Rev: 1.1, 23-May-2022
© 2022 Thorlabs
Page 7
SPDMHx Series
3 Operation
·
Focusing Quality: For optimum timing resolution the light should be focused to a small spot
(<70
m
m) in the center of the sensor. Off-center focusing or overfilling the sensor area might
lead to an increased FWHM of the photon timing resolution. This is particularly important for
the free space detectors.
·
Count Rate: High count rates decrease the timing resolution. Especially at count rates above
1 MHz the FWHM may be double the value compared to low count rates.
Temporal Stability
The temporal stability of the pulse output depends on the count rate. High count rates lead to a
relative shift of the pulse to later times. The total shift may reach 800 ps at count rates above 1
MHz.
Saturation Level
The dead time limits the measurable count rate at high incoming light levels. The count rate at
which the signal does not change significantly by increasing incident photon numbers is called
saturation level. Precautions should be taken to avoid prolonged excessive light levels that may
damage the SPDMHx Series detector.
Correction Factor
Every SPDMHx Series detector has an inherent dead time of approx. 43 ns after detecting a
photon. The dead time is also noted in the included production report. During this dead time,
the SPDMHx Series detector is "blind" and can not detect further photons. As a consequence,
the measured count rate is lower than the true incident photon rate.
The photon rate can be calculated from the measured counting rate as follows:
R
R
1-R
T
measured
photon
measured
D
=
g
where:
R
photon
: actual incident photon rate
R
measured
: measured count rate
T
D
: detector dead time
The correction factor can be used to correct the non-linearity especially at high light levels.
The following plot shows the effect of the dead time as the measured count rate does not pro-
portionally increase with the real photon rate for high count rates due to the dead time effect.
The correction factor is required to receive the real photon rate.