THORLABS APD410 Series, Руководство по эксплуатации, Страница: 12 / 30

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APD410x
3.4 Recommendations
Please always remember that the Avalanche Photodetectors are extremely sensitive devices.
Carefully shielding the Avalanche Photodetectors from any unwanted light sources is essential.
Common techniques include baffling or other opaque barriers like black cloths or lens tubes.
It is highly recommended to use appropriate band pass filters in front of the detector to mini-
mize the influence of stray light.
Since stray light has its strongest frequencies at DC and line frequency or harmonics, optical
chopping and Lock-In detection can further improve measurement sensitivity.
It is not necessary to switch off the Avalanche Photodetectors when it is exposed to ambient
light. The amplifier will saturate but unlike Photomultiplier Tubes it will not be damaged or satu-
rated for a long period of time.
Another critical point can be electrostatic coupling of electrical noise associated with ground
loops. In most cases an electrically isolated post (see Thorlabs parts TRE or TRE/M) will sup-
press electrical noise coupling. You should always try to identify the electrical noise sources
and increase the distance to the Avalanche Photodetector. If possible, you can also rotate the
Avalanche Photodetector input away from the noise source. Different common ground points
can also be tested.
The amplifier offset voltage is factory set to zero at 23°C ambient temperature. Due to the
very high transimpedance gain, even small temperature changes may affect offset voltage.
Therefore it is recommended to use the Avalanche Photodetector in a constant temperature
environment after a short warm up period (~5min) for exact DC light level measurements. The
output offset voltage is dependent on the M factor. As the intrinsic noise of InGaAs APD is sig-
nificant higher than Si APDs, it causes a higher impact on the output offset voltage.
The M factor is factory set at 23°C ambient temperature. The APD410x are operated at an in-
ternal reverse bias voltage that is temperature-compensated, and their actual M factor will re-
main nearly constant within the specified ambient temperature range of (23±5) °C.
It is recommended to set the GAIN to minimum before applying the optical signal to the
APD410x. This is the condition when the APD410x is most insensitve and can tolerate its
highest optical input level.
For any level of the optical input signal there is an optimal M factor that provides the best SNR
(Signal-to-Noise Ratio). To change the M factor, turn the
GAIN knob on the side panel.
A non-linearity caused by intrinsic effects of the APD may appear at a too high power, as well
as at a too high power density (too small beam diameter) on the APD chip.