6.3 Theory

regenerated Bragg grating based on annealing a conventional Type-I FBG to create a new, more robust one seems to be a promising candidate to achieve better sustainability at high temperature. However, in standard photosensitive silica fiber, RG

CGCRI, Kolkata, India in collaboration with RRCAT, Indore, India has taken up the development of a new glass composition-based photosensitive fiber to fabricate RG for temperature 1400°C. The fiber is likely to be based on yttrium-stabilized zirconia-calcium-alumina-phospho silica glass. The motivation of the choice of such kind of multi-material glass-based optical fiber is to increase the photosensitivity along with thermal stability of fabricated RG. The regeneration takes place near the fiber glass transition temperature, in which the transformation of the glass from monoclinic structure to tetragonal structure occurs. The ultra-high temperature sustainability of RG will be evaluated for the special composition through material study, which definitely is not achievable in a standard germano-silicate fiber. This work is expected to provide a new degree of freedom in the design of optical fiber sensor for ultra-high temperature sensing. This will open up opportunities in sectors such as power plants, turbines, combustion, and aerospace engineering where often the environments are too harsh for existing FBG sensor technology and will offer a

A long period fiber grating sensor in photonic crystal fiber with a strain sensitivity of 2.0 pm/με and negligible temperature sensitivity is fabricated by use of CO2 laser beam. Such a strain sensor can effectively reduce the cross sensitivity between strain and temperature. Due to single material (pure silica) construction, they have been shown to be resistant to nuclear radiation and are thus useful for applications in secondary loops of nuclear reactors. The authors' lab has designed

Photonic crystal fibers (PCFs) also known as holey fibers are a new class of optical fibers that have attracted intense scientific research during past few years. Typically, these fibers incorporate a number of air holes that run along the length of the fiber, and the size, shape, and distribution of the holes can be designed to achieve various novel wave-guiding properties that may not be possible in conventional fibers. Various PCFs have been demonstrated so far that exhibit remarkable properties such as endlessly single mode fiber, large mode area, and highly nonlinear performance. Temperature-insensitive long period gratings have

attracted much attention because of their potential applications in achieving stable optical filters and gain flatteners as well as in realizing temperature-insensitive sensors for industrial and nuclear applications. Conventional fibers contain at least two different glasses, each with a different thermal expansion coefficient, thereby giving rise to high temperature sensitivity. PCFs are virtually insensitive to temperature because they are made of only one material (and air hole). This property can be used to obtain temperature-insensitive PCF-based devices. Long period gratings (LPGs) in PCF fibers have not yet been reported in India. Besides, the effect of high nuclear radiation on such PCF-based grating sensors has not been

gratings were found to be stable only below 950°C.

Applications of Optical Fibers for Sensing

new degree of freedom in the design of optical fiber sensors.

6. Strain sensors for nuclear environment

reported by any group to the best of our knowledge.

6.1 Wavelength encoded strain sensors

and developed such sensor devices.

6.2 Design principle

66

An LPG is formed by introducing periodic modulation of the refractive index along a single mode fiber. Such a grating induces light coupling from the fundamental guided mode to co-propagating cladding modes at discreet resonant wavelengths. LPGs in conventional fibers have been extensively used as band rejection filters, gain flattening filters, tunable couplers, and sensors. In general, as fiber devices and sensing elements, LPGs offer low back reflection, insensitivity to electromagnetic interference, and low insertion loss and cost effectiveness. For a long period grating with periodicity Λ, the wavelength λ(m) at which mode coupling occurs is given by

$$
\lambda^{(m)} = \begin{pmatrix} n\_{\text{eff}} - n\_{\text{cl},m} \end{pmatrix} \Lambda \tag{5}
$$

where neff is the effective refractive index of the propagating core mode at wavelength λ, and ncl,m is the effective refractive index of the mth cladding mode. The variation in the grating period and modal effective indices due to strain and temperature causes the coupling wavelength to shift. This spectral shift is distinct for each loss band and is a function of the order of corresponding cladding mode.

The axial strain sensitivity of LPGs may be examined by expanding Eq. (5) to yield

$$\frac{d\lambda}{d\varepsilon} = \frac{d\lambda}{d\left(\delta n\_{\text{eff}}\right)} \left(\frac{dn\_{\text{eff}}}{d\varepsilon} - \frac{dn\_{cl}}{d\varepsilon}\right) + \Lambda \frac{d\lambda}{d\Lambda} \tag{6}$$

where δneff = (neff � ncl) is the differential effective index; ordinal m has been dropped for the sake of simplicity. The two terms on the right side can be divided into material (first term) and waveguide (second term) contributions. The temperature sensitivity of LPG grating is given by

