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A new optical configuration is proposed for ground stations designed to receive information from modulating-retroreflector-based terminals in a free-space optical communication link. This configuration is based on the polarization discrimination of the ground station laser sent to the remote terminal. It achieves the optimal usage of the laser power, which is a key parameter in modulating-retroreflector-based links. A simulation has been made in order to model the system in terms of power losses and states of polarization and an experimental setup has been implemented to verify the validity of the simulation. A gain of over 6 dB has been measured using the new setup compared with a simple trajectory discrimination setup, achieving a very good agreement with the prediction. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2520–2522, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27116

The mass and the power consumption are the most constrictive restrictions in an Aerostatic Platform. We present in this work a bidirectional link based on a liquid crystal retro-modulator (LCRM). The use of this device displaces to the terrestrial station the tasks of powering the laser, and the pointing mechanism. Also, due to the non-conductive characteristic of the LCRM, the power requirements are low. The beam aperture of the upward link can be tailored with the compromise between the aperture losses, and the tracking mechanics. The range of the distances for the system presented is from 2 to 20 km, being the lower limit for the tests and checking of the system, and the higher the final system for a stratospheric survey Aerostat. The system will be modified to fulfill the requirements associated to the increasing distances, although the main characteristics (modulation schemes, modulator, etc.) will be maintained. The system uses two modulation schemes for each direction: in the uplink a carrier constant envelope FSK modulation is used. For the down link, part of the optical signal is polarization modulated and reflected to the earth. For increasing the data rate, a multi-polarization modulation is used. Actually, 32 polarization levels are used, increasing the data rate in a 5-fold factor. The use of differential scheme, i.e. the State of Polarization (SOP) change defines the data, not the SOP, avoids the necessity of SOP maintenance in the transmitter.

Free-space quantum key distribution links in urban environment have demanding operating needs, such as functioning in daylight and under atmospheric turbulence, which can dramatically impact their performance. Both effects are usually mitigated with a careful design of the field of view of the receiver. However, a trade-off is often required, since a narrow field of view improves background noise rejection but it is linked to an increase in turbulence-related losses. In this paper, we present a high-speed automatic tracking system to overcome these limitations. Both a reduction in the field-of-view to decrease the background noise and the mitigation of the losses caused by atmospheric turbulence are addressed. Two different designs are presented and discussed, along with technical considerations for the experimental implementation. Finally, preliminary experimental results of beam wander correction are used to estimate the potential improvement of both the quantum bit error rate and secret key rate of a free space quantum key distribution system., Comment: 16 pages, 8 figures

The signals that will be received on Earth from deep-space probes in future implementations of free-space optical communication will be extremely weak, and new ground stations will have to be developed in order to support these links. This paper addresses the feasibility of using the technology developed in the gamma-ray telescopes that will make up the Cherenkov Telescope Array (CTA) observatory in the implementation of a new kind of ground station. Among the main advantages that these telescopes provide are the much larger apertures needed to overcome the power limitation that ground-based gamma-ray astronomy and optical communication both have. Also, the large number of big telescopes that will be built for CTA will make it possible to reduce costs by economy-scale production, enabling optical communications in the large telescopes that will be needed for future deep-space links.