Teledyne Princeton Instruments PI-MAX 3, System Manual

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Chapter 15 Tips and Tricks 185
The signal timing will probably be more difficult to measure. Typically, you might divert
a small portion of the laser beam using a pellicle mirror located near the sample
position. By directing the beam to a PIN diode module, you could obtain an electrical
signal that could be monitored with the oscilloscope to accurately indicate the arrival
of the laser beam at the sample position. Note that the indicated time would have to be
corrected for the insertion delay of the path from the pellicle mirror to the oscilloscope,
including the insertion time of the PIN diode, which might be on the order of the 10 ns.
This correction would have to be compared with the delays that would normally exist
between the sample position and the detector to determine the actual time the signal
would arrive at the detector. Also, the oscilloscope will have its own insertion delay,
perhaps 20 ns, and an uncertainty of nominally 1% of the time base.
Another consideration is how to trigger the oscilloscope. If there is a common trigger
source for the sample position signal and for the gating, that trigger could also be used
to trigger the oscilloscope, allowing both signals to be observed simultaneously.
Another possibility is to trigger the oscilloscope from the PIN diode signal to observe
the Monitor signal, or to trigger the oscilloscope from the Monitor signal to observe the
PIN diode signal. The signal that occurs first would have to be used as the oscilloscope
trigger.
NOTE:
This is not necessarily always the case. Digital oscilloscopes
can display signals that occur before the trigger.
15.2.3 Adjusting the Signal Delay
The PI-MAX3 internal timing generator gives the user wide latitude with respect to
adjusting the delay between the time the timing generator is triggered and the time the
Gate On and Off edges are generated. This being the case, as long as the light signal
applied to the detector occurs after the minimum delay time of the timing generator
(25 ns), there will be no problem establishing the necessary coincidence.
On the other hand, if the light signal applied to the detector occurs before the
minimum delay time of the timing generator, then no amount of adjusting the delay at
the timing generator can rectify the problem. The light signal itself will have to be
delayed.
If a common source is triggering both the timing generator and the laser, a very
convenient solution is to insert electrical delay (long cable) between the trigger source
and the laser. This is generally preferable to establishing the necessary delay optically
via mirrors or fiber optic cable.
Alternatively, pass the laser output through a length of optical fiber cable. By using
different lengths of fiber, almost any desired signal delay can be achieved. Yet another
solution would be to set up two separated parallel mirrors with a small angle between
them. Typically, it will be easily possible to bounce the laser beam back and forth
between the mirrors half a dozen times to obtain the necessary delay. In any case, once
the light signal is arriving at the detector after the minimum gate time, the timing
generator delay adjustments can be used to bring them into coincidence. Keep in mind
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.