Fiber Optic Bragg Grating Sensors

Abstract

This paper describes a new method that we have developed to generate Bragg grating filters in germania-doped communication fibers and discusses their applications as sensors. The gratings are formed by exposing a short section of the core, through the side of the fiber, to an interference pattern of intersecting coherent beams of UV light. A permanent periodic index modulation is produced by the interference fringes. It forms a phase grating which acts as a band rejection filter, passing wavelengths that are not in resonance with the grating and strongly reflecting wavelengths which satisfy the Bragg condition. The grating wavelength is sensitive to changes temperature and strain. Measurements of the shift in Bragg wavelength with these quantities are reported and compared with computed estimates of the sensitivity. A large number of independent gratings can be easily written along a length of fiber to make a distributed sensor system. We have also demonstrated that Bragg gratings can wavelength tune laser diodes, which could be used for detecting the wavelength shift of the Bragg sensors.