Optical waveguides – Optical waveguide sensor
Reexamination Certificate
1998-11-12
2001-10-09
Font, Frank G. (Department: 2877)
Optical waveguides
Optical waveguide sensor
C356S465000, C250S227140
Reexamination Certificate
active
06301400
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates in general to the field of fiber optic sensors. In particular, the invention relates to a fiber optic interferometric sensor. More particularly, it relates to magnetic field and current sensing with a Sagnac loop, not having rotation sensitivity.
Over the past decade, Sagnac interferometers have received attention in the application of magnetic field sensing and current sensing. Fiber optic current sensors are particularly advantageous over iron-core current transformers, since fiber optic sensors are non-conductive and lightweight. Furthermore, fiber optic sensors also do not exhibit hysteresis and provide a much larger dynamic range and frequency response.
Fiber optic current sensors work on the principle of the Faraday effect. Current flowing in a wire induces a magnetic field that, through the Faraday effect, rotates the plane of polarization of the light traveling in the optical fiber wound around the current carrying wire. Faraday's law, stated as
I=H·dL
where I is the electrical current, H is the magnetic field and the integral is taken over a closed path around the current. If the sensing fiber is wound around the current carrying wire with an integral number of turns, and each point in the sensing fiber has a constant sensitivity to the magnetic field, then the rotation of the plane of polarization of the light in the fiber depends on the current being carried in the wire and is insensitive to all externally generated magnetic fields such as those caused by currents carried in nearby wires. The angle, &Dgr;&phgr;, through which the plane of polarization of light rotates in the presence of a magnetic field is given by
&Dgr;&phgr;=
V∫H·dL
where V is the Verdet constant of the fiber glass. The sensing optical fiber effectively performs the line integral of the magnetic field along its path, which is proportional to the current in the wire, when that path closes on itself. Thus we have that &Dgr;&phgr;=VNI where N is the number of turns of sensing fiber wound around the current carrying wire. The rotation of the state of polarization of the light due to the presence of an electrical current is measured by injecting light with a well defined linear polarization state into the sensing region, and then analyzing the polarization state of the light after it exits the sensing region.
A typical related-art optical fiber current sensor is disclosed in U.S. Pat. No. 5,644,397 issued Jul. 1, 1997 to inventor James N. Blake and entitled “Fiber Optic Interferometric Circuit and Magnet Field Sensor”, which is incorporated herein by reference. Optical fiber current sensors are also disclosed in U.S. Pat. No. 5,696,858 issued Dec. 9, 1997 to inventor James N. Blake and entitled “Fiber Optics Apparatus and Method for Accurate Current Sensing”, which is incorporated herein by reference.
The related-art sensing of current suffers from a number of difficulties. Exceptionally stable optical components are required for measuring the polarization state changes with the accuracy needed for certain applications such as revenue metering. In addition, birefringence present in the sensing region rotates the plane of polarization as well as current, yielding an indistinguishable signal. Mechanical disturbances such as vibrations in the sensing fiber can yield a time varying birefringence, which yields signals indistinguishable from time varying currents. Similarly, thermal disturbances may also produce erroneous results.
Accordingly, a need has arisen for a fiber optic current and magnetic field loop sensor that is insensitive to rotation and time-varying effects such as changing temperature and vibration, and does not require exceptionally stable optical components and low birefringence in the sensing region.
SUMMARY OF THE INVENTION
In accordance with the present invention, an optical interferometric current sensor configuration is provided which eliminates or substantially reduces the disadvantages associated with prior optical sensors.
The current sensor includes a polarization maintaining optic fiber forming a sensing loop and has a rotation-compensating fiber coil. Counter-propagating circularly polarized light waves propagate along the optical path and pass through a magnetic field-sensitive sensing medium. A differential phase shift induced in the light waves in the presence of a magnetic field, is then detected by a photo detector and the value of the sensed parameter is determined by the associated signal processing electronics.
An all-guided wave version of the current sensor has fiber quarter wave converters that are used to convert linearly polarized light into circularly polarized light. The quarter wave converter is preferably constructed from a short section of long beat length polarization maintaining fiber. The sensing medium or loop may be made from a low linear birefringence optical fiber or fiber with high circular birefringence. The latter can be made by spinning the fiber during fabrication.
The loop sensor configuration detects a magnetic field of a current conducted by a wire. However, the sensor known as a Sagnac interferometer is also sensitive to the rotational movement of the loop. This rotation effect is compensated with a fiber loop or coil inserted in the optical circuit. The compensation is for errors due to rotation or angular vibration of the sensing loop. The compensation loop or coil has the same rotation sensing product (RSP) of number of winding turns times the area enclosed by the loop or coil, as that of the current-sensing loop or coil. N
s
A
s
=−N
c
A
c
, where N
c
A
c
is the RSP of the compensating coil and N
s
A
s
is the RSP of the sensing coil. The minus (−) sign indicates that the compensating coil has a sensing axis polarity opposite to that of the current-sensing loop or coil. Thus, the rotation sensitivity of the compensating coil is equal and opposite of that of the sensing coil, resulting in no rotation sensitivity by the overall current sensor.
Another aspect of the invention is to fabricate the rotation compensating (RC) loop or coil with a fixture so that the coil or loop can be flipped over, i.e., the axis polarity of the compensation coil in the optical circuit is reversed. This permits the calibrating of the non-reciprocal optical phase shift of the sensor by measuring, and then removing or nulling out the effects of rotation rate for precise current sensing. A first reading of the sensor is made to measure or calibrate sensitivity to rotation and then the RC coil can be flipped over for a second reading of current only. This reveals the effects of rotation or vibration. Thus, the sensor can be first calibrated and then used for accurate current sensing without the rotation or vibration sensitivity. Alternatively, the compensating coil may be just left in place to continuously eliminate sensitivity to rotation.
The optical current sensor may have a sensing loop or coil with a small RSP and a delay line coil to lower the proper frequency of the Sagnac sensing loop or coil so that the bias modulation frequency can be low enough for low drift loop closure electronics. The delay line coil may have a large RSP relative to that of the sensing loop or coil and has only rotation sensitivity that is of concern. One may use, and oppositely wind a second delay line coil to null the rotation sensitivity of the first delay line coil.
REFERENCES:
patent: 5644397 (1997-07-01), Blake
patent: 5696858 (1997-12-01), Blake
patent: 0 074 465 (1983-03-01), None
Arditty, H. et al., “Current sensor using state-of-the-art fiber-optic interferometric techniques,” IOOC '81: Third International Conference on Integrated Optics and Optical Fiber Communications, San Francisco, CA, USA. pp. 128-130, Apr. 1981.*
PCT search report, PCT/US 99/26499.
Bergh et al, geometrical Fiber Configuration for Isolators and Magnetometers, Proceedings of teh First Intl. Conference on Fiber-Optic Rotation Sensors and Related Technologies, Springer Series in Optical Sciences,
Font Frank G.
Lee Andrew H.
Nxtphase Technologies SRL
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