THORLABS SPDMA, Operation Manual

Page: 9 / 21

Rev: 1.0, 08-Dec-2021 © 2021 Thorlabs Page 5
SPDMA 3 Operating Instructions
During the quenching time, which is known as the dead time of the diode, the APD is insensit-
ive to any other incoming photons.
Spontaneously triggered avalanches are possible while the diode is in a metastable state. If
these spontaneous avalanches occur randomly, they are called
dark counts . An integrated
TEC element stabilizes the diode’s temperature below the ambient temperature to reduce the
dark count rate. This eliminates the need for a fan and avoids mechanical vibrations.
In case the spontaneously triggered avalanches are correlated in time with a pulse caused by a
photon, it is called an
afterpulse .
Note
Due to APD properties, not all single photons may be detected. Reasons are the APD's intrinsic
dead time during quenching and the APD's nonlinearity.
Gain Adjustment
Using the gain adjustment screw, an overvoltage beyond the breakdown voltage can be adjus-
ted to the SPDMA. This increases the photon detection efficiency but also the dark count rate.
Please be aware that the probability of afterpulsing slightly rises with higher gain settings and
that adjusting the gain also affects the time between photon detection and signal output.
The dead time increases with decreasing gain.
3.5 Block Diagram and Trigger IN
The current pulse generated by an incoming photon passes a pulse shaping circuit, which is
shortening the APD's output TTL pulse duration. On the "Pulse Out" terminal the signal from
the pulse shaper is applied so that counts can be viewed on an oscilloscope or registered by an
external counter. In the absence of a Trigger, the gate is closed and allows signal out.
The Gain changes the Bias (overvoltage) on the APD. The Bias is physically guided through the
active quenching element but does not impact the active quenching.
TTL Trigger:
The TTL Trigger allows for selective activation of the pulse output: At high Trigger Input (spe-
cified in the
Technical Data ) the signal arrives at Pulse Out. This is the default whenever no ex-
ternal TTL signal is applied as a trigger.