Glossary
GLOSSARY
This is a disorganised hotch-potch of definitions, links and comments. It's the place we put things we've just been asked or have only just thought of.

PRF
Pulse repetition Frequency

Clear aperture input
The GOI cameras use a transmitting mesh over the input window to couple the gate pulse to the cathode. Although this mesh is highly transmitting it can produce weak spurious diffraction patterns in the image when imaging coherent light. We are able to provide a clear aperture option specifically designed for coherent applications. Please email us for details.

Delay generator
delay_generators

ECL
Emitter Coupled Logic

TTL
Transistor Transistor Logic

EMP
Avalanche_pulsers Electromagnetic pulse, usually used for large electromagnetic pulses as occur with lightning or electrical discharges. More recently also used for machines capable of simulating aspects of these effects.

Pulse generators
Pulsers This is the highest performance pulser we think we can make without some more development effort.

Fluorescence lifetime imaging
High Repetition Rate Imagers - GOI This is a diagnostic for obtaining chemical information with spatial resolution. Applications in biomedical fields are likely to be the most common.

GOI18
Gated optical imager with 18mm diameter cathode.

100MHz, 0.5ns exposure gated intensifier [Also called HRI]
A GOI with a 12mm cathode diameter which can run asynchronously at up to 100MHz with a 0.5ns gate width. This will work well with many laser systems that have a PRF in the 80MHz range.

Intensifier
A device for intensifying an image. They range in complexity and may contain one or more Microchannel plates for more gain. There are three generations of electrostatic intensifier. Gen 1, are devices that use an electrostatic lens to image the cathode onto the phosphoror or MCP.
Gen 1 devices with MCPs suffer from image distortion due to the MCP input face being flat. The len simage plane should be curved. Some long tube designs with extra electrodes can reduce this. Gen 1 tubes cannot be gated fast as the cathode structure does not lend itself to being driven fast. Gating the MCP requires high voltages and low inductance connections to it.
Gen 2 devices are wafer tubes that use proximity focussing of the cathode onto an MCP. These devices cvan be gated quickly (down to 50ps for a small tube). The cathode is generally gated.
Gen3 devices use Gallium Arsenide cathodes. They can be gated fairly quickly but require much more gate drive power than Gen 2 devices due to the prescence of a barrier between the cathode and MCP input. Photoelectrons have to accelerated to enough energy to penetrate the barrier.

LINAC
Linear Accelerator
Special pulsers

Free electron laser LINAC pulser
Linacs operate at high PRF and pulsers to drive them need to deliver high PRF, short high voltage pulses.

lppmm
Line Pairs per mm

Avalanche
The process of electron multiplication that leads to breakdown of an insulator. If the energy dissipated is controlled carefully the breakdown can occur in a solid without causing permenant damage. Such a process is used in avalanche transistors to make fast high voltage pulsers.

FET
Field Effect Transistor. These devices are suitable for moderately fast pulsers and are capable of delivering high average power. Devices can be cascaded to give very high voltages and currents.

MCP
Micro Channel Plate. A device consisting of a flat glass pate with a matrix of holes used for electron multiplication with spatial resolution. The inner surface of the holes is coated with a material with a high secondary electron emission coefficient. The plate is placed in an electric field across the thickness of the plate such that electrons are accelerated through the plate. Electrons hitting the sides of the tube cause the emission of many electrons which are further accelerated and again hit the tube sides. In this way many electrons emerge from the tube. Typical parameters are the length to thickness ratio (L/D) which is of the order of 40 in many common plates and determines the gain response to an applied voltage; also the pore size which can vary from around 5µm to tens of µm. Plates are usually made round disks but can be ground to other shapes. Plates come with either hard or soft edges. Hard edges have no holes in them and are good for clamping electrodes. Soft edges have holes to the edge and this means that they are more easily damaged by clamping but they are less prone to distortion when absorbing moisture and also maintain the effective dielectric constant to the edge, this is important for fast gating as it enable transmission line matching to be better. Plates also come with or without conducting layer laid down on their faces. A variety of materials and surface resistivities are available. MCP is also often a misused expression for an optical wafer intensifier which uses an MCP.

PFN
Pulse forming Network. An electronic network, often purely passive, that can convert one pulse shape into another. The network will often use capacitors, inductor, resistors and transmission lines to achieve this.

