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We are presenting the results of the study of the Single Photon Avalanche Diode (SPAD) pulse response risetime and its
dependence on several key parameters. We were investigating the unique properties of K14 type SPAD with its high
delay uniformity of 200 μm active area and the correlation between the avalanche buildup time and the photon number
involved in the avalanche trigger. The detection chip was operated in a passive quenching circuit with active gating. This
setup enabled us to monitor the diode reverse current using an electrometer, a fast digitizing oscilloscope, and using a
custom design comparator circuit. The electrometer reading enabled to estimate the photon number per detection event,
independently on avalanche process. The avalanche build up was recorded on the oscilloscope and processed by custom
designed waveform analysis package. The correlation of avalanche build up to the photon number, bias above break,
photon absorption location, optical pulse length and photon energy was investigated in detail. The experimental results
are presented. The existing solid state photon counting detectors have been dedicated for picosecond resolution and
timing stability of single photon events. However, the high timing stability is maintained for individual single photons
detection, only. If more than one photon is absorbed within the detector time resolution, the detection delay will be
significantly affected. This fact is restricting the application of the solid state photon counters to cases where single
photons may be guaranteed, only. For laser ranging purposes it is highly desirable to have a detector, which detects both
single photon and multi photon signals with picoseconds stability. The SPAD based photon counter works in a purely
digital mode: a uniform output signal is generated once the photon is detected. If the input signal consists of several
photons, the first absorbed one triggers the avalanche. Obviously, for multiple photon signals, the detection delay will be
shorter in comparison to the single photon events. The detection delay dependence on the optical input signal strength is
called the "detector time walk". To enable the detector operation in both the single and multi photon signal regime with a
minimal time walk, a time walk compensation technique has been developed in nineties.
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