A QUITE SPECTACULAR RAINBOW

Overwhelmed by the amount of information flying around the airwaves? Sit back and relax, because technologies are emerging that will make our current information highways look like footpaths. At Lucent Technologies, scientists have made one laser do the work of 206, generating the largest number of communications channels ever.

The technique used is called wavelength division multiplexing (WDM), the transmission of data over multiple wavelengths, which many believe to be the transmission technology of the future. Each wavelength adds a transmission lane to the information superhighway, so more traffic can be carried on a single fibre. While a single light source can generate hundreds of communications channels, conventional WDM systems are usually limited to dozens because the task of combining a large number of single frequency lasers is complex and expensive.

The team of scientists at Lucent have taken a femtosecond laser transmitter, a spectrally broadband source that covers a multitude of channels and combined it with a new kind of data encoding scheme.

To record data on all 206 channels, they took advantage of the fact that different wavelengths in a light pulse travel through optical fibre at different speeds; for example, red light propagates faster than blue. Usually, this spreading of the light signal – called dispersion – creates a problem, but here it is the essential to the operation.

Each wavelength arrives at the end of a fibre at a slightly different time so that the data can be encoded sequentially onto the WDM channels using a single data modulator: a series of rainbows, each about 20 billionths of a second long, flying across the optical fibre.

The technique requires a very broad colour spectrum and a short optical pulsewidth, both characteristic of a femtosecond laser. The shorter the light pulse, the wider the colour spectrum. The wider the spectrum, the more channels can be generated.

The femtosecond light source is a pulsed laser made of a length of optical fibre containing the rare-earth element erbium. Each colour, or channel, in the experimental system carries 36.7 million bits of information per second.

The scientists believe that the `spectral slicing’ of a spectrally broadband source is a simple way to generate a very large number of WDM channels, thereby paving the way to ultrafast applications in the area of communications, medicine, metrology as well as industrial process control.

Figure 1: A photolithographic mask used in making dense wavelength division multiplexers

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