|
Wavelength Division Multiplexing
Wavelength Division Multiplexing (WDM) is a fancy term for shining multiple colors (wavelengths) of light down the one fiber, whilst modulating each with some data and then separating the light at the other end and detecting each color separately to recover its data.
The most obvious approach is to use two lasers - at 1310 and 1550 nm - to double the capacity of existing fibers. However most of the excitement about WDM involves using 8, 16 or even 32 closely spaced wavelengths in the 1520 to 1560 'window' in which silica is most transparent. With each wavelength carrying 622Mb/s, 2.5Gb/s or 10Gb/s, the benefits of WDM for new submarine cables, or for retrofitting to existing terrestrial fiber systems - including some old 140Mb/s PDH fibers - are obvious!
The physics and engineering required to achieve WDM is fascinating and subtle, but beyond the scope of this report. However some of the technical principles and characteristics of WDM will be described because they bear directly on the performance and business case for such systems. These characteristics also directly affect the costs and performance of high-speed global communications.
The first requirement is for highly stable and reliable laser-diodes, modulators to turn their light on and off, and fiber combiners to efficiently mix these multiple wavelengths into a single fiber. Modulators and optical combiners are well developed, but it is a challenge to tightly control the wavelength of semiconductor lasers which must last for years. The light of each wavelength is modulated using standard SDH techniques - so an 8 wavelength WDM system would consist of sets of 8 standard SDH 2.5 or 10Gb/s Add Drop Mulitplexers (or two sets or the simpler SDH Terminals for a point-to-point link) with each ADM modulating and receiving one of the eight wavelengths. Thus an 8 x 2.5Gb/s WDM point-to-point system consists of eight separate 2.5Gb/s systems carried over the one pair of fibers.
Secondly there must be a reliable mechanism at the receiving end for splitting each wavelength into separate beams to be sent to individual photodiodes. This is daunting to say the least, with the wavelengths differing by only a fraction of a percent - but elaborate diffraction gratings and other optical techniques enable it to be achieved reliably.
If that was all there was to WDM, it would be an exciting and valuable technology - with the SDH regenerator stations every 100Km, and the full set of splitters, photodiodes, electronics, lasers and modulators. In fact WDM is even more exciting due to the serendipity of nature and some careful engineering which makes it possible to amplify light directly in the fiber - rather than splitting the light into separate wavelengths, turning the pulses into electrical signals, recovering the data bits and regenerating each wavelength's signal with a laser-diode.
The 'magic-bullet' which multiplies WDM attractiveness is the 'Erbium Doped Fiber Amplifier' (EDFA). An EDFA is a small, simple device containing a few meters of silica fiber which has been doped with a few parts-per-million of the element erbium. The light to be amplified is passed through this fiber while the erbium atoms are energized with shorter wavelength light from a high powered laser-diode, which is coupled in via a fiber splice. Each energized erbium atom amplifies a passing photon of light by creating an identical photon which travels in step with the original. An EDFA is capable of amplifying multiple wavelengths of light at once and so replaces the complex, power-hungry and expensive opto-electronics of a regenerator station.
Consequently, with relatively simple, and highly reliable EDFAs in amplifier stations every 100Km or so, it is possible to construct terrestrial and submarine fiber cables which use no regenerator stations for many thousands of kilometers. Sending 2.5Gb/s light pulses - which are only 80mm long in the fiber - for these distances requires special attention to some dispersion and distortion problems in the fiber, but once these are solved, a WDM cable is practical. This link uses eight wavelengths, each carrying a 2.5 Gb/s SDH data stream. Dispersion is perhaps the most intractable problem in multi-gigabit-per-second fiber communications. It causes light of differing wavelengths to travel at different speeds. Since modulated light consists of slightly different wavelengths (the different frequencies are sidebands several Gigahertz above and below the approximately 200,000 GHz frequency of the infra-red light itself), dispersion causes the various frequency components of the pulses to 'smear' and so arrive at slightly different times. This prevents the data signal being detected, so dispersion sets a fundamental limit on the data-rate a fiber can support. Longer fibers lead to greater dispersion and a given level of dispersion is a proportionally greater problem for short-pulse 10Gb/s systems than for 2.5Gb/s. There are techniques for reducing dispersion in the fiber, and for correcting for it, but achieving this when all components must last for a decade or so without maintenance at the bottom of the ocean is not easy.
Back to Main Menu Previous Page Next Page
|