Lensless Magneto-optic speed sensor

• US 5,719,497 A
• Filed: 05/09/1996
• Issued: 02/17/1998
• Est. Priority Date: 05/09/1996
• Status: Expired due to Fees

• Current Assignee: Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Rodriguez, Patrick J., Forman, Peter R., Veeser, Lynn R.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US08/647,364
• Time in Patent Office: 649 Days
• Field of Search: 324/96, 324/173-175, 324/207.13, 324/207.25, 324/244.1, 250/225, 250/227.11, 250/227.14, 250/227.21, 250/231.1, 250/231.13, 250/231.18, 250/230, 359/280, 359/281, 359/284, 359/484, 385/6, 385/12
• US Class Current: 324/174
• CPC Class Codes: G01P 3/486 delivered by photo-electric detectors
• CPC Class Codes: G01P 3/487 delivered by rotating magnets
• CPC Class Codes: G01R 33/0322 using the Faraday or Voigt effect

Abstract

Lensless magneto-optic speed sensor. The construction of a viable Faraday sensor has been achieved. Multimode fiber bundles are used to collect the light. If coupled directly into a 100 or 200 μm core fiber, light from a light emitting diode (LED) is sufficient to operate the sensor. In addition, LEDs ensure that no birefringence effects in the input fiber are possible, as the output from such light sources have random polarization. No lens is required since the large diameter optical fibers and thin crystals of materials having high Verdet constants (such as iron garnets) employed permit the collection of a substantial quantity of light. No coupler is required. The maximum amount of light which could reach a detector using a coupler is 25%, while the measured throughput of the fiber-optic bundle without a coupler is about 42%. All of the elements employed in the present sensor are planar, and no particular orientation of these elements is required. The present sensor operates over a wide range of distances from magnetic field sources, and observed signals are large. When a tone wheel is utilized, the signals are independent of wheel speed, and the modulation is observed to be about 75%. No sensitivity to bends in the input or output optical fiber leads was observed. Reliable operation was achieved down to zero frequency, or no wheel rotation.

Claims

1. A Faraday effect magnetic field detector, which comprises in combination:

a. a magneto-optic material having a high Verdet constant and further having a first planar face and a second planar face, the first face and the second face being substantially parallel;

b. a light source for generating light having wavelengths which are transmitted by said magneto-optic material;

c. a polarizer disposed in front of the first planar face of said magneto-optic material;

d. lensless means for directing the light through said polarizer and into said magneto-optic material through a portion of the first parallel face thereof, such that the divergence of the light emerging from said light directing means is comparable when compared to diffraction of the light by domains within said magneto-optic material;

e. reflective means disposed behind the second parallel face of said magneto-optic material for reflecting light which has passed through said magneto-optic material back therethrough, through the first parallel face thereof and through said polarizer;

f. means for collecting part of the light returning through and analyzed by said polarizer which has emerged from the first face outside of the portion thereof into which the light was directed by said lensless directing means, said collecting means having a collection aperture which is large when compared to the diffraction of the light by the domains; and

g. means for detecting the collected light, producing thereby a signal;

whereby the signal may be related to the strength of the magnetic field.

2. The Faraday effect magnetic field detector as described in claim 1, wherein said magneto-optic material is selected from the group consisting of bismuth iron garnet and yttrium iron garnet.

3. The Faraday effect magnetic field detector as described in claim 1, wherein said lensless light directing means includes an optical fiber.

4. The Faraday effect magnetic field detector as described in claim 3, wherein said optical fiber is a multimode fiber.

5. The Faraday effect magnetic field detector as described in claim 1, wherein said reflective means is a reflective material deposited onto the second planar of said magneto-optic material.

6. The Faraday effect magnetic field detector as described in claim 1, wherein said light collecting means includes at least one optical fiber.

7. The Faraday effect magnetic field detector as described in claim 6, wherein said optical fiber is a multimode fiber.

8. The Faraday effect magnetic field detector as described in claim 1, wherein the distance between the first face and the second face of said magneto-optic material is chosen such that the polarization of the light emerging from the first face of said magneto-optic material is rotated not more than 90°

from the polarization of the light entering the first face thereof.

9. A Faraday effect wheel rotational speed sensor comprising:

a. a magneto-optic material having a high Verdet constant and further having a first planar face and a second planar face, the first face and the second face being substantially parallel;

b. a light source for generating light having wavelengths which are transmitted by said magneto-optic material;

c. a polarizer disposed in front of the first planar face of said magneto-optic material;

d. lensless means for directing the light through said polarizer and into said magneto-optic material through a portion of the first parallel face thereof, such that the divergence of the light emerging from said light directing means is comparable when compared to diffraction of the light by domains within said magneto-optic material;

e. reflective means disposed behind the second parallel face of said magneto-optic material for reflecting light which has passed through said magneto-optic material back therethrough, through the first parallel face thereof and through said polarizer;

f. means for collecting part of the light returning through and analyzed by said polarizer which has emerged from the first face outside of the portion thereof into which the light was directed by said lensless directing means, said collecting means having a collection aperture which is large when compared to the diffraction of the light by the domains;

g. means for detecting the collected light, producing thereby a signal; and

h. a tone ring having regions of alternating magnetic poles which interact with said magneto-optic material, said tone ring having an axis collinear with the axis of the wheel and being affixed to a face thereof;

whereby the signal may be related to the speed of rotation of the wheel.

10. The Faraday effect wheel rotational speed sensor as described in claim 9, wherein said magneto-optic material is selected from the group consisting of bismuth iron garnet and yttrium iron garnet.

11. The Faraday effect wheel rotational speed sensor as described in claim 9, wherein said lensless light directing means includes an optical fiber.

12. The Faraday effect magnetic field detector as described in claim 11, wherein said optical fiber is a multimode fiber.

13. The Faraday effect wheel rotational speed sensor as described in claim 9, wherein said reflective means is a reflective material deposited onto the second planar face of said magneto-optic material.

14. The Faraday effect wheel rotational speed sensor as described in claim 9, wherein said light collecting means includes at least one optical fiber.

15. The Faraday effect magnetic field detector as described in claim 14, wherein said optical fiber is a multimode fiber.

16. The Faraday effect wheel rotational speed sensor described in claim 9, wherein the distance between the first face and the second face of said magneto-optic material is chosen such that the polarization of the light emerging from the first face of said magneto-optic material is rotated not more than 90°

from the polarization of the light entering the first face thereof.