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Chapter 6 LightField and Gated Operation 99
6.4.1 Bracket Pulsing in LIF Measurements
Most experiments using laser-induced fluorescence to probe combustion flows are
performed with UV probe/lasers. Atomic emission from flames also has significant UV
content. If the flame is continuous, the UV background will also be continuous. Even where
a flame is transient (e.g. internal combustion engine,) its lifetime can be many milliseconds,
compared to the nanosecond time scale of the laser used. This background can be a million
times as long. If the background is bright, then a UV on/off ratio of 20,000:1 will be
overwhelmed by the duty cycle and will not be adequate for extracting a signal of 10
-5
. In
high dynamic range quantitative measurements, backgrounds must be kept to an absolute
minimum. MCP bracket pulse gating dramatically improves the rejection of CW and even
millisecond time-scale background.
An alternative to suppressing background for imaging has been the use of very narrow
spectral bandpass filters. In the UV, these filters are expensive and they can have low
transmission at their central wavelength. An additional filter is required for each wavelength
to be imaged. The use of electronic temporal rejection of CW or quasi-CW background may
make it unnecessary to use these filters, thus increasing the optical throughput, sensitivity,
and quantitative precision of the measurement.
6.4.2 Bracket Pulsing in Nanosecond Pump Probe Experiments
Some nanosecond pump-probe experiments combine a nanosecond or faster pump with a
flashlamp probe. The duration of the probe flash can be 10-50 µs and a gate is used to select
the specific nanosecond-scale time slice to be observed within the much longer probe flash.
In these absorbance experiments, accurate measurement of absorbance values depends
critically on the lack of stray light contamination, particularly at moderate to high optical
density levels. Selecting a 5 ns time window out of a 10 µs pulse is already one part in
2,000. If UV leakage gives an on/off ratio of only 20,000:1, contamination could be 10% of
higher. This would limit the optical density to 1.0, and it could make linear quantitation
difficult beyond 0.1 OD.
MCP bracket pulse gating can substantially improve the on/off ratio in such an experiment.
Even with a 1 µs MCP pulse, the rejection of flash-lamp leakage can add more than an order
of magnitude of range, to 2.0 OD.
6.4.3 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 (Raleigh,
MIE, 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-MAX4.
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.