A sensor that uses thin optical fibers to carry light to and from a location to be probed. In performing the sensing, light can be lost from the fibers or modified in velocity by the action of the phenomena on the fiber. Fiber-optic sensors are ideal for probing in remote or hostile locations, where miniature sensors are required such as in the body, or where extreme sensitivity is required. Two classes of fiber sensors have evolved: intensity sensors, in which the amplitude of light in the fiber is changed during sensing, and interferometric sensors, in which the velocity of light or its phase is modified during sensing. The latter class has proved to be extremely sensitive; intensity sensors are used where moderate performance is acceptable and lower cost is important. Depending on their design, intensity sensors can respond to pressure, temperature, liquid level, position, flow, smoke, displacement, electric and magnetic fields, chemical composition, and numerous other conditions.
Optical interferometry is one of the most sensitive means of detecting displacements as small as 10-13 m. Interferometric fiber sensors apply this technology to sense many physical phenomena. A fiber gyro based on the Sagnac effect is formed by making a fiber loop which, when rotated, causes the light traveling in both directions in the loop to experience different velocities with or against the rotation. A second type of interferometric sensor is constructed by using the Mach-Zehnder interferometer. Sensitivities equal to or surpassing the best conventional technologies have been achieved in these sensors. See Interferometry