Operating Instruction; Operating Principle; Optical Input - THORLABS APD110 Series Operation Manual

APD110x/120x

3 Operating Instruction

3.1 Operating Principle

The Thorlabs APD110x/120x series Avalanche Photodetectors combine a high sensitivity Si or
InGaAs Avalanche Photodiode with a specially designed ultra-low noise transimpedance ampli-
fier for detection of optical signals from DC to 50 MHz. The buffered output stage can deliver up
to 1.8 V into a 50 W impedance load. The ultra-low noise design includes an active low-pass fil-
ter to effectively suppress out-of-band noise. No external high voltage power supply is required
for operation.
APD110x/120x series Avalanche Photodetectors have an exceptionally low NEP, making them
ideal for fast low-level light detection applications, such as spectroscopy, fluorescence mea-
surements, laser radar and optical rangefinders. Due to their very high sensitivity the
APD110x/120x 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 second-
ary 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 de-
scribed by the M (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 gen-
eral, with lower temperatures the M factor will increase, with higher temperatures - decrease.
Please see section M Factor Temperature Dependency for typical curves.
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The APD110x/120x Series is powered by the included external power supply (±12 V, 250 mA)
via a PICO M8 power connector.

3.1.1 Optical Input

The APD110C uses an InGaAs Avalanche Photodiode with a detector active area diameter of
0.2 mm, operating from 900 to 1700 nm nm.
The APD120A uses a Silicon Avalanche Photodiode with a detector active area diameter of
1 mm, operating from 400 to 1000 nm.
The APD120A2 uses an UV-enhanced Silicon Avalanche Photodiode with a detector active
area diameter of 1 mm, operating from 200 to 1000 nm.
The typical responsivity curves can be found in the appendix .
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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 op-
tics, filters, apertures or fiber adapters.
For fiber coupled applications, in case of detectors with an active area of 1 mm, fiber adapters
like Thorlabs S120-xx series can be easily mounted on the optical input. The fiber adapter will
accommodate multi-mode as well as single-mode fiber. For detectors with smaller active areas,
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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Please note, that coupling losses may occur due to small detector size, which will result in a re-
duced output signal. If angled connectors are used the fiber adapter can be rotated from its ori-
ginal 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-
frequency measurement device.
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