Method of eliminating the effects of birefringence from the detection of electric current using Faraday rotation

• US 5,008,611 A
• Filed: 03/14/1989
• Issued: 04/16/1991
• Est. Priority Date: 03/14/1989
• Status: Expired due to Fees

• Current Assignee: BBA Canada Limited
• Original Assignee: Square D Company (Schneider Electric SE)
• Inventors: Ulmer, Edward A. Jr.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): Burns, William
• Application Number:
  US07/323,599
• Time in Patent Office: 763 Days
• Field of Search: 250/225, 250/231 R, 324/96, 350/374, 350/376
• US Class Current: 324/96
• CPC Class Codes: G01R 15/246 based on the Faraday, i.e. linear magneto-optic, effect

Abstract

An improved method and apparatus for measuring a target electric current utilizing a Faraday effect in which an optical medium is magnetically coupled with the target electric current and light is passed through the optical medium at an orientation for which the effects of birefringence on the optical medium can be disregarded in comparison with the Faraday effect, and the light departing from the optical medium is measured and analyzed in a manner permitting the target current to be accurately determined.

Claims

1. A method for minimizing the effect of stress birefringence within the optical apparatus of an optical current transducer providing an optical current signal proportional to a time-varying target current of electricity to be measured, said optical current transducer including at least one optical medium having a Faraday effect, said optical medium receiving a light beam of polarized light and transmitting the received light to produce an optical current signal, comprising the steps of:

geometrically selecting a geometric configuration of said optical medium to be placed within a magnetic field produced by said target current;

orientatedly selecting said optical medium to have an entrance surface, and an exit surface, said optical medium having fast and slow known characteristic directions in which if polarized light is oriented in the absence of a magnetic field the polarized light is transmitted without a change in polarization angle, a known Verdet constant, a known permeability, a known approximate birefringence comprising the difference between a fast index of refraction and a slow index of refractions, each index associated with an associated said characteristic direction and a known path length with respect to said received and transmitted light from said entrance surface to said exit surface from which said optical current signal exits;

mathematically determining the existence and identity of at least one non-birefringent angle of polarization with respect to said characteristic direction of said optical medium at which a polarized beam of light may be received by and transmitted through said optical medium along said path length and produce said optical current signal with a time-varying component, said optical current signal having a current polarization angle proportional to and produced by the target current and proportional to the path, length, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current;

aligning said received light with said non-birefringent angle; and

measuring said current polarization angle.

2. The method described in claim 1, in which said optical medium is a bulk optical medium and annealed in such a manner as to cause the characteristic directions within said optical medium to substantially line up with the major geometric faces of said optical medium prior to its placement within said magnetic field produced by said target current.

3. The method described in claim 1, in which comprises the additional step of experimentally verifying that said light beam introduced into said optical medium at said non-birefringnet angle may be passed through said optical medium along said path length and produce an optical signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current within a range of temperatures when subjected to certain selected and fixed target currents, said selected and fixed target currents being selected to test the range of potential target currents to be measured and said range of temperatures selected to test the range of temperatures to which said optical medium may be exposed while said target currents are to be detected.

4. The method described in claim 2, which comprises the additional step of experimentally verifying that said light beam introduced into said bulk optical medium at said non-birefringent angle may be passed through said bulk optical medium along said path length and produce an optical signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current within a range of temperatures when subjected to certain selected and fixed target currents, said selected and fixed target currents being selected to test the range of potential target currents to be measured and said range of temperatures selected to test the range of temperatures to which said bulk optical medium may be exposed while said target currents are to be detected.

5. The method described in claim 1, in which the step of mathematically determining said non-birefringent angles further comprises the steps of;

a. mathematically calculating the Faraday effect rotations on said beam of light passing through said optical medium as a result of several different hypothetical target currents assuming an absence of birefringence at several hypothetical angles between the linear polarization of the said beam of light and the selected characteristic direction of said optical medium;

b. mathematically calculating the net rotation of a beam of light passing through said optical medium as a result of the Faraday effect rotation caused by said hypothetical target currents together with other hypothetical values of birefringence at said hypothetical angles between the polarity of said beam of light and the characteristic direction of said bulk optical medium;

c. comparing the mathematical calculations of the rotation due to the Faraday effect alone with the mathematical calculations of the optical signal resulting from the Faraday effect together with stress birefringence; and

d. repeating steps a.-c. until one or more said angles are found for which said Faraday rotation produces an optical signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current.

