Architecture overview and data summary of a 5.4 km free-space laser communication experiment

Abstract

MIT Lincoln Laboratory designed and built two free-space laser communications terminals, and successfully demonstrated error-free communication between two ground sites separated by 5.4 km in September, 2008. The primary goal of this work was to emulate a low elevation angle air-to-ground link capable of supporting standard OTU1 (2.667 Gb/s) data formatting with standard client interfaces. Mitigation of turbulence-induced scintillation effects was accomplished through the use of multiple small-aperture receivers and novel encoding and interleaver hardware. Data from both the field and laboratory experiments were used to assess link performance as a function of system parameters such as transmitted power, degree of spatial diversity, and interleaver span, with and without forward error correction.

Reciprocity-enhanced optical communication through atmospheric turbulence - part I: reciprocity proofs and far-field power transfer optimization

“…We limited our attention to power-in-fiber operation, but this restriction is consistent with current practice in high data-rate free-space optical communication, as in the Lincoln Laboratory experiments. 1,2 We have assumed quasimonochromatic operation with transmitter modulation whose pulse durations greatly exceed the ∼ps turbulence-induced multipath spread and are much shorter than the ∼ms-duration scintillation fades, but this too is typical of high data-rate free-space optical communication as carried out at Lincoln Laboratory. Most critically, our proof requires that the propagation delay between the two communication terminals is such that the turbulence state can be considered frozen-in.…”

“…1,2 Each of the tested systems incorporated high-speed beam tracking, channel diversity, and data interleaving to mitigate scintillation-induced signal fading. Tracking beams were propagated from both link nodes, and the unidirectional high-bandwidth data link functioned at 2.7 Gbps.…”

Air-to-ground lasercom system demonstration

“…This technique was demonstrated in a 5.4 km horizontal link experiment in September 2008 and reported previously. [7] For the FOCAL air-to-ground experiment described here, the aircraft aperture was increased to 2.5 cm to accommodate longer ranges; however, the four 1 cm ground apertures were not changed. Using fiber add-drop multiplexers, separate beacon wavelengths were multiplexed into the ground receive apertures to serve as uplink tracking beacons.…”

Design of a 40 Watt 1.55μm uplink transmitter for Lunar Laser Communications

“…The uplink uses a variable-duty cycle 4-PPM waveform [6][7][8] with ½-rate SCPPM forward error correction 9 at ~10 and ~20 Mbps. The UL TX resides in the control-room trailer operating nominally at room temperature and communicates with the LLCD space terminal Uplink RX 5 Using techniques developed in the late 1990s using photon-efficient variable duty-cycle pulse-position modulation (PPM) waveforms and average-power-limited erbium-doped fiber amplifiers (EDFAs) 6,7 , the transmitter generates four copies of the uplink signals on separate wavelengths, with each wavelength transmitted through a separate telescope aperture or spatial channel, implementing spatial diversity at the TX in order to reduce far-field atmospheric-turbulenceinduced fading [10][11][12][13][14] . Due to diffraction, the four channels overlap in the far field so that the single-spatial mode LLST RX receives four wavelength division multiplexed (WDM) copies of the uplink signals, which are preamplified, optically filtered with a single ~10 GHz narrow band filter, and converted to an electrical signal using a single photodetector.…”