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When is Optical Transmission Your Best Choice for Data Communications?



Introduction and product focus


This article will look at the various requirements of data communication systems to understand where optical transmission and the use of fiber optic cable and its associated transceivers can provide a superior solution to conventional electrical signaling over copper cables. To illustrate this, the article references a number of fiber optic transceiver products available from Hotenda including Finisar's Gigabit Interface Converters (FTL1319 and FTL-8519-3D), Toshiba's Toslink Series for plastic optical fiber (TODX2402), Avago’s AFBR-5978Z Fast Ethernet module, and an interesting, non-fiber optical isolation device from Avago, the HFBR-3810Z.

Fiber optic communications – a quick technology refresh

At the most basic level, fiber optic communications is simply about sending data, as pulses of light, along thin strands of glass. These optical fibers, often no thicker than a human hair, function like a waveguide in the way they transmit light from one end to the other. This relies on the light being contained within the fiber through total internal reflection, something that is achieved by surrounding the core fiber with a cladding that has a lower refractive index and acts like a mirror. Core and cladding are then covered with a protective outer coating, and the resulting optical fiber may then be bundled with other fibers in a single sheath to form the final fiber optic cable. Despite their glass construction, optical fibers are both strong and flexible, as well as being lightweight, all benefits we will return to later.

Optical fibers come in two distinct types, defined by their propagation mode, in turn determined by the diameter of the fiber core relative to the wavelength of the transmitted light. Fibers with a core diameter of 50 micrometers (μm) or more accept light (in the near-infrared band) at various angles that then propagates along the fiber in multiple transverse modes – hence such fibers are referred to as multi-mode. Single-mode fibers (also known as mono-mode) typically have a core diameter of 8-10 μm for use with light wavelengths in the range 1300-1550 nm.

Single-mode fibers at these longer wavelengths are generally made from high-quality, low-attenuation silica glass, which offers the highest performance in terms of signal bandwidth and operating range, e.g. in telecommunications systems where 2.4 Gbit/s SDH links can have a range of over 100 km. By contrast, the range of multi-mode fibers is limited by absorption (where light is not completely reflected and passes into the cladding) and dispersion (where light rays at different angles have different path lengths and therefore take different times to reach the receiver). Nevertheless, systems using multi-mode fiber operating at 850 nm can still achieve rates of 1 to 2 Gbit/s over distances of several hundred meters, sufficient for most industrial applications. Many applications do not even require this range and, increasingly, plastic optical fiber (POF) is being deployed in systems operating over just a few tens of meters at 850 nm or even with visible light at 650 nm.

Beyond the cable itself, the other key components in a fiber optic communication system are the transmitter and receiver circuits that provide the vital conversion between electrical and optical signals. The transmitter modulates a light source, either an LED or laser diode, using the incoming data together with any additional encoding (e.g. Manchester coding) that may be required to ensure clocking information can be recovered by the receiver. LEDs can be used both at 850 nm with GaAs devices and at 1300 nm with GaAsP technology. However, vertical cavity surface-emitting laser (VCSEL) devices cover these same wavelengths and offer improved performance over LEDs (speed and power) at a similar cost. VCSELs also have the further advantage that lasers produce coherent light, which reduces the effects of dispersion and couples more efficiently into single-mode fiber.

Fiber optic receivers use a photodiode coupled to a transimpedance amplifier to convert the incoming optical signal back into an electrical signal and correct for the attenuation and distortion suffered during transmission. The output from the transimpedance amplifier usually requires further processing with a limiting amplifier and decoder or clock recovery circuits to restore the original data.

Single-mode fiber optic transmitters and receivers are generally more expensive than multi-mode equivalents because of higher component costs, particularly as the connectors have to incorporate greater precision to couple light in and out of the smaller fiber core. Where the ultimate in speed or range is not required, cheaper multi-mode solutions tend to win out. Curiously, multi-mode fiber itself is more expensive than single, but with the shorter distances involved, this is not likely to be a key decision factor.

When is fiber better than copper for data rate and range?

As we have already noted, fiber optic technology enables communication at gigabit-plus data rates over distances of 100 km or more. Single-mode operation achieves these speeds without the use of repeaters. Rates in excess of 10 terabits can be achieved over similar range links using optical amplification and wavelength-division multiplexing (i.e. the parallel transmission of data using light carriers at different wavelengths), but this is outside the scope and purpose of this article. Similarly, the use of repeaters, which regenerate the optical signal, to extend transmission range is not something we will consider here, although the principle is easy enough to understand.

A characteristic of data transmission over conventional electrical (copper wire) cables, also true for fiber optic connections, is the trade-off between bandwidth and range. The difference is that the product of bandwidth x range is two to three orders of magnitude higher for fiber than copper. These comparisons often need to be qualified. For example, twisted pair copper cables, as used for regular telephone lines, typically achieve ADSL data rates of 10 Mbit/s over 1 km while the co-axial cables used for cable TV are fine for 250 Mbit/s over 200-300 m. A good point of comparison is where a communications standard, like Gigabit Ethernet, defines the range at a given data rate (in this case 1 Gbit/s) for different cable types. So for twisted-pair copper cable (Cat-5 or higher specification), it specifies a 100 m range whereas for fiber the 1000BASE-SX specification offers 550 m over 50 μm multi-mode fiber (or 220 m over 62.5 μm fiber), and the 1000BASE-LX specification provides for up to 5 km over 10 μm single-mode fiber.

