THORLABS SPCM A Series, Operation Manual

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6 Appendix
© 2019 Thorlabs
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Dark Count Rate : This is the average rate of registered counts in the absence of any incident
light and determines the minimum count rate at which the signal is dominantly caused by real
photons. The false detection events are mostly of thermal origin and can therefore be strongly
suppressed by using a cooled detector.
Geiger Mode: In this mode, the diode is operated slightly above the breakdown threshold
voltage. Hence, a single electron-hole pair (generated by absorption of a photon or by a
thermal fluctuation) can trigger a strong avalanche.
Pulse Blind Time : As explained in section Operating Principle , spontaneously triggered ava-
lanches may be correlated in time with a pulse caused by a photon, which then is called after-
pulse. To ignore such afterpulses in the measurement, an additional
Pulse Blind Time can be
set in the software, which will cause the internal counter of the SPCM to ignore all pulses occur-
ring during the set
Pulse Blind Time . This additional Pulse Blind Time starts at the moment
when a photon was registered by the counter.
Saturation of the APD :
The photon count by an APD is not exactly linearly proportional to the incident optical CW
power, the deviation increases smoothly with increasing optical power. This non-linearity leads
to wrong photon count at high input power levels. At a certain input power level, the photon
count begins even to decrease with further raise of optical power.
As for this reason, a saturation alert is displayed as soon as the photon count is no more in-
creasing with increasing incident CW power.
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