6. The method described in claim 5, which comprises the additional step of experimentally verifying that said light beam introduced into said bulk optical medium at said non-refringent angle may be passed through said optical medium along said path length and produce an optical signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current within a range of temperatures when subjected to certain selected and fixed target currents, said selected and fixed target currents being selected to test the range of potential target currents to be measured and said range of temperatures selected to test the range of temperatures to which said optical medium may be exposed while said target currents are to be detected.

7. The method described in claim 2, in which the step of mathematically determining said non-birefringent angles further comprises the steps of;

a. mathematically calculating the Faraday effect rotations (hereinafter Faraday rotations) on said beam of light passing through said bulk optical medium as a result of several different hypothetical target currents assuming an absence of birefringence at several hypothetical angles between the linear polarization of the said beam of light and the selected characteristic direction of said bulk optical medium;

b. mathematically calculating the net rotation of a beam of light passing through said bulk optical medium as a result of the Faraday effect rotation caused by said hypothetical target currents together with other hypothetical values of birefringence at said hypothetical angles between the polarization of said beam of light and the characteristic direction of said bulk optical medium;

c. comparing the mathematical calculations of the rotation due to the Faraday effect alone with the mathematical calculations of the optical signal resulting from the Faraday effect together with stress birefringence; and

d. repeating steps a.-c. until one or more said angles are found for which said Faraday rotation produces an optical signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current.

8. The method described in claim 7, which comprises the additional step of experimentally verifying that said light beam introduced into said bulk optical medium at said non-birefringent angle may be passed through said bulk optical medium along said path length and produce an optical signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current within a range of temperatures when subjected to certain selected and fixed target currents, said selected and fixed target currents being selected to test the range of potential target currents to be measured and said range of temperatures selected to test the range of temperatures to which said bulk optical medium may be exposed while said target currents are to be detected.

9. A method for detecting and measuring alternating electric target current passing through a conductor proximate to an optical medium, comprising the steps of:

geometrically selecting a particular form of an optical medium to be placed within the magnetic field produced by said target current, said optical medium having an entrance surface, an exit surface, said optical medium having a fast and slow known characteristic directions in which if polarized light is oriented in the absence of a magnetic field the polarized light is transmitted without a change in polarization angle, a known permeability, a known Verdet constant, an approximately known birefringence comprising the difference between a fast index of refraction and a slow index of refraction, each index associated with an associated said characteristic direction, and a known path length for a light beam between said entrance surface to said exit surface;

p1 mathematically determining the existence and identity of at least one non-birefringent angle of polarization with respect to said characteristic direction of said optical medium at which birefringence is effectively reduced to zero and a polarized light beam may be passed through said optical medium along said path and produce a processed final signal whose value is equivalent to the value of the final signal that would be produced in the absence of stress birefringence in said optical medium;

placing said optical medium within the proximity of the magnetic field produced by said target current;

projecting said polarized light beam through said entrance surface at said non-birefringent angle, along said path, and out of said optical medium at said exit surface, and through a analyzing polarizer to produce a departing beam of light; and

measuring the intensity of said departing beam of light and processing said measured intensity to produce a final signal proportional to said target current.

10. The method described in claim 9, in which said optical medium is a bulk optical medium and annealed in such a manner as to cause the characteristic directions within said optical medium to substantially line up with the major geometric faces of said optical medium prior to its placement within said magnetic field produced by said target current.

11. The method described in claim 9, which comprises the additional step of experimentally verifying that said light beam introduced into said optical medium at said non-birefringent angle may be passed through said bulk optical medium along said path length and produce a final signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current within a range of temperatures when subjected to certain selected and fixed target currents, said selected and fixed target currents being selected to test the range of potential target currents to be measured and said range of temperatures selected to test the range of temperatures to which said optical medium may be exposed while said target currents are to be detected.

12. The method described in claim 10, in which comprises the additional step of experimentally verifying that said light beam introduced into said bulk optical medium at said non-birefringent angle may be passed through said bulk optical medium along said path length and produce a final signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current within a range of temperatures when subjected to certain selected and fixed target currents, said selected and fixed target currents being selected to test the range of potential target currents to be measured and said range of temperatures selected to test the range of temperatures to which said bulk optical medium may be exposed while said target currents are to be detected.

