Arrangement for the magneto-optical measurement of currents

An arrangement for the magneto-optical measurement of currents with a linearly polarized light beam in which the measuring path contains an analyzer with two orthogonal pass directions, over which the partial beams are conducted, has a light deflector which directs one partial beam alternatingly at a high frequency to a detector which is arranged in the other partial beam so that the detector receives alternatingly the measurement signal with the noise component and the noise component only.

BACKGROUND OF THE INVENTION 
This invention relates to the magneto-optical measurement of currents in 
general and more particularly to an arrangement for the magneto-optical 
measurement of currents utilizing a rotation of the plane of polarization, 
dependent on the current to be measured, of a linearly polarized light 
beam, which is split into partial beams with orthogonal directions of 
polarization. 
Measuring devices for currents in high voltage conductors and for large AC 
currents with a DC component wherein a light beam is fed via a polarizer 
and a magneto-optical measuring sensor, as well as through an analyzer, to 
a detector which is followed by an electronic circuit are known. The plane 
of polarization of the light beam is subjected to a rotation corresponding 
to the magnitude of the current in the measuring sensor, which is 
influenced by the magnetic field of the current to be measured. In an 
evaluator arranged at low-voltage potential, the magnitude of the rotation 
is converted, in an analyzer, into a corresponding intensity signal which 
can be picked up by a photo detector. The output signal of the detector is 
fed to the electronic circuit. 
In one known arrangement, the evaluator contains an analyzer in which the 
light coming from the measuring sensor is split into two partial light 
beams, the planes of polarization of which are orthogonal and which change 
their intensity in opposite directions with the angle of rotation of the 
plane of polarization of the incident beam. The two partial light beams 
are fed to separate detectors which can preferably be semiconductor photo 
diodes and the outputs of which are fed to a differential amplifier. The 
difference voltage at the amplifier input is a measure for the Faraday 
rotation of the measurement signal. With this method, the noise components 
of the measurement signal, which are caused by intensity variations of the 
light beam, preferably a laser beam, can be eliminated. However, the photo 
diodes used have a small spread in their sensitivity. In addition, it is 
unavoidable, in a technical realization of the design, that the laser 
beams on the semiconductor photo cathode vary locally. These diodes have a 
production related location dependence of their photo sensitivity. Beam 
displacements of a few .mu.m can cause signal variations of up to several 
percent. The detectors associated with the two partial beams therefore 
furnish different signal variations which cannot be eliminated by the 
known differential measuring method (German Pat. No. 2,130,047). 
Another known method (Rogers in "Optical Methods for Measurement of Voltage 
and Current at High Voltage", A.I.M., Leige, Traitment des donnes--1977, 
page 6, para. 3.2(c), Intensity Distribution Noise) therefore, works with 
a modulated light beam which is fed, via the measuring sensor and an 
analyzer, to a single photo diode, the output signal of which is processed 
in an electronic circuit. 
The detector measures the intensity of the arriving measurement signal 
which also contains the superimposed modulation signal. Demodulation takes 
place in the following electronic circuitry. The intensity is influenced 
by the Faraday rotation in the measuring sensor as well as by the noise 
components, namely, the intensity fluctuations of the radiation source, as 
well as intensity variations which are caused by mechanical vibrations of 
the optical components in the ray path, and in addition, by the 
sensitivity of the photo diode itself. With this measuring method, the two 
intensity components can be separated from each other and the noise 
component on the measurement signal is suppressed. However, the linearity 
between the useful signal and the measurement signal is sufficient only at 
small angles of rotation up to about 2.degree.. For larger angles of 
rotation, with a correspondingly larger signal to noise ratio, a 
nonlinearity is obtained which depends on the signal amplitude. 
SUMMARY OF THE INVENTION 
It is now an object of the present invention to provide a measuring 
arrangement for very accurate measurements with a measuring error of, for 
instance, less than 0.2%, which permits a substantially larger rotation of 
the plane of polarization. 
