Photon counting receiver for the laser time transfer, optical design, and construction

We are presenting the design of an input optics of the single photon counting detector. This device is under construction in our lab and will be used in the European Laser Time (ELT) transfer space project. The main goal was to design an input optics, which is easy to manufacture and which will be suitable for space application. The narrow bandwidth optical filter of bandwidth of 3nm cantered at 532nm has to be used to reduce background photon flux. Such a filter has a narrow field of view in order of several degrees; however the entire optics must have larger field of view from 5° to at least 60° and must keep the detection timing delay, timing jitter and signal energy constant for different incident angles in the interval. To enlarge optics field of view the front aperture consists of ground glass, however the energy level behind ground glass is strongly angle dependent and together with increasing satellite distance the angle dependence is event higher. The three effective methods suitable for space applications of incident angles dependence reduction will be discused.