I am using the experimental scheme presented in the image below. It seems that no matter what I do, I cannot obtain coincidence counting higher than 350/s, although the single count at each detector is about 500000/s. The crystal used is a 2 cm long 10 um period PPKT collinear type II crystal.
I'd double check that the mirrors after the PBS (supposing this is a polarizing beam splitter) reflect the differently polarized beams equally enough. Probably, you could even get rid of them completely, displacing instead the SPDs for alignments.
Then, check that the SPDs have the same QY, e.g. by exchanging them.
Also alignments of the PBS is always suspected, for every of the 6 degrees of freedom.
I would suggest to verify the temporal synchronization of the two detectors. For a too narrow coincidence window and different latency of the detectors, the true coincidences might be outside of the coincidence window. Only false (random) coincidences are present. The first step would be to increase the coincidence window to dozens of ns to surely cover any difference in latency. Then you can start decreasing the coincidence window and adding a delay to the fastest detector channel, e.g. using time tagger functionality or simply extending the length of a coaxial cable.
Also, it might be useful to add cut-off filters transmitting the red signal and stopping the blue pump in front of the detectors. Using the filters you make sure that your signal comes from the SPDC process and not from the laser. Filters https://www.semrock.com/filterdetails.aspx?id=blp01-633r-25 are very good but you can use any low-cost alternative.
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