PEM200 User Manual
57
The required setting for quarter-wave retardation is:
( )
( )
( )
=
2
2
4
2
2
2
2
λ
π
π
λ
f
f
V
J
J
V
(7)
( )
515
.
2
2
=
λ
f
V
( )
2
2
λ
f
V
(8)
( )
2
2
λ
f
V
is the lock-in reading for half-wave retardation, as set by the oscilloscope.
Parallel Polarizer Calibration at a Single Wavelength
The primary calibration technique discussed in “Calibration Using an Oscilloscope” is
observation of the waveform at half-wave retardation. This gives calibration only at half-
wave retardation or integral multiples of half-wave retardation. Other methods such as
the Bessel function zero method give retardation only at specific values of retardation
(e.g. 3.877 radians or 5.136 radians). Another method of accurately measuring
retardation at arbitrary values of modulator drive (:MOD:DRV) is much desired.
Calculating the ratio of V
2f
/V
DC
is in principle a good approach. However, if calibration is
being done with the PEM between crossed polarizers at 45º there are problems,
especially at low retardation levels. From theoretical analysis of the resulting
waveforms we learn that:
DC
V
α
( )
0
0
1
A
J
−
(9) and
f
V
2
α
( )
0
2
2
A
J
(10)
If one plots the ratio V
2f
/V
DC
the graph shown in Figure A.9 is obtained.
Figure A.9 Ratio V2f/VDC vs retardation, radians
V
2f
/ V
DC
A
0
