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86 PI-MAX
®
3 System Manual Version 1.A
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.
Triggered Lasers
Timing Generator as Trigger Source: Using the PI-MAX3’s SyncMASTER1
signal to trigger the laser allows you to get rid of the propagation delay for the External
Trigger (25 ns) and to set up all timing relative to
T0 . You will still need to consider the
delays from the cable to the laser (1.5 ns/ft), internal delay from trigger to firing (laser
dependent, 50 ns for example), the PI-MAX3's internal minimum allowable gate delay
(25 ns).
External Source Triggers Both Timing Generator and Laser:
This is the more
complex case because it contains many sources of delay that would have to be
considered. A carefully prepared Time Budget could prove invaluable in determining
what steps need to be taken to bring the gate and signal into coincidence at the detector.
In addition, actually measuring delays with a fast oscilloscope as previously described
could be very helpful. If the laser provides a
Pre-Trigger output that can be used to