One of the interesting pieces of technology which has grown in popularity during recent years is the optical free space datalink. In this month's feature Carlo Kopp explores technology issues in modern optical datalink design.

Conceptual Design

In the simplest of terms an optical datalink comprises a modulated optical source, an optical package to focus the beam, a transmission path through the atmosphere, a receiving optical package, and optical detector, receiver and demodulator. A digital modulation is imposed at the transmitter end, propagates through the channel and upon reception the digital data stream is reconstituted for use.

In concept the idea seems very simple, but in practice the engineering of such links is often anything but simple. The writer in a previous life completed most of the design of a 32 Megabit/s optical datalink, and can comment from direct experience.

Modern optical datalinks have been in the market for about one and a half decades at this time, since high power LEDs and semiconductor lasers became available at reasonable costs. Data rates have followed Moore's Law, insofar as early designs ran at speeds of the order of a Megabit/s, and contemporary products can deliver Gigabits/s.

While the speed performance of datalinks, driven by receiver and transmitter technology, has grown and arguably will continue to grow, the physics of the optical hardware and of propagation through the atmosphere have not changed. In many respects these are the key constraints to what optical datalinks will be able to achieve. Much marketing literature for datalink products conveniently glosses over what frequently is the biggest show stopper in the game.

To better understand the limitations and the future potential of optical datalinks, we will explore in more detail the design issues in transmitters, receivers and take a brief look at propagation impairments to datalink operation.

Datalink Transmitters

The transmitter is in many respects the simplest part of any datalink design. A typical datalink, apart from any router or bridging hardware which may be embedded in the same package, comprises a line interface, power supply, modulator, optical driver, optical source and some optical arrangement for focussing the output from the source into a tightly collimated beam.

While the line interface is arguably trivial, for many optical datalink designs, the design can be quite demanding. Because optical datalinks are often located in relatively inhospitable environments like rooftops, or upper portions of building structures, they must be built to survive and reliably operate under harsh conditions of high temperatures, low temperatures, rapidly changing temperatures, often high humidity, nearby lightning strikes and interference from mobile telephones and other reasonably strong RF sources. Often the line interface cables must double up as remote DC power feeds for the equipment proper.

A robust design typically uses a shielded cable, twisted pair for low speed designs, coax for high speed designs and preferably optical fibre for very fast designs. Where copper is used, a good high voltage isolation scheme is advisable.

The data format going up the line interface (or down at the receiver end) can be either in the format which goes over the optical channel, to reduce the complexity of the hardware in the actual optical head, or it can be another arbitrary format designed about the line interface. Synchronous streams with an embedded clock are usually preferred.

Data arriving at the line interface of the optical head is used to drive the optical transmitter proper.

Transmitter designs vary widely depending upon the bit rates in use and the type of optical source being used.

The simplest designs, operating at low speeds, will use a high efficiency, high power Light Emitting Diode or LED. LEDs are non-coherent sources, which produce a frequently fairly wide infrared colour spectrum, usually centred around 0.8 microns with a Gaussian spread in colour. Peak output powers can be up to Watts, and some designs may employ ganged LEDs in an array to increase output power levels. LEDs are highly capacitive loads for driver circuits, and usually have to be driven very hard to deliver the required optical output. As a result, the drive circuits usually pulse the LED with a high current spike. Most LEDs do like this type of treatment and will usually experience a decline in deliverable optical peak power over the life of the LED. Therefore, if your elderly datalink on the rooftop is beginning to show an increased bit error rate, odds are the LED has followed the inevitable aging curve and is delivering a fraction of the optical power it did when new. Time for an overhaul and new LED installation.

Newer and faster designs typically employ solid state lasers, a favoured colour is the eye-safe 1.55 microns. Lasers designed for industrial automation are frequently a favoured component in datalink designs.

