A novel method for simultaneous measurement of current and voltage using one low-birefringence fiber

Abstract

A novel method for simultaneous measurement of current and voltage employing a single monomode low-birefringence (Lo-Bi) fiber is described. This uses the Faraday effect for detection of current and the Kerr effect for detection of voltage. The waveform of the output light intensity due to the Kerr effect has been detected and the Kerr constant was measured as 5.4×10⁻¹⁶ m/V⁻² at 25°C. Theoretical analysis shows that the current component can be measured free from the influence of the voltage component and the two components can be easily separated.

Introduction

Over the last few years, current and voltage measurement systems based on optical devices have been developed. A number of quite different optical current transducer (OCT) and optical voltage transducer (OVT) systems are now on the market or undergoing field trails1, 2; most of these can detect only a single system parameter, namely current or voltage.

To measure both current and voltage with an optical fiber, the best method is simultaneous measurement using one sensor; this can make the measuring system very simple, cheap and reliable and can allow the phase difference between current and voltage to be measured correctly.

In 1976, A.J. Rogers constructed a system for simultaneous measurement of current and voltage using crystalline quartz[3]. This used a combination of a magneto-optic and an electrogyration effect in the material. This proposal proved not to be practical as a result of the effect of the common mode electric field appearing on the current reading (and vice versa) and the electrogyration effect of crystalline quartz is very weak.

Another method for simultaneous measurement of current and voltage is to use a single monomode low-birefringence (Lo-Bi) fiber as the transducing element, employing the magnetic stress effect for current measurement and the piezoelectric effect for voltage[4]. The forces caused by the current and the voltage act on the Lo-Bi fiber simultaneously and induce birefringence. The fundamental component of the phase perturbation is proportional to the voltage and the second harmonic component is proportional to the current. Thus the two parameters can be separated with the help of suitable frequency filters. In fact, this scheme modulates the light with mechanical force, which can easily be corrupted by the environment.

In this paper we describe an experimental system for simultaneous measurement of current and voltage using a single monomode Lo-Bi fiber. This uses the Faraday effect for detection of current and the Kerr effect for detection of voltage. The system is very attractive owing to its simplicity and its convenience.