Optical Input; Electrical Output - THORLABS APD440A Operation Manual

APD440x
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 APD440x,
resulting in a variation of the M-factor that is equivalent to a gain variation.

3.5.1 Optical Input

The APD440A uses a Silicon Avalanche Photodiode with a detector active area diameter of
1.0 mm, operating from 400 to 1000 nm and the APD440A2 uses an UV-enhanced Silicon
Avalanche Photodiode with a detector active area diameter of 1.0 mm, operating from
200 to 1000 nm. The typical responsivity curves can be found in the
An open beam should be carefully aligned with the detector. External optics, filters or apertures
can be easily attached to the Avalanche Photodetectors because the housing is compatible
with any number of Thorlabs 1" and ½" threaded accessories.
For detectors with smaller active areas, as it is the case for APD440x with 1 mm active area
diameter, it is recommended to focus the optical signal out of the fiber onto the detector.
For this and other fiber coupled applications, fiber adapters like Thorlabs S120-xx series can be
mounted on the optical input. The fiber adapter will accommodate a multi-mode as well as
single-mode fiber. For detailed information on fiber coupling, please see the
Note
Coupling loss may occur due to small detector size, which will result in a reduced output sig-
nal. If angled connectors are used, the fiber adapter can be rotated from its original position
to check for an improved alignment. For this process use an optical input power below the
saturation power while observing OUTPUT voltage on a digital voltmeter or other low-fre-
quency measurement device.
The maximum OUTPUT voltage swing is 4.1 V at High-Z termination. Saturation of the
OUTPUT will occur at optical input power greater than CW Saturation Power listed in
tions . If necessary, use external neutral density filters or attenuators to reduce the input light
9
level. Please note that Avalanche Photodetectors are extremely sensitive to unwanted stray
light. Careful shielding of the Avalanche Photodetectors from any unwanted light sources is es-
sential. Common techniques to minimize the influence of stray light include baffling or other
opaque barriers like black cloths, beam tubes or using appropriate band pass filters in front of
the detector.
Attention
The optical damage threshold is 1 mW. Exceeding this value will permanently destroy the
Avalanche Photodetector!

3.5.2 Electrical Output

Thorlabs APD440x Avalanche Photodetectors deliver an output voltage. The ouput voltage V
is a function of the incident light power P
wavelength and M-factor, and the transimpedance gain
6
, the detector's responsivity
opt
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appendix
15
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(l) at a given
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© 2020 Thorlabs
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