PEM200 User Manual
55
which could be made to the optical system, such as the angular position of the second
polarizer. The DC intensity can therefore be used for “normalization” of the AC signals,
by forming the ratio
DC
AC
V
V
. This is now independent of fluctuations in intensity in the
light source, changes in optical transmission, etc.
To establish precisely the controller setting for which
0
0
=
J
, graphs of
DC
V
vs controller
setting were made for several different angular positions of the second polarizer. A
typical set of graphs is shown in Figure A.8. Notice that the graphs should intersect at
one particular controller setting for which the Bessel function
0
J
vanishes.
Figure A.8 Determining the Controller Settings for which J
0
= 0
If a lock-in amplifier is available, and if the modulator is capable of providing adequate
peak retardation, then the AC signals can provide similar retardation calibration points.
Consider the second harmonic signal, at twice the modulator frequency. At a peak
retardation or amplitude of
136
.
5
0
=
A
radians,
( )
0
0
2
=
A
J
. This setting can be
established with great accuracy using a lock-in at frequency 2f.
Addition of a birefringent element as shown in figure A.1, so that a significant 1f signal
at the modulator frequency is obtained, will enable determining the controller setting for
which
( )
0
0
1
=
A
J
. (This occurs for
872
.
3
0
=
A
radians.) Detection of the null point is
made with a lock-in amplifier at the modulator frequency.
V
DC
Controller Setting
Polarizer
Orientation
Angle
