Array-based architectures for deep-space photon counting lasercom links offer a powerful mechanism to
lower the cost and improve the scalability of Earth- or space-based optical receivers. In this architecture, a
large area receive telescope is constructed by using an array of small, inexpensive telescopes that are
networked together. However, a limitation on how many small telescopes can be used comes from the
minimum SNR requirement for synchronization. In general, the synchronization requirements are not difficult
to meet for systems with enough SNR to achieve >Mbps performance. However, deep-space links often have
large variations in their operational parameters due to varying link distances from orbital considerations as
well as atmospherics. If the system is required to operate under such stressing conditions, albeit with a low
(100's of Kbps) data rate, it is important to consider required SNR for synchronization as a design parameter.
Furthermore, for very remote systems (e.g. beyond Mars), expected data rates may only be 100's of Kbps, in
which case synchronization will be a critical design parameter. In this paper, we will examine the design trade
space between number of arrayed telescopes and synchronization parameters. We will focus on the low SNR/
low data rate case as it is the most stressing.
NASA anticipates a significant demand for long-haul communications service from deep-space to Earth in the near
future. To address this need, a substantial effort has been invested in developing a novel free-space laser
communications system that can be operated at data rates that are 10-1000 times higher than current RF systems. We
will focus here on the receiver design which consists of a distributed array of telescopes, each with a Geiger-mode
Avalanche Photo Diode (APD) array capable of detecting and timing individual photon arrivals to within a fraction of a
nanosecond. Using an array of telescopes has the advantage of providing a large collection area without the cost of
constructing a very large monolithic aperture. A key challenge of using a distributed array receiver is combining the
detected photons from each of the telescopes so that the combined system appears as a single large collector.
This paper will focus on the techniques employed by the receiver to spatially acquire a deep-space downlink laser
signal, synchronize the timing of all the photon arrivals at each telescope, and combine the photon detections from each
telescope into a single data stream. Results from a hardware testbed utilizing this receiver concept will be shown that
demonstrate an efficiency of less than one incident photon per bit at data rates up to 14 Mbps, while operating within 1
dB of the channel capacity.
NASA anticipates a significant demand for long-haul communications service from deep-space to Earth in the near future. To address this need, a substantial effort has been invested in developing a free-space laser communications system that can be operated at data rates that are 10-1000 times higher than current RF systems. We have built an end-to-end free-space photon counting testbed to demonstrate many of the key technologies required for a deep space optical receiver. The testbed consists of two independent receivers, each using a Geiger-mode avalanche photodiode detector array. A hardware aggregator combines the photon arrivals from the two receivers and the aggregated photon stream is decoded in real time with a hardware turbo decoder. We have demonstrated signal acquisition, clock synchronization, and error free communications at data rates up to 14 million bits per second while operating within 1 dB of the channel capacity with an efficiency of greater than 1 bit per incident photon.