Diploma thesis

dark counts, afterpulsing, stray light and remaining pump photons. To distinguish
all these effects we constructed a coincidence setup where we employed the strict
photon number correlation between signal and idler photons.
The setup sketched in Figure 4.42 is pumped at 760 nm with a pump FWHM
around 10 nm. We used a long wave cutoff filter of about 1000 nm and a short
wave cutoff filter at 700 nm to get rid of unwanted light from the laser system. Our
goal is to excite the type-II process in the crystal. Precise control over the pump
polarization is necessary therefore we controlled it with a Glan-Thompson polarizer.
The first half-wave plate in front of the polarizer served as a power control, the
second half-wave plate after the polarizer turned the pump polarization in the yplane.
Afterwards the beam is coupled into a polarisation maintaining fiber and fed
into the crystal. After the downconversion a long wave pass filter is applied to get
rid of the remaining pump light. Signal and idler beams are separated by a PBS,
coupled into single mode fibers and guided into the APDs. The signals have been
post processed with a time-to-digital converter (TDC), which evaluated the number
of coincidence counts [27].
Figure 4.42: Measurement setup to detect signal and idler photon-pairs
We achieved count rates around 3200 events per second for each detector and a
coincidence level of 94 coincidences per second (see table 4.5).
This setup may further be improved by the use of lenses to couple into the SMfibers.
At the moment we apply microscope objectives to couple into the single mode
fibers. The coupling efficiency is around 60%. The challenge is to couple the guided
modes emitted by the rectangular waveguide into the emission modes of a circular
single mode fiber. These two different modes don not perfectly match each other,
therefore the incoupling efficiency is limited and will never reach 100%.
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