Abstract

A fiber optic tachometer is described that uses the Faraday effect in a ferrite to modulate light from a multimode fiber. A yttrium iron garnet (YIG) crystal is used as the magnetooptic modulator. A permanent magnet is placed near the crystal, such that a nearby rotating steel toothed wheel changes the magnetic field inside the crystal, causing modulation of the intensity of the 1.3-μm light. Modulation depths up to 50% have been achieved with signal-to-noise ratios of over 15 dB. A single multimode fiber connects the remote sensor head to the LED light source and detector.

© 1989 Optical Society of America

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References

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  1. R. Soohoo [Theory and Application of Ferrites (Prentice-Hall, Englewood Cliffs, N.J., 1960)] gives a good derivation of the Faraday effect for the microwave regime. In the optical regime, his derivation reduces to find that the rotation angle is proportional to the magnetization in the ferrite and the distance traveled. For nonsaturating fields, the magnetization is proportional to the applied magnetic field through a scalar initial permeability.
  2. Quarter-pitch graded-index (GRIN) rods are compact lenses that have the property of collimating a point source of a specified wavelength of light. They are especially useful for collimating light emitted by an optical fiber, or conversely, focusing a collimated beam onto an optical fiber.
  3. T. Nakano, H. Yuri, I. Sasaki, U. Kihara, “Evaluation of Optical Isolator Applied YIG Single Crystal Grown by the TSFZ Method,” in Advances in Ceramics, Vol. 16, F. F. Y. Wang, Ed. (American Ceramic Society, Columbus, OH, 1985), pp. 223–229.
  4. W. J. Tabor, “Magneto-Optic Materials,” in Laser Handbook, Vol. 1, F. T. Arecchi, E. O. Shulz-Dubois, Eds. (North-Holland, Amsterdam, 1972).
  5. Deltronic Crystal Industries, Inc., YIG product literature.
  6. J. Helszajn, YIG Resonators and Filters (Wiley, New York, 1985).
  7. W. H. von Aulock, Handbook of Microwave Ferrite Materials (Academic, New York, 1965).
  8. Directional fiber couplers are essentially an optical fiber analog of optical beam splitters. They are directional in that the light continues to travel in the same direction although the power has now been distributed into two fibers. In our sensor, the coupler is used to connect the single probe fiber to two fibers, one input fiber from the source and the other the output fiber to the detector.

Helszajn, J.

J. Helszajn, YIG Resonators and Filters (Wiley, New York, 1985).

Kihara, U.

T. Nakano, H. Yuri, I. Sasaki, U. Kihara, “Evaluation of Optical Isolator Applied YIG Single Crystal Grown by the TSFZ Method,” in Advances in Ceramics, Vol. 16, F. F. Y. Wang, Ed. (American Ceramic Society, Columbus, OH, 1985), pp. 223–229.

Nakano, T.

T. Nakano, H. Yuri, I. Sasaki, U. Kihara, “Evaluation of Optical Isolator Applied YIG Single Crystal Grown by the TSFZ Method,” in Advances in Ceramics, Vol. 16, F. F. Y. Wang, Ed. (American Ceramic Society, Columbus, OH, 1985), pp. 223–229.

Sasaki, I.

T. Nakano, H. Yuri, I. Sasaki, U. Kihara, “Evaluation of Optical Isolator Applied YIG Single Crystal Grown by the TSFZ Method,” in Advances in Ceramics, Vol. 16, F. F. Y. Wang, Ed. (American Ceramic Society, Columbus, OH, 1985), pp. 223–229.

Soohoo, R.

R. Soohoo [Theory and Application of Ferrites (Prentice-Hall, Englewood Cliffs, N.J., 1960)] gives a good derivation of the Faraday effect for the microwave regime. In the optical regime, his derivation reduces to find that the rotation angle is proportional to the magnetization in the ferrite and the distance traveled. For nonsaturating fields, the magnetization is proportional to the applied magnetic field through a scalar initial permeability.

Tabor, W. J.

W. J. Tabor, “Magneto-Optic Materials,” in Laser Handbook, Vol. 1, F. T. Arecchi, E. O. Shulz-Dubois, Eds. (North-Holland, Amsterdam, 1972).

von Aulock, W. H.

W. H. von Aulock, Handbook of Microwave Ferrite Materials (Academic, New York, 1965).

Yuri, H.

T. Nakano, H. Yuri, I. Sasaki, U. Kihara, “Evaluation of Optical Isolator Applied YIG Single Crystal Grown by the TSFZ Method,” in Advances in Ceramics, Vol. 16, F. F. Y. Wang, Ed. (American Ceramic Society, Columbus, OH, 1985), pp. 223–229.

Other

R. Soohoo [Theory and Application of Ferrites (Prentice-Hall, Englewood Cliffs, N.J., 1960)] gives a good derivation of the Faraday effect for the microwave regime. In the optical regime, his derivation reduces to find that the rotation angle is proportional to the magnetization in the ferrite and the distance traveled. For nonsaturating fields, the magnetization is proportional to the applied magnetic field through a scalar initial permeability.

Quarter-pitch graded-index (GRIN) rods are compact lenses that have the property of collimating a point source of a specified wavelength of light. They are especially useful for collimating light emitted by an optical fiber, or conversely, focusing a collimated beam onto an optical fiber.

T. Nakano, H. Yuri, I. Sasaki, U. Kihara, “Evaluation of Optical Isolator Applied YIG Single Crystal Grown by the TSFZ Method,” in Advances in Ceramics, Vol. 16, F. F. Y. Wang, Ed. (American Ceramic Society, Columbus, OH, 1985), pp. 223–229.

W. J. Tabor, “Magneto-Optic Materials,” in Laser Handbook, Vol. 1, F. T. Arecchi, E. O. Shulz-Dubois, Eds. (North-Holland, Amsterdam, 1972).

Deltronic Crystal Industries, Inc., YIG product literature.

J. Helszajn, YIG Resonators and Filters (Wiley, New York, 1985).

W. H. von Aulock, Handbook of Microwave Ferrite Materials (Academic, New York, 1965).

Directional fiber couplers are essentially an optical fiber analog of optical beam splitters. They are directional in that the light continues to travel in the same direction although the power has now been distributed into two fibers. In our sensor, the coupler is used to connect the single probe fiber to two fibers, one input fiber from the source and the other the output fiber to the detector.

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Figures (6)

Fig. 1
Fig. 1

Faraday rotation shown for a small amount of rotation. Note that the direction of rotation follows the right-hand rule about the magnetic field vector, that is, clockwise when looking in the direction of H (+z-direction). The electric field is polarized in the xy plane and is propagating in the +z-direction.

Fig. 2
Fig. 2

Methods of magnetic field modulation (side views): (a) magnet mounted on the shaft and (b) magnet mounted with the Faraday probe with a steel tooth modifying the field.

Fig. 3
Fig. 3

Bias point located on the cosine-squared curve.

Fig. 4
Fig. 4

Diagram of actual sensor tested.

Fig. 5
Fig. 5

Actual output of the sensor tested.

Fig. 6
Fig. 6

Alternative sensor using two fibers.

Equations (2)

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I = I 0 2 cos 2 ( θ ϕ pol + Δ θ )
I = I 0 2 cos 2 [ 2 ( θ + Δ θ ) ] .

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