All of the above simply says that if your application requires a combination of bandwidth and range (without repeaters) that exceeds the capability of copper cabling then a fiber optic connection may be your only option. Conversely, for low-bandwidth short-range applications, electrical transmission is commonly the default solution, often because the possibility of using fiber optics has not been considered or it is assumed to be more expensive.

In some applications, the choice of communication technology may be predetermined by a system specification that requires a piece of equipment to interface to a wider network. For example, high-capacity storage devices, of the type commonly found in computer data centers, may call for Fibre Channel connectivity. There is a grey area in between where, with all other things being equal, the decision is not so straightforward and some of the other features and benefits of fiber optics need to be considered, as discussed below.

The less obvious advantages of fiber optic data communications

Here we will look at situations where the choice of a fiber optic solution may have benefits that are not always so readily appreciated, and at applications that are enabled by the more unique features of optical transmission. Some of these are particularly relevant in certain industries, including industrial automation, automotive, aerospace, petrochemical, and utilities.

Solution cost
The cost of the copper and other metals (aluminum, tin, gold, etc.) used in electrical cables and connectors continues to rise faster than most other materials, making the choice of fiber optic cable (whether glass or plastic) more and more attractive for all but the shortest cable runs. While bandwidth considerations are also a factor, the deployment of fiber in the “last mile” for local telecoms and cable TV distribution is now very much a reality.

Figure 1: Finisar’s 1.25 Gbit/s single-mode fiber-optic transceiver module.

The other key cost consideration is that of the transmitters and receivers. For short-range communications, the cost of electrical line driver/receiver devices, in moderate volumes, ranges from 20 cents for the simplest devices to around 50 cents for USB and CAN bus transceivers, and upwards of a couple of dollars for devices that incorporate additional circuitry, such as encoding and control for various networking protocols like 100 Mbit/s Fast Ethernet. It is no wonder there is a preconception that a fiber optic solution cannot compete on cost. While this might be true for a high-end, single-mode fiber optic transceiver like Finisar’s FTL-1319-3D Gigabit Ethernet transceiver, this is not a like for like comparison. A fairer and more realistic alternative might be the fully-connectorized modules intended for use with low-cost plastic optical fiber, such as Toshiba’s TODX2402(F) Toslink transceiver, which is about a quarter the price. When viewed as part of the total solution cost, this is not so expensive, especially if there are other factors to consider such as cost of installation, which we will return to shortly.

Figure 2: Toshiba’s 250 Mbit/s Toslink plastic optical fiber transceiver.

Power consumption
The superior range performance of fiber, when compared to electrical cable, stems from fiber’s low attenuation. Consequently, it does not require much to power a laser diode or LED that is capable of transmitting a light signal over considerable distances. Consider again the Finisar FTL-1319-3D Gigabit Ethernet transceiver, which offers a range of 10 km with a power budget of just 660 mW (based on the typical supply current of 200 mA and operation at 3.3 V). Finisar’s 850 nm transceiver (FTL-8519-3D) has a similar power specification, which reflects the higher attenuation and hence shorter range / lower efficiency of multi-mode fiber. Both these transceivers consume less than half the power of Finisar’s own Cat-5 electrical cable equivalent Gigabit Ethernet transceiver, which only has a 100 m range.

Figure 3: Finisar’s 2.125 Gbit/s multi-mode fiber-optic transceiver module.

Visible-light (650 nm), multi-mode transceivers, like Avago’s AFBR-5978Z Fast Ethernet product, use an LED rather than a laser diode and are intended for industrial automation applications, supporting both plastic optical fiber (POF) and hard-clad silica (HCS) fiber. Despite the need to achieve a rugged specification, this device still only requires 825 mW of power.

Figure 4: Avago’s 650 nm transceiver for Fast Ethernet (10/100 Mbit/s).

Size, weight and ease of installation
Even ignoring the far higher data-handling capacity, optical fibers are significantly thinner than copper wires. This allows more fibers to be bundled into a given-diameter cable, so when replacing cabling in cable ducts, multiple fiber optic cables can be run in place of a twisted pair bundle. Similarly, a city’s infrastructure can be more readily upgraded using fiber, which can fit in smaller or more crowded spaces than electrical cables, especially underground, e.g. in subway tunnels, down sewers and alongside utility pipes and cables (taking advantage of fiber’s immunity to electrical noise as covered in the next section).

