements ranging from 1 kGy to 1 MGy.

We have investigated the gamma radiation effects on parameters of electricarc-induced long period fiber gratings in high Ge doped and B/Ge co-doped single mode fibers. The grating resonance wavelength shifts and amplitude of the dips of various cladding modes were monitored on-line to study the role of grating fabrication and fiber chemical composition. These studies lead to identification of boron as a critical core dopant for high radiation sensitivity. After a Co-60 gamma dose of 1 MGy, the optimized gratings show radiation-induced changes of their transmission dip wavelength up to 20 nm which is comparable to CO2 laser-induced gratings reported by us previously [39]. These gratings also show very high temperature sensitivity specially when operated in dispersion turn-around-point (TAP) mode [38].

Fibers doped with different boron contents in SiO2-GeO2-B2O3 host were fabricated indigenously under collaboration with CGCRI, Kolkata, India. The gratings in such fibers were modeled and analyzed. We have also designed and fabricated a stable and robust sensor package unit for remote gamma dose measurements up to a dose of 1 MGy. Lab trials of such units have been carried out, and the experience in using such devices for dose estimation is discussed. These devices make arc-induced LPGs and CO2 laser-induced LPGs in Boron doped fibers a strong candidate for applications in super Large Hadron Collider (LHC) and International Thermonuclear Experimental Reactor (ITER). Table 2 shows the experimental results of such online measurements.

Figure 13 shows our on-line gamma dose effect measurements using specialty turn-around-point (TAP) long period fiber grating.


### Table 2.

Gamma radiation exposure data for LPG of a 400 micron grating period inscribed in Fiber Logix, SM G652 fiber. Total dose: 65 kGy.

The spectral region of mid-IR is also a finger print region for most of the volatile

Index-guided photonic crystal fibers are made with stack-and-draw process in fiber draw tower. They contain a periodic array of air holes running in cladding region and light is confined to solid core due to reduced effective index of the cladding. A fraction of evanescent field of power is extended into the holey region where it is absorbed by gas species and the gas concentration is obtained from intensity attenuation through the Beer-Lambert law. It is possible to design an IG-PCF that has a large fraction of evanescent power located within air-holes. This provides a good platform for gas sensing applications. Gas detectors based on absorption spectroscopy with IG-PCFs were experimentally demonstrated by Hoo

One concern in using IG-PCF as evanescent field sensors is the limited response time due to time required for sample gas to diffuse into holes from two open ends. Hoo et al. [46] have numerically calculated response time for acetylene gas in IG-PCF fiber (Λ = 1.55 μm, d = 1.4 μm) of length l with two ends open. When the fiber sensor length is 1 m, the time required to reach 90% concentration of exposed gas in surrounding is 200 min. This shows that we have to take a very short fiber if we require a response time of 1 min governed by diffusion time. However, this will reduce the sensitivity of the device due to limited path length. It is also observed that the relative sensitivity is also about 6% of that of open path cell per equal length. To achieve fast response time, one has to introduce periodic openings along the sensing fiber and measures like micro-pumping of sampling gases. To improve sensitivity, hollow core fibers may be a better choice as efficient interaction of

Hollow core photonic crystal fiber is composed of a hollow core and a cladding with holes. The light is guided through the bandgap effect due to a proper design of hole sizes and gaps. They allow simultaneous confinement of optical mode and

compounds and nerve gases which is under investigation due to availability of compact tunable lasers in this band. The general principles of quantitative measurement of gas concentration have been described extensively in the literature [40, 41]. The basic principle is that the absorption is wavelength-dependent and compound specific. Initial systems used open path gas absorption cells comprising a pair of graded index collimator lenses with fiber pigtails [42]. The optical fibers here perform a passive role like transfer of light to and from absorption cell but play no active role in gas sensing. The National Aeronautics and Space Administration (NASA) reported a multi-point fiber optic multi-point hydrogen micro-sensor system for leak detection for launch vehicles [43]. The system consists of a multiplexed system having palladium coated micro lens attached to fiber tips. These chemically reactive coatings undergo changes in reflection in proportion to hydrogen concentration and are used as leak detectors. The invention of PCF or Holey fiber in 1990 opened new opportunities for exploiting the interaction of light with gases either through evanescent field or through hollow core region [44, 45]. The PCF family includes index guided PCF, hollow core photonic bandgap-fiber, and suspended

core fiber.

et al. [46].

