138 PI-MAX/PI-MAX2 System Manual Version 5.F
that using optical cable or mirrors to delay the signal will carry some intensity penalty,
which might have an adverse affect on measurement results in some experiments.
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.
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.
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
Pretrigger Output, it can be used to trigger the timing generator. If the interval between
the Pretrigger and the laser output is long enough, the timing generator delay can then be
adjusted to catch the laser pulse following each pretrigger. If the interval between the
Pretrigger and the laser output isn’t long enough to accommodate all the 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 doesn’t have a Pretrigger 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.
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