13. The method described in claim 9, in which the step of mathematically determining said non-birefringent angles further comprises the steps of;

a. mathematically calculating the Faraday effect rotations on said beam of light passing through said optical medium as a result of several different hypothetical target currents assuming an absence of birefringence at several hypothetical angles between the linear polarization of the said beam of light and said characteristic direction of said optical medium;

b. mathematically calculating the net rotation of a beam of light passing through said optical medium as a result of the Faraday effect rotation caused by said hypothetical target currents together with other hypothetical values of birefringence at said hypothetical angles between the polarization of said beam of light and said characteristic direction of said optical medium;

c. comparing the mathematical calculations of the rotation due to the Faraday effect alone with the mathematical calculations of the optical signal resulting from the Faraday effect together with stress birefringence; and

d. repeating steps a.-c. until one or more said angles are found for which said Faraday rotation produces a final signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current.

14. The method described in claim 13, in which comprises the additional step of experimentally verifying that said light beam introduced into said optical medium at said non-birefringent angle may be passed through said bulk optical medium along said path length and produce a final signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current within a range of temperatures when subjected to certain selected and fixed target currents, said selected and fixed target currents being selected to test the range of potential target currents to be measured and said range of temperatures selected to test the range of temperatures to which said optical medium may be exposed while said target currents are to be detected.

15. The method described in claim 10, in which the step of mathematically determining said non-birefringent angles further comprises the steps of;

a. mathematically calculating the Faraday effect rotations (hereinafter Faraday rotations) on said beam of light passing through said bulk optical medium as a result of several different hypothetical target currents assuming an absence of birefringence at several hypothetical angles between the linear polarization of the said beam of light and said characteristic direction of said bulk optical medium;

b. mathematically calculating the net rotation of a beam of light passing through said bulk optical medium as a result of the Faraday effect rotation caused by said hypothetical target currents together with other hypothetical values of birefringence at said hypothetical angles between the polarization of said beam of light and said characteristic direction of said bulk optical medium;

c. comparing the mathematical calculations of the rotation due to the Faraday effect alone with the mathematical calculations of the optical signal resulting from the Faraday effect together withstress birefringence; and

d. repeating steps a.-c. until one or more said angles are found for which said Faraday rotation produces a final signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current.

16. The method described in claim 15, in which comprises the additional step of experimentally verifying that said light beam introduced into said bulk optical medium at said non-birefringent angle may be passed through said bulk optical medium along said path length and produce a final signal with a time-varying component, the root=mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current within a range of temperatures when subjected to certain selected and fixed target currents, said selected and fixed target currents being selected to test the range of potential target currents to be measured and said range of temperatures selected to test the range of temperatures to which said bulk optical medium may be exposed while said target currents are to be detected.

17. An optical current transducer apparatus for detecting and measuring time-varying electric target current passing through a conductor, the apparatus comprising;

an optical sensor medium in the magnetic field produced by said target current, said optical sensor medium having a specific characteristic direction which in the absence of the magnetic field can receive polarized light oriented with said characteristic direction and transmit the polarized light without changing the polarization angle of the polarized light, and Verdet constant,

said optical sensor medium further being adapted with an entrance surface receives a beam of light, a fixed path having a known path length through which said beam of light may travel and an exit surface through which said beam of light may exit said optical medium;

a first linear polarizer which orients a linear polarization of a single beam of light such that said linear polarization is oriented at a non-birefringent angle with respect to said characteristic direction to produce a single non-birefringent light beam received by said entrance surface;

a second polarizer analyzer which is adapted to receive beam of transmitted light upon its departure from said optical sensor medium following receipt of said non-birefringent light beam at said entrance surface and the consequent transmission of light through said optical sensor medium and out of said exit surface and to allow the passage through said analyzer a single analyzer beam, said analyzer beam being only that component of said beam of light which is polarized at a predetermined angle from the angle between said polarizer angle with respect to said characteristic direction,

the combination of said non-birefringent angle of said first polarizer and said predetermined angle of said analyzer producing an optical signal of said analyzer beam from said analyzer with a time-varying polarized angular component, the root-mean-square average value of said polarized angular component being proportional to the root-mean-square average value of said target current; and

a photoelectric detector which is adapted to receive and measure the intensity of said analyzer beam and produce a corresponding electrical signal.

