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High single count rates but very view coincidences
It sometimes happens that the setup ends up in a state, where both arms receive a high
number of single counts, but the expected coincidence rate is very low. It is important to
understand that whenever two reasonable high single rates
and
occur, there is a certain
probability, that two events from the two rates are accidentally closer together then the
coincidence window width nm and thus are considered as coincident events. The rate
of these accidental coincidences
can be estimated by
Especially, when there are very view coincidences, it has to be checked whether they result from
actual signal or rather are purely accidental.
If there is actually a true coincidence rate from signal events, the setup is most probably aligned
asymmetrically and the fastest solution is the full alignment procedure detailed in section 3.4.3.
If the observed coincidences are entirely accidental, there can be several reasons for this
behavior which can be distinguished in the following way:
1. First make sure, the polarizers are set to actually allow for coincidences. So, to check for
photon pairs always set the polarizers to be both vertically and subsequently
horizontally.
2. If there are still no coincidences, the couplers might actually not collect light from the
crystal but be aligned for fluorescence light emitted from the long pass filter in the
window of the white housing. Again, this can be checked easily: Remove the wave plate
from the pump assembly. Thereby, only one sort of photon pairs are generated in the
BBO crystal, both horizontally or vertically polarized. Now, try for each arm separately if
there is high contrast visible in the single count rates when turning the polarizer, a full
revolution. If so, this means the light is polarized and the respective coupler actually
collects down-conversion light. If there is no or only very weak contrast, the light is
unpolarized which is a hint that the coupler collects fluorescence light from the filter.
The procedure for both these cases is explained in the following.
4.2.1 There is polarization contrast in both arms
This means, the single count rates actually are down-conversion signal. If there are no real
coincidences at all, the setup is aligned severely asymmetrically and you need to perform the
full alignment procedure explained in section 3.4.3.
There is no polarization contrast in at least one arm
This means, the rates are due to fluorescence in the long pass filter. The situation for both arms
is sketched in Figure 12.
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