Ap1115
8/28/2001 Application
Notes AP1115
A
multiple-pass optical adapter for high resolution in turbulent environment
I.
What is the problem?
For high resolution
and high accuracy measurement in a
shop environment, the measurement is effected by
air turbulence. For a
laser interferometer type
measurement, typical
resolution is 0.000001 in
or 0.00001 mm.
However, because of the air
circulation, or turbulence, the effective laser beam path length
(OPD) is
fluctuating. This fluctuation
limited the accuracy of
the laser measurement. Long time-average has
been used to minimize the effect of
air turbulence.
However, too much
averaging may cause
a time-lag and
inconvenience in the measurement.
Another method is to control the environment to minimize the air circulation and temperature gradient or to
cover the laser beam path completely.
However, in a shop environment, both of these are difficult to
achieve. II. What is the multiple-pass optical
arrangement?For a typical laser interferometer,
the laser beam is reflected by a
retroreflector target, and the
displacement of the target is determined by measuring the change
in the optical path
length. A multiple-pass is an optical
arrangement, with the laser beam reflected back and
forth between the retroreflector target and
some mirrors or prisms mounted
stationary with the laser head. It has been shown in Ref [1] and [2] that multiple-pass optical arrangement can
increase the resolution, and reduce the
effect of air turbulence. III. How the
multiple-pass optical adapter work? Conventional multiple-pass optical
arrangements using mirrors and prisms are
rather complex and
difficult to align. The multiple-pass
optical arrangement developed
by Optodyne (patent pending) can easily be achieved by attaching an
optical adapter (an
accessory) to the single aperture laser
head (MCV-500 laser calibration system) and using a
1" diameter retroreflector as target as
shown in Fig. 1. The
number of passes between the optical adapter and the retroreflector is
increased by a factor of 6. That is,
a one inch displacement of the retroreflector will become an 6 inches increment of the effective optical path
length. Hence the resolution is
increased by a factor of 6 and the air turbulence is averaged over the
6 parallel paths. For a MCV-500 laser
system, the standard range is more than 400
inches. Hence the range becomes 66 inches with the 6-pass optical adapter.
Since the maximum velocity is 200
in/sec, the maximum velocity with the 6-pass optical adapter becomes 33 in/sec.
