A fiber optic relative humidity (RH) sensor based on the tilted fiber Bragg grating (TFBG) coated with graphene oxide (GO) film was presented. Amplitudes of the cladding mode resonances of the TFGB varies with the water sorption and desorption processes of the GO film, because of the strong interactions between the excited backward propagating cladding modes and the GO film. By detecting the transmission intensity changes of the cladding mode resonant dips at the wavelength of 1557 nm, the maximum sensitivity of 0.129 dB/%RH with a linear correlation coefficient of 99% under the RH range of 10–80% was obtained. The Bragg mode of TFBG can be used as power or wavelength references, since it is inherently insensitive to RH changes. In addition, the proposed humidity sensor shows a good performance in repeatability and stability.

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A low frequency testing technology based on eddy current technique is proposed for detecting defects in some special equipment surface. A two-dimension model is built to simulate the distribution of low frequency (10 Hz) magnetic flux density nearby the surface of a metal plate. The influence of lift-off effect, coil diameter, crack shape on the measurement are discussed. And the crack measurement sensitivity of 0.6 pm was obtained.