It has been almost 20 years since DWDM came on the scene with Ciena’s introduction of a 16 channel system in March of 1996, and in the last two decades it has revolutionized the transmission of information over long distances. DWDM is so ubiquitous that we often forget that there was a time when it did not exist and when accessing information from the other side of the globe was expensive and slow. Now we think nothing of downloading a movie or placing an IP call across oceans and continents. Current systems typically have 96 channels per optical fiber, each of which can run at 100Gbps, compared to the 2.5Gbps per channel in the initial systems. All of this got me thinking about how it often takes two innovations coupled together to make a revolution. Personal computers did not revolutionize office life until they were coupled with laser printers. Similarly, the benefits of DWDM were enormous because of erbium doped fiber amplifiers (EDFAs).
DWDM stands for Dense Wavelength Division Multiplexing, which is a complex way of saying that, since photons do not interact with one another (at least not much) different signals on different wavelengths of light can be combined onto a single fiber, transmitted to the other end, separated and detected independently, thus increasing the carrying capacity of the fiber by the number of channels present. In fact non-Dense, plain old WDM, had been in use for some time with 2, 3 or 4 channels in specialized circumstances. There was nothing particularly difficult about building a basic DWDM system. The technology initially used to combine and separate the wavelengths was thin film interference filters which had been developed to a high degree in the 19th Century. (Now a ’days photonic integrated circuits called Arrayed Waveguide Gratings, or AWGs are used to perform this function.) But until the advent of EDFAs there was not much benefit to be had from DWDM.
Fiber optic data transmission began in the 1970s with the discovery that certain glasses had very low optical loss in the near infrared spectral region, and that these glasses could be formed into fibers which would guide the light from one end to the other, keeping it confined and delivering it intact, although reduced by loss and dispersion. With much development of fibers, lasers and detectors, systems were built which could transmit optical information for 80km before it was necessary to “regenerate” the signal. Regeneration involved detecting the light, using an electronic digital circuit to reconstruct the information and then retransmitting it on another laser. 80km was much farther than the current “line of sight” microwave transmission systems could go, and fiber optic transmission was adopted on a wide scale. Although 80 km was a significant improvement, it still meant a lot of regeneration circuits would be needed between LA and New York. With one regeneration circuit needed per channel every 80 km, regeneration became the limiting factor in optical transmission and DWDM was not very practicable. The then expensive filters would have to be used every 80 km to separate the light for each channel before regeneration and to recombine the channels after regeneration.
Since full regeneration was expensive, researchers began to look for other ways to extend the reach of an optical fiber transmission system. In the late 1980s Erbuim Doped Fiber Amplifers (EDFAs) came on the scene. EDFAs consisted of optical fiber doped with Erbium atoms which, when pumped with a laser of a different wavelength, created a gain medium which would amplify light in a band near the 1550nm wavelength. EDFAs allowed amplification of the optical signals in fibers which could counter the effects of optical loss, but could not correct for the effects of dispersion and other impairments. As a matter of fact, EDFAs generate amplified spontaneous emission (ASE) noise and could cause fiber nonlinearity distortions over a long transmission distance. So EDFAs did not eliminate the need for regeneration completely, but allowed the signals to go many 80 km hops before regeneration was needed. Since EDFAs were cheaper than full regeneration, systems were quickly designed which used 1550nm lasers instead of the then prevailing 1300nm.
Then came the “ah ha” moment. Since EDFAs just replicated the photons coming in and sent out more photons of the same wavelength, two or more channels could be amplified in the same EDFA without crosstalk. With DWDM one EDFA could amplify all of the channels in a fiber at once, provided they fit within the region of EDFA gain. DWDM then allowed the multiple use of not only the fiber but also the amplifiers. Instead of one regeneration circuit for every channel, there was now one EDFA for each fiber. A single fiber and a chain of one amplifier every 40~100 km could support 96 different data streams. Regenerators are still needed today, every 1,200~3,500km, when the accumulated EDFA ASE noise exceeds a threshold that a digital signal processor and error correction codec can handle.
Of course, since the gain region of the EDFA was limited to about 40 nm of spectra width, great emphasis was placed on fitting the different optical wavelengths as close together as possible. Current systems place channels 50GHz, or approximately 0.4 nm, apart, and hero experiments have done much more.
In parallel, new technologies have increased the bandwidth per channel to 100 Gbps using coherent techniques that we have discussed in other blog posts. So a single fiber that in the early 1990s would have carried 2.5Gbps of information, now can carry almost 10 Terabits/sec of information, and we can watch movies from the other side of the globe.
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