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LASER
d = = P
RETROREFLECTOR
INTERFEROMETER
BEAM
SPLITTER
RECEIVER
trgure
2-36.
Two-Axis Measurement System with Unequal Measurement Paths
2.31
Beam Path Loss Computation
In general, multiaxis systems must be designed with sufficient safety margin in the power
received by the 10780A Receiver. Computation of beam path loss is straightforward. The
minimum output power of the laser head is 120 microwatts; most exceed this figure. The out-
put power is relatively constant over the usable life of the tube and tends to drop off slightly
toward the end. The minimum necessary power at the receiver is four microwatts. For a three-
axis system, the power safety margin is 40 pW +
4 pW
or 10:l on each axis.
Relating
this
to
system
configuration,
the
following
considerations
are
important.
Thirty-
three
percent
Beam
Splitters
are not
actually
33%
but
more
on
the
order
of
33%
&5%.
The
10700
Series
Beam
Benders
with
dielectric
coatings
reflect
99%
of
the
light.
Some
other
beam
benders
(not
supplied
by
HP)
are
only
80-95%
reflective,
and
using
more
than
one
causes
the
losses
to
multiply.
Cube-corners
are
typically
80%
to
90%
reflective.
Dirt
on
the
optics
reduces
the
amount
of
light
at
the
receiver.
Poor
alignment
of
the
optics
or
the
receiver
reduces
the
amount
of
light
detected
by
the
receiver
photodiode.
This
specifically
includes
misalignment
of
the
optics
causing
the
position
of
the
beam
at
the
receiver
to
wander
a s
the
object
being
measured
runs
down
its
travel.
Fluctuations
of
the
refractive
index of
air
in
the
path
of
the
interfering
beams
(which
can
be
caused
by
local
temperature
differences)
cause
the
laser
beam
to
lose
some
of
its
coherence
and
could
even
break
it
for
an
instant. This
is
detected
by
the
electronics
which
causes
an
error
signal
to
be
generated.
The
smaller
the
received
signal
safety
margin,
the
more
likely
the
fluctuations
are
to
break
the
beam.
Techniques
of
protecting
the laser
beam
to
minimize
these
fluctuations
are
discussed
later
in
this
section.
One thing that must be kept in mind when calculating the path loss in an axis of the laser is that
the optics split the two frequency components of the laser beam into two separate paths for
each axis and the losses are normally computed separately for each of the two components.
Since the laser beam is detected by a mixing process and the result is proportional to the
product of the powers of the two frequencies, the loss bookkeeping can be handled by calcu-
lating a transmission factor for each path. The overall transmission factor is then the product
of all the individual transmission factors. The transmission factors refer to usable signal trans-
mitted and not to light intensity.
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