Diploma thesis

kcounts per second
180
160
140
120
100
80
60
40
20
808nm pulsed laser 1nW, Width: 2.5 ns
Probability: 10%, Deadtime: 0 µs
Probability: 10% ,Deadtime: 1 µs
Probability: 10% ,Deadtime: 2 µs
Probability: 25%, Deadtime: 0 µs
Probability: 25%, Deadtime: 1 µs
Probability: 25%, Deadtime: 2 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 4.9: 808 nm laser, 2.5 ns gating
kcounts per second
1800
1600
1400
1200
1000
800
600
400
200
808nm pulsed laser 1nW, Width: 5 ns
Probability: 10%, Deadtime: 0 µs
Probability: 10% ,Deadtime: 1 µs
Probability: 10% ,Deadtime: 2 µs
Probability: 25%, Deadtime: 0 µs
Probability: 25%, Deadtime: 1 µs
Probability: 25%, Deadtime: 2 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 4.10: 808 nm laser, 5 ns gating
leading to the detector. Actual PDC pump powers are in the mW regime but already
pump light over six magnitudes smaller than that introduces a significant level of
counts (figure 4.9 and 4.10). Evidently, pump filters with a suppression of 10 −6 or
higher have to be applied to cope with this dark count source. All measurement
data are again displayed in Appendix A.3.4.
4.2.4 Summary
Taking all different dark count sources into account, it is important to apply sufficient
deadtimes when the id201 is operated above 1MHz trigger rate. A narrow gating is
crucial to get rid of noise. Our laser systems produce pulses in the fs and ps regime
and enables us to use the narrowest gating of 2.5 ns. Also in the process of PDC it
is necessary to filter the remaining pump photons, as opposed to stray light, which
has no noticeable effect. All experiments can be done without the need for total
darkness in the laboratory.
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