Uncoated Fiber Bragg Gratings are in general insensitive to humidity.
However, FBGs are in general covered with a coating such as Polyimide, Acrylate or Ormocer, which can affect the wavelength of the FBG. This can be explained as follows:
The coating absorbs moisture from the air which causes the coating to swell and which results in apparent strain on the fiber and FBG. The magnitude of this humidity effect is strongly dependent on the type of coating, the stiffness of the fiber (or the diameter of the glass) and the thickness of the coating.
The graph below shows the effects of this influence for different types of DTG® fibers with glass diameters of 80 and 125 µm and an Ormocer coating varying between 140 and 195µm.
As can be seen, the sensitivity of a standard DTG®-fiber (125/195) is around 0.9 pm / % RH. When the coating diameter increases, this value also increases significantly.
As a consequence, a varying humidity level will also change the nominal wavelength of a FBG. Therefore, in the case of a coated fiber applied in a free position for temperature measurements, the effect of the humidity needs to be taken into account. FBGS has developed technologies to eliminate this effect. For more information please contact us: firstname.lastname@example.org .
It should be emphasized tat the influence of humidity on the FBG wavelength is only visible when the fiber is in a free position. If the FBG is attached to a structure (for example for strain measurements), this influence of the coating is no significant as the structure will impose the strain state on the fiber and not the coating.