PEM200 User Manual
58
For low values of retardation, this ratio is approximately a constant! This means that
use of this ratio for determining PEM retardation is quite insensitive.
The situation may be corrected if the PEM is placed between parallel polarizers. In
this case:
DC
V
α
( )
0
0
1
A
J
+
(11)
The ratio V
2f
/V
DC
vs. retardation is shown in Figure A.10.
Figure A.10 Ratio V2f/VDC vs. retardation (radians) for parallel polarizers
This method gives monotonically increasing values of the ratio V
2f
/V
DC
for low values of
A
0
(3 radians and less). It lacks sensitivity for retardation values near 3.5 radians, but
this is where the flat-topped half-wave waveform occurs (
π
radians). The graph is used
in reverse, for a given value of
DC
f
V
V
2
, A
0
may be determined.
The method is conceived as a systematic calibration of retardation vs. :MOD:DRV for a
sequence of retardations, beginning with low values. Such a study should avoid
ambiguities in A, since the functional relationship is an inverse one. The method,
however, requires considerable attention to detail.
First, care must be taken to ensure that V
DC
is proportional to the average intensity of
the modulated light signal. Other contributions to the experimentally measured DC
A
0
V
2f
/ V
DC
