This invention relates generally to sensors for monitoring extremely small mechanical displacements, and in particular to an improvement in a Fiber Optic Bragg Grating (FOBG) sensor consisting of a unique demodulator for detecting changes in an optical signal induced by the sensed mechanical displacement. The invention has broad utility in applications in which small mechanical displacements are to be detected, for example in monitoring stresses in advanced materials used in aircraft components and detecting the onset of corrosion.
A typical FOBG consists of a Bragg grating formed within the core of an optical fiber. The grating consists of a region within the core, having a periodic spatial variation in its refractive index along the direction of the axis of the fiber core. Because of the spatial variations in the refractive index, the core of the optical fiber effectively has "lines," i.e. subregions of alternating higher and lower refractive index. These gratings may be formed by exposing a doped optical fiber to an interference pattern formed by two laser beams. The varying energy density in the interference pattern induces permanent changes in the index of refraction of the core of the optical fiber.
In practice, a coherent, but relatively broad band, light source is coupled to an optical fiber having one or more Bragg gratings. Each of the Bragg gratings reflects light in a narrow band of wavelengths which depends on the spacing of the lines of the grating. The reflected light beam is transmitted through a two-way optical coupler to a spectrum analyzer which, in effect "demodulates" the light beam modulated by the gratings. The spectrum analyzer is able to extract useful information concerning the condition of the gratings. The optical fiber may be coupled to an article under mechanical stress so that strain in the article effects changes in the spacing of the grating elements. The resulting changes in the spectrum of the returning light beam are detected and interpreted to obtain information on the strain in the article under stress.
A single optical fiber can incorporate several Bragg gratings, and the gratings can have different line spacings so that they reflect different bands of wavelengths. This makes it possible to distinguish between the optical signals returned by several gratings in a single optical fiber.
Previously known demodulation schemes include broadband optical filtering, and interferometry using a Fabry Perot cavity. These have various deficiencies such as high cost, bulkiness and sensitivity to thermal fluctuations and/or mechanical vibrations.