High-availability free-space laser communications
are demonstrated

In a five-month rapid development effort, the Advanced Lasercom Systems and Operations Group at MIT Lincoln Laboratory modeled, designed, built, integrated, and fielded two custom 2.7 gigabaud (2.7 billion symbols per second) lasercom terminals at field sites located in the western suburbs of Boston and separated by 5.4 kilometers of wooded countryside. The field-site effort, which culminated with data collection throughout much of September 2008, verified high-availability communications over the link by using modest, eye-safe levels of optical power. Potential applications for these lasercom terminals include networking for forces on the move and transmission of aggregated tactical data in theater. Because the characteristics of the ground-to-ground link are similar to those of a much longer air-to-ground link, the demonstration results can be extrapolated to applications such as the offload of remote sensing data from aircraft to the ground.

Transmit site, a fire tower The transmit site for the demonstration was a fire tower in Groton, Massachusetts.
Receive site, a telescope domeThe receive site was the telescope dome at the Laboratory's Firepond facility in Westford, Massachusetts. The transmit and receive sites were chosen on the basis of simulations that identified ground-to-ground links that would experience the desired turbulence.

The communications link used in Lincoln Laboratory's demonstration was especially challenging because both terminals were close to the ground and not far above sea level, where atmospheric turbulence is by far the most disruptive to laser beams. Turbulence will typically transform the cross section of a beam into a complex, time-varying spatial pattern of bright and dark spots at the receiver. At any point in the cross section of this speckled beam, the intensity grows and decays over time, occasionally varying by a factor of 10,000 over just a few milliseconds. This phenomenon, known as fading, is typically the biggest challenge facing free-space optical links in fair-weather conditions.

Pattern for strong turbulence Turbulence patterns for weak  intensity
Diurnal turbulence variations. These false-color, logarithmic measured-intensity patterns were captured at an 8" × 8" receive site. The same false-color scale is used for both images. The image on the left, with deep intensity variations, was captured in late morning with strong turbulence. The image on the right, with far less intensity variability, was captured in the late afternoon, during the quiescent period of mild turbulence.

Lead system engineer Dr. John Moores describes the uniqueness of the demonstration. "There has been a reasonable amount of theoretical work describing methods to overcome the challenges of fading lasercom links, but limited experimental validation of these techniques over aggressive channels. We were able to repeatably demonstrate high availability for durations of hours over a challenging ground-to-ground link using modest transmitted laser power—less than 50 milliwatts. Success at such low power levels was made possible through the use of spatial and temporal diversity techniques. These mitigation techniques also enabled the use of standard terrestrial network protocols for our client interfaces—Ethernet over SONET. These protocols were designed for links with steady, consistent received power levels not for the wildly fluctuating received power from a fading channel. Tolerance to fades was achieved using the optical transport network protocol with minor modifications and temporal interleaving. Our claims of high availability are based on rigorous bit-level qualification using protocol testers, rather than on a mere qualitative assessment of received video images."

At the start of the program, the Laboratory's terminal design team used numerical modeling to predict the channel (path of transmission) conditions that would be encountered. (Data collected at the end of the program verified that these predictions were accurate.) Once the channel conditions were understood, the design engineers chose to use a single transmit aperture (opening through which light is transmitted) and four receive apertures (openings through which light is collected), each just a half-inch in diameter, to mitigate the effects of turbulence. A small aperture samples only a tiny portion of the complex spatial pattern of intensity and of beam phase at the receiver site—the intensity and phase vary nearly linearly across the aperture. This linearity greatly facilitates coupling the received light into an optical fiber, tracking the effective direction of an incoming beam, and pointing an outgoing beam directly at a small target far away.

A disadvantage of a small receive aperture is that it collects only a small fraction of the beam at the receive site; in the demonstrations, the beam at the plane of the receiver was roughly 1 meter in diameter versus the 12-millimeter aperture diameter. Another disadvantage is that the light received through the aperture undergoes a random fading process: for durations on the order of a millisecond, the light in the aperture can become so dim that it is difficult to know where the light is coming from (difficult to track and point) and to correctly recover digital communications bits modulated onto the laser beam.

In order to overcome these two disadvantages, the design engineers chose to invoke spatial diversity, incoherently combining the power from four apertures. Relative to a single receive aperture, this configuration increased the collected power by a factor of four. However, the real benefit of using multiple apertures was the dramatic net reduction in received-power fluctuations. By placing the apertures far enough apart (a square 10 centimeters on a side was used in the field demonstrations), the fading in the four apertures became largely uncorrelated. The reduced power fluctuations of the incoherent sum greatly facilitated clock and data recovery. The following example illustrates the benefits of incoherently summing uncorrelated signals: on a link in which fading imposes a 20 dB (a factor of 100) communications penalty (meaning that an additional 20 dB of optical power would have to be transmitted relative to a link with no fading in order to achieve the same communications performance), the use of four receive apertures reduces this penalty to just 5 dB (a factor of 7). The incoherent summing technique allowed the Laboratory's team to transmit far less optical power than would be needed with a brute-force approach.

Transmit site optics and hardwareFree-space optics and tracking hardware were mounted on a tripod at the transmit site, the Groton fire tower.

For cooperative tracking purposes, four unmodulated milliwatt-class beacon laser beams at different wavelengths were sent to the transmit site. The incoherent optical summation of these four beams reduced the fading seen by the tracking system at the transmitter, analogous to the spatial diversity used at the communications receiver. The tracking systems at both sites were fully automated under real-time computer control and were designed not only to keep the optical beams coupled into optical fibers, but also to automatically keep the outgoing free-space beams coaligned. Except for the lack of gimbal-based coarse pointing systems, the tracking systems at both sites are fully capable of operating on maneuvering aircraft.

Receive site optics and tracking hardwareA dramatic reduction in received-power fluctuations was achieved by using the variably separated four apertures of the free-space optics and tracking hardware mounted on a tripod at the receive site. Four fast steering mirrors (FSM), key elements in tracking systems, were employed in this system.
Image of telescope dome taken by infrared cameraView of Firepond telescope dome with beacon lasers turned on. The image was captured using the tracking system's filtered infrared camera at the Groton fire tower site. During operation, the camera integration time was reduced so that only the beacons were visible.

Because of the aggressive schedule for the program, the engineers chose a modulation format based on simplicity of implementation—return-to-zero, on-off keying (RZ-OOK) with adaptive thresholding. At the receiver, each aperture fed into a single-mode preamplified receiver. The detected light from the four receivers was combined electrically and then fed into clock and data recovery circuitry. Temporal interleaving and standard low-overhead forward error correction (FEC) were applied to the data to ensure proper operation of terrestrial network protocols over links with fading. Interleaving is a technique of reordering the transmitted symbols so that FEC is effective on a fading channel. All temporal processing—interleaving and deinterleaving, and coding and decoding—was performed in real time at transmitted line rate by using field-programmable gate array–based processing boards.
 
To follow the success of this field demonstration and to leverage the valuable experience gained, the team hopes to have the opportunity to fly terminals on aircraft, to test spatial diversity and other turbulence mitigation schemes on even more aggressive links, and to package lasercom terminals into smaller, more fieldable units.

 

Posted February 2009

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