Chapter 11 Tips and Tricks 139
Triggered Lasers
Timing Generator as Trigger Source: Using the PTG’s T0 signal or the DG535’s
T0 output to trigger the laser allows you to get rid of the propagation delay for the
External Trigger (25 ns for PTG and 85 ns for DG535) 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), cabling from
the ST-133 to the PI-MAX (1.5 ns/ft), the PI-MAX's internal delay (15 ns), and the
minimum allowable gate delay (21 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 Pretrigger output that can be used to trigger
the Timing Generator, it may not be necessary to use mirrors or fiber-optic cable to delay
the laser pulse. If there is no pretrigger, then taking steps to delay the arrival of the laser
pulse at the sample would likely be necessary. The easiest solution would be to insert
electrical delay between the external trigger source and the laser.
Jitter
Jitter, uncertainty in the timing of the laser output, is a critical laser performance
parameter in gated experiments. If the jitter is significant relative to the duration of the
signal pulse, the gate width will have to be wide enough to accommodate it, and the
temporal discrimination against unwanted signal will be reduced. Some types of high
power laser pulse have considerable jitter, even using a pretrigger. Where this is the case,
there is no choice but to trigger from the actual laser pulse. One way of doing that is to
use a pellicle mirror and PIN diode as previously described and then to delay the light
(usually by multiple reflections between mirrors or in an optical fiber) until the gate
“opens.”
Inhibiting the Pulser during Readout
In Gate mode operation, if gating pulses are applied to the detector during a readout, it
will cause undesirable artifacts in the data. In experiments where the time between cycles
is longer than the readout time, there is no possibility of this happening and it is not a
matter of concern. If the experiment is such that it is possible for a new gate to be applied
before the readout of the previously gathered data set is complete, preventive action will
be required. With a PTG, the timing generator is inhibited internally by the Controller.
With a DG535, it is necessary to connect a cable from the Shutter
*
output signal
(provided at the ST-133’s Output) to the Inhibit input of the DG535 (requires
that the DG535 have the Inhibit Option installed).
Lens Performance
Imaging applications require that a lens be mounted to the detector. Because the lens
characteristics affect system performance, it may be helpful to review some basic lens
concepts. Basically, light from an object enters the front of the lens and is focused to a
*
NOT SCAN and SHUTTER are two different signals and must be set by jumper or software.
To Next Page
Loading...