Radar
Radio Detection and Ranging. More recently High voltage fast pulsers have been used for Ultra Wideband radar. Such Radars can operate asynchronously and are virtually impossible to jam. They operate with a very wide bandwidth making them suitable for detecting a wide range of materials or penetrating a wide range of materials. Applications are for example, personnel detection, foliage penetration or land mine detection.

Spectroscopy
XRSC

Synchrotron
High PRF gated imaging in synchrotrons
interesting_things

Time of flight
Special pulsers

Three dimensional imaging
interesting_things GOI
Transillumination    Turn off time (optical gating)
The turn off time in a gated intensifier is shorter than the gate width. The gain in the intensifier is a very strong function of cathode voltage close to zero so the transition from relatively high to zero sensitivity can be extremely quick. In a GOI we believe this transition to be between 10 and 20ps. In the high repetition rate cameras it is considerably less than 500ps. It is a function of various parameters and can be optimised.
This effect will be beneficial in fluorescence lifetime imaging and transillumination as in both measurements the detection of first light is important.

X-ray, X-ray streak cameras
XRSC

Field emission pulse X-ray source
interesting things

Pulse technical considerations
 The critical parameters for the pulser are:
*Risetime
*Peak output power
*Repetition rate
*Pulse shape

The risetime generally determines the technology, be it either avalanche, FET or FET hybrid. Avalanche pulsers will provide rise times to <100ps however the higher voltage FET pulsers are restricted to a couple of nanoseconds. FET hybrid designs can operate with 0.5ns risetime at amplitudes approaching 1kV at PRFs up to 1MHz.

The peak power output determines the number of switching elements. The peak power is peak voltage squared over R, where R is the load resistance. The actual voltage is rather less important as we are able to make wideband transformers to match odd load impedances. Please tell us the load when you ask for a particular voltage.

The repetition rate is usually limited by the average output power. FET pulsers are able to provide MUCH more average power than avalanche pulsers. Avalanche pulsers are usually restricted to 1kHz although in some applications <10kHz is possible. FET pulsers can operate at many MHz.

The pulse shape is usually determined by passive pulse forming components and the amplitude is therefore lower when a particular pulse shape is provided.

Voltage doubling
In certain applications a significant economy on power may be made by exploiting the fact the the pulse voltage in a cable will double if it hits an open circuit or small capacitive load. An example of such an application is a pockels cell extracting a short pulse from a laser cavity. In this case the voltage at the pockels cell will be approximately double that launched into the connecting cable. The penalties are greater risetime and ringing noise which occurs a double transit of the cable after the first pulse. Such applications may be insensitive to this noise however and the peak pulser power required may be reduced by a factor of four.
The pulsing of microchannel plates if another situation in which this technique may sometimes be used.

Triggering
All our pulsers may be triggered electrically although fibre optic triggering arrangements can be provided for isolation. When triggering from a photodiode be aware that the trigger circuits integrate for a few nanoseconds so changes in the amplitude of the trigger signal will result in timing changes.

We are frequently asked for TTL compatible inputs however it is not always clear whether a 50Ω, termination is required. We usually provide a TTL input with a few kΩ load to ground so the signal in a 50Ω cable is actually 2.5V and this doubles at the open cicuit termination. It is usual to drive such a load from a 50Ω reverse terminated source. Please ask if you are unsure.

Jitter
The shot to shot jitter for all our pulsers is small, typically less than 20ps RMS. There are warm up drifts, particularly when there is logic circuitry included such as the \D delay options, but these stabilise after a short initial warm up period.
In situations where several pulsers must be synchronised with equal or sequential timing we have highly stable techniques for matching the timing of many channels. This is useful in framing cameras and phased array pulse transmitters.

Pulse shape
The basic pulsers produce a double exponential output with a fast rise and slow fall. We are able to build internal or external pulse forming networks to turn the basic shape into a square pulse, monocycle or ramp. These networks always reduce the output amplitude. When specifying a special pulser be sure to consider the constraints on pulse flatness and noise.

Pulse measurements
The high voltages from our pulsers will destroy many (most!) types of wideband attenuators. Please contact us is you would like advice on pulse shape and amplitude measurements.

Some common pulser applications
Laser cavity dumping
Regenerative amplifier switching
Laser pulse chopping
Time domain radar
Driving fast imaging diagnostics
Time of flight mass spectroscopy
Streak camera deflection drive
EMC testing