Teledyne Princeton Instruments PI-MAX 3, System Manual

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130 PI-MAX
®
3 System Manual Issue 2
9.3.3 Limitations of Bracket Pulse Gating
MCP bracket pulse gating is most useful in rejecting background that lasts
microseconds up to CW. Fast transient backgrounds can be in the form of stray laser
light scattering (e.g., Raleigh, MIE, or Raman,) or unwanted fast fluorescence. Because
these usually fall below the MCP bracket pulsing 35 ns delay restriction, these
measurements cannot be improved much by MCP bracket pulsing in the PI-MAX3.
Electrically, gating the MCP will only reduce leakage at wavelengths where the MCP has
photoelectric response (primarily in the UV.) Thus, for visible and NIR wavelengths
where leakage is primarily optical, the improvement will be minimal (although the
on/off ratio is already very good in these regions.) Note that in some spectroscopic
applications, visible leakage may appear to be reduced by MCP pulsing. This is because
the second order UV spectrum overlays the first order visible spectrum in a grating
spectrograph. MCP pulsing can eliminate unwanted sensitivity to CW or quasi-CW
second order UV, causing the apparent improvement.
Also, keep in mind that MCP bracket pulsing adds 10 ns delay to the photocathode
gating. Even though the bracket timing is controlled automatically by the software, in
an experiment where it is necessary to delay the arrival of the laser pulse at the sample,
this will mean inserting an additional delay of 10 ns (min gate delay = 25 ns, with
bracket = 35 ns) to accomplish coincidence at the detector. MCP bracketing should
only be used in experiments where it is going to make a difference.
Also, MCP bracket gating limits the repetition rate to 6.25 kHz. Without it the gate
repetition rate can be up to 1 MHz (more with option boards.) This is not a significant
limitation in most cases. Note that with a 10 ns gate at 6.25 kHz, the on/off ratio =
16,000, close to the non-bracket value.
Note that background light need not be the limiting factor in measurements where
MCP bracket pulsing is unable to provide the required degree of rejection. In such
measurements, the option remains of installing an external shutter ahead of the
PI-MAX3.
9.3.4 Impact of Bracket Pulsing on Delay
If operating in the UV when bracket pulsing is activated (Gen II Intensifier only,) the
MCP gate automatically brackets the photocathode gate pulse to further enhance the
on/off ratio. There is, however, a limitation of bracket pulsing that can complicate the
coincidence of the signal and gate at the camera. Because MCP bracket gating is slower
than photocathode gating (35 ns is required to gate the MCP on, and another 1

s to
gate the MCP off at the end of the photocathode gate.) As a result, MCP bracket pulsing
should not be used in experiments where the delay between the trigger and the
photocathode gate is less than 35 ns.