A Comparative Study of Radiation-Tolerant Fiber Optic Sensors for Relative Humidity Monitoring in High-Radiation Environments at CERN

In this paper, we report a comparative study of fiber optic sensors for applications of relative humidity (RH) monitoring in high-radiation environments. In particular, we present investigations carried out since 2011 by our multidisciplinary research group, in collaboration with the European Organization for Nuclear Research (CERN) in Geneva. Our research has been first focused on the development of polyimide-coated fiber Bragg grating (FBG) sensors, and recently, it has been extended to nanoscale metal oxide-coated long-period gratings (LPGs). Experimental tests in the [0-70] %RH range at different temperatures, before and after γ-ionizing radiation exposures, have been carried out to assess the sensors' performances in conditions required in experiments running at CERN. The advantages and disadvantages of the two proposed technologies are discussed in this paper in light of their possible application in high-energy physics environments. In particular, reported results suggest that LPG-based sensors can be preferred in some applications (particularly in presence of very low humidity levels) mainly because they are able to provide very high RH sensitivity (up to 1.4 nm/% RH), which is up to three orders of magnitude higher than that exhibited by FBG-based hygrometers. On the other side, compared with FBGs, LPGs are more difficult to multiplex due to limitations in terms of available bandwidth.