Simultaneous transmission of faint laser pulses and of synchronization signal at 1.55 μm for secured optical transmissions

The purpose of this paper is to describe the conception of the detectors developed for a system dedicated to quantum keys distribution on optical fiber network. Because there is still no commercial single photon sources quantum keys distribution is usually based on faint laser pulses transmission. Detection of such signals is very difficult but it can be improved by the use of a clock signal. Detectors based on APD can be designed to use this clock signal. In order to have a very good synchronization of both quantum keys and clock signals it is necessary to minimize chromatic dispersion in the optical fibers. The innovation presented in this paper is to use modulators effects leading to a very small change in the wavelength of the modulated signal. So it is possible to get two very close wavelengths dedicated to both signals, avoiding dispersion in the optical fiber. The system is designed at the 1.55 μm and is based on an acousto-optic modulator and an optical filter to separate both wavelengths at the reception. The detector is realized with an InGaAs APD, working in gated mode at liquid nitrogen temperature in order to reduce dark counts.