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This invention dynamically reduces the effects of birefringence and ellipticity in optical recording.
English (United States)
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Automatic Phase Retardation Control for Optical Recording Systems
This invention dynamically reduces the effects of birefringence and ellipticity
in optical recording.
Birefringence and other apparitions induce elliptically polarized light, which
interferes with data detection. In MO, this induced ellipticity can be of the same
magnitude as the Kerr Rotation. A quarterwave plate can be inserted into the
readback path to compensate for the induced ellipticity, but the plate must be
tuned manually for each type of substrate. Also, since the amount of
Birefringence can vary circumferentially as well as radially, the setting of the
quarterwave plate is either a compromise for best average performance, or must
be continually retuned as track position is changed.
This proposed device uses an LCD as a voltage controlled phase retardation
plate to turn elliptically polarized light into linearly Birefringence. The difference
signal is low-pass filter whose cutoff frequency is approximately 5 to 20 times the
RPs of the disc. This allows the phase retardation to be set dynamically
regardless of media type.