Development of acoustic optics tunable filter and its application to strain sensing system

A highly accurate strain sensing system has been developed by combining an acoustic optic tunable filter (AOTF), an optical fiber, and fiber Bragg gratings (FBGs). The AOTF is composed of an interdigital transducer (IDT) to generate surface acoustic wave (SAW), a SAW guide, and polarization beam splitters (PBSs) for separating TE and TM modes from a light with both modes and optical waveguides consisting of a Li-diffused layer on a Y-X LiNbO₃ substrate. The AOTF outputs only the wavelength of light corresponding to the applied SAW frequency. The AOTF operates as an optical spectrum analyzer by applying the swept SAW frequency to the AOTF. A highly accurate strain sensing system has been contributed by combining the optical fiber, the fiber Bragg gratings (FBGs), and the AOTF used instead of a conventional optical spectrum analyzer. The each FBG has each period of the grating in the fiber. The only wavelength of the light corresponding to the period of grating of FBG reflects when the light with wide range wavelength is inputted. The AOTF measures the reflected wavelength. When the strain is applied to the FBG, the only reflected wavelength corresponding to the strained grating period of its FGB shifts. The strength of the strain can be obtained from the shift value of the wavelengths measured by the AOTF. The place, where the strain is received, can also be measured at the same time by setting up a lot of FBGs having different period of grating. The strain accuracy of 0.9 ppm was measured by using this method at the room temperature. But a temperature characteristic of the AOTF is not good, it is necessary to improve the temperature characteristic of this sensing system. Adopting additional FBGs for reference, measured accuracy of standard deviation of 0.63 pm was obtained under the temperature change from -20 to 70degC. This strain sensing system would be able to apply to monitoring or watching a fall of rock, a landslide, an earthquake, a vibration of a- building or a bridge, a trespasser, a water level of river, and so on.