Avalanche photodiode as two photon correlation detector

The future possibility of single photon counters development is presented. The solid state photodetectors based on silicon avalanche photodiode has been optimized for detection of echo signal in millimeter laser ranging measurements. The structures are up to 200 um in diameter, they exhibit an excellent uniformity of detection delay, sensitivity and timing resolution. The avalanche process nonlinearity enhance the influence of starting conditions to avalanche grow rapidity. This is the reason why the internal delay of this type of detectors is depended on detected signal intensity, i.e. in case of weak signal depended on number of detected photons. The dependence is in the range of 0-200 ps for photon numbers 1-1000 photons. The active quenching and gating circuit with time walk compensation has been constructed to eliminate this effect. We have used the outputs of the compensation circuit to estimate the photon numbers detected on a shot by shot basis. The mutual time difference between the compensated and uncompensated output pulses corresponds to the photon number. Monitoring this time difference by the Pico Event Timer with picosecond timing resolution enabled to monitor the echo signal strength fluctuation on a shot by shot basis in laser ranging experiments. The goal in the field of correlation detection is to enhance the primary delay dependence and to use it for initial photon number discrimination. The experimental status quo will be presented.