William Angerer - Department of Physics and Astronomy - University ...

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the chopper. The photon counter counted the number of photons in both gates and
reported the difference between the two gates, i.e. A-B.
Although the A Gate and B Gate parameters may be set independently, accurate
A-B counting requires that the gates have identical characteristics. In a typical measurement.
we amplitude modulated the Ti:Al 2 0 3 laser light at 1 kHz. Both the A
Gate and B Gate widths were set to 400 J-lS and the gates were delayed 50 J-lS from the
start of the open and closed positions of the chopper, respectively. By setting the gate
durations less than the period of the open and closed chopper positions, we ensured
that signal photons were always counted in A Gate. However, signal photons can be
counted in the B Gate due the finite width of the Ti:Al 2 0 3 beam. For a finite sized
beam, the transmission of light through the chopper is not a series of step functions
but rather is a series of step functions with tails. These tails represent the partially
blocked beam. and the length of the tail is proportional to the size of the beam. Thus.
a fraction of light is transmitted through the nonlinear optical microscope when the
optical chopper has a closed phase. We eliminated this systematic error by placing
the chopper at the focal point of the telescope lenses.
vVe now discuss the photon generated current pulses and their relationship to the
measured counts. The number of detected counts depends on two parameters: the
discriminator level and the shape of the current pulse. Our PMT was ideally suited
for photon counting because it generates high amplitude current pulses when a single
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