Teledyne Princeton Instruments PI-MAX 3, System Manual

Page: 186 / 250

186 PI-MAX
®
3 System Manual Issue 2
15.2.4 Optimizing the Gate Width and Delay
When the basic delay questions have been answered, the next consideration is
optimization of the Gate Width and Delay. The goal is to have the gate just bracket the
signal event. One effective approach is to:
1. Begin with minimum delay and a gate width far wider than the optical signal pulse
to be measured.
2. While observing the data signal at the computer monitor, gradually increase the
delay until the event vanishes. This will mark the point at which the gate is opening
just after the signal, causing the signal to be lost.
3. Reduce the delay until the signal reappears.
4. Then begin reducing the gate width (not the delay.) As the gate is narrowed, the
amount of EBI generated will decrease so the signal-to-noise ratio should improve.
When the point is reached where the gate becomes narrower than the signal being
measured, the observed signal data will degrade. You may have to adjust the delay
to keep the signal in view.
5. From there increase the width slightly for maximum signal and optimum
signal-to-noise.
15.3 Lasers
Pulsed lasers are used in many experiments where a gated intensified detector might be
used to recover the signal. For example, in combustion measurements, a laser pulse
might be applied to a flame and the resulting fluorescence studied as the signal to be
analyzed. Because this short-term signal is much weaker than the integrated light
emitted by the flame, an intensified gated detector should be used to do the
measurement.
Because available lasers differ so widely with respect to their characteristics and
features, there is no way to discuss specifically how to incorporate your particular laser
into a measurement system. It is necessary that users be familiar with the features,
operation, and limitations of their equipment. Nevertheless, the following observations
might prove helpful.
15.3.1 Free Running Lasers
These lasers behave essentially as oscillators. They typically exhibit little jitter from
pulse to pulse and are very easy to synchronize with the experiment.
• If the laser has a Pre-trigger Output, it can be used to trigger the timing
generator.
— If the interval between the pre-trigger and the laser output is long enough,
the timing generator delay can then be adjusted to catch the laser pulse
following each pre-trigger.
— If the interval between the pre-trigger and the laser output is not long
enough to accommodate all insertion delays, the timing generator delay
can be adjusted to catch the next laser pulse.
As long as the laser's jitter relative to the period is small, this is a perfectly
valid way to operate.
• If the laser does not have a Pre-trigger Output, one option is to use a pellicle
mirror and a PIN diode to obtain the timing generator trigger.
Again, the timing generator delay could be adjusted to catch the next laser
pulse to achieve the necessary synchronization between the optical signal and
the photocathode gate at the detector, although this would cause at least every
other laser pulse to be lost.