h) Opto-Electronic Components for Metrological Applications
Laser
based
systems
were
developed
to
measure
velocities
of
gases
and
liquids
with
a
laser
anemometer
or
to
monitor
smallest
shifts
and
vibrations
by
laser-triangulation
and
interfero
-
meters and lon
ger distances with laser range finders.
Most of these devices were realized and tested by students working for their master thesis.
The physical background and operation of respective devices can be found in the lecture note:
giving
an
introduction
to
lasers
and
explaining
the
different
measuring
applications
(starting
at
page 64, in German).
Polarization Sensitive Interferometer with Direction Identification
Laser
interferometers
measure
distances,
shifts
or
vibrations
in
units
of
a
half
or
a
quar
ter
wavelength,
and
thus,
they
belong
to
the
most
accurate devices in metrological studies.
In
a
slightly
modified
Michelson
interferome
ter
configuration
the
incident
radiation
is
split
into
a
reference
and
a
measuring
beam.
After
pas
-
sage
through
the
reference
and
measuring
arm
the
beams
are
again
superimposed
and
the
in
-
terference
structure
is
observed
as
a
function
of
the
displacement
of
one
of
the
mirrors
or
re
-
tro-reflectors.
To
allow
practical
measurements
with
forward
and
backward
scanning
of
objects
and
to
avoid
a
malfunction
caused
by
smallest
vibrations,
an
identification
of
the
moving
direction
is
neces
-
sary. One possible realization is a polarization sensitive detection of the superimposed beams.
Frequency Scan of a Piezo Translator
An
example
for
interferometrically
recording
the
frequency
characteristic
of
a
piezo
element
is
shown
here.
One
arm
of
the
interferometer
is
designed
to
stabilize
the
set-up
to
a
working
point,
around
which
faster
displacements
measured in the other arm can be captured.
In
this
way
the
frequency
dependent
dis
place
-
ment
of
a
piezo
translator
can
be
wobbled
with a resolution in the nano meter range.
Laser Triangulation Set-Up
For measurements of sub-mm displacements
and
diffusely
reflecting
surfaces
a
laser
trian
-
gulation
set-up
has
proven
to
moni
tor
shifts
or
vibrations
down
to
µm
resolution,
as
this
is
re
-
quired
for
laser
cleaning
and
structuring
of
surfaces.
On
the
other
hand
with
a
triangulation
system
designed
to
measure
vibrations
of
the
red-hot
glowing
graph
ite
electrodes
of
an
electric
arc
melting furnace allow to measure the oscillations with sub-mm resolution from 15 m distance.
Diff
erence Doppler Anemometer in Backward Direction
Two
parallel
laser
beams,
which
are
focused
to
a
common
spot,
form
a
pattern
of
parallel
stripes
in
the
focus
of
a
lens.
Scattering
parti
-
cles
of
µm
size
in
a
flowing
gas,
a
liquid
or
from
a
solid
surface
passing
this
spot
are
illu
-
minated
periodically
and
radiate
a
modula
ted
bunch.
The
scattered
light
can
be
detected
in
forward
or
backward
direction
and
its
modu
-
lation
is
directly
proportional
to
the
velocity
of
the
flowing
medium.
In
this
way
contactless
flow
profiles
in
tubes,
around
the
wings
of
aero
-
planes or from glowing surfaces can be monitored without perturbing a flux by the sensor.
Laser Range Finder
Lidar System
For
measurements
of
longer
distances
from
km
up
to
several
100,000
km
laser
range
finders
based
on
the
pulse-echo
principle
are
applied.
A
short
laser
pulse
of
sub-ns
or
only
a
few
ps
duration
is
generated
and
by
a
transmission
optic
directed
to
a
target,
from
which
the
reflected
signal
is
collected
by
a
Cassegrain
optic
and
directed
to
a
fast
photo
diode
or
photo-multiplier.
Then
the
measured
distance
L
is
given
by
L
=
c
.
t/2
with
c
as
the
speed
of
light
and
t
as
the
measured time.
Publications
J. Pfuhl, H. Harde
Mikroprozessor gesteuerter Laserentfernungsmesser mit zeitlicher und statistischer Rauschdiskriminierung
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Fachausschuss Kurzzeitphysik, Regensburg,
17. März 1983, Verhandl. DPG (VI) 18, 370 (1983)
H. Harde
Lasers for Range Finders and Detection of Trace Gases
Script, Carl-Cranz-Gesellschaft, Hamburg, 1991
