A typical FSO communications system terminal.

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.

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.

Bandwidth

In addition to being covert, more precise, and more compact, the latest FSO systems offer operating speeds up to 10 Gbps at high service reliability with link distances from two to five kilometers.

FSO systems are typically used alongside RF systems, offering three orders of magnitude better bandwidth on top of the base RF bandwidth, which is maintained in areas where available to ensure no data is lost in transit. The FSO system is most effective at a range of three to five kilometers, in which the small beam and PAT system offer a significant performance advantage over systems that consume more power while maintaining the link.

Resistance to Adverse Atmospheric Effects

FSO communications terminal mounted on a building rooftop.

FSO systems remain mildly susceptible to adverse weather conditions, though they can maintain a secure link through smoke, dust, snow, and heavy rain. Newer systems can support a stable, reliable link even over a wide temperature range and thermal gradients and during high winds or on unstable mounts.

The small beam system of newer FSO systems resists weather events because it opens orders of magnitude more bandwidth and is often combined with RF technology. These hybrid systems link an RF system to the FSO system to support critical data in the event of a disconnection, offering soldiers at least as much bandwidth as they have under traditional RF systems.

Portability and Safety

Because they rely on PAT rather than gimbals and mirrors, the latest FSO systems are more portable and can be initiated more easily and quickly. One unit in particular weighs just 10 pounds, measures 1.4 cubic feet, and operates on 15 watts. Its size and weight allow it to be quickly set up anywhere to add bandwidth to any existing infrastructure or towers without interfering with existing communications.

For the protection of the warfighters and intelligence officers operating them, FSO systems use an infrared laser at an “eye-safe” wavelength of 1550 nanometers, which is also ideal for atmosphere propagation.

The Future of FSO

Proposed systems would create broadband data links beyond 10 Gbps. One target application of the future development of FSO systems is PAT capabilities for ground-to-air communication. Volume (and bandwidth/data rates) continues to exponentially expand as more airborne surveillance vehicles, with multi-sensor packages and communications links/relays, are added to support the U.S. military’s intelligence gathering mission.

Covert airborne surveillance data, and the real-time information that can be processed from it, is limited by the availability of networks to downlink the raw collected data. A solution to this data transfer challenge is the development of advanced FSO communications links with PAT resources. While currently used for building-to-building, tower-to-tower, and vehicle-to-vehicle applications, the lightweight, internal-beam PAT could enable accurate tracking for airborne applications without the bulk or power consumption of gimbal- and mirror-based systems.

Effective communications obviously play a crucial role in soldiers’ success, and in environments where RF frequencies are jammed or unavailable, FSO systems have emerged as a higher-bandwidth option for transmitting the rapidly expanding intelligence gathered during military operations. As warfighters, intelligence officers and emergency responders require increasing bandwidth to relay rapidly expanding intelligence quickly and covertly, the new generation of FSO systems extends the channel and enables more effective missions throughout the defense and emergency responder communities.

This article was written by Ann Kutsch, Lighting and Imaging Defense Manager, SCHOTT Defense (Arlington, VA). For more information, Click Here .