In October 2013 the Lunar Laser Communications Demonstration (LLCD) made communications history by successfully demonstrating 622 megabits per second laser communication from the moon’s orbit to earth. The LLCD consisted of the Lunar Laser Communication Space Terminal (LLST), developed by MIT Lincoln Laboratory, mounted on NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft and a primary ground terminal located in New Mexico, the Lunar Laser Communications Ground Terminal (LLGT), and two alternate ground terminals. <p> </p>This paper presents the optical layout of the LLST, the approach for testing the optical subsystems, and the results of the optical qualification of the LLST. Also described is the optical test set used to qualify the LLST. The architecture philosophy for the optics was to keep a small, simple optical backend that provided excellent boresighting and high isolation between the optical paths, high quality wavefront on axis, with minimal throughput losses on all paths. The front end large optics consisted of a Cassegrain 107mm telescope with an f/0.7 parabolic primary mirror and a solar window to reduce the thermal load on the telescope and to minimize background light received at the sensors.
Lasercom terminals often scan an area of uncertainty during acquisition with a wide-divergence beacon beam. Once the terminal has established cooperative tracking with the remote terminal, a narrow divergence beam is used for communication. A mechanism that enables continuous beam divergence control can provide significant size, weight, and power (SWaP) benefits to the terminal. First, the acquisition and the communication beams can be launched from the same fiber so only a single high-power optical amplifier is required. Second, by providing mid-divergences, it eases the remote terminal’s transition from the acquisition phase to the communication phase. This paper describes a mechanism that provides gradual, progressive adjustment of far-field beam divergence, from wide divergence (> 300 μrad FWHM) through collimated condition (38 μrad FWHM) and that works over a range of wavelengths. The mechanism is comprised of a variable-thickness optical element, formed by a pair of opposing wedges that is placed between the launch fiber and the collimating lens. Variations in divergence with no beam blockage are created by laterally translating one wedge relative to a fixed wedge. Divergence is continuously adjustable within the thickness range, allowing for a coordinated transition of divergence, wavelength, and beam power. Measurements of this low-loss, low-wavefront error assembly show that boresight error during divergence transition is maintained to a fraction of the communication beamwidth over wavelength and optical power ranges.
Free space optical communication systems require robust pointing and tracking to establish and maintain lineof-
sight (LOS). Atmospheric scintillation can present a challenge to the LOS tracking systems located at each end of the
link. This paper describes a pointing, acquisition, and tracking (PAT) approach for single-mode fiber coupling, which
was successfully demonstrated over a 5.4 km lasercom link that was subject to severe turbulence conditions. One of the
primary advantages of the scheme described is its compensation for thermo-mechanical drift, which simplifies optomechanical
design and allows use of simple COTS hardware. An overview of the PAT system and performance data are
This paper describes a lasercom terminal using spatial diversity to mitigate fading caused by atmospheric scintillation.
Multiple receive apertures are separated sufficiently to capture statistically independent samples of the incoming beam.
The received optical signals are tracked individually, photo-detected, and summed electrically, with measured diversity
gain. The terminal consists of COTS components. It was used in successful demonstrations over a 5.4km ground-ground
link from June through September 2008, during which it experienced a wide temperature range. Design overview and
hardware realization are presented.
This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002.
Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by
the United States Government.