Unlike the LED, which is a simple and robust component which can be driven by a simple high current transistor switch, lasers are much more finicky devices. A semiconductor laser is in simplest terms a refined type of LED which incorporates a Fabry-Perot optical resonant cavity in the design of the die. Current flowing through the device causes LED-like light emission, which bounces back and forth between the mirrors of the cavity. At some threshold current level and thus light intensity, stimulated emission or lasing occurs, and the device outputs a highly coherent high power light emission.

The threshold current of the laser is a parameter which varies with the temperature and the age of the device. In a typical pulsed laser circuit design, a continuous current, which is just below the threshold current, passes through the laser to keep it biased just below the point at which it lases. To initiate lasing, a signal current is then added to the bias current to push the laser into full power emission. To turn the beam off, the signal current is removed.

In practice lasers typically require a dedicated power control loop, which senses the optical power level out of the laser and controls the bias current. This compensates for device temperature and age. Since lasers can dissipate respectable amounts of power, and optical heads can be installed on very hot roofs, a good power control loop will include an over-temperature crowbar to prevent the device from be converted into a little pool of molten Gallium Arsenide.

Like LEDs, lasers are capacitive loads, albeit usually smaller. This can complicate the design of driver circuits considerably, if speeds of hundreds of Megabits/s are required in pulsed operation. Designs running at high speeds are usually very difficult to operate in a simple pulsed mode, and may instead employ a more complex analogue modulation which is imposed by varying a drive current through the laser. A good laser has an electrical to optical transfer function (i.e. relationship between lasing current and output optical power) which is fairly linear, so a modest modulation depth may produce very little distortion in the modulation. Since lasers usually do not have to share bandwidth with other services in adjacent bands, second and third harmonic distortion is not the show stopper it is in radio broadcast and communications.

In principle, a laser transmitter can exploit any technology used in optical fibre communications, with the caveat that higher power levels must be produced.

The modulated light produced by the LED or laser must then be focussed by suitable optics into a beam which can be aimed at the receiver, which is typically hundreds of yards away, or perhaps even a kilometer or further away, depending on the design. Various optical designs are possible, although in practice the most common approach is some lensing arrangement. An LED with a large emission angle is commonly focussed using a large lens which acts as a collimator. Lasers usually produce very narrow coherent beams, and some designs will get by with little more than a small cylindrical lensing package attached to the laser device proper. If the system uses an EDFA optically pumped amplifier, then a suitable lensing arrangement to capture the light leaving the end of the fibre will be required.

The width of the beam produced is an issue in its own right. The optical head which is bolted to a mast on a roof, or attached to the side of a building, is ideally perfectly rigid. In reality it is never perfectly rigid, since masts and even tall buildings to sway in the wind, and almost may carry some structural low frequency vibrations. Therefore the line-of-sight of the optical head will drift with the mechanical movement of the mast or building.

If the beam produced had an infinitesimal width then a deflection of X degrees at hundreds of yards would simply cause the beam to drift off its aimpoint and away from the intended receiver, causing the signal to be lost entirely.

Therefore the optical designer faces an inherent contradiction - the narrower the beam the better in terms of optical power delivered to the receiver, yet the narrower the beam the greater the difficulty in keeping it properly pointed at the receiver. This problem is not unique to rooftop datalinks - military lasers installed on jets, helicopters and vehicles, used for guiding bombs and missiles, frequently incur huge cost overheads in the provision of highly accurate beam stabilisation hardware to avoid this sightline jitter problem.

Since rate gyros and servo driven mirrors are much too expensive for rooftop datalinks, the usual solution is to make the beamwidth such that it is wide enough to maintain receiver in the beam even if the sightline of the optical head does drift around by a degree or so. The price to be paid is in optical power delivered, since the wider the beam, the larger the spot produced by the optics at the distance of the receiver and the smaller the proportion of the transmitted power coupled into the receiver optics. Indeed this is one of the big limitations to optical datalink range.

