The creation of optical fibers with a small extinction coefficient has encouraged the development of sensors operated entirely by optical signals and requiring no external electrical supplies. These sensors are often called passive sensors. This means that the sensitive elements of such sensors do not have any electrical circuits, semiconductors, etc., and consist, in many cases, of only elements such as silicon and silicon dioxide. These materials are electrically passive, i.e. they do not distort the surrounding electric and magnetic fields, they do not radiate electromagnetic waves and are not sensitive to electromagnetic interference. Passive sensors can remotely operate in the presence of strong electromagnetic interference, a hostile environment, explosiveness, and at high temperatures. These conditions often appear in space, plasma, on oil platforms, power stations, and in many other situations. The applications of fiber optic sensors are widely known for the measurement of temperature, pressure, force, displacement, acceleration, etc.

The passive sensors are usually developed as intensity-encoded sensors, and therefore require special means such as a reference channel in order to minimize the influences of long-term aging of their source characteristics, or short-term fluctuations (including optical power loss in the cable) on the accuracy of measurements. Using frequency as the information parameter for a quantity to be measured presents two major advantages: it can be transmitted through extended systems and over large distances without error, and secondly, it can be easily digitized by counting its periods.

One concept for frequency-encoded fiber-optic sensors makes use of micromechanical resonant structures that are anisotropically etched in silicon and are more frequently in a form of a rectangular beam clamped at both ends on a comparatively massive substrate. The basic principle is that the parameter of interest (such as acceleration) deforms the substrate, the mechanical stress appears inside the resonant beam, and consequently, the resonance frequency of the fundamental flexural mode is changed proportionally to the applied acceleration.