Browse Prior Art Database

Enhanced Optical Disk Access

IP.com Disclosure Number: IPCOM000099507D
Original Publication Date: 1990-Jan-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 4 page(s) / 135K

Publishing Venue

IBM

Related People

Korth, HE: AUTHOR [+2]

Abstract

An interferometric device suitable for detecting the phase variations of reflected light is employed for the ultrasensitive detection of magnetically stored data. Given an appropriate phase shift, the interferometer is tuned to produce zero output in one beam path in response to symmetric reflection (common mode rejection), whereas in the other beam path there is maximum output, from which track and focus servo signals are derived.

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Enhanced Optical Disk Access

       An interferometric device suitable for detecting the phase
variations of reflected light is employed for the ultrasensitive
detection of magnetically stored data.  Given an appropriate phase
shift, the interferometer is tuned to produce zero output in one beam
path in response to symmetric reflection (common mode rejection),
whereas in the other beam path there is maximum output, from which
track and focus servo signals are derived.

      When the symmetry of the reflected beam is sensed along the
data track, a light pulse is generated in response to each phase
change. Compared to commonly used encoding schemes defining a data
bit by the location of the transition between two surface states, the
enhanced access technique makes the data bit pattern directly
visible.  The resultant pulses may be handled by a significantly
reduced transmission frequency band.  This means that most of the
laser and amplifier noise is suppressed.

      The basic set-up for enhanced optical disk access is very
similar to well-known CD players (Fig. 1).

      A laser diode transmits linearly polarized light to the disk
surface.  The light is collimated into a parallel circular beam with
a Gaussian intensity profile.  After transmission through a
polarizing beam splitter, a quarterwave plate produces circularly
polarized light.  The objective lens then focuses the light through
the disk onto the data surface.

      A good match between lens aperture and beam exists when the
beam amplitude at cut off is about 1/e of the value at the center.
This minimizes the spot size, whereas the spot shape does not deviate
too much from the Gauss profile.

      The reflected light is recollimated by the objective lens.
After transmission through the quarterwave plate, the beam is again
linearly polarized but rotated by 90o to the incident beam.
Therefore, the beam is reflected by the polarizing beam splitter.
The "autocorrelating interferometer" then separates the common mode
free data beam and the servo beam which are focused on appropriate
photodiodes.

      Fig. 2 is a schematic view of the transmission of a laser beam
through the optical system (the light path being shown unfolded for
clarity).  The shift of the reflected beam with respect to the lens
aperture corresponds to a tilt of the object surface of about 1/4 of
the aperture angle, i.e., about 7.5o for a numerical aperture of 0.5.
 With tan(7.5) = 0.13, this corresponds to a phase change of 130 nm
across a 1-micron laser focus.  A similar diffraction pattern is
observed for a phase step of 65 nm in the center of the focus.  The
asymmetry of the reflected beam is so high that only minor portions
of the beam and its mirror image interfere, most of the light being
picked up by the detector.

      A simple arrangement for an autocorrelating interferometer uses
the well-kn...