73

6.6.1 Index guided photonic crystal fiber

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

guided light with gas molecules would be possible.

6.6.2 Hollow core photonic crystal fiber

### Figure 13.

Real-time resonance spectral measurements of TAP-LPG in gamma field [38].

The requirement of such sensors is divergent: for radiotherapy, we need very compact needle-like disposable tip sensors with very weak annealing properties. Once a patient is given a certain dose, the tip is discarded and a new dose sensing tip is used for other patients. For high dose sensing applications, the probe with high or low annealing may be desirable depending on application. For example, for longterm monitoring of integrated dose in waste storage facility will need very low annealing dose sensors while for repetitive dose measurements like gamma pencil fabrication, a high annealing rate sensor will be required.

### 6.6 Distributed gas sensors

The detection and identification of hazardous volatile compounds with low false alarm probability in large areas such as airports, underground metro stations, mines, bus stands, and major event arenas are a very daunting task. Other strategic applications include multi-point hydrogen leak detection systems for commercial and military launch vehicles that use cryogenic hydrogen as the main propellant. This gas is highly volatile, extremely flammable, and highly explosive. Hundreds of point gas sensors with electronic circuits are needed to cater big areas. Current detection systems use transfer tubes at a small number of locations through which gas samples are drawn and stream analyzed by a mass spectrometer. They are complex and costly systems and do not provide leak location.

Among several sensing systems, absorption-based systems are extensively explored and successfully employed. There have been serious efforts to develop absorption-based fiber optic systems for gas detection with higher sensitivity, fast response, and distributed sensing capability. They are based on measurement of VIS-IR-mid IR absorption by intended chemical species. The characteristic absorption spectra can be used as a fingerprint to identify a particular gas species. For example, many gas molecules have absorption lines in 0.8–1.8 μm band which is also a low loss transmission window of the silica fiber.

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

The spectral region of mid-IR is also a finger print region for most of the volatile compounds and nerve gases which is under investigation due to availability of compact tunable lasers in this band. The general principles of quantitative measurement of gas concentration have been described extensively in the literature [40, 41]. The basic principle is that the absorption is wavelength-dependent and compound specific. Initial systems used open path gas absorption cells comprising a pair of graded index collimator lenses with fiber pigtails [42]. The optical fibers here perform a passive role like transfer of light to and from absorption cell but play no active role in gas sensing. The National Aeronautics and Space Administration (NASA) reported a multi-point fiber optic multi-point hydrogen micro-sensor system for leak detection for launch vehicles [43]. The system consists of a multiplexed system having palladium coated micro lens attached to fiber tips. These chemically reactive coatings undergo changes in reflection in proportion to hydrogen concentration and are used as leak detectors. The invention of PCF or Holey fiber in 1990 opened new opportunities for exploiting the interaction of light with gases either through evanescent field or through hollow core region [44, 45]. The PCF family includes index guided PCF, hollow core photonic bandgap-fiber, and suspended core fiber.

### 6.6.1 Index guided photonic crystal fiber

Index-guided photonic crystal fibers are made with stack-and-draw process in fiber draw tower. They contain a periodic array of air holes running in cladding region and light is confined to solid core due to reduced effective index of the cladding. A fraction of evanescent field of power is extended into the holey region where it is absorbed by gas species and the gas concentration is obtained from intensity attenuation through the Beer-Lambert law. It is possible to design an IG-PCF that has a large fraction of evanescent power located within air-holes. This provides a good platform for gas sensing applications. Gas detectors based on absorption spectroscopy with IG-PCFs were experimentally demonstrated by Hoo et al. [46].