$$\frac{d\lambda}{d\varepsilon} = \frac{d\lambda}{d\left(\delta n\_{\rm eff}\right)} \left(\frac{dn\_{\rm eff}}{dT} - \frac{dn\_{\rm cl}}{dT}\right) + \Lambda \frac{d\lambda}{d\Lambda} \frac{1}{L} \frac{dL}{dT} \tag{7}$$

where λ is the central wavelength of the attenuation band,T is the temperature, L is the length of the LPG, and Λ is the period of the LPG. For standard long period gratings with periodicity of hundreds of micrometers, the material effect dominates the waveguide contribution. Hence, only the first term in Eqs. (6) and (7) is considered for evaluation of sensitivity. For photonic crystal fibers which are single material fibers, the first term in Eq. (7) becomes negligible, resulting in very low temperature sensitivity. This term is an order smaller than that of B-Ge-doped photosensitive fiber. This opens-up the field for PCF-based temperature-insensitive sensors.

### 6.4 Device designs

Inscription of LPGs has been demonstrated using various techniques such as UV treatment, heat treatment with a CO2 laser, or by applying mechanical pressure. Formation of LPG in pure-silica core PCF fibers is not straightforward because there is no photosensitivity provided by Ge-O2 vacancy defect centers. The LPGs in PCF are primarily formed due to modification of glass structure. However, any geometrical deformation results in flaws or cracks that result in fracture of the fiber, and therefore, LPGs in PCF require high precision systems. Our fully automated CO2 laser-based grating writing system can set the grating period in the range of

200–800 μm with a precision of 1 μm while laser intensity can be stabilized within 5%. Figure 10 shows the schematic diagram of our grating writing system. The fiber is exposed to CO2 laser for a predetermined period and the beam is scanned repeatedly over the fiber until grating of sufficient strength is formed. This operation is performed through an AutoCAD program in which the period and length of the grating are selected as per the design requirement. This method is more accurate and free from vibration related uncertainties in the grating period. The spectral response is recorded using an optical spectrum analyzer (OSA) (86142B, Agilent) which is connected to LPG through patch-cords as shown in Figure 10.

During application of LPG-based strain sensors, one of the main difficulties is the cross sensitivity between strain and the temperature [34]. The common methods for cross sensitivity reduction are using temperature compensation and simultaneous strain and temperature measurement. Conventional fibers contain at least two different glasses, each with a different thermal expansion coefficient, thereby giving rise to high temperature sensitivity. By use of the CO2 laser method, an LPG sensor with strain sensitivity of 0.45 pm/με and a temperature sensitivity of 59.0 pm/°C was written in corning SMF-28 fiber 2. Another LPG with a strain sensitivity of 0.19 pm/με and a temperature sensitivity of 10.9 pm/°C was described in PCF fiber. In this paper, we present a LPG-PCF sensor fabricated in ESM-PCF with a high strain sensitivity (2.0 pm/με) and negligible temperature sensitivity.

For the preparation of LPFG in an endless-single-mode photonic crystal fiber (ESM-PCF), both ends of the PCF are fusion spliced to SMFs [34]. The loss for each splice is about 0.74 dB. An X-Y scanning CO2 laser is used for the fabrication of LPGs in the ESM-PCF. The CO2 laser operates at a frequency of 2 kHz and has a maximum power of 10 W. The laser power is controlled by the mark-speed of the laser pulses. The typical grating length and period in our experiment is 23.4 mm and 450 μm, respectively. Figure 11 shows the transmission characteristics of a LPG fabricated on an ESM-PCF. Attenuation bands in the range of 1300–1700 nm have been investigated by an Optical spectrum analyzer.

The device has been tested on a standard strain calibration platform. Figure 12 shows the strain-dependent wavelength shift of the fabricated device.

6.5 Strain sensors for cryogenic environment

Strain-dependent wavelength shift of LPG [34].

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

Figure 11.

Figure 12.

69

The tendency of superconducting magnet coils to quench prematurely, at relatively low fractions of the critical current, or to exhibit training behavior, is often attributed to mechanical issues. Knowledge of stress, strain, and displacement of the windings is therefore central to the design of the superconducting magnet. The resistive foil strain gauge has remained the device most commonly used for measuring the strain on cryogenic structures. The nonlinear thermal apparent strains and measurement sensitivity to electromagnetic noise remain the most significant limitations to its successful implementation. FBG sensor has a number of distinct advantages over other sensors, such as EMI immunity, high sensitivity, and compact size. Furthermore, the wavelength-encoded nature allows the distributed sensing of strain. Fiber Bragg gratings are used to monitor temperature and strain in

Transmission characteristics of a LPG fabricated on an ESM-PCF with a period of 450 μm [34].

