Definitions - THORLABS SPDMA Operation Manual

SPDMA 5 Appendix

5.2 Definitions

Active Quenching occurs when a fast discriminator senses the steep onset of the avalanche
current, released by a photon, and quickly reduces the bias voltage so that it is below break-
down momentarily. The bias is then returned to a value above the breakdown voltage in prepar-
ation for detection of the next photon.
Afterpulsing: During an avalanche, some charges can be trapped inside the high field region.
When these charges are released, they can trigger an avalanche. These spurious events are
called afterpulses. The life of those trapped charges is on the order of 0.1 µs to 1 µs. Hence, it
is likely that an afterpulse occurs directly after a signal pulse.
Dead Time is the time interval the detector spends in its recovery state. During this time, it is ef-
fectively blind to incoming photons.
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.
Gain Adjustment Factor: This is the factor by which the gain can be increased.
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 the wrong photon count at high input power levels. At a certain input
power level, the photon count begins even to decrease with further increase in optical power.
Each delivered SPDMA is tested for appropriate Saturation behavior to resemble this example.
Rev: 1.0, 08-Dec-2021 © 2021 Thorlabs Page 9