Influence of Humidity on Fiber Bragg Grating Sensors

We demonstrate the influence of the relative humidity (RH) on the wavelength of fiber Bragg grating sensors (FBGS), performing tests with five FBGS at different humidity and temperature conditions. These tests were performed in a climate chamber whose RH changes according to a scheduled profile from 30% to 90%, in steps of 10%. These profiles were repeated for a wide range of temperatures from to , in steps of . Two different types of instrumentation methods have been tested, spot welding and epoxy bonding, in two different materials, steel and carbon fiber reinforced polymer (CFRP). We discuss the results for each type of sensor and instrumentation method by analyzing the linearity of the Bragg wavelength with RH and temperature. 1. Introduction Optical fiber sensors are one of the most suitable options for aircraft structural health monitoring (SHM) systems due to the advantages they offer: immunity to electromagnetic noise, with the consequent increase of safety and a better protection against loss of information; little weight compared with traditional monitoring systems, leading to a reduction of fuel consumption; and wide working temperature ranges [1]. At present, the application areas for fiber sensors are increasing; they can be used for corrosion monitoring in metallic components, carbon fiber reinforced polymers (CFRP), aircraft components health monitoring, sensing pressure, temperature, or microelectromechanical systems (MEMS) accelerometers [2], impact detection, vibration and strain measurement, or static strain temperatures. An optical fiber is composed by a core, a thin strand of glass material that transmits light with very low loss, a cladding that reflects stray light back into the core, and a buffer coating that protects the fiber. A fiber Bragg grating (FBG) sensor consists of an ultraviolet (UV) periodic inscription, where a short length of photosensitive fiber is exposed to a periodic distribution of light intensity. As a consequence, the refractive index is altered according to light intensity, resulting in a periodic variation of the refractive index along the core. When a broad-spectrum light is sent to an FBG core, a certain wavelength of this light is reflected, called Bragg wavelength, allowing the rest of the incoming light to pass through without additional modification [3]. This reflected Bragg wavelength changes according to the applied strain and to temperature conditions. In order to separate the temperature effect from the strain we have used a not bonded FBG as a reference so that we could subtract this temperature