THORLABS APD430 Series, Operation Manual

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© 2015 Thorlabs 6
APD430x
3 Operating Instruction
3.1 Operating Principle
The Thorlabs APD430x series of temperature-compensated, variable gain Avalanche
Photodetectors combine a high sensitivity Si or InGaAs Avalanche Photodiode with a specially
designed ultra-low noise transimpedance amplifier for detection of optical signals from DC to
400 MHz. The buffered output stage can deliver up to 2.0 V into a 50
W impedance load. No
external high voltage power supply is required for operation.
APD430x series Avalanche Photodetectors have an exceptionally low NEP, making them ideal
for fast low-level light detection applications, such as spectroscopy, fluorescence
measurements, laser radar and optical rangefinders. Due to their very high sensitivity the
APD430x series Avalanche Photodetectors can replace Photomultiplier Tubes (PMT) in many
applications. The Avalanche Photodetectors cannot be damaged by unwanted ambient light,
which is critical for many Photomultiplier Tubes.
Avalanche Photodiodes use an internal gain mechanism to increase the sensitivity. Incident
photons generate electron-hole pairs, like in a normal photo diode. By applying a high reverse
voltage, a strong electric field appears that accelerates these electrons and produces
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 (multiplication factor). Due to internal processes, the M factor is
temperature dependent. At a fixed reverse bias voltage the M factor will change with
temperature: in general, with lower temperatures the M factor will increase, with higher
temperatures - decrease.
The APD430x series Avalanche Photodetectors are temperature-compensated. A thermistor
senses the temperature inside the APD430x enclosure, and a special electronic circuit controls
the applied to the APD reverse voltage in accordance with the temperature change. As the M
(multiplication) factor depends on the applied reverse voltage, the temperature dependency of
the M factor can reduced drastically.
These detectors have a rotary GAIN knob that allows the reverse bias voltage of the avalanche
photodiode to be adjusted within a certain range, resulting in a variation of the M (multiplication)
factor that is equivalent to a gain variation.
The APD430x Series is powered by the included external power supply (±12 V, 200 mA) via a
PICO M8 power connector.
3.1.1 Optical Input
The APD430C uses an InGaAs Avalanche Photodiode with a detector active area diameter of
0.2 mm, operating from 900 to 1700 nm nm.
The APD430A uses a Silicon Avalanche Photodiode with a detector active area diameter of
0.5 mm, operating from 400 to 1000 nm.
The APD430A2 uses an UV-enhanced Silicon Avalanche Photodiode with a detector active
area diameter of 0.2 mm, operating from 200 to 1000 nm.
The typical responsivity curves can be found in the appendix .
An open beam should be carefully aligned to the detector. Additional focusing lenses can be
easily attached to the Avalanche Photodetectors. The housing is compatible with any number
of Thorlabs 1” and ½” threaded accessories. This allows convenient mounting of external
optics, filters, apertures or fiber adapters.
For fiber coupled applications, in order to meet specifications, it is recommended to focus the
optical signal out of the fiber onto the detector. Details please see in the
appendix .
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