Diploma thesis

deadtime may be applied. This deadtime extends the time electrons and holes get
to recombine thus suppressing afterpulsing. The overall single photon detection
efficiency of these detectors can be set up to 25%, by adjusting the base voltage
(parameters in Figure 4.1, efficiency in Figure 4.2).
❏ Trigger Rate (0 - 8 MHz)
❏ Detector Pulse Width (2.5
ns, 5 ns, 20 ns, 50 ns, 100
ns)
❏ Detector Efficiency (10%,
15%, 20%, 25%)
❏ Detector Deadtime (None,
1 µs, 2 µs, 5µs, 10 µs)
Figure 4.1: id201 parameters Figure 4.2: id201 quantum efficiency
Before we used the APDs in an actual PDC experiment, we extensively tested
them. We investigated the dark count level, afterpulsing and additional noise from
remaining pump light guided into the detector.
4.2.1 Dark counts
First, we investigated the effects of stray light and thermal fluctuations. We monitored
the dark counts with a simple setup (see Figure 4.3). We blocked the input
to the detector, a single mode fiber for the near infrared, and applied an external
trigger source. The data acquisition for this experiment has been fully automated
and the trigger generator together with the APDs are remotely controlled (see Appendix
A.2). Our goal was to investigate the effects of trigger rates, pulse width,
detector efficiency and deadtime on the dark count rate. The results are visualised
in Figure 4.4 and 4.5 (for all measurement data, see Appendix 4.5).
As expected, the dark count rate rises linearly with the applied trigger frequency.
The critical parameter for the noise is the gating width. A doubling in gating
time from 2.5 ns to 5 ns (see Figure 4.4 and 4.5) leads to an increased count rate
over one order of magnitude. It is therefore crucial to apply the narrowest gating
possible. The nonlinear increase in the case of 5 ns gatings in Figure 4.5 is a result of
afterpulsing events. As also plotted in Figure 4.5 the afterpulsing can be significantly
reduced by deadtimes longer than 5 µs. For all data, please refer to Appendix A.3.1.
4.2.2 Afterpulsing
To further investigate the effect of afterpulsing, we used two cw-lasers centered
around 1310 nm and 1555 nm. These laser diodes where attenuated with a neutral
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