Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
2002-03-01
2004-12-14
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
Reexamination Certificate
active
06831749
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a current sensor using a Sagnac interferometer including an optical fiber coil set within a magnetic field generated by a current and through which a clockwise and counter-clockwise light are propagated to undergo a Faraday effect to have their polarization planes rotated in mutually opposite directions to produce a phase difference between them, whereby the detection of the phase difference allows the current to be determined.
BACKGROUND OF THE INVENTION
The measurement of a current through a transmission line used in the art of power transmission and distribution generally employs a transformer comprising an iron core and a coil winding thereon. Since the transformer represents a purely electrical instrument, it is required that the transformer satisfies electrical noise resistance and dielectric strength requirements, and depending on the location where it is installed, a consideration must be paid to the outer profile and dimension.
Sagnac interferometer comprising an optical fiber coil is being investigated and developed for its use in a current sensor which is free from influences of electrical noises and which does not require a dielectric strength to be secured. Sagnac interferometer comprising an optical fiber coil has been used in the art of detecting the rotation of a moving body in an optical fiber gyro application. In addition to detecting the rotation, Sagnac interferometer exhibits a response to a magnetic field generated by a current and such response can be utilized to determine a current. Specifically, when a magnetic field is applied to an optical fiber coil which comprises a transparent material, the Faraday effect causes a rotation of a polarization plane, and an angle of rotation of the polarization plane is proportional to both the strength of the magnetic field and a distance through which a light passes in the magnetic field. A phase difference is produced between dextrorotatory and levorotatory light passing through the optical fiber coil due to a rotation of the polarization plane. The detection of the phase differences allows the magnitude of the current which generated the magnetic field to be determined. A conventional example of a current sensor using a Sagnac interferometer will now be described with reference to FIG.
1
.
In
FIG. 1
, light emitted from a light source
1
passes through an optical directional coupler or a first optical branch unit
2
, and further passes through a first polarization filter
3
to a second optical branch unit
4
where it is split into levorotatory and dextrorotatory light to impinge on a current sensing coil
6
. The levorotatory light is modulationed in a phase modulator
5
, whereupon it passes through a quarter-wave plate
16
, impinges on one end of the coil
6
, proceeds through the coil
6
as levorotatory light to be emitted therefrom, impinges on a second quarter-wave plate
17
, and then successively passes through the second optical branch unit
4
and the first polarization filter
3
to impinge on the first optical branch unit
2
where it is branched into a light receiver
7
to be received thereby. On the other hand, dextrorotatory light from the second optical branch unit
4
passes through the second quarter-wave plate
17
to impinge on the current sensing coil
6
, proceeds through the coil
6
clockwise to be emitted therefrom, impinges on the first quarter-wave plate
16
where its optical phase is modulated in the phase modulator
5
. The phase modulated dextrorotatory light successively passes through the second optical branch unit
4
and the first polarization filter
3
to impinge on the first optical branch unit
2
where it is branched into the light receiver
7
to be received thereby. It is to be understood that each of the first quarter-wave plate
16
and the second quarter-wave plate
17
converts a linearly polarized light which is incident from the polarization filter
3
into a circularly polarized light which is emitted, and also converts a circularly polarized incident light into a linearly polarized emitted light. It will be noted that a modulation input is input to the phase modulator
5
from an oscillation circuit
9
in order to perform an optical phase modulation of dextrorotatory and levorotatory light.
When an electric wire
10
is brought close to an end of the current sensing coil
6
which is subject to a magnetic field such that the diametrical direction of the coil is on an extension of the wire
10
, a phase difference is produced between the dextrorotatory light and the levorotatory light after passing through the coil
6
, and the dextrorotatory light and the levorotatory light emitted from the coil are subject to a synthesizing interference in the second optical branch unit
4
, with consequence that the light receiver
7
receives phase modulated light having an optical strength which varies in accordance with the phase difference. A change in the strength of the interfered light has a frequency which coincides with the frequency of the modulation signal from the oscillation circuit
9
, and a phase which corresponds to the phase difference between the levorotatory light and the dextrorotatory light. Upon reaching the light receiver
7
, the phase modulated light is converted into an electrical signal having an amplitude which varies in accordance with the optical strength. The electrical signal which is obtained by the photoelectric conversion is input to a synchronous detector
8
. The modulation signal which is supplied to the phase detector
5
is input to the synchronous detector
8
from the oscillation circuit
9
as a reference signal, thus performing a synchronous detection of the output from the light receiver
7
which is input thereto. The synchronous detection output corresponds to the phase difference which is in turn proportional to the magnetic field applied to the current sensing coil
6
. (For details of the phase modulation, see Japanese Laid-Open Patent Applications No. 99/351883 and 01/21363.)
As mentioned above, a current sensor using a Sagnac interferometer determines the magnitude of a current which generated a magnetic field applied to a current sensing coil, by causing circularly polarized lights to impinge on opposite ends of the coils and propagate therethrough as a levorotatory light and a dextrorotatory light and causing the both lights having a phase difference therebetween to interfere with each other so that the resulting interfered light has a varying optical strength which can be used to determine the magnitude of the current.
In order for Sagnac interferometer to operate as a current sensor, it is necessary that circularly polarized lights be incident on the current sensing coil
6
as mentioned above. To satisfy this requirement, in the conventional example shown in
FIG. 1
, optical fibers or fiber portions shown in thick lines are constructed by polarization maintaining optical fibers. Specifically, except for the optical fiber which forms the current sensing coil
6
, an optical fiber extending from the light source
1
to the first optical branch unit
2
and having a length on the order of one meter, an optical fiber extending from the first branch unit
2
to the first polarization filter
3
and having a length on the order of one meter, an optical fiber extending from the first polarization filter
3
to the second optical branch unit
4
and having a length on the order of one meter, and an optical fiber extending from the second optical branch unit
4
to the phase modulator
5
and having a length on the order of one meter are each formed by a polarization maintaining optical fiber. Assuming a total length of the optical fiber which forms the current sensing coil
6
to be ten meters, the length of each of optical fibers extending from the second optical branch unit
4
to the first quarter-wave plate
16
and extending from the second optical branch unit
4
to the second quarter-wave plate
17
has a length which is chosen to be about fifty meters, each optic
Ohno Aritaka
Sasaki Kinichi
Takahashi Masao
Terai Kiyohisa
Usui Ryuji
Gallagher & Lathrop
Japan Aviation Electronics Industry Limited
Lathrop, Esq. David N.
Lee Andrew H.
Smith Zandra V.
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