Optical Propagation

At this point our notional optical datalink has put a spot of laser (or LED) light over the receiver and its immediate surrounds at some nominal distance of several hundred yards, or perhaps even the odd kilometre. Our receiver, which we are yet to discuss in detail, captures some fraction of the impinging light and converts it into an electrical signal for further processing.

How does the atmosphere affect the behaviour of our transmitted beam? The simple answer is in various deleterious ways!

Let us first assume that we have a nice hot dry summer day, with no pesky raindrops or fog droplets to complicate life. We are shining our laser beam close to parallel with the earth's surface, since the curvature of the earth is negligible at these ranges and thus refraction due to curvature is also negligible.

As the earth's surface heats under the sun, we get thermals, or heated air rising upward. Assume our beam is travelling above the roofs of various houses, commercial buildings, carparks and other such artifacts of industrial age civilisation. What happens as a result?

Different surfaces heat at different rates, and also transfer heat to the surrounding air at different rates. The result is that the columns of rising air above each of our artifacts will be at slight different temperatures, which also means slightly different densities and thus refractive indices. Since the boundaries between these pockets or cells of air will be continuously undulating, as all thermals do, we end up with a time variant change in the shape of the boundary.

Geometrical optics tells us that light impinging on a boundary between two areas with different refractive indices will bend. For pockets of air with small differences in temperature, this bending may only be a tiny fraction of a degree. But our beamwidth may only be of the order of a degree. What this means in practical terms is that our beam will be randomly zig-zagging along the general transmission path it is following, the zig-zags varying in time with the behaviour of the thermals.

The consequences of this are twofold. The first is that the beam will become defocussed, reducing power at the receiver slightly. The second is that the length of the transmission path changes, and multiple transmission paths may exist for various parts of the beam. The former effect can be compensated by pumping more power out of the transmitter, but the latter cannot be easily handled (unless we borrow the adaptive mirror technology the US Air Force is putting into its multi-MegaWatt burn the ballistic missile out of the sky 747 mounted Chemical Oxygen Iodine laser). Why does it matter? If we aim to pump Gigabits through our datalink beam, this effect may cause us considerable heartache since it is no different in principle to the modal dispersion effects we see in a step index or graded index optical fibre. Except, unlike the fibre, the severity of the dispersion will vary with the time of day and day in the year, and weather on the day, since it is a result of local atmospheric temperature changes.

This is a non-issue for the older generation optical datalinks, but will become a serious impediment once people attempt to pump multiple Gigabits/s or more through a free space link with distances of kilometres. How serious an impediment will depend on the distance and local atmospheric environment, needless to say this problem is not one which is trivially calculated or modelled.

To be trivial, though, let us hypothesis that half of our beam gets delayed relative to its sibling by 0.01%, over a 1 km transmission path. Let's assume the light seen by the receiver is a mix of 50% from each half. Then the receiver gets a mix of equal proportions, time shifted by 1/3 of a nanosecond. A bit cell at 3 GHz occupies about - 1/3 nanosecond.

Dispersion effects due to atmospheric density pockets will be an issue for transmission of multi-Gigabit/s data streams over optical datalinks, in the same manner as modal dispersion is becoming an issue in fibre LANs. How severe an issue will depend on the speed of the link and the distances involved, as well as the local atmospheric environment.

Is this the only headache the datalink designer faces?

Fog is without doubt the big show-stopper for optical datalinks, since the water droplets are similar in size to the wavelength and the their density is sufficiently high to completely scatter all of the beam energy in a matter of metres. If you live in a foggy neighbourhood, don't bother with an optical link unless the downtime doesn't matter.

Areas which might experience high levels of atmospheric pollution with dust and aerosols are also not the best environment for an optical datalink. While the density may not be such as to blind the link, it may be enough to cause a power loss and thus increased bit error rate.

