Article
Optical APD Receiver Modules: Detection Of Extremely Small Amounts Of Light
June 30, 2011
•Tech Note: Optical APD Receiver Modules: Detection Of Extremely Small Amounts Of Light
By LASER COMPONENTS
For a single photon to trigger a considerable photocurrent, avalanche photodiodes (APDs) have to come into play. APDs are particularly well suited to improve sensitivity. Their internal amplification can be increased until the signal-to-noise ratio (SNR) is restricted by the so-called "shot" noise. The maximum performance of an APD, therefore, primarily depends on a correct, optimally adjusted electronic driver.
APDs are used when the noise of the amplifier is larger than the inherent noise of the detector. This occurs at high speeds (MHz range). The transimpedance amplification is then restricted by the RC time constant. The SNR of an APD amplifier module is better than that of a PIN detector amplifier module.
In general, the quality of a photodiode is assessed according to its noise ratio. The inherent noise of the APD is relatively low. Much more crucial is the total noise after subsequent amplification. At high bandwidths, the amplifier noise is even the dominant noise source. To reduce the total noise of the detector-amplifier module, the capacitance of the amplifier has to be kept as low as possible.
To fulfill this requirement, APD receiver modules were developed. All components are mounted next to the semiconductor compactly on a ceramic substrate that has a low capacitance. This reduces stray capacitance and thus noise. Compared to common APDs with added amplifiers, the receiver modules are extremely compact.
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