Free space optics (FSO) is a technology which has been repeatedly hailed as an alternative to wireline fiber for backhaul and high speed connectivity. But despite its claims to provide a solution to rising capacity demands and spectrum shortage, the laser-based technology has never quite made it to the commercial mainstream of the telco sector, and there has been a string of casualties along the way, most recently AOptix in 2016. The latest organization to try to revive it is LGS Innovations, a former subsidiary of Alcatel-Lucent, which is now independent.
Its technical director of new business development, Tom Wood, said in an interview with FierceWirelessTech: “We’re seeing a lot more interest in commercial free space optical communications now than we did a few years ago.” The company has worked in FSO for 20 years, but its main customers have been in government rather than the commercial sector.
With FSO, signals are sent through the air rather than over a physical cable, in frequencies around 193 THz. It can be used to communicate between satellites and the ground, or other satellites, and has often been mooted as a solution for high speed cellular backhaul or very high performance point-to-point links. The FSO spectrum is unlicensed and plentiful, at a time when operators are seeking new capacity, and the units are generally smaller and lighter than an RF product for the same application.
Some companies have combined it with millimeter wave radio technology to create a solution that uses the best qualities of optical and radio – for instance, with the radio mitigating the weaknesses of FSO, such as poor performance in cloud and rain.
However, it has been hard to get such solutions to market, as the demise of AOptix – which had designed a hybrid FSO/mmWave platform – highlighted. Some start-ups, such as fSona, have survived, while others, like Clearmesh, have not.
But acceptance of technology is often a question of timing, and LGS feels FSO’s day has come at last because of the congestion of conventional spectrum. “These free space optical communications systems can support the kind of bandwidth that users are coming to expect,” Wood said.
Jay Moorman, group president of wireless solutions at LGS, even thinks FSO could achieve ‘five 9s’ reliability – despite its sensitivity to weather – by placing multiple ground stations, and that in years to come, it could be an alternative to ground-based terrestrial systems, though he admitted: “I don’t see it happening in the short term. We’re still a little bit a ways from that.”
It remains to be seen whether there will be a sustained interest in commercial FSO, or whether this is just another in a string of periodic surges of interest in the system as a backhaul option. FSO has remained a niche technology since it was originally developed by space agency NASA in the 1970s. It uses line of sight lasers or LEDs to transmit voice, data and video at up to 2.5Gbps over distances of 4-5 kilometers in 60 GHz frequencies, carrying any protocol. Through use of wavelength division multiplexing (WDM), speeds can increase to 10Gbps, rivalling physical and wireless fiber rates, delivered by competing technologies like millimeter wave, point-to-multipoint microwave and high capacity microwave.
After 2003, the clutch of small FSO suppliers largely failed in the dream of targeting the broadband access market (NTT of Japan is one of the few majors that has used FSO for last mile applications). FSO is estimated to account for only 1% of the global wireless industry, even though it is part of development programs at a few major operators, such as British Telecom. The most prominent specialist, Lightpointe, has had its own financial travails, and another pioneer, Terabeam, exited the market to focus on WiFi and WiMAX (and was later acquired).
AT&T did much to bring FSO to telcos’ attention, when in 2003 it initiated a flagship project to overlay FSO with its own fixed wireless technology to target backhaul, fixed wireless access and enterprise applications. But AT&T’s interest in combining the best attributes of radio and FSO waned (though Lightpointe succeeded in creating a similar hybrid platform). WiMAX appeared to present a new opportunity for combining with FSO to deliver ‘wireless fiber’, but again, the business did not scale up.
Interest rose again around 2010, when fSona conducted trials with several operators, tempted by the 2.5Gbps peak performance of FSO, which is particularly suited to dense urban areas, where the main backhaul crunch is experienced. MNOs have little experience with non-radio technologies, and are often suspicious of them, and they remember the poor performance of many early FSO experiments. These were overhyped, and did not deliver on their promises, often because of the natural downsides of optical technologies – the signal’s vulnerability to fog, humidity and other air conditions, plus the need for full line of sight.
These disadvantages drove the interest in a hybrid radio system, where a failed FSO signal could fall back to more robust microwave or millimeter wave links. But these had their own downsides, notably the expense and complexity of supporting and optimizing two technologies for one link, and the potential need for licensed spectrum.
Thus companies like LGS and fSona have focused on overcoming FSO’s problems without the use of radio hybrids. fSona did this in 2011, claiming link availability of 99.999% over ranges from 50 to 5,000 meters using 1550nm wavelength lasers, which allow for higher power and signal intensity while remaining safe to eyes. The higher transmit power levels extend range and data rates, and also penetrate fog and other environmental obstacles.
Of course, the need for FSO may be lessened as mmWave radio technologies move into the mainstream for backhaul and fixed wireless access.