Operating Principle - THORLABS SPDMA Operation Manual

SPDMA
status LED will return to red when the diode temperature is too high. If the LED is red, no
signal is sent to pulse output.
b. Green: Detector is ready for operation. The diode is at operating temperature and signal
arrives at the pulse output.
Note
The Status LED will turn red whenever the operating temperature is too high. Please ensure
sufficient air ventilation.
4. Push the cover back in front of the status LED to prevent LED light from disturbing the mea-
surement.
5. To increase the photon detection efficiency, turn the Gain Adjustment Screw with a slotted
screw driver (1.8 to 2.4 mm, 0.07" to 3/32"). For more information on the gain, please refer
to the chapter Operating
a. Use Minimum Gain when a low dark count rate is critical. This comes at the cost of low
photon detection efficiency.
b. Use Maximum Gain when it is desirable to collect a maximum number of photons. This
comes at the cost of a higher dark count rate.
c. Because the time between photon detection and signal output changes with the gain set-
ting, please reevaluate this parameter after changing the gain setting.
Note
"Trigger In" and "Pulse Out" are of a 50
is capable to work on a 50
W
50 input impedance.

3.4 Operating Principle

The Thorlabs SPDMA uses a silicon avalanche photo diode (Si APD), operated in reverse di-
rection and biased slightly beyond the breakdown threshold voltage V
point A), also known as the avalanche voltage.
This operating mode is also known as "Geiger mode". An APD in Geiger mode will remain in a
metastable state until a photon arrives and generates free charge carriers in the PD's junction.
These free charge carriers trigger an avalanche (point B), leading to a significant current. An
active quenching circuit integrated into the APD limits the current through the APD in order to
avoid destruction and lowers the bias voltage below the breakdown voltage V
mediately after a photon released an avalanche. This enables high count rates with dead time
between counts down to the
stored.
Page 4
Principle.
W
impedance. Make sure that the trigger pulse source
W
load and that the device connected to "Pulse Out" operates at a
specified dead time at max gain. Afterwards the bias voltage is re-
3 Operating Instructions
(see diagram below,
BR
(point C) im-
BR
MTN025859_D02