Fiber optic cable is often much easier to install, especially in existing buildings, as a direct benefit of the physically smaller cable size. Holes drilled through walls can be smaller and more discrete conduits or trunking can be used. The longer range of fiber optics may avoid installing repeaters or at least reduce the number required and allow them to be more conveniently situated. By contrast, electrical cables providing similar capacity will be much thicker and more rigid, increasing the difficulty of installation and requiring significant space, which may be challenging in large office complexes or data centers. The ‘Safety’ section below highlights some other installation benefits.

An optical cable also weighs less than a comparable copper wire cable. This not only helps with installation, but also with the transportation of cable, which can be a significant aspect of the logistics and cost of large projects. More important than this is the benefit of lighter weight in specific applications, such as in aircraft and increasingly in automobiles where reduced weight translates to performance improvements, especially in fuel savings and reduced emissions. Temporary and portable cabling solutions also benefit from the small size, light weight, and flexible fiber optic cable. Further physical benefit that is easily overlooked is the corrosion resistance of fiber by virtue of its non-metallic construction.

Noise Immunity
A common problem in electrical communications is the issue of electrical interference or, more accurately, electromagnetic interference (EMI). Interference arises from a number of sources, and is commonly referred to as noise. It can be picked up from electrical motors and similar equipment and from power lines, but also as crosstalk from adjacent electrical signal cables. This is true even for twisted pair, coaxial, and other types of shielded cable since these only reduce rather than eliminate crosstalk and noise pickup. Noise, from whatever source, will ultimately degrade the wanted signal in an electrical cable to the point where the signal-to-noise-ratio (SNR) limits the transmission range or where more spurious interference causes an unacceptable level of errors in the data received (even allowing for protocols that provide error correction or re-transmission of data).

Fiber optic cable is immune to all these sources of EMI and even to more severe forms of electrical disruption like lightning strikes or nuclear electromagnetic pulses (NMP). This means fiber can be installed in environments with naturally high EMI, such as alongside power lines or railroad tracks. Indeed, utility companies have taken advantage of this benefit by using their power distribution networks to carry fiber optic communication links.

Security
Not only is fiber immune to interference from electromagnetic signals, neither does it radiate any electromagnetic energy. So where an electrical signal can leak from even a shielded cable, light in a fiber optic cable is entirely contained within it. This makes it impossible to intercept data being carried by a fiber optic cable without physically attempting to tap into it, which is very difficult to do without being detected. This makes fiber the ideal medium for the transmission of sensitive data in high-security environments.

Safety
By not conducting electricity, fiber optic cables are not only immune to EMI but also to the problems of ground loops, where the ground (or earth) potential can differ by a small amount between different buildings, or even between different parts of a larger building. This difference in ground potential may only be a few volts, but this can give rise to stray currents that contribute to electrical noise. Higher currents can damage sensitive electronic components leading to equipment failure or even the risk of fire. By the same argument, fiber cables also avoid the problem of electrical surges from power lines or induced by lightning, which makes fiber safe to use near high voltage equipment and outdoors.

Fiber optic cables also eliminate the spark hazard that is inevitably present in any electrical system – even low voltages can give rise to small sparks. Ordinarily these may pose no danger, but they can be extremely dangerous in certain environments such as an oil refinery, where potentially explosive, flammable vapors may be present.

One option for installing communication cables in an existing building is to run the cables through ventilation ducts. However, building regulations and fire codes require such cables to meet very stringent fire-retardant standards to limit the emission of smoke. This is another instance where the smaller physical size of fiber optic over electrical cable provides the further benefit of offering a lower-cost solution, simply by requiring less of the costly fire-retardant material in the outer jackets of these cables.

Finally, and perhaps slightly deviating from the subject of fiber optics but certainly a form of optical communication and very relevant to safety, there are some interesting transceiver products that purely provide electrical isolation. Avago’s HFBR-3810Z is a good example. Unlike all the other transceivers we have looked at in this article, which provide one end of a bi-directional communications link, this one comprises a transmitter and receiver that communicate with each other across an enclosed 25 mm air gap, using visible light at 650 nm. This device enables isolation across a printed circuit board, achieving 12 kV transient voltage suppression while supporting data transmission at signal rates from DC to 10 MBaud.

Figure 5: Avago’s 650 nm optical air-gap Tx/Rx isolation link for DC to 10 MBaud.

Summary

The decision to use fiber optics as the medium for a data communications link may be dictated by system requirements which offer little choice, such as if the data rate or range exceed that which can be achieved with electrical cable, or if a particular fiber standard is mandated (e.g., Fibre Channel). Where this is not the case, a range of other factors will need to be considered. Some of these are cost or performance trade-offs, but in other instances, the unique features and benefits of fiber optics and optical signaling may provide the only solution to a particular challenge.

This article has looked at cost from a solution perspective (not just the ticket price of individual components) and other performance criteria such as power consumption, size, and weight. The latter has a bearing on ease of installation, which in turn may factor back into the cost equation. There are some more interesting features of optical communications that all essentially derive from the fact that it is not electrical. So its immunity to electrical interference, the fact that a fiber optic cable cannot generate a spark or give you a shock, and that it is so much more secure than an electrical cable, are all benefits that cannot be ignored.

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