5. Ultra-high temperature distributed sensors

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

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

6.3 Theory

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

occurs is given by

yield

sensors.

67

6.4 Device designs

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

<sup>λ</sup>ð Þ <sup>m</sup> <sup>¼</sup> neff – ncl,m

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

> dneff <sup>d</sup><sup>ε</sup> � dncl dε

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 temper-

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

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

dneff dT � dncl dT <sup>þ</sup> <sup>Λ</sup> <sup>d</sup><sup>λ</sup>

dλ <sup>d</sup><sup>ε</sup> <sup>¼</sup> <sup>d</sup><sup>λ</sup> d δneff

ature sensitivity of LPG grating is given by

dλ <sup>d</sup><sup>ε</sup> <sup>¼</sup> <sup>d</sup><sup>λ</sup> d δneff Λ (5)

<sup>þ</sup> <sup>Λ</sup> <sup>d</sup><sup>λ</sup>

dΛ 1 L dL

<sup>d</sup><sup>Λ</sup> (6)

dT (7)

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

## 6. Strain sensors for nuclear environment

### 6.1 Wavelength encoded strain sensors

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 and developed such sensor devices.

### 6.2 Design principle

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 reported by any group to the best of our knowledge.
