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2.14
SINGLE
BEAM
INTERFEROMETER
LASER
BEAM
PATH.
A
polarizing
beam-splitter
reflects
f,
to
the
reference cube
corner
and
transmits
f,
to
the
10704A
Retroreflector
or
other
surface
whose
displacement
is
being
measured
(Figure
2-74).
The
return
path
is
superimposed
on
the
outgoing
path.
Since
both
beams
leaving
the
beam-splitter
pass
through
a
quarter-
wave
plate
the
returning
polarizations
are
rotated
through
90
degrees.
This
causes
f,
to
be
transmitted
and
f,kAf
to
be
reflected
so
that
they
are
directed
coaxially
to
the
receiver
along
a
path
perpendicular
to
the
measurement
path.
REFERENCE
10704A
RETROREFLECTOR
10705A
SINGLE
BEAM
INTERFEROMETER
Figure
2-14.
Single
Beam
Interferometer
Laser
Beam
Path
2.15
1070614
PLANE
MIRROR
l
NTERFEROMETER
(Figure
2-75).
The
plane
mirror
inter-
ferometer
has
a
unique
feature
in
that
the
retroreflector
can
be
a
flat
mirror and
it
has
a
par-
ticular
advantage
in
that
interference
fringes
can
still
be
detected
even if
the
measurement
beam
is
not
at
a
perfect
right
angle
to
the
mirror.
It
is
an
advantage
to
use
plane
mirrors
a s
retroreflectors
because
(in
a
two-axis
system
for
example)
the
X
retroreflector
can
be
allowed
to
move
in
the
Y
direction
without
affecting
the
signal
strength
or
the
X
measurement.
There-
fore
both
retroreflectors of
a
two-axis
system
can
be mounted
on
the
same
moving
part.
This
makes
it
very
easy
to
eliminate
Abbe
offset
on
a
two-axis
system.
If
the
measuring
point
is
defined
to
be
where
the
two
axis
beams
cross,
the
measurement
is
essentially
independent
of
yaw
of
the
moving
stage.
Such
a
design
is
shown
in
Figure
2-44.
Contrasting
this
system
to
a
two-axis
system using
standard
interferometers,
we
see
that
(if
standard
interferometers
are
used)
the
X-axis
retroreflector
must
be
mounted
on
a
part
of
the
stage
which
moves
only
in
the
X
direction
and
never
in
the
Y
direction.
Also,
the
Y-axis
retroreflector
must
be
mounted
on
a
different
part
of
the
stage
which
is
al-
lowed
to
move
only
in
the
Y
direction
and
never
in
the
X
direction.
Therefore,
the
two-axis
measure-
ments
cannot
be
made
on
the
same
part
of
the
stage
and
there
is
by
necessity
some
geometry
error
in
the
system
if
it
is
not
perfectly
rigid.
Another
difference
between
the
plane
mirror
interferometer
and
the
previous
two
types
is
that
with
a
plane
mirror
inter-
ferometer
the
measurement
beam
travels
between
the
interferometer
and
the
retroreflector
twice
and
therefore
the
resolution is
twice
that
of
the
other
inter-
ferometers.
To
be
specific,
the
standard
interfero-
meter
and
single
beam
interferometer have
a
reso-
lution
of
0.16
micrometre
(6
pinches)
without
elec-
tronic
resolution
extension,
whereas
the
plane
mirror
interferometer
has
a
resolution
of
0.08
micrometre
(3
pinches)
without
electronic
resolution
extension.
All
three
of
these
interferometers
are
used
to
measure
linear
displacement.
Figure
2-15.
70706A
Plane
Mirror
Interferometer
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