6
Kentech Instruments Ltd., Unit 9, Hall Farm Workshops, South Moreton, Didcot, Oxon, OX11 9AG, U.K.
10th. July 1999
3
Orientate the cell for maximum extinction. Ideally this should be done by looking at the isogyre
pattern obtained by scattering the incoming light over a range of angles. This can be done with
suitable tape over the input face of the cell. The laser should pass through the centre of the pattern.
Don’t forget to remove the tape!
4
Repeat 3 with the second cell.
5
Rotate the output polariser so that it is parallel to the input polariser.
6
Apply the relevant half wave voltage DC to on cell and repeat 3. The position should be near to the
existing position. Adjust the voltage so that the extinction is best. Note that misalignment and a
higher voltage can achieve an extinction minimum also and one is looking for the minimum
transmission near the existing position and at the lowest voltage.
7
Remove the voltage from the cell and repeat 3 on the second cell.
8
Remove the voltage from the second cell and now connect the quarter wave voltage to both cells.
Now work with small movements of the cells to obtain the best extinction ratio. The ratio is easily
obtained from the transmission obtained when the voltage is removed to the transmission when the
voltage is applied. Note that some power supplies will not discharge the cells when they are turned
off. It may be necessary to discharge the cell capacitance with a resistor (note that the capacitance
of any cabling to the cell will be much bigger than the cell and will store a significant amount of
energy.)
9
Remove the DC supply and connect the pulser. The light source may now be pulsed or DC to look
for the relevant timing of the pulsers, the laser source and the detector display.
Figure 2
A plot of DC transmission against applied voltage. The voltage is
applied to both cells simultaneously.
5.2
TIMING ADJUSTMENT
The relative timing of the four channels of the pulser must be set to obtain the best overall risetime.
It is assumed that the above alignment has been done and that the pulser, light source and detector are all suitably
timed for fine adjustment work.
The KD*P material used in the cells has a dielectric constant of approximately 80 at the electrical drive frequencies
which are up to around 500MHz. This results in a speed of propagation in the cell of around 7 times slower than the
light. In addition as we are looking for transitions of the order of 700ps or less the separation of the cells in space will
affect the required timing of each cell.
