Intensity noise measurement of strongly attenuated laser diode pulses in the time domain

Developing the ability to characterize photon statistics of light sources has been one of the important driving forces of Quantum Optics. Photon statistics is also a crucial parameter to evaluate of quantum key distribution security. As practical quantum cryptographic systems encode information on faint laser pulses, we present a simple method to measure and calibrate their intensity noise with respect to shotnoise reference. The technique is based on the record of photodetection timetags in the photon counting regime. Two different methods are considered to produce light pulse: first, direct pulsing of a laser diode driving current, second, chopping the CW laser beam emitted by a laser diode with an acoustc-optical modulator. As predicted by basic Quantum Optics theory, levels of attenuation used in practical quantum key distribution systems lead in both cases to Poissonian photon number distribution in the generated light pulses.