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

200–800 μm with a precision of 1 μm while laser intensity can be stabilized within 5%. Figure 10 shows the schematic diagram of our grating writing system. The fiber is exposed to CO2 laser for a predetermined period and the beam is scanned repeatedly over the fiber until grating of sufficient strength is formed. This operation is performed through an AutoCAD program in which the period and length of the grating are selected as per the design requirement. This method is more accurate and free from vibration related uncertainties in the grating period. The spectral response is recorded using an optical spectrum analyzer (OSA) (86142B, Agilent)

During application of LPG-based strain sensors, one of the main difficulties is

For the preparation of LPFG in an endless-single-mode photonic crystal fiber (ESM-PCF), both ends of the PCF are fusion spliced to SMFs [34]. The loss for each splice is about 0.74 dB. An X-Y scanning CO2 laser is used for the fabrication of LPGs in the ESM-PCF. The CO2 laser operates at a frequency of 2 kHz and has a maximum power of 10 W. The laser power is controlled by the mark-speed of the laser pulses. The typical grating length and period in our experiment is 23.4 mm and 450 μm, respectively. Figure 11 shows the transmission characteristics of a LPG fabricated on an ESM-PCF. Attenuation bands in the range of 1300–1700 nm have

The device has been tested on a standard strain calibration platform. Figure 12

shows the strain-dependent wavelength shift of the fabricated device.

been investigated by an Optical spectrum analyzer.

Schematic diagram of long period grating fabrication set-up [34].

which is connected to LPG through patch-cords as shown in Figure 10.

Applications of Optical Fibers for Sensing

sensitivity.

Figure 10.

68

the cross sensitivity between strain and the temperature [34]. The common methods for cross sensitivity reduction are using temperature compensation and simultaneous strain and temperature measurement. Conventional fibers contain at least two different glasses, each with a different thermal expansion coefficient, thereby giving rise to high temperature sensitivity. By use of the CO2 laser method, an LPG sensor with strain sensitivity of 0.45 pm/με and a temperature sensitivity of 59.0 pm/°C was written in corning SMF-28 fiber 2. Another LPG with a strain sensitivity of 0.19 pm/με and a temperature sensitivity of 10.9 pm/°C was described in PCF fiber. In this paper, we present a LPG-PCF sensor fabricated in ESM-PCF with a high strain sensitivity (2.0 pm/με) and negligible temperature

Figure 11. Transmission characteristics of a LPG fabricated on an ESM-PCF with a period of 450 μm [34].

Figure 12. Strain-dependent wavelength shift of LPG [34].

### 6.5 Strain sensors for cryogenic environment

The tendency of superconducting magnet coils to quench prematurely, at relatively low fractions of the critical current, or to exhibit training behavior, is often attributed to mechanical issues. Knowledge of stress, strain, and displacement of the windings is therefore central to the design of the superconducting magnet. The resistive foil strain gauge has remained the device most commonly used for measuring the strain on cryogenic structures. The nonlinear thermal apparent strains and measurement sensitivity to electromagnetic noise remain the most significant limitations to its successful implementation. FBG sensor has a number of distinct advantages over other sensors, such as EMI immunity, high sensitivity, and compact size. Furthermore, the wavelength-encoded nature allows the distributed sensing of strain. Fiber Bragg gratings are used to monitor temperature and strain in engineering structures; to date, however, their use has been limited to ambient and high temperatures, typically in the range of 273–773 K.

To cover a broad dose range from few Gy to 1 MGy, novel sensor systems like gratings are desirable. For most fibers, the increase in attenuation with dose saturates near few kGy which is accumulated within a relatively short time at certain critical locations and so they need to be replaced frequently. Even space-based

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

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

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

Figure 13 shows our on-line gamma dose effect measurements using specialty

Gamma radiation exposure data for LPG of a 400 micron grating period inscribed in Fiber Logix, SM G652

Wavelength measured after 4.30 h of dose (dose of 3.6 kGy) after removal from gamma chamber (nm)

systems are qualified for a dose up to 100 kGy.

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

1 kGy to 1 MGy.

(TAP) mode [38].

online measurements.

Wavelength before exposure (nm)

fiber. Total dose: 65 kGy.

Table 2.

71

turn-around-point (TAP) long period fiber grating.

1161 1165.4 1229.6 1231 1250 1253.6 1364 1366.4 1546.7 No significant dip

There exist very few published reports on FBG strain sensors that have been functional at liquid nitrogen temperature. Zhang et al. have reported one such FBG sensor used to strain sensing at 77 K and used in high temperature superconducting magnet [29]. To date, an FBG sensor used to strain sensing at 4.2 K and used in low temperature superconducting magnet has not been reported. RRCAT, Indore is also working on FBG strain and temperature sensors for cryogenic applications. The sensors show linearity in strain range from 50 to 500 micro-strain at liquid nitrogen temperature.

