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Making light work

Optoelectronics is enabling even higher data rates. By Graham Pitcher.

Despite the current drop in demand for telecommunications equipment, the world is generating more and more data. There are two ways to handle this: lay more fibre or make better use of the existing system. With the cost of laying fibre currently approaching $100,000 per kilometre, it is no surprise to find that attention is focused on how to increase the capacity of the existing system.
One way in which higher data capacity is being achieved is through dense wavelength division multiplexing, or dwdm. This technique allows up to 128 closely spaced, but distinct, wavelengths to be ‘squeezed’ into one fibre. The technology in now routinely supporting data rates of 10Gbit/s, with 40Gbit/s links beginning to be considered.
However, achieving these goals requires some sophisticated technology. The transmit side needs lasers with precise, stable wavelengths. The fibre itself must be low loss, whilst at the receiving end, photodetectors and optical demultiplexers using thin film filters or diffractive elements are needed. And, to link with other fibres, high performance optical cross connects are required.
Dr Steven Dye is Agere Systems’ senior manager for optoelectronics in Europe. He said most links were running at 2.5 and 10Gbit/s. “Most 10Gbit/s links are based on lithium niobate (LN) modulation technology; at 2.5Gbit/s, LN was the enabling technology. But it is being caught by electroabsorbtion module (eam) technology, particularly over shorter reach links.”
LN modulators are used to minimise the effects of chirp – a major obstacle in long distance multigigabit systems. Chirp is the tendency of signals to spread out and cause interference, limiting the system’s span. External modulators offer a way to reduce or eliminate chirp because the laser source is held in a steady state, constant wave (CW) mode. The CW output laser has the narrowest linewidth possible, which minimises dispersive effects. Operating in the CW mode also frees the laser driving circuitry from complicated feedback loops. The continuous beam is then acted upon by an external form of modulation to generate usable data.
According to Dr Dye, eam uses a different technology to the Mach Zender phase interference method used with LN. “Normally, a laser and modulator are integrated onto one chip, but eam doesn’t have the laser,” he said. The benefit is size: an eam can be housed in a 14pin butterfly package. Even so, LN based systems are likely to be the basis for 40Gbit/s links, with eam playing a role in shorter span systems.
Agere also sees significant growth in demand for optical cross connect. Dr Dye noted these are increasingly based upon microelectronic mechanical systems (mems) technology and devices are being produced which handle 256 channels in and out. These use mirrors to redirect the communications stream. In a 2d device, n^2 mirrors are needed: 64k in the case of a 256 x 256 cross connect. But 3d technology is being developed which reduces the mirror requirement to 2n, or 512. “For lower channel counts,” he continued, “2d will probably be the better solution, but 3d will be the way to go for larger devices.”
Its 5200 series is a 64 x 64 optical switch component. Its 3d mems architecture allows it to be packaged in an enclosure measuring 9 x 9 x 4in. The switch uses moving micromirrors, doing away with the need for optical - electrical - optical conversions, bringing with it bit rate and protocol independence.
Flexibility is also being provided by tunable lasers, which can produce multiple optical wavelengths from the one device. Dr Dye expanded: “If you had 16 lasers for 16 wavelengths, you’d also need 16 spares. Tunable lasers can be used for whichever frequency you need.”
The approach requires the use of distributed Bragg reflector lasers. Dr Dye: “In wavelength division multiplexing, not only do you have to tune the laser quickly, but you also need high stability otherwise it won’t work.”
Tunable lasers also allow what Dr Dye calls dynamic wavelength allocation. “This allows wavelengths to be varied according to which customer needs it. For example, if a bank needs 2.5Gbit/s bandwidth, then it can be allocated.”
But the technology is only of limited use without fibre. Alcatel has announced a new optical fibre for telecommunication applications. Enhanced single mode fibre (E-SMF) is said to add about a third more wavelengths to the usable spectrum and to extend transmission reach by 50%. It has also improved polarisation mode dispersion performance, increasing the usable system distance by more than half.
According to Alcatel, E-SMF offers a 12% improvement in attenuation at key operating wavelengths and its attenuation performance from 1350 to 1450nm makes about 30% more wavelengths available for use on a single fibre.

Vanessa Knivett

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