18. The apparatus described in claim 17, in which said target current is substantially surrounded by a ferromagnetic ring, said ferromagnetic ring adapted to concentrate the magnetic field produced by said target current;

said ferromagnetic ring being further adapted with a ring gap of sufficient volume to permit the insertion of said optical medium which further comprises a bulk optical member and subject said bulk optical member to said concentrated magnetic field, but permit said beam of light to be introduced into said entrance surface at said non-birefringent angle and to depart said bulk optical member through said exit surface and enter said analyzer.

19. The invention described in claim 17, in which said optical medium further comprises three or more separate elongate optical members, said elongate optical members each such optical member adapted with the identical characteristic direction and Verdet constant used to predetermine said non-birefringent angle;

said 3 or more elongate optical members further comprising a first elongate optical member having a first end which is adapted with said entrance surface and another end, a last said optical member having a last end which is adapted with said exit surface and another end, and one or more successive elongate intermediate optical members, each said successive elongate intermediate optical members having two ends;

the said other ends of said first and last elongate optical members and both ends of each said successive intermediate elongate optical members being further adapted with coupling means to optionally couple with said successive intermediate elongate optical members such that each said optical coupling comprises a first reflecting surface and a second reflecting surface, said resulting reflections oriented so as to cause the electric field component of said beam of light to pass from the said other end of said first elongate optical member into each said next successive intermediate elongate optical member and finally the said other end of the said last elongate optical member at an orientation which is perpendicular to its orientation upon each said first reflecting surface;

each said elongate optical member further oriented such that said identical characteristic direction of each said elongate optical member is aligned to be perpendicular with each said coupled elongate optical member,

said path length further comprising the length of said first optical member, each said successive intermediate elongate optical member, and said last elongate optical member and further defining an integral number of loops around said target current.

20. The invention described in claim 17, in which said optical medium further comprises a single mode optical fiber, said single mode optical fiber being looped around said target current.

21. The invention described in claim 17, said apparatus further comprising a signal processing means, said signal processing means being adapted to receive said corresponding signal from said photodetector and further comprising;

electronic filtering means for separating the time-varying current and direct current components of said photodetector output signal;

electronic conversion means for converting the time-varying component of said corresponding signal from said photodetector into a final signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current.

22. The invention described in claim 18, said apparatus further comprising a signal processing means, said signal processing means being adapted to receive said corresponding signal from said photodetector and further comprising:

electronic filtering means for separating the alternating current and direct current components of said corresponding signal from said photodetector; and

electronic conversion means for converting the time-varying component of said corresponding signal from said photodetector into a final signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current.

23. The invention described in claim 19, said apparatus further comprising a signal processing means, said signal processing means being adapted to receive the signal from said photodetector and further comprising;

electronic filtering means for separating the alternating current and direct current components of said photodetector output signal;

electronic conversion means for converting the time-varying component of said corresponding signal from said photodetector into a final signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-means-square average value of said target current.

24. The invention described in claim 20, said apparatus further comprising a signal processing means, said signal processing means being adapted to receive the signal from said photodetector and further comprising;

electronic filtering means for separating the alternating current and direct current components of said photodetector output signal;

electronic conversion means for converting the time-varying component of said corresponding signal from said photodetector into a final signal with a time-varying component, the root-mean-square average value of said time-varying component being proportional to the root-mean-square average value of said target current.

25. The invention described in claim 18, in which said bulk optical medium is annealed in such a manner as to cause the characteristic directions within said optical medium to substantially line up with the major geometric faces of said optical medium prior to its placement within said magnetic field produced by said target current.

26. The invention described in claim 19, in which each said elongate optical member is annealed in such a manner as to cause the characteristic directions within said optical medium to substantially line up with the major geometric faces of said optical medium prior to its placement within said magnetic field produced by said target current.

27. The invention described in claim 22, in which said bulk optical medium is annealed in such a manner as to cause the characteristic directions within said optical medium to substantially line up with the major geometric faces of said optical medium prior to its placement within said magnetic field by said target current.

28. The invention described in claim 23, in which each said elongate optical member is annealed in such a manner as to cause the characteristic directions within said optical medium to substantially line up with the major geometric faces of said optical medium prior to its placement within said magnetic field produced by said target current.

