Patent ID: 8841596

Claim:
A photon detection system comprising: an avalanche photodiode and a voltage source for said avalanche photodiode, wherein said avalanche photodiode comprises silicon or its heterostructures and wherein said voltage source is configured to bias said avalanche photodiode with a voltage component which is static with respect to time and a voltage component which varies with time such that an alternating bias is provided that in successive periods biases the avalanche photodiode above and below its breakdown voltage, wherein the voltage component which is static with respect to time is a DC bias and the alternating bias is an AC modulation bias which varies periodically in time and the AC modulation has a period less than 100 ns, the DC bias being larger than the breakdown voltage of the avalanche photodiode, the AC bias resulting in the avalanche photodiode being biased above its breakdown voltage at the highest values of the bias applied to the avalanche photodiode and below the breakdown voltage at the lowest values of the bias applied to the avalanche photodiode, the system further comprising, an output circuit configured to receive an output signal from said avalanche photodiode and process said output signal to remove a time varying component from said output signal, the output circuit comprising, a signal divider to split the output signal into two parts, an electrical line to delay one of the two parts relative to the other so as to produce a delayed part and a non-delayed part, and a signal differencer to output the difference between the delayed part and the non-delayed part, such that a time varying component of the output signal which appears in each period is removed from the output signal; and a circuit configured to discriminate an output signal from said avalanche photodiode, with the time varying component removed by the output circuit, between multiple predetermined levels to determine the number of photons absorbed by the avalanche photodiode.