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Chapter 12 Picosecond Gating Option 181
12.5 Timing
When using optical gate widths from a few nanoseconds to a fraction of a nanosecond,
timing is obviously critical. The PI-MAX4 is calibrated with respect to the optical input
plane (located on the front mounting plane) and the rear panel. All other propagation paths
must be accounted for by the user. These are significant, considering a 1 meter coaxial cable
represents typically 4.5 ns delay, or 9 times the gate width, assuming the gate width is set to
500 ps. To get the best representation of the
MONITOR output, the user should use a high
bandwidth oscilloscope set at 50 input impedance. The rise time of the
MONITOR pulse is
typically less than 500 ps when terminated in 50 .
12.6 Methods for Finding a Short Optical Pulse
Two methods of identifying a short optical pulse are described in this section.
1. The first method is to calculate all delays in the optical and trigger paths, and configure
the PI-MAX4 delay accordingly. If all arithmetic is correct and accurate numbers have
been used for all of the delays involved, this method will work. In practice, however, it
seldom works because either some of the delays are not accurately known or something
gets overlooked. The sum of the optical delays must be greater than the trigger delay
(including the PI-MAX4 minimum delay.) Summing after the fact is still a valuable
check on the system, even if the timing is achieved by method 2.
2. A more direct and usually more convenient method is to start with a gate pulse much
wider than the optical pulse and set the PI-MAX4 so the optical gate is wide enough to
be sure it encompasses the optical pulse. This method works well if the pulse is
conveniently repetitive, such as one derived from a repetitive laser. Once the pulse is
found, it is an easy matter to reduce the pulse width and adjust the delay until the
precise timing needed is achieved. The PI-MAX4 repetitive and sequential gating can
be used to good advantage in this method.
Example
This example is simplified in that it does not address all of the possible settings for Gate
mode setup. It is intended as a sense of how to locate the signal of interest by successively
decreasing gate pulse parameters.
1. Starting with a 1 s gate and delay set to the minimum value for the PI-MAX4 (~25 ns,)
the pulse is located. It is then known that the pulse arrives sometime between 25 ns and
1.025 s.
WinX LightField
Number of Images/Spectra: 1
Gate Mode: Repetitive
Number of Frames: 1
Gate Delay: 25 ns
Gate Width: 1 µs
4411-0137_0110