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Problem
Avalanche photodiodes (APDs) are highly sensitive single-photon counters and are used in fluorescence lifetime measurements, medical devices, safety equipment, and optical communication devices, among many other applications. They function by placing a high voltage across an optically sensitive semiconductor. When a photon strikes the semiconductor, excited charge carriers initiate an avalanche of current, which results in an electrical pulse at the output. A potential problem with using APDs is the possibility of overloading the input, resulting in excessive dead time and/or damage to the photodiode. This invention provides a way to protect APDs by sensing the count rate, comparing it to a reference frequency, and then rapidly shutting down the device before damage can be done if the reference frequency is exceeded.
Invention
This invention describes a device and method to help protect a given avalanche photo diode (APD) from excessive light or photon counts without modification to the APD itself or the associated power for the APD. High photon count rate to a reference frequency and rapidly shutting down the APD if a threshold is exceeded. The design can also be interfaced with a laser interlock /label warning system, in which a laser shutter can isolate the APD from an excessive source.
Features
Protects photon counters from bright light damage; Works on multiple device types – APDs, Photomultipliers, Multi-Pixel Photon Counters; Novel concept: compares random photon rate to adjustable periodic reference frequency; If photon rate > ref frequency --> protect!