The sensor characteristics of a coated Fiber Bragg grating (FBG) cryogenic temperature sensor depends mainly on the coating materials. The sensitivity of the coated FBG can be improved by enhancing the effective thermal strain transfer between the layers and the bare FBG. The dual coated FBG’s has a primary layer and the secondary coating layer. The primary coating layer acts as an intermediate buffer layer between the secondary coating layer and the bare FBG. The outer secondary coating layer is normally of higher thermal expansion sensitive metals. In this work, a detailed study is carried out on chromium and titanium intermediate buffer layers with various coating thickness and combinations. To improve the sensitivity, the secondary coating layer was tested with Mercury, Indium, Lead and Tin. The sensors were then calibrated in a cryogenic temperature calibration facility at Institute of Technical Physics (ITEP), Karlsruhe Institute of Technology. The sensors were subjected to several thermal cycles between 4.2 and 80K to study the sensor performance and its thermal characteristic. The sensor exhibits the Bragg wavelength shift of 14pm at 20K. The commercially available detection equipment with a resolution of 1pm can result into the temperature resolution of 0.071 K at 20K.
Dr Francisco. Araujo (FiberSensing) Prof. João Pedro Araújo (Department of Physics and Astronomy and IFIMUP-IN, University of Porto) Mr Romao Freitas (Department of Physics and Astronomy and IFIMUP-IN, University of Porto)