According to the present invention, this problem is solved with a two beam 
arrangement of the type mentioned at the outset by providing a light 
deflector in one partial light beam which directs that partial light beam 
alternatingly, at a high frequency, to a detector which is arranged in the 
other partial light beam. The direction of polarization of the measurement 
signal is switched back and forth between the two orthogonal directions of 
polarization at the switching frequency which is at least one order of 
magnitude and, preferably, at least two orders of magnitude higher than 
the signal frequency. The detector therefore receives alternatingly the 
intensity of the measurement signal in one state of polarization and 
subsequently the sum of the intensities of both partial light beams with 
the two states of polarization.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1 there is shown a radiation source 2, a measuring sensor 6, an 
analyzer 8 and a detector 10. The detector 10 is followed by an electronic 
circuit 12. The arrangement also includes a light deflector 16. The 
radiation source 2, e.g., a laser, furnishes a beam, not specifically 
designated, of linearly polarized light, preferably a laser beam, the 
direction of polarization of which is rotated in the measuring sensor 6 as 
a function of the magnitude of a magnetic field which surrounds an 
electric conductor, not shown in the figure, preferably a high voltage 
conductor, the current of which is to be measured. Measuring sensor 6 can 
comprise a light guide fiber (e.g., a monomode fiber) which is looped 
around the current carrying conductor and is provided with a feed from the 
light source and a return to the analyzer. The analyzer 8 acts as a 
polarization beam divider and can preferably consist of the type of 
analyzer prism which is known as a Wollaston prism. Such analyzers contain 
mutli-part prisms of calcite or quartz. A Foster polarization beam divider 
can also be used. In the analyzer 8, two separate partial light beams P1 
and P2 are formed, the directions of polarization of which are orthogonal, 
i.e., include an angle of 90.degree.. The partial beam P2 strikes the 
light deflector 16 and is either passed by the latter alternatingly at a 
high switching frequency f.sub.m, as is indicated in the figure by the 
dashed line, or it is directed toward the detector 10. Light deflector 16 
can take any of a number of forms. For example, there are mechanical light 
deflectors which work with a rotating mirror, as well as acousto-optical 
light deflectors and also piezoelectric light deflectors. Mechanical light 
deflectors are relatively slow (kHz range). On the other hand, 
acousto-optical light deflectors can be used up into the MHz range. The 
detector therefore receives alternatingly, at the switching frequency of, 
say 50 kHz, which is far higher than the signal frequency of, for 
instance, 50 Hz, the partial light beam P1 with the one direction of 
polarization and subsequently, the sum of the two partial light beams P1 
and P2. 
With a direction of polarization P of the linearly polarized light beam, 
the orthogonal directions of polarization of the two partial light beams 
P1 and P2 always include, according to FIG. 2, a positive and a negative 
angle .alpha., respectively, of 45.degree.. For the partial light beam P1, 
which is fed to the detector 10 directly, the detector signal is then, for 
.alpha.=+45.degree.: 
EQU D.sub.- =(1/2) p(t).multidot.I(t) (1 -sin 2F(t)) 
where p(t) is the sensitivity of the detector, I(t) the intensity of the 
light beam and F(t) the measured Faraday rotation signal. 
This signal is received by the detector 10 as long as the light deflector 
16 passes its partial light beam P2. As soon as the light deflector 16 
directs its partial light beam toward the detector 10, the latter also 
receives the additional detector signal. 
EQU D.sub.+ =(1/2) p(t).multidot.I(t) (+sin 2F(t)) 
The detector signal therefore consists, during this time, of the sum of the 
individual signals and one obtains 
EQU D.sub.+ +D.sub.- =P(t).multidot.I(t) 
which corresponds to the pure noise component of the measurement signal. 
The detector 10 therefore delivers, in the rhythm of the switching 
frequency f.sub.m, alternatingly, the total measurement signal with the 
rotation component and the noise component and, subsequently, only the 
noise component. 
In a similar manner, the partial light beam P2 can be directed toward the 
detector 10 directly and the other partial light beam P1 fed additionally 
to the detector 10 in the rhythm of the switching frequency f.sub.m. 
In the electronic circuit 12 according to FIG. 3, the output signal of the 
detector 10 is alternately fed, via an electronic switch 20, to an 
electronic memory 22, e.g., a sample and hold circuit and an electronic 
computer 24 in the rhythm of the switching frequency f.sub.m predetermined 
by a clock 18. The output of clock 18 also drives the light deflector 16 
so that switch 20 and light deflector 16 are synchronized. The memory 22, 
which operates, for instance, as a sample and hold circuit, samples the 
detector signal D.sub.-, stores it and passes it on to one input of the 
computer 24 during the following time interval, after the switch 20 has 
switched over to the position shown on the drawing. During this time, the 
sum signal D.sub.+ +D.sub.- is fed directly to the other input computer 
24. 
The computer forms the quotient of the detector signal D.sub.- and the 
detector signal D.sub.30 +D.sub.-, representing the sum of the partial 
light beams P1 and P2, and one obtains, at the output 26, the signal: 
EQU U.sub.26 =D .sub.- /(D.sub.30 +D.sub.-)=(1/2) (1 -sin 2F(t)). 
The computer module 24 can, in principle, be implemented with an 
operational amplifier. However, a two-quadrant analog divider is 
preferred. 
The constant factor 1 can be eliminated, for instance, by electronic 
filtering, and the sine can be linearized in a manner known per se with an 
electrical arcsin module.