1. Field of the Invention
The present invention relates to a fiber-optic sensor for measuring electric field strengths or voltages and having a quartz body in accordance with the preamble of claim 1.
2. Discussion of Background
Fiber-optic sensors of this type are disclosed, for example, in EP-A1-0,316,619, EP-A1-0,316,635 and EP-A1-0,433,824, as well as in K. Bohnert, J. Nehring, Appl. Opt. 27, (1988), pages 4814-4818 or K. Bohnert, J. Nehring, Opt. Lett. 14, (1989), pages 290-292.
In the known sensors, the measurement principle is based on the fact that a circular disk which is made from a piezoelectric material (for example, quartz) and wound around with an optical fiber experiences in the electric or E-field a change in circumference which can be measured interferometrically by the resulting change in length of the optical fiber. In this case, it is chiefly the two-mode interferometer which has proved itself as a simple type of interferometer, cf. B. Y. Kim et al., Opt. Lett. 12, (1987), pages 729-731. The signal acquisition is performed, for example, by compensating the differential phase shift which is to be measured between the fiber modes.
Quartz is very well suited as a piezoelectric sensor material, because it has a sufficiently large piezoelectric effect and a small dielectric constant. Furthermore, quartz disks which are cut at right angles to the crystallographic x-axis (definition of axis in accordance with IRE Standard 1949, see further below) can be used for selective measurement of E-field components in the x-direction, that is to say to form the scalar product (E, x). By winding such a quartz body, which extends in the x-direction from the ground electrode to the high-voltage electrode, in an approximately uniform fashion extended along the cylinder axis in the x-direction, or by connecting in series a plurality of such quartz disks along an arbitrary integration path between ground and the high-voltage electrode, it being the case that the x-axis is aligned tangentially to the integration path and all the quartz disks are wound approximately uniformly with a continuous fiber, it is possible to approximate the line integral of the electric field
.intg.E dx
and thus to measure the voltage independently of the E-field distribution (EP-A1-0,316,619, EP-A1-0,316,635).
For measurement signal compensation, use is mostly made of a piezoelectric ceramic cylinder (EP-A1-0,433,824) whose piezoelectric effects are at least two orders of magnitude stronger by comparison with quartz and thereby permit a substantial step-down in voltage.
It has proved to be disadvantageous in the known sensors that their measurement sensitivity exhibits a disturbing temperature dependence. A decrease in the measurement signal by typically a few percent is found between -20.degree. C. and +70.degree. C. with the type of fiber mostly used to date. The reasons for this are, on the one hand, the temperature dependences of piezoelectric coefficients and, on the other hand, the temperature dependences of the fiber interferometer.
It has been proposed in EP-A1-0,433,824 to eliminate the temperature dependence by producing the piezoelectric sensor and the compensator from the same material and keeping them at the same temperature. The temperature-induced falsification of the measurement signal (E-field or electric voltage) is actively eliminated or reduced in this case, which requires a not inconsiderable outlay on apparatus and control.
Manifold quartz sections are known from the chronological industry, where quartz crystal oscillators are used for the purpose of frequency stabilization in order to compensate for the temperature dependence of the piezoelectric resonant frequency of the crystals in the 1st, 2nd or 3rd order (cf., for example, J. C. Brice, Review of Modern Physics 57 (1), (1985) pages 105-146; U.S. Pat. No. 3,766,616; U.S. Pat. No. 4,503,353; U.S. Pat. No. 4,447,753; U.S. Pat. No. 4,453,105). In the fiber-optic sensor considered here, however, the issue is not the temperature dependence of a resonant frequency of a quartz crystal oscillator, but how a change in the dimension of a quartz body which is caused piezoelectrically is converted into a change in length of a length of an optical fiber connected to it, and the temperature dependence of this effect.