Abstract
Use of Fiber Bragg Grating (FBG) sensor is very appealing for sensing low temperature and strain in superconducting magnets because of their miniature size and the possibility of accommodating many sensors in a single fiber. The main drawback is their low intrinsic thermal sensitivity at low temperatures below 120 K. Approaching cryogenic temperatures, temperature changes lower than a few degrees Kelvin cannot be resolved, since they do not cause an appreciable shift of the wavelength diffracted by a bare FBG sensor. To improve the thermal sensitivity and thermal inertia below 77 K, the Bare FBG (BFBG) sensor can be coated with high thermal expansion coefficient materials. In this work, different metal were considered for coating the FBG sensor. For theoretical investigation, a double layered circular thick wall tube model has been considered to study the effect on sensitivity due to the mechanical properties like Young’s modulus, Thermal expansion coefficient, Poisson’s ratio of selected materials at a various cryogenic temperatures. The primary and the secondary coating thickness for a dual layer metal coated FBG sensor have been determined from the above study. The sensor was then fabricated and tested at cryogenic temperature range from 4-300 K. The cryogenic temperature characteristics of the tested sensors are reported.
Highlights
Selection of right sensors and an appropriate measurement system has become a challenging task as the operational requirements of the new technologies demands to measure extreme temperatures and conditions
In the most advanced research centers, such as those using large particle accelerators [1], where cryogenic temperatures are essential for the proper operation of sensitive equipment, sensors capable of operating in these temperature ranges are essential
In this work, different metal were considered for coating the Fiber Bragg Grating (FBG) sensor
Summary
Selection of right sensors and an appropriate measurement system has become a challenging task as the operational requirements of the new technologies demands to measure extreme temperatures and conditions. 2. Coating thickness determination Figure 1 shows the model of the dual layer metal recoated FBG sensors (DMCFBG) where material 2 serves as primary coating layer while material 1 severs as secondary coating layer. To determine the coating thickness, the thermal stresses acting in the bare fiber and the coatings should be known These stresses can be calculated by analyzing a simple theoretical model of FBG sensor shown in figure 2.In figure 2, r0 = radius of core, r1 = radius of the primary material coating (C1),r2 = radius of the secondary material coating (C2),C1 = Primary coating material,C2 = Secondary coating material,P1 = Lateral Pressure at the glass fiber and primary coating interface,P2 = Lateral pressure at the primary coating and secondary coating interface. Using Lame formula [19], the stress components, at radius r in a circular thick walled tube subjected to internal pressure (pi) and external pressure (pe) can be expressed as [19]
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