Fixed and tactical information networking, especially for defense applications, has begun to require increasing data transmission capacity as the volume of information available to warfighters grows through more sophisticated sensors and capabilities. The breadth of the information grid is growing beyond the capacity of radio frequency (RF) or microwave broadband direct line of sight (DLoS) links. RF systems in particular are gridlocked by a congested spectrum, can be limited by licensing issues, and in hostile environments, can be jammed.

A typical FSO communications system terminal.
Ultimately, the need for more covert, secure, and survivable communications has led to further evolutions of free space optical (FSO) communications systems. FSO communications systems do not rely on RF frequencies, they offer wider bandwidth, and they have a low probability of intercept and detect (LPI/D). A number of innovations, both available now and in the near future, are making FSO systems a more viable option for field communications.

Smaller, Highly Collimated Beam

Past FSO systems were unable to withstand any number of environmental conditions. Wind, fog, dust, rain or snow could shift the platform of the system and result in beam transmission lost. Vibrations, ambient temperatures, the angle of the sun or even the amount of shadow could cause the beam to wander and lose the connection. In virtually all of these scenarios, a technician would have to go out to the site and manually reset the system.

More concerning was the fact that these older FSO systems used a large infrared beam to communicate from terminal to terminal to better maintain the connection, despite any shifts in the terminals. While the large beam size was necessary to preserve a consistent link, it introduced a security risk, as anyone with a similar terminal could intercept the beam in such a way that the original link was still connected. In such a situation, the soldiers transmitting the data would be none the wiser.

As FSO technology advanced, many solutions to address the problems were developed. The most commonly raised solution was to simply make the beam bigger and upgrade the gimbal upon which the system is mounted. In addition to the security issues raised, a larger beam caused other problems. While a larger beam can help compensate for a swaying platform, it comes at a high cost in power density and sometimes affects the fidelity of the beam. Moreover, a larger beam, as well as the gimbal, significantly increases the system’s power consumption and results in much higher costs.

More successful advancements in FSO technology have, instead, narrowed the beam so that it’s no larger than the receiving terminal, minimizing the instantaneous field of view and preventing the jamming or interception of communications otherwise made vulnerable by a larger beam. Today FSO systems make use of narrow, low-power infrared lasers with beams nearly impossible for adversaries to detect. Even if they are detected, the smaller beam can’t be intercepted unless it’s interrupted, and in that case, the system automatically halts data transmission. The smaller, highly collimated beam used in newer FSO systems also reduces signal noise and offsets 1/r2 losses by improving power density, both of which further advance FSO communications.

Pointing, Acquisition, and Tracking

The wide beams needed in previous iterations of FSO technology to maintain alignment and integrity between terminals introduced not only security issues, but size, weight, and power (SWaP) problems as well.

In introducing narrow beams to FSO communications, a more precise means of linking terminals and maintaining the link was required. An automated pointing, acquisition, and tracking (PAT) system was developed to maintain the link without affecting bandwidth, improving performance in adverse weather conditions across comparable distances, and increasing motion tolerances. Beam steering technology enables the beam to be guided completely internally with a steering field of up to 30 degrees in two axes. Its tracking and pointing capability can correct for up to 1.5 m/s of terminal motion for each kilometer of range, while maintaining a bit error rate (BER) of 1E-10.

Utilizing optical fiber beam steering, this new generation of FSO systems can establish surface-to-surface data links in less than 15 minutes for initial acquisition and in seconds for repeated acquisition. The absence of gimbals, gears, or steering mirrors for precision pointing and tracking reduces the SWaP of the system and allows it to be rapidly transported and easily set up to establish communications.