<title>Fiber optic electric-field microsensor</title>

Abstract

A new method and apparatus are presented by which electric fields (AC and DC) can be measured in high voltage (HV) environments above ground using a variable gap, Fabry-Perot micro-cavity transducer which is mounted on the tip of a multimode optical fiber. By constructing the Fabry-Perot cavity as a conductive Faraday cage, external electric fields can be detected by the electrostatic forces they exert on the top surface of the cavity where a flexible, corrugated, silicon diaphragm is incorporated. Under the action of the electrostatic forces, the diaphragm deflects varying the gap of the cavity which is measured as a change in the backreflected light. To make the device insensitive to bending and transmission losses in the fiber, a dual wavelength referencing technique is employed. Thus, the magnitude of the electric field can be related to a change in the intensity ratio at two different wavelengths. The sensor is characterized by being small, lightweight, unobtrusive, accurate and immune to electromagnetic interference (EMI), temperature or pressure effects. DC electric fields in the range of 0 to 300 KV/m have been successfully measured. The minimum field intensity detected was of the order of 40 KV/m. This relatively low sensitivity is due to the high stiffness of the diaphragm arising from the high boron-diffusion. However, higher sensitivities are possible by thinning the diaphragm, increasing the radius, reducing the boss ratio or decreasing the corrugation depth.