Interferometer
INTERFEROMETERS
Interferometers
are optical instruments used for measuring flatness and determining the length
of slip gauges by direct reference to the wavelength of light. The
interferometer incorporates the extension of the application of the optical
flat.
It overcomes
some of the disadvantages of the optical flats used in ordinary daylight or
diffused source of monochromatic light by having some refined arrangements.
In
interferometers the lay of the optical flat can be controlled and the fringes
can be oriented to the best advantage. Also there is arrangement to view the
fringes directly from top and above the fringes thus avoiding any distortion
due to incorrect viewing.
Brief
Description of components
(i) Two frequency Laser source
It is generally
He-Ne type that generates stable coherent light beam of two frequencies, one
polarized vertically and another horizontally relative to the plane of the
mounting feet. Laser oscillates at two slightly different frequencies by a
cylindrical permanent magnet around the cavity. The two components of
frequencies are distinguishable by their opposite circular polarization.
Beam containing
both frequencies passes through a quarter wave and half wave plates which
change the circular polarizations to linear perpendicular polarizations, one
vertical and other horizontal. Thus the laser can be rotated by 90°about the
beam axis without affecting transducer performance. If the laser source is
deviated from one of the four optimum positions, the photo receiver will
decrease. At 45° deviation the signal will decrease to zero.
(ii) Optical elements
a) Beam splitter
Sketch shows the
beam splitters to divide laser output along different axes. These divide the
laser beam into separate beams. To avoid attenuation it is essential that the beam
splitters must be oriented so that the reflected beam forms a right angle with
the transmitted beam. So that these two beams are coplanar with one of the
polarisation vectors of the input form.
b) Beam benders
These are used
to deflect the light beam around corners on its path from the laser to each
axis. These are actually just flat mirrors but having absolutely flat and very
high reflectivity. Normally these are restricted to 90° beam deflections to
avoid disturbing the polarizing vectors.
c) Retro reflectors
These can be
plane mirrors, roof prism or cube corners. Cube corners are three mutually
perpendicular plane mirrors and the reflected beam is always parallel to the
incidental beam. Each ACLI transducers need two retro reflectors. All ACLI
measurements are made by sensing differential motion between two retro
reflectors relative to an interferometer. Plane mirror used as retro reflectors
with the plane mirror interferometer must be flat to within 0.06 micron per cm.
(iii) Laser head’s measurement
receiver
During a
measurement the laser beam is directed through optics in the measurement path
and then returned to the laser head is measurement receiver which will detect
part of the returning beam and a doppler shifted frequency component.
(iv) Measurement display
It contains a
microcomputer to compute and display results. The signals from receiver and
measurement receiver located in the laser head are counted in two separate pulse
converter and subtracted. Calculations are made and the computed value is displayed.
Other input signals for correction are temperature, co-efficient of expansion, air
velocity etc., which can be displayed.
