Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2002-11-15
2004-06-29
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C359S484010
Reexamination Certificate
active
06756781
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to optical sensors that detect the Faraday effect in crystal media, and more specifically, the class of transmissive optical sensors that rotate the plane of polarization of light traveling through a crystal media that is under the influence of external quantities such as magnetic fields, electrical currents which give rise to magnetic fields, or temperature fluctuations.
2. Related Art
More than 150 years ago, Michael Faraday discovered that when linearly polarized light travels through flint glass that is exposed to a magnetic field, its plane of polarization rotated. This property, now known as the Faraday effect, is widely used in the fiber optic telecommunications field, specifically to prevent reflected light energy from coupling back into a light source and changing source parameters such as frequency or power output. In sensor systems that exploit the Faraday effect, a sensor assembly is placed into a magnetic field. By monitoring the rotation of the incident polarization state, a direct measurement of the magnetic field intensity can be inferred. The relationship governing this phenomenon is best stated as:
&THgr;=
VHl
EQN(1)
where &THgr; is the measured angle of rotation of the field, V is a constant known as Verdet's constant, H is the applied magnetic field, and l is the optical path length. All materials exhibit the Faraday effect, but the magnitude varies greatly. For example, the Verdet constant for a SiO
2
crystal is approximately 3.2e−4 (deg/cm-Oe), while in ferromagnets such as the value can be on the order of 6.0e+5 (deg/cm-Oe).
When an optical path completely encircles a conductor, a numerical integration can be performed about the optical path, which results in the ability of relating the Faraday rotation directly to the current flowing through the optical path. In this instance, the rotation is related to current I by a form of Ampere's Law:
&THgr;=
V
∫Hdl
EQN(2)
&THgr;=
VI
EQN(3)
Finally, if N optical paths exist around the conductor, the total current in the conductor is
I
=
Θ
VN
EQN
(
4
)
Optical fiber is one material that exhibits a small Faraday effect. Based upon this, devices have been known and used for measuring the amount of current flowing through a conductor. By wrapping multiple turns of optical fiber around the conductor and applying Ampere's Law, the amount of current can be directly measured. Sensitivity is controlled in this fashion: applications requiring higher sensitivity wrap a higher number of turns around the conductor being monitored.
Unfortunately, using optical fiber as a sensor is often impractical in many applications because it is not feasible to interrupt power by disconnecting the conductor, installing the fiber coil assembly, then reconnecting the conductor. Another disadvantage of an all-fiber sensor is that in practical use, the loops that encircle the conductor can be no smaller than 4-5 cm in diameter. Violation of this condition typically results in tremendous temperature sensitivity, which then appears as an undesired rotation of the state of polarization of the desired signal.
Bulk glass is another material that exhibits a Faraday effect. An advantage of the use of bulk glass is that the sensor can be fabricated from materials with a higher Verdet constant, which improves the sensitivity to the influencing magnetic field. These bulk crystals can be annealed, which can release internal stresses, thereby reducing linear birefringence. By themselves, bulk-glass sensors are relatively mechanically stable in both temperature and mechanical handling. Bulk glass can be made relatively inexpensively, which portends well for mass production concepts using these sensors.
Bulk-glass sensors suffer from their own set of limitations. The transducers manufactured from bulk glass are large, relatively on the same order as the all-optical fiber sensors previously described. Bulk glasses are not ferromagnetic, hence their Verdet constants are lower, which restricts their applications to extremely high current measurement. Obtaining multiple circular paths around a bulk-glass arrangement in order to increase the sensitivity of the sensor has been accomplished by some researchers, but there are limitations of using this configuration in applications that experience tremendous temperature fluctuations. Finally, assembly and alignment of bulk-glass sensors has historically been performed by hand, resulting in tremendous labor costs that preclude their widespread use.
Ferromagnetic materials, such as bismuth- and terbium-doped yttrium-iron-garnet (BiTb
2
Y
3
Fe
5
O
12
) for example, have much larger Verdet constants per unit thickness. This results in a much smaller Faraday rotator to measure a given magnetic field strength, and the outcome is that a whole class of reduced size magneto-optical transducers is enabled. Methods to grow these materials are well established and directly support other markets, specifically optical telecommunications, hence tremendous economies of scale are realized that surpass that of bulk-glass and rival the cost of optical fiber. Packaging of the transducer becomes smaller with the introduction of high-Verdet constant materials, and thus manufacturing costs are significantly less than what is available with all-fiber or bulk-glass designs.
Applications for a reduce-sized magneto-optic transducer continue to grow. For example, the electric utility industry is experiencing tremendous pressures as consumer and regulatory demands upon the industry increase. Consumers, with expanding telecom, data processing, and other energy needs, are demanding “high-nines” reliability. Utilities are attempting to respond, but are doing so with an antiquated infrastructure that has an average age of 31 years. Regulatory pressures have created large uncertainties in the future ownership of assets, and hence infrastructure improvements have fallen sharply since the mid 1990's. Additionally, many utilities operate under rate caps and cannot pass costs onto consumers. Not surprisingly, the industry is looking to conserve capital, and is doing so by pushing equipment harder without fully understanding the long-term consequences, as well as deferring maintenance until corrective action is required. Even small percentage changes in distribution system operating efficiencies can result in hundreds of millions of dollars a year in savings. Hence, many utilities are reviewing technologies that can provide efficiency and reliability improvements.
Optical sensor technologies for utility applications promise to deliver lower-cost monitoring solutions to the industry. These technologies provide an entirely new means of measuring electrical current, conductor temperature, voltage, and combustible gasses. When combined with the latest wireless and network topologies, automated data delivery and control is possible, resulting in improved operations. Optical sensory systems which cost less than current state-of-the-art transformer-based systems gives utilities the key to unlocking information by which they can manage their systems much more efficiently, resulting in improved reliability and improved system efficiencies. Furthermore, widespread use of these technologies will result in the immediate notification and location of power faults and outages, potentially saving the utility industry and it's customers 100's of millions of dollars in outage costs.
U.S. Pub. No. US2001/0043064A1 to Bosselmann et al. discloses a pi-shaped transmissive polarimetric sensor that is comprised of a polarizer, a sensor element, and an analyzer. An output optical waveguide with a core diameter of at least 100 &mgr;m is used. The input light is uncollimated and unfocused. The sensor requires the use of a prism to steer the light from the input fiber to the sensor element, and correspondingly, from the sensor element to the output optical waveguide.
U.S.
Duncan Paul Grems
Schroeder John Alan
Airak, Inc.
Greenberg & Traurig, LLP
Kurtz II Richard E.
Snow Walter E.
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