5
Kentech Instruments Ltd., Unit 9, Hall Farm Workshops, South Moreton, Didcot, Oxon, OX11 9AG, U.K.
10th. July 1999
Figure 1
The front panel
causes half wave relative retardation of the extraordinary ray and the polarisers are either parallel or perpendicular. In
particular the system is intended for use with a pair of cells between one pair of polarisers. By using two cells only half
the pulse power is needed as compared with achieving twice the voltage on one cell. (c.f E = 0.5CV
2
).
4
CIRCUIT DESCRIPTIONS
The circuit consists of four high voltage FET switches. These is driven by a single trigger pulse. The leading edge
is speeded up using the non linear capacitance of silicon diodes. This is a proprietary circuit and details are not normally
available to the user.
The HT is obtained from an array or small floating 30 volt supplies which are either switched in or out as the
“Amplitude” switch is turned.
5
USING THE PULSER WITH POCKELS CELLS FOR LASER PULSE CHOPPING
The main application of this pulser is to chop the falling edge off a laser pulse. In order to achieve this the polarisers
are mounted parallel and the cell arranged to switch the plane of polarisation of the light when energised.
The pockels cells work by using a material which is bifringent when placed in an electric field (transverse in this
case). Plane polarised light entering the cell is split into two waves of equal amplitude polarised at 90
°
to each other.
These travel at slightly different speeds in the excited medium and emerge with a relative phase shift of
λ
/2. In the
unexcited medium there is no relative change in the phase and the polarisation is not affected. This means that the exit
plane of polarisation is at 90
°
to the incident plane. The light can then be rejected on a polariser.
These conditions only exist for a small range of angles for the light through the crystal. At other angles the light
may not be split equally or there may be some intrinsic bifringence leading to partial or complete rejection in the absence
of the electric field.
It is hard to set up this system in pulsed mode. Looking for extinguished light is far easier than looking for a peak
of transmission. With the pulser running most of the time the system is still transmitting.
5.1
STATIC SETTING UP
A simple way to set up the pockels cells is to use a DC power supply. This can be set at the half wave voltage
(quarter wave on each cell) and the cells and polarisers orientated for maximum extinction. A CW laser and a
photodiode running into a high impedance (high sensitivity) connected to a DVM will suffice.
In order to keep the pulser power down the system is intended for use with two cells each being driven to the quarter
wave voltage. Ideally these need to be set up independently. Here is a suggested method.
1
Align two polarisers perpendicular by looking for the maximum extinction.
2
Place one cell between the polarisers with the straight edge of the square aperture crystal at 45
°
to
the plane of polarisation of the incoming light.
