Browse Prior Art Database

Advanced Flight Height Sensor

IP.com Disclosure Number: IPCOM000120532D
Original Publication Date: 1991-May-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 4 page(s) / 135K

Publishing Venue

IBM

Related People

Korth, HE: AUTHOR

Abstract

A white light interferometer system is described which allows measuring the air gap between a rapidly spinning glass disk and the air-bearing surface of a magnetic read/write head. The interference pattern between head and disk is intermodulated with light from a second beam reflected at the disk. A special demodulation technique allows extracting the flight height information with height-independent sensitivity and a precision better than 1 nm for a flight height from 0 to 1000 nm.

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Advanced Flight Height Sensor

      A white light interferometer system is described which
allows measuring the air gap between a rapidly spinning glass disk
and the air-bearing surface of a magnetic read/write head.  The
interference pattern between head and disk is intermodulated with
light from a second beam reflected at the disk.  A special
demodulation technique allows extracting the flight height
information with height-independent sensitivity and a precision
better than 1 nm for a flight height from 0 to 1000 nm.

      Fig. 1 shows the interference mechanism for optical flight
height testing.  An intermodulated interference signal S may be
produced by superimposing an additional light beam on the beams
reflected at slider and disk.  A fixed path difference of a few
microns produces a number of interference fringes in the spectrum.
The presence of the slider leads to modulation of both phase and
amplitude.

      For demodulation, a reference spectrum T is required. This is
produced when the slider is removed and only the two beams reflected
at the disk interfere.  The signal and the reference spectrum are
passed through a high-pass filter to produce symmetrical sinusoidal
signals.  A quadrature signal Tq is derived from the filtered signal
Tp having the same phase as T.  The product S$Tp yields a signal
corresponding to the cosine of the phase shift between S and T plus
the cosine of twice the modulation frequency.  A low-pass filter
removes this second term.  Similarly, the product S$Tq yields the
sine of the phase shift.

      The phase shift / between S and T is implied in the low-pass
output Sp and Sq.  The phase shift p between disk and slider can be
calculated from / and the ratio of the amplitudes of S and T.  p
therefore follows from Sp and Sq as:
   tan (p) = Sq / (Tm/2 - Sp)
where Tm denotes the modulation intensity of T which can be
calculated from Tp and Tq:
      Tm = ! Tp2 + Tq2 .

      The flight height d, i.e., the optical path between disk and
slider is encoded in the slope of the phase vs. wave number (w) line
   p = 4 f d w
and can be extracted by a simple averaging procedure.

      Fig. 2 shows a simple set-up of a 3-beam white light
interferometer for flight height sensing.  A polarizing beam splitter
sends polarized light through a Wollaston prism located in the rear
focal plane of the objective lens.  The Wollaston prism produces an
orthogonally polarized beam pair.  A small tilt of the Wollaston
prism allows introducing an asymmetry into the optical path of the
beams. The beams are...