Coherent optical turns 10: Here’s how it was made
By Bo Gowan – This is the story of how a team of over 100 people in Ciena’s R&D labs pulled together an impressive collection of technology innovations that created a completely new way of transporting data over fiber…and in the processes helped change the direction of the entire optical networking industry.
Back in 2008, many in the industry had serious doubts that commercializing coherent fiber optic transport was even possible, much less the future of optical communications. That left a team of Ciena engineers to defy the naysayers and hold the torch of innovation.
“What we first began to see at Telecom 99 was that we could achieve these high speeds the brute force way, but it was really, really painful,” said Dino DiPerna in an interview. Dino, along with many in his team, were brought on by Ciena as part of the company’s 2010 acquisition of Nortel’s optical business. He now serves as Ciena’s Vice President of Packet-Optical Platforms R&D and is based in Ottawa.
By ‘brute force’ Dino is referring to the traditional time-division multiplexing (TDM) method that had been used until then to speed up optical transmission – basically turning the light on and off at increasingly faster speeds (also called the baud or symbol rate). “But once you start pushing past 10 billion times per second, you begin running into significant problems,” said DiPerna.
Those complexities had to do with the underlying boundaries of what you can do with light. The fundamental issue at hand was the natural spread and propagation of light as it travels along the fiber – created by two phenomenon called chromatic dispersion and polarization mode dispersion, or PMD. As you push past 10G speeds, the tolerance to chromatic dispersion goes down with the square of the baud. Due to PMD and noise from optical amplifiers, a 40 Gbaud stream will lose at least 75% of its reach compared to a 10 Gbaud stream.
This reach limitation had two consequences. First, it meant adding more costly regenerators to the network. Second, it meant that the underlying fiber plant required a more expensive, high-quality fiber to operate properly at 40G transmission speeds. more>
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