(in collaboration with Frank Rayal)
Free space optical (FSO) technology in commercial applications has been around for a couple of decades now. During this time, significant developments happened to improve the utility and reduce cost. Today’s systems pack more capacity in smaller volume and at lower price while measures to improve reliability are integrated into the solutions to increase the robustness of the link. Yet, the commercial applications of FSO remain in niche segments without a major breakthrough into mainstream markets such as mobile backhaul. In this article, I like to review the basic elements of the FSO market to shed some light on this segment of backhaul that has lived in the shadow of RF technologies.
How it’s done: in FSO systems, visible or infrared beams are modulated with the information bits, collimated with lenses to focus the beam, and transmitted through the air. The receiver is equipped with lenses that collate the beams which are detected by a photodetector and demodulated to extract information bits.
The pros: Perhaps one of the most important advantages of FSO systems is the low latency which measures on the order of tens of nanoseconds. Compare this to the fastest wireless technologies which measure in the order of hundreds of microseconds and the advantage is clear. Another advantage is security. FSO light beams have very small diameter which makes it virtually impossible to ‘tap’ say by placing a mirror which would have to be in the middle of the link to be able to intercept both directions of communications. If such a thing happens, communication would be interrupted and the operator would investigate immediately the cause. FSO systems are not subject to interference as wireless systems would be. Finally, FSO does not require a license to operate: anyone can deploy FSO systems.
The cons: The main disadvantage is atmospheric effects on link reliability. FSO systems are highly susceptible to fog, rain, sand or other particles in the air that can block the path between the two end points of the system. Light fog results in attenuation on the order of 20 dB/km which is similar to what a 60 GHz wireless system would encounter due to oxygen absorption at sea level. The attenuation increases to a few hundred dBs per kilometer, which effectively block all communication between the two end points. Atmospheric effects are factored into the design of the link – reducing distance to reach the operational requirements with the constraint of capacity and atmospheric absorption.
This is effective in many areas where fog is not common. Otherwise, FSO systems are backed-up by RF transmission system (generally in 5 GHz band) to transport the highest priority traffic. Other disadvantages include background light, beam dispersion, line-of-sight disruptions, and signal scattering.Figure 1 International Visibility Code weather conditions and precipitation along with their visibility.
Not all FSO systems are the same: FSO systems can be differentiated by the type of beams they use.
Traditionally, lasers in the 700 – 800 nm were used but this has expanded over time to comprise additional bands on both side of the spectrum. Some systems use lasers or light emitting diodes (LEDs) in the visible spectrum (wavelength between 390 nm and 700 nm which corresponds to frequency range between 430 – 790 THz). Other systems use lasers or infrared emitting diodes (IREDs) that operate higher in the band to reach 1550 nm far-infrared wavelength. At this wavelength, the water absorption is higher than lower wavelength, such as 800 nm which is commonly used in many FSO systems, but it is also it is possible to use higher power light source. The 1550 nm laser technology is considered eye-safe as it is absorbed by the cornea and does not focus on the retina.
The marketplace: According to industrial forecasts and estimates, the market in non-military and space applications is estimated at $33.5 million in 2013 up by 11% from 29.8 million in 2012. It is forecasted to double in size to $60 million in 2018. The EMEA and Americas regions are currently leading sales, but Asia will pick up to overtake EMEA by 2018. Comparing this to millimeter wave solutions which are the closest competitor, the FSO market is less than half the size today of that market. Applications mostly center on enterprise connectivity to bridge building in close proximity with high speed link.
One application that many FSO companies have angled for is servicing financial trading companies where latency is critical for arbitrage purposes. Applications in mobile backhaul are few although recently at least one FSO company (Polewall) has developed a system for small cell backhaul. Notable companies in this field include Aoptix, fSONA Networks, Lightpoint, LSA, Mostcom, Polewall, pureLiFi, Skyfiber, and decibel loss per kilometer attenuation .
Stacking up against the competition: The main competition to FSO comes from millimeter wave 60 GHz V-band and 70-90 GHz E-band solutions. The 60 GHz band is unlicensed and includes up to 7 GHz of spectrum. It is characterized by high oxygen absorption of RF energy that is about 20 dB/km. This makes the performance of systems in this band vulnerable in areas of high rain fall. The E-band is typically licensed on non-exclusive basis. From a capacity perspective, V and E-band systems provide order of hundreds of Mbps up to a couple of Gbps of capacity with promise for higher capacity (e.g. up to 10 Gbps in some instances and solutions) in the future by increasing channel bandwidth and modulation scheme. FSO systems provide orders of a few hundred Mbps of capacity today with some systems capable of reaching a couple of Gbps and promise of 10 Gbps in the future. These characteristics make up for an environment of fierce competition between these technologies where the specific location and path details playing an important and decisive factor. A myriad of other factors can be the decisive factor on which to use which includes the spectrum regulatory regime, latency, and size (FSO systems can be large in size reaching tens of liters and a few kilograms). Both FSO and millimeter wave technologies feature pricing along similar lines, but this is specifically a tricky area as performance in the area of deployment has to be factored in for proper analysis.
The way forward: The FSO market leverages innovations in the optical space. Over the last few years, there has been significant developments that reduce the cost of optical transceivers and increased their utility and functionality which translates positively into the FSO equipment market. But for the market to really take off, there is a need to fit the requirements of a major application that is highly in demand. In the mobile backhaul space, FSO faces stiff competition from millimeter wave solutions, which today have a significantly higher market size (more than double that of FSO and projected to grow to $600 million in 2017 according to recent market forecasts). But there is one emerging area where FSO can play an important role – the fronthaul. FSO already possesses the low latency required in fronthaul applications. Should FSO deliver the high capacity and price points required for fronthaul, it would achieve a major milestone towards further growth.
 Kim et. al., “Wireless optical transmission of fast Ethernet, FDDI, ATM, and ESCON protocol data using
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