OPTICAL TACHOMETER BASED ON FARADAY ROTATION. Andrew Koller, Harold Metcalf and John Noe, Laser Teaching Center, Department of Physics and Astronomy, SUNY at Stony Brook.

This research has focused on the design of an optical sensor capable of measuring the rotational speed of a wheel or a shaft. The sensor exploits the Faraday effect, in which applying a magnetic field to certain materials rotates the polarization of light passing through the material. The Faraday effect is useful because it reveals changes in magnetic field intensity without the use of any conducting material. Some of its applications in addition to the tachometer are optical current sensors and Faraday isolators.

The tachometer measures changes in the polarization of laser light as a piece of magnetically permeable material attached to a rotating shaft modulates the magnetic field in a specific type of Faraday material. To get the greatest amount of rotation, special rare-earth garnets were used. I have worked with a Terbium Gallium Garnet (TGG) crystal, and have recently begun using a smaller piece of Yttrium Iron Garnet (YIG). Using the TGG crystal, a distinct signal was generated every half-cycle of rotation, with an observed modulation depth of about 40 mV. By finding the light intensity peaks visually, or with an electronic device, the frequency of rotation can be computed. Although data for a YIG based tachometer has not been recorded, the signal has been seen on an oscilloscope. Although the signal produced by the YIG-based device appears to be less intense than when TGG was used, the signal to noise ratio can be made high enough to compensate for this difference. The thinness of the YIG sample (less than one millimeter) makes it more desirable than the TGG crystal, because it allows for a more compact design and makes it easier to induce Faraday rotation by modulating an applied magnetic field.

This research was made possible by the Simons Fellowship Program and the Laser Teaching Center at SUNY Stony Brook.

Andrew Koller August 2000