Tests of optical communications for deep space show promise

Abstract

A free-space optical beam can reliably transfer more than one bit of data per photon received, even in the presence of background light.Communication is vital for any spacecraft. After all, an instrument is of little use if it cannot return data. Space communications is currently dominated by radio-frequency (RF) links. Optical communication, however, promises higher data rates while adding less mass and volume to the spacecraft and consuming less power. Optical links have two main advantages over radio communications. First, the beam's shorter wavelength means less diffraction, so it spreads less as it travels through space and can be more tightly directed towards the receiver. Second, the optical system's available modulation bandwidth is much larger than that of an RF system, especially when considering the regulatory restrictions on RF allocation.While spacecraft optical-communications systems have been demonstrated in Earth orbit, 1-3 links to deep space at interplanetary distances have yet to be deployed. The improved optical-link performance is highly beneficial for these massand power-constrained spacecraft, yet the extreme distances involved pose a new set of challenges. For example, a link from Mars orbit to Earth would operate at up to 10,000 times longer range than one from geosynchronous orbit to the ground, corresponding to an additional loss of 80dB. Deep-space optical communications therefore require new paradigms and innovative technologies. 4 To reduce mission risk prior to flight operations, we have used elements of the Jet Propulsion Laboratory's deep-space optical-communications technology for a series of demonstrations aimed at validating subsystem models and operations. They evolved from initial in-fiber laboratory validations to current day-and-night operational free-space links that functionally validate transmitter and receiver systems in real time at data rates of over 44Mbps with efficiencies of approximately two bits per detected photon. 5 We have also found that testing with pseudo-random data is not always sufficient to validate robust system operations, e.g., temporal acquisition and tracking issues may be masked by repetitive data sequences. Thus, we included a channel for live transmission of high-definition television at a compressed nominal data rate of 30Mbps.The testbed implements a building-top to building 100m-range link with the receiver placed indoors behind a window (see Figure 1). A custom telescope, used to test prototype low-cost, large-aperture optics, collects a small part of a wide Continued on next page