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Pre-amplifier
One essential element of the laser interferometer is the secure
detection of the bright / dark transitions even for variable
contrast conditions which may be established due to
modifications of the initial adjustment during the displacement
of the measuring reflector. To compensate for these influences a
signal C phase shifted by 1800 with respect to signal A is
produced. By means of a comparator disturbing DC-offset parts
are eliminated from the resulting signal which at the same time
is converted into a TTL signal. To detect the direction of the
displacement of the measuring reflector a signal B, phase
shifted by 90° with respect to signal A, is created. To also get
this channel independent of variations in contrast, the signal D
is used.
Evaluation electronics
The phase shifting of 90° are created by means of a quarter wave
plate which has to be adjusted correspondingly. For visualising
the signals, the monitor outputs of the pre-amplifier have to be
connected to the oscilloscope. The recording and evaluation of
the fringes can be done in different ways:
A) The fringes are counted with the counter. The directional
identification is done by the counter (UP-DOWN). The actual
displacement is obtained by multiplying the number of counted
fringes with the wavelength (λ) divided by the selected
interpolation factor.
B) If the PC-interface card is used in connection with a PC, the
conversion into units of length can be performed by the
software.
Set-up and functioning of the technical interferometer
During the discussion of the optical beam paths, further optical
components will be discussed such as quarter wave plates and
triple reflectors. Since the states of polarisation are mutually
orthogonal polarised within the two interferometer arms there
will be no spatial interference pattern as for the Michelson
interferometer. Only by use of a λ/4 plate with polarising beam
splitting cube, visible interference pattern are generated.
Signal generation and processing
The interferometer set-up provides the four interference signals
A, B, D and E. They are available at the monitor outputs of the
pre-amplifier. The quadratic signals C and F are created by
these signals. Through various adjustments, these signals can be
influenced. The λ/4 plate, for instance, can be rotated in its
holder. In consequence, the position of the angle of incidence
of the light with regard to the optical axis of the crystal can
be investigated. By means of these signals, the basic facts of a
homodyne interferometer as used in daily life become evident.
Furthermore this type of signal generation and processing is
typical for incremental angle encoder or linear encoder for CNC
machines and last but not least also for modern calliper gauges.
Signal representation
If an oscilloscope is used in XY-mode, a figure as shown on the
right appears on the screen, provided the sine-signal is
connected to the X-channel and the cos-signal is connected to
the Y - channel. A closed ellipse or circle is received if the
reflector of the interferometer is displaced. Only a point is
visible in the state of rest. One rotation of the point
corresponds to a path difference of λ/2. This kind of
representation gives important information about the state of
adjustment of the interferometer. Theoretically, a circle is
expected but in reality the optical components are never ideal
and a perfect state of adjustment can-not be realised either.
But for good functioning of the interferometer this is not
required.
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