Rain is another headache for optical datalinks. Rain droplets are much larger than the wavelength of the beam and thus will cause most of the energy impinging on them to be scattered and lost. Whether enough energy penetrates for the link to remain open will depend on the rainfall rate across the beam. If the raindrops are large and sparse, as in a summer shower, odds are some impairment to bit error rate will occur but the link will remain up. A nice run of winter drizzle from a Stratus or Altocumulus layer will most likely produce the same effect as fog will. The author recalls a certain 256 kilobit/s datalink which resolutely refused to run in rainy weather.

In practical terms, the propagation environment is the big limiter to optical datalink reliability and performance. Therefore if you live in a hot and humid climatic environment, this is not the best choice in technology. On the other hand, if you live in a climatic environment which is relatively dry and cold, and not hampered by fog, then optical datalinks can produce excellent uptime.

For potential users of optical datalinks, some careful study of the satellite literature on regional rainfall and fog behaviour might just be a very good idea before investing heavily in this technology.

Optical Receivers

No discussion of an optical datalink would be complete without some exploration of the behaviour of receivers. Like transmitters, the same caveats concerning optics, optical head mechanical stability, line interfaces, and environmental conditions apply.

In a receiver, a light gathering optical system, very frequently a lens package, focusses the incoming light on to a PIN diode or avalanche diode optical detector. In turn it produces a faint electrical current which is amplified by a front end receiver and then handled accordingly to demodulate the waveform and extract the data stream. The same technology base used with fibre links can be exploited quite effectively here.

Unlike a fibre system, a free space link has one very nasty problem to cope with - background emission current.

The noise performance of a top end optical receiver depends mostly upon the noise produced in the detector element and the front end receiver. In a detector element, the dominant noise sources are thermal effects and shot noise. Shot noise is produced mostly by the tiny trickle of dark current, essentially leakage through the photodiode structure of the detector. In a fibre system, designers invest much effort into suppressing shot noise and may even cool down the detector element with a Peltier thermo-electric refrigerator to get better performance.

In a free space optical datalink, the light impinging on the detector comprises the received signal from the transmitter, plus whatever infrared light is emitted and reflected by the objects or scenery within the field of view of the receiver optics. The easiest way to visualise this is to imagine putting your eye up to the receiver optics, where the detector is placed. You would see what the detector sees, in effect the same image as seen through a telescope with similar optical strength - the transmitter optical head and its surrounds.

The electrical current produced in the detector is thus a sum of the signal current and the background current. If the scene is very bright and reflects enough, the transmitter may be completely swamped.

Since the background emissions and illumination around the transmitter are not modulated, the result is a largely DC current through the detector. In effect this current is no different from dark current, except it is much greater in magnitude. Accordingly it produces a lot more shot noise than dark current does. Therefore the same detector used in a free space link will never match the noise performance in a fibre application.

Are there any tricks a designer can play to beat this problem? Infrared optical filters, the same technology used in heat-seeking missiles, have been and continue to be widely used for this purpose. Typically a rare earth doped glass filter with an additional multilayer interference filter can knock out most of the infrared outside the immediate colour of the transmitter. As a result, a very substantial proportion of the infrared background can be removed and the background current reduced to a manageable level, but it can never be eliminated.

Pushing the Envelope

The big thrust in optical datalink technology today is exploitation of recently devised technologies used in fibre applications. These are techniques such as direct optical amplification using spools of doped, optically pumped fibre, and Wavelength Division Multiplexing (WDM).

Both of these technologies will provide important gains in achievable transmitter and receiver bit rates. However, the idiosyncrasies of the optical and transmission environment will not be beaten that easily. The basic physics which result in beam jitter, signal time dispersion and background current are implicit parts of free space transmission and cannot be easily managed or constrained.

For the end user the important caveat is to match the datalink technology with the local environment. If some careful thought is applied, optical datalinks can be an exceptionally useful adjunct to fibre or copper links. However, the predictions often seen in the prospectus' of some vendors are likely to remain marketeers' fantasies for the foreseeable future.