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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.
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