29. A method for minimizing the effects of stress birefringence and changes in said stress birefringence, such changes in said stress birefringence frequently being produced by changes in ambient temperature, within the optical apparatus of an optical current transducer, thereby making said optical current transducer substantially free of the effects of said ambient temperature, said optical current transducer based on the Faraday rotation effect and useful in measuring a target electric current carried in a target electric conductor comprising the steps of:

selecting a suitable light source such as a light emitting diode emitting a light beam in the near infrared portion of the optical spectrum at a wavelength hereinafter referred to as the operating wavelength;

determining said operating wavelength through the use of such equipment as a monochrometer;

providing a means to drive a substantially constant current through said light source so as to provide a substantially constant output intensity in said light beam;

arranging the principal optical elements of said optical apparatus;

said light beam to be transmitted along the main optical axis of said optical apparatus;

said principal optical elements being a first polarizer with its first transmission axis and its first polarizer extinction coefficient, Tp, an optical medium described hereinbelow which might consist of one or more physically distinct pieces, and a second polarizer with its second transmission axis and its second polarizer extinction coefficient, said second polarizer being termed an analyzer and said second transmission axis being termed an analyzer transmission axis, and said second polarizer extinction coefficient being termed an analyzer extinction coefficient, Ta;

said analyzer transmission axis to be oriented at an analyzer angle of 45 degrees from said first polarizer transmission axis;

said analyzer angle to be measured in a plane perpendicular to said main optical axis of said optical apparatus;

arranging said principal optical elements so said light beam propagates first through said first polarizer, then through said optical medium, and finally through said analyzer;

said light beam after exiting said analyzer being referred to as the returning light beam;

the polarization of said returning light beam no longer being important;

selecting a single optical transmission means among air, vacuum, and optical fibers by which to transmit said light beam from said light source to said first polarizer;

selecting intermediate optical elements such as an achromatic lens, or lenses, as may be necessary to efficiently transmit said optical intensity in said light beam from said light source to said polarizer and hence through the other said principal optical elements;

said light beam exiting said polarizer as a plane polarized light beam;

arranging said principal optical elements so as to transmit said light beam through said principal optical elements with a pre-selected efficiency so as to preserve as much of said optical intensity as possible;

selecting a particular material with which to form the optical medium, said particular material being essentially transparent at said operating wavelength and exhibiting a measurable said Faraday rotation effect whereby in the absence of any disturbing effects such as those from said stress birefringence, the plane of said plane polarized light beam is continuously rotated thereby ever increasing the Faraday rotation angle φ

by the action of the component of a magnetic field parallel to said main optical axis of said optical apparatus as said plane polarized light beam propagates along a known path length along said main optical axis of said optical apparatus;

said particular material with said measurable Faraday rotation effect having a known magnetic permeability;

said particular material with said measurable Faraday rotation effect having a known Verdet constant at said operating wavelength;

said Verdet constant being the constant of proportionality between said known path length, said known magnetic field, said known magnetic permeability and said Faraday rotation angle;

adapting or selecting said optical medium to have an entrance surface, an exit surface, and other such reflective surface or surfaces as may be necessary to direct said light beam along a suitable main optical axis of said optical apparatus;

adapting or selecting said optical medium to have two distinct surfaces which are approximately parallel to each other;

said two distinct surfaces being defined as a first distinct surface and a second distinct surface;

said first distinct surface and said second distinct surface being also parallel to said main optical axis of said optical apparatus;

arranging said particular material by resting said first distinct surface on a first surface of high thermal conductivity material such as aluminum, and bringing said second distinct surface in close proximity to a second surface of high thermal conductivity material such as aluminum which itself is parallel to said first surface of high thermal conductivity material such as aluminum;

annealing said optical medium by raising its temperature as well as the temperature of said first surface of high conductivity material and said second surface of high conductivity material to just above the glass transition temperature of said particular material which was used to form said optical medium, soaking said optical medium for several hours at said temperature just above said glass transition temperature, then slowly cooling said optical medium to approximately 100 degrees centigrade below said glass transition temperature of said particular material which was used to form said optical medium;

then rapidly or slowly cooling said optical medium to room temperature and removing said optical medium from the annealing apparatus;

measuring said optical medium using different orientations of said first polarizer transmission axis in the absence of said magnetic field to determine the direction of the two characteristic directions;