One concern in using IG-PCF as evanescent field sensors is the limited response time due to time required for sample gas to diffuse into holes from two open ends. Hoo et al. [46] have numerically calculated response time for acetylene gas in IG-PCF fiber (Λ = 1.55 μm, d = 1.4 μm) of length l with two ends open. When the fiber sensor length is 1 m, the time required to reach 90% concentration of exposed gas in surrounding is 200 min. This shows that we have to take a very short fiber if we require a response time of 1 min governed by diffusion time. However, this will reduce the sensitivity of the device due to limited path length. It is also observed that the relative sensitivity is also about 6% of that of open path cell per equal length. To achieve fast response time, one has to introduce periodic openings along the sensing fiber and measures like micro-pumping of sampling gases. To improve sensitivity, hollow core fibers may be a better choice as efficient interaction of guided light with gas molecules would be possible.

### 6.6.2 Hollow core photonic crystal fiber

Hollow core photonic crystal fiber is composed of a hollow core and a cladding with holes. The light is guided through the bandgap effect due to a proper design of hole sizes and gaps. They allow simultaneous confinement of optical mode and

The requirement of such sensors is divergent: for radiotherapy, we need very compact needle-like disposable tip sensors with very weak annealing properties. Once a patient is given a certain dose, the tip is discarded and a new dose sensing tip is used for other patients. For high dose sensing applications, the probe with high or low annealing may be desirable depending on application. For example, for longterm monitoring of integrated dose in waste storage facility will need very low annealing dose sensors while for repetitive dose measurements like gamma pencil

The detection and identification of hazardous volatile compounds with low false alarm probability in large areas such as airports, underground metro stations, mines, bus stands, and major event arenas are a very daunting task. Other strategic applications include multi-point hydrogen leak detection systems for commercial and military launch vehicles that use cryogenic hydrogen as the main propellant. This gas is highly volatile, extremely flammable, and highly explosive. Hundreds of point gas sensors with electronic circuits are needed to cater big areas. Current detection systems use transfer tubes at a small number of locations through which gas samples are drawn and stream analyzed by a mass spectrometer. They are complex and costly systems and do not provide leak

Among several sensing systems, absorption-based systems are extensively explored and successfully employed. There have been serious efforts to develop absorption-based fiber optic systems for gas detection with higher sensitivity, fast response, and distributed sensing capability. They are based on measurement of VIS-IR-mid IR absorption by intended chemical species. The characteristic absorption spectra can be used as a fingerprint to identify a particular gas species. For example, many gas molecules have absorption lines in 0.8–1.8 μm band which is

fabrication, a high annealing rate sensor will be required.

Real-time resonance spectral measurements of TAP-LPG in gamma field [38].

also a low loss transmission window of the silica fiber.

6.6 Distributed gas sensors

Applications of Optical Fibers for Sensing

location.

72

Figure 13.

gas phase materials within the hollow core. This provides an excellent means for strong light/molecular interaction inside the fiber core over long distance.

Acknowledgements

List of acronyms

AS anti-Stokes

DE dual-ended

DNA deoxyribonucleic acid DWT discrete wavelet transform EMI electromagnetic interference

FBG fiber Bragg gratings FIR finite impulse response IIR infinite impulse response

LHC Large Hadron Collider LPGs long period gratings

OSA optical spectrum analyzer

PCFs photonic crystal fibers RGs regenerated gratings

SNR signal-to-noise ratio

TAP turn-around-point

TDM time division multiplexing TLD thermoluminescent dosimeter

WDM wavelength division multiplexing

St Stokes

UV ultraviolet

75

OTDR optical time domain reflectometry

conducting various experiments.

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

The authors are thankful for the help received from Shri Jai Kishore, Smt. Smita

Chaubey, and Shri Sanjai Kumar of Fiber Sensors Lab., RRCAT, Indore, in

CGCRI Central Glass & Ceramic Research Institute

ESM-PCF endless-single-mode photonic crystal fiber

ITER International Thermonuclear Experimental Reactor

MOSFET metal-oxide semiconductor field-effect transistor NASA National Aeronautics and Space Administration

RRCAT Raja Ramanna Centre for Advanced Technology

ROFDTSs Raman optical fiber-based distributed temperature sensors
