Fiber Bragg gratings (FBGs) are beginning to be widely used for non-destructive evaluation (NDE) of structures. We explore here a novel and alternate optical scheme for NDE involving planar thin film gratings. These gratings could be fabricated directly on surface-coated thin-film or fabricated on a suitable substrate separately which is then attached to the surface. These gratings are fabricated in azo-dye-doped organic polymers using a holographic technique similar to that used for the fabrication of FBGs. Sensitivity of this technique for detecting surface acoustic waves is compared to the relatively traditional technique with FBGs.

Figure 1 below shows the schematic of an experimental set-up to compare thin film gratings with FBGs for detecting acoustic waves.

A FBG and a thin-film grating are adjacently attached to the surface while an acoustic transducer is mounted on the other end of a planar structure. FBG is fabricated for a Bragg wavelength around 1300 nm and needs a relatively more sophisticated detection scheme with a high-resolution spectrometer, a spectrum analyzer or a narrow-band tunable laser. By comparison, the thin-film grating diffraction can be monitored with a cheap He-Ne or any other low-power laser source together with a knife edge. Signal processing for both techniques is done with a lock-in amplifier. Figure 2 shows the characteristic derivative acoustic signal from the FBG for a Bragg wavelength centered around 1284.3 nm. Figure 3 shows the acoustic signal from the thin-film grating. Diffracted He-Ne beam is intensity modulated at the transducer frequency by the location of the knife edge. The transducer is alternately turned on and off to record the acoustic signal from the thin film grating. Both figures 2 and 3 are for a dynamic strain amplitude of 100 micro-strain. A full comparison will be presented for FBG and thin-film grating techniques.