10
SPCMxxA
© 2019 Thorlabs
3
Operating Instructions
3.1
Operating Principle
The Thorlabs SPCMxxA uses a silicon avalanche photo diode (Si APD), operated in reverse di-
rection and biased slightly beyond the breakdown threshold voltage
V
BR
(see diagram below,
point
A
), also known as 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
BR
(point
C
) im-
mediately after a photon released an avalanche, this way enabling high count rates (dead time
between counts down to 35 - 45ns). Afterwards the bias voltage is being restored.
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
Peltier element stabilizes the diode’s temperature below the ambient temperature to reduce the
dark count rate. For long-term temperature stability, a fan is built in.
In case the spontaneously triggered avalanches are correlated in time with a pulse caused by a
photon, it is called an
afterpulse
. 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 occurring during the set pulse blind time. This additional
Pulse Blind Time
starts at the moment when a photon was registered by the counter.
Note
Due to APD properties, single photons may be not counted, i.e., lost. Possible reasons are the
APD's intrinsic dead time during quenching, an additionally set blind time of the SPCM and the
APD's nonlinearity.
Summary of Contents for SPCM A Series
Page 1: ...Single Photon Counting Module SPCMxxA Operation Manual 2019 ...
Page 2: ...Version Date 1 2 27 May 2019 ...
Page 42: ...40 SPCMxxA 2019 Thorlabs ...
Page 43: ...5 Maintenance and Service 2019 Thorlabs 41 ...
Page 54: ...52 SPCMxxA 2019 Thorlabs 6 5 Dimensions Dimensions of the SPCMxxA imperial version ...
Page 55: ...6 Appendix 2019 Thorlabs 53 Dimensions of the SPCMxxA M metric version ...
Page 61: ...www thorlabs com ...