said two characteristic directions being orthogonal to each other and defined as those directions measured perpendicular to said main optical axis of said optical apparatus, and defined by their action on said plane polarized light beam, or a probe beam of plane polarized light created in a test measurement independent of said optical current transducer, whereby said plane polarized light beam or said probe beam of plane polarized light remains plane polarized as said plane polarized light beam or said probe beam of plane polarized light propagates parallel to either one of said two characteristic directions;

said plane polarized light beam or said probe beam of said plane polarized light remaining plane polarized because either of said two characteristic directions are characterized by a single albeit different index of refraction;

said characterized directions thereby becoming a known first characteristic direction and a known second characteristic direction;

a first index of refraction being associated with said first known characteristic direction, and a second index of refraction being associated with said second known characteristic direction;

measuring the difference between said first index of refraction being associated with said first known characteristic direction and said second index of refraction being associated with said second known characteristic direction;

such measurement done at said operating wavelength using for example such equipment as a Soliel-Babinet compensator;

said difference arising as it does from small mechanical stresses in said particular material being defined as the stress birefringence;

said stress birefringence being largely determined by the thermal history of said particular material is defined as residual stress birefringence;

said annealing of said optical medium producing said first characteristic direction perpendicular to both said first distinct surface of said optical medium, and to said second distinct surface of said optical medium, and said second characteristic direction parallel to both said first distinct surface of said optical medium, and to said second distinct surface of said optical medium; and

as a result said stress birefringence now being a known residual stress birefringence;

said characteristic directions thereby remaining relatively fixed as said ambient temperature changes produce small mechanical stresses parallel to said first and second distinct surfaces;

positioning said optical medium in known proximity to said target electric conductor;

said target electric current creating said magnetic field;

said geometry of said known proximity permitting apriori approximate knowledge of said magnetic field along said main optical axis of said apparatus to be expected from said target current;

selecting a suitable light detector such as a silicon photodiode and sensitive to said returning light beam at said operating wavelength;

connecting said light detector to electronic signal processing means;

selecting an optical transmission means such as air, vacuum, and/or optical fibers by which to transmit said returning light beam from said analyzer to said light detector;

calculating the Faraday rotation per unit length as the total said Faraday rotation angle produced by the action of said magnetic field along all of said optical path length within said optical medium, divided by said optical path length;

calculating the birefringence per unit length as the total said known residual stress birefringence produced by said optical medium along all of said optical path length within said optical medium, divided by said optical path length;

calculating a parameter phi as twice the sum of the square of said Faraday rotation angle per unit length and the square of half said birefringence per unit length;

defining an angle alpha that said first transmission axis of said first polarizer makes with either said first known characteristic direction or said second known characteristic direction, the particular characteristic direction chosen to be known as the selected characteristic direction, and letting the angle psi define said analyzer angle and be therefore equal to said angle alpha plus 45 degrees;

arranging said optical medium so said main optical axis of said optical apparatus forms one or more integral loops around said target electric conductor and therefore said target electric current passes through the plane formed by said optical medium;

calculating said expected root mean square, rms, response from said optical current transducer by using well known formulae describing said Faraday rotation effect and said stress birefringence when they co-exist in the same said optical medium;

said expected response to be calculated using said known magnetic permeability, said known path length, said angle alpha, said angle psi, said parameter phi, said first polarizer extinction coefficient, said analyzer extinction coefficient, said known Faraday rotation per unit length, said known residual stress birefringence per unit length, said known magnetic field;

repeating said calculation of said expected rms response from said optical current transducer, and varying during these repetitions of said calculations said angle alpha associated with the first polarizer, with the intention of defining a specific angle alpha such that said expected rms response in the presence of stress birefringence does not differ significantly from the ideal response when said known residual birefringence is set equal to zero as would be the case in an ideal birefringence-free optical medium;

said repetitions defining a non-birefringent angle alpha at which to set said first polarizer with respect to said selected characteristic direction;

repeating said calculations of said expected response for different hypothetical target electric currents to be sure that said non-birefringent angle alpha calculated at one hypothetical target electric current does not differ significantly from said non-birefringent angle alpha calculated at another hypothetical target electric current;

these second sets of calculations producing a final non-birefringent angle at which said expected response does not differ significantly from said ideal response as either said hypothetical target current or said stress birefringence is varied;

adjusting the actual position of the actual said first transmission axis of the actual said first polarizer to said final non-birefringent angle;

creating an output signal from said signal processing means substantially proportional to and substantially in phase with said target electric current, said output signal being thereby essentially free from said changes in ambient temperature.