Examples of investigation and measurement

Preamplifier
One essential element of the laser interferometer is the reliable 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 180° with respect to signal A is produced. By means of a comparator DC-offsets are eliminated and at the same time 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 get this channel independent of variations in contrast, the signal D is used.

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 preamplifier. The quad 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 technical interferometer 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.

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 preamplifier are connected to an oscilloscope. For recording the fringes are summed up with the counter. The directional identification is done by the up-down signal controlling the counter. The actual displacement is obtained by multiplying the number of counted fringes with the wavelength (λ) divided by the selected interpolation factor.

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 cannot be realised either. But for good functioning of the interferometer this is not required.