Recommendations; Operating Principle - THORLABS APD450C Operation Manual

Turning off the APD450C
· Move the power slider to O after finishing the measurements.
Note
Avoid saturating the amplifier! Therefore, make sure that the optical input power does not ex-
ceed the CW saturation power level listed in

3.4 Recommendations

· Ambient or stray light
o Avalanche Photodetectors are extremely sensitive devices. To achieve precise results,
shield the Avalanche Photodetectors from light sources during measurements to reduce
the background. Use opaque light barriers like light boxes, cloth or lens tubes.
o It is highly recommended to use appropriate band pass filters in front of the detector to
minimize the influence of stray light.
o It is not necessary to switch off the Avalanche Photodetector when it is exposed to ambient
light. The amplifier might saturate but, unlike Photomultiplier Tubes, it will not be damaged
or saturated for a long period of time.
· High Optical Input Power
Upon excessive optical input power or at excessive power density (i.e. very focused light and
very small beam diameter), a non-linearity caused by intrinsic effects of the APD may ap-
pear. Please keep the input power below the maximum input power stated in the
Data .
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· Electrostatic Coupling
Electrostatic coupling of electrical noise associated with ground loops can be critical. In most
cases an electrically isolated post (see Thorlabs parts TRE or TRE/M) will suppress electrical
noise coupling. Electrical noise sources should always be identified and the distance to the
Avalanche Photodetector should be increased. If possible, rotate the Avalanche Photode-
tector input away from the noise source.
· Impact of Temperature
o The M-factor is factory set at 23°C ambient temperature. The APD450C is operated at an
internal reverse bias voltage that is temperature-compensated. Therefore, the actual M
factor will remain nearly constant within the specified ambient temperature range of
(23±5)°C. Please see the Appendix for the

3.5 Operating Principle

In general, Avalanche Photodiodes use an internal gain mechanism to increase the sensitivity.
Incident photons generate electron-hole pairs, as in normal photo diodes. By applying a high
reverse voltage, a strong electric field is generated that accelerates these electrons and pro-
duces secondary electrons by impact ionization. This leads to an electron avalanche producing
gain factors of up to several hundreds. The amplification depends on the reverse bias voltage
and is described by the M-factor.
The reverse bias voltage can be adjusted with the rotary gain knob on the side of the
APD450C, resulting in a variation of the M-factor that is equivalent to a gain variation.
© 2020 Thorlabs
Technical Data .
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Typical M factor Temperatur Dependency
3 Operating Instruction
Technical
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