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F iber-optic sensors are being used in a wide variety of applications. 1-5 Sagnac interferometers, which can determine movement by measuring the shift in interference fringes from two counterpropagating coherent beams in a ring, support fiber gyros on aircraft, missiles, rockets, and robots. Michelson interferometers, in which interference fringes indicate the length relationship between two legs of the interferometer, support strain and acoustics measurements for civil structures and underwater applications. Another type of fiber sensor, fiber-grating sensors, is emerging as a potentially low-cost solution to a wide variety of point-sensor measurements such as axial strain and temperature, transverse strain, shear strain, moisture, pressure , acoustics, vibration, and chemical content. Applications areas for these sensors include aerospace and structural monitoring. gratings in fiber UV light shining into the germanium-doped core of an optical fiber induces a local change in the index of refraction. One can create a grating in the core of the fiber by imaging an interference pattern (created either holographically or by a phase mask) through the side of the fiber. The resulting grating acts as a wavelength filter. The period of the induced index of Gratings written into fiber provide a mechanism for sensing strain. tutorial tutorial GOOD SENSE