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

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PMT
to photon counter
snubber
Figure 7.7: Snubber schematic to reduce current pulse ringing. A signal is reflected
from the short coaxial cable and reshapes the current pulse from the P:\IT. The
length of the cable determines the delay of the reflected pulse and the resistance of
the variable resistor determines the magnitude of the reflected signal. We adjusted
the variable resistor while observing the current pulse on an oscilloscope to minimize
pulse ringing. This figure was adapted from reference [lllJ.
a main pulse followed by several smaller pulses. If the height of these secondary
oscillations had been above the discriminator level, the pulse was counted twice. By
placing a variable resistor at a distance of 6 inches from the PMT and in parallel with
the signal cable to the photon counter [lllJ, we introduced electrical feedback into
the signal (see Fig. 7.7). This feedback reshaped the pulse and damped the pulse
ringing. Hence, we ensured each signal pulse was counted only once.
7.2.3 Registering and Positioning Samples
In contrast to second-harmonic generation from a Si wafer, which is homogeneous.
many applications of second-harmonic microscopy require accurate and reproducible
control of the sample position. Reproducibility and accuracy of the sample position
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