PEM200 User Manual
54
so that the light beam traverses the center of the modulator multiple times. For
example, by traversing the modulator optical element twice, a flat-topped waveform
(half-wave retardation) would indicate quarter-wave PEM retardation (for a single pass).
See figure A.7.
Figure A.7 Setup for Double Pass Modulation
Static Waveplate Techniques
Another technique for establishing a fractional retardation would be to use a static
waveplate with known retardation. The waveplate would be placed as shown in Figure
A.1, with the fast axis of the waveplate parallel to the modulator axis. If the sum of the
static waveplate retardation and the modulator peak retardation is one half-wave, then a
flat-topped oscilloscope waveform will result. The modulator retardation would then be
half-wave minus the retardation of the fixed waveplate. The use of a calibrated Soleil-
Babinet Compensator would give a means of calibrating a modulator for continuous
values of peak retardation.
Bessel Function Zero Methods
An examination of equation 2 suggests a method for precise setting of certain values of
retardation.
Note, for example, the “DC” term in equation 2. For certain values of
0
A
(e.g. 2.405
radians),
( )
0
0
0
=
A
J
, and therefore the DC term becomes a constant, independent of the
birefringence B. The DC intensity also becomes independent of many other changes
Light Source
Polarizer
+45º
Modulator
0°
Polarizer
-45°
First
Surface
Mirror
