MICRORESONATOR FIBER-OPTIC SENSORS

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.

Mechanical resonators are well recognized in the general transducer industry as providing the potential for accurate measurements; their frequency-coded signals may be readily transmitted without distortion. In fiber optics, the resonators are set in motion and interrogated optically. The excitation of the mechanical oscillation is accomplished photothermally using an intensity-modulated laser radiation, while the displacements of the resonant beam are detected with the aid of the Fabry-Perot interferometer formed by the partially reflecting surface of the vibrating beam and the output end surface of the optical fiber. Oscillation of the beam modulates the reflectivity of a cavity, so that an intensity-modulated light propagates back, when the Fabry-Perot cavity is illuminated with a CW laser source.

Animation shows the microresonator in the form of microbridge illuminated by pulsed light from the optical fiber. Microresonator is coated by metal. Because of different linear expansion coefficients of the microresonator and coating any heating of such a structure will lead to bend of it. As a result, when the pulsed optical radiation is absorbed by the microresonator, the temperature of the illuminated region is changed and the flexural oscillation of microresonator is exited. We can see from the animation that the microresonator oscillation occures in phase with the light modulation.

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