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Browse Prior Art Database

Differential Laser Interferometer/Vibrometer

IP.com Disclosure Number: IPCOM000108524D
Original Publication Date: 1992-Jun-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 7 page(s) / 288K

Publishing Venue

IBM

Related People

Lee, CK: AUTHOR [+2]

Abstract

Disclosed is a differential laser interferometer/vibrometer that adopts the Doppler principle, phase decoding algorithm, and balanced mechanical design, etc., to achieve non-contact differential and absolute displacement measurement of opaque objects. Unlike the traditional laser interferometer, displacement of untreated surfaces can be measured because of the newly designed optical configuration. Displacement resolution as high as 1.0 nm can be achieved due to the fringe interpretation schemes used. A novel balanced mechanical design was also developed for flexibility in alignment and for insensitivity to interferometer arm fixture thermal deformation induced measurement noise.

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Differential Laser Interferometer/Vibrometer

       Disclosed is a differential laser
interferometer/vibrometer that adopts the Doppler principle, phase
decoding algorithm, and balanced mechanical design, etc., to achieve
non-contact differential and absolute displacement measurement of
opaque objects.  Unlike the traditional laser interferometer,
displacement of untreated surfaces can be measured because of the
newly designed optical configuration.  Displacement resolution as
high as 1.0 nm can be achieved due to the fringe interpretation
schemes used.  A novel balanced mechanical design was also developed
for flexibility in alignment and for insensitivity to interferometer
arm fixture thermal deformation induced measurement noise.

      The schematic layout of the newly developed
interferometer/vibrometer is shown in Fig. 1.  A He-Ne laser was used
as the optical source due to its wavelength stability.  A
polarization beam splitter, a half-wave plate and a right angle prism
was used to initiate the two interferometer arms.  The incident light
intensity of the two interferometer arms can be varied continuously
in order to compensate the reflectivity difference on the measurement
surfaces such as N-58 sliders and thin film disks.  A Bragg cell was
used to shift the laser light frequency in order to remove the
directional ambiguity of the measurement.  The two optical heads were
assembled such that both the output and the input fibers were located
at the focal plane of the whole optical head.  Since the optical head
turns the output laser beam into a point source located at the
surface of the measurement object, the phase information will not be
destroyed no matter if the measurement surface is reflective or
diffusive. Furthermore, this focused optical configuration does not
place a significant constraint on the static pitch allowed on the two
measurement surfaces, i.e., this new configuration provides us with a
way to measure the flying height of high static pitch sliders.

      Since the optical laser beam remained linearly polarized, a
polarization beam splitter can be used to place the two measurement
spots anywhere from zero to several inches, i.e., the two measurement
spots were separated by using the polarization states of the light
beams.  This flexibility in varying the separation distance is
important for slider flying height measurement as the disk profile
effect on the measured flying height (noise) becomes more significant
when the relative distance of the two measurement spots increases.
Two optical fibers were fused together to form the optical fiber
combiner.  Both return fibers were aligned so that the polarization
states of the two returned laser light beams aligned with respect to
each other to maximize the interference efficiency.  Alignment effort
is greatly reduced in this configuration.  The interference signal
generated by interfering the returned signals from the two
interferometry arms are...