### 6.5.1 Nuclear radiation sensors

Optical fibers offer a unique capability for remote monitoring of radiation in hazardous locations such as nuclear reactors and waste storage sites. Increase of attenuation, luminescence, and radiation-induced index change have been used to design dose sensors for dose ranges up to 100 kGy. The attenuation-based sensors based on specialty doped fibers reach a saturation level above 10 kGy. To overcome this limitation, alternative techniques such as changes in fiber gratings are explored. The wavelength-encoded operation of fiber gratings can solve many measurement problems such as radiation-induced broadband transmission loss in optical fibers, source fluctuation, etc. Most Bragg grating-based sensors, reported till date, are either less sensitive or reach a saturation level near 50–150 kGy depending on the composition and grating writing technique [29–33]. Recent publications have reported measurements only up to 100 kGy. The authors Henchel et al. [35] used specialty chiral gratings and reported measurements up to 100 kGy. However, the mode orders and fiber composition in sensitive gratings were not known. Rego et al. [36] have performed gamma dose measurements on arc-induced long period fiber gratings up to 500 kGy but found no measurable shift in the resonance wavelength. Gusarov et al. [16, 17, 37] have conducted high dose measurements on FBGs but did not find high sensitivity. We have discovered sensitive gratings in commercially available single mode fibers with known composition and mode orders [38, 39]. Our results and approach are described. These are believed to be the first studies of CO2 written long period gratings up to 1 MGy.

6.5.2 Optical fiber composition optimization for high gamma dose and temperature sensing applications

Following requirements explain the need for novel radiation dose sensors:


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

engineering structures; to date, however, their use has been limited to ambient and

There exist very few published reports on FBG strain sensors that have been functional at liquid nitrogen temperature. Zhang et al. have reported one such FBG sensor used to strain sensing at 77 K and used in high temperature superconducting magnet [29]. To date, an FBG sensor used to strain sensing at 4.2 K and used in low temperature superconducting magnet has not been reported. RRCAT, Indore is also working on FBG strain and temperature sensors for cryogenic applications. The sensors show linearity in strain range from 50 to 500 micro-strain at liquid nitrogen

Optical fibers offer a unique capability for remote monitoring of radiation in hazardous locations such as nuclear reactors and waste storage sites. Increase of attenuation, luminescence, and radiation-induced index change have been used to design dose sensors for dose ranges up to 100 kGy. The attenuation-based sensors based on specialty doped fibers reach a saturation level above 10 kGy. To overcome this limitation, alternative techniques such as changes in fiber gratings are explored. The wavelength-encoded operation of fiber gratings can solve many measurement problems such as radiation-induced broadband transmission loss in optical fibers, source fluctuation, etc. Most Bragg grating-based sensors, reported till date, are either less sensitive or reach a saturation level near 50–150 kGy depending on the composition and grating writing technique [29–33]. Recent publications have reported measurements only up to 100 kGy. The authors Henchel et al. [35] used specialty chiral gratings and reported measurements up to 100 kGy. However, the mode orders and fiber composition in sensitive gratings were not known. Rego et al. [36] have performed gamma dose measurements on arc-induced long period fiber gratings up to 500 kGy but found no measurable shift in the resonance wavelength. Gusarov et al. [16, 17, 37] have conducted high dose measurements on FBGs but did not find high sensitivity. We have discovered sensitive gratings in commercially available single mode fibers with known composition and mode orders [38, 39]. Our results and approach are described. These are believed to be the first studies of CO2

6.5.2 Optical fiber composition optimization for high gamma dose and temperature

Following requirements explain the need for novel radiation dose sensors:

a. Measurement of precise dose delivery is very crucial for treatment of cancer

b. In the case of gamma source misplacement by universities or hospitals, it is important that state-of-the-art sensors away from the source are required.

c. Accelerators, fusion reactors, nuclear waste sites, and accidental leaks in reactors all require a sensitive, large area but remote dose sensors. Typically, the dose in various conditions and installations are: Tokamak Fusion reactor system, Japan (behind coils: 2 kGy, behind tiles: 200 MGy, 1.1 m behind port plug: 15 Gy). For space-based systems, total 10 year dose is around 100 kGy.

high temperatures, typically in the range of 273–773 K.

temperature.

6.5.1 Nuclear radiation sensors

Applications of Optical Fibers for Sensing

written long period gratings up to 1 MGy.

(40–50 Gy in about 20 sittings).

sensing applications

70

To cover a broad dose range from few Gy to 1 MGy, novel sensor systems like gratings are desirable. For most fibers, the increase in attenuation with dose saturates near few kGy which is accumulated within a relatively short time at certain critical locations and so they need to be replaced frequently. Even space-based systems are qualified for a dose up to 100 kGy.
