Diploma thesis

kcounts / second
0.14
0.12
0.1
0.08
0.06
0.04
0.02
Figure 4.3: Dark counts measurement setup
Gating width: 2.5 ns Detector efficiency 25%
Deadtime: 0 µs
Deadtime: 1 µs
Deadtime: 2 µs
Deadtime: 5 µs
Deadtime: 10 µs
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
trigger frequency [MHz]
Figure 4.4: Dark counts, 2.5 ns gating
kcounts / second
4
3.5
3
2.5
2
1.5
1
0.5
Gating width: 5 ns Detector efficiency 25%
Deadtime: 0 µs
Deadtime: 1 µs
Deadtime: 2 µs
Deadtime: 5 µs
Deadtime: 10 µs
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
trigger frequency [MHz]
Figure 4.5: Dark counts, 5 ns gating
density filter to simulate single photon sources. After the attenuation their light was
coupled into the detectors (see setup in Figure 4.6).
In this experiment incoming photons constantly hit the detector material. Therefore,
the trigger rate should be related to the count rate linearly. Any nonlinear
increase can be traced back to afterpulsing events. The measurement data plotted
in Figure 4.7 and 4.8 (all data in Appendix A.3.2 and A.3.3) shows that these assumption
is only valid in the case of low trigger rates. For trigger rates above 1
MHz, an additional deadtime is needed to restore the linear increase and discard
afterpulsing effects. This is very apparent in Figure 4.8. The count rate for no
applied deadtime (red graph) increases in a nonlinear manner above 1MHz. The
blue graph (1µs deadtime) exhibits a sharp drop at exactly 1 MHz trigger rate.
The reason for this behaviour is the deadtime of 1 µs. After each avalanche, the
deadtime overwrites the following gating pulse. After each detection the following
gating and hence the following possibility to detect an incoming photon is lost. As
a result, the effective detection efficiency of the APDs decreases. In this extended
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