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'Pure Tone' Laser Tops Long-Haul Comms Bandwidth of Current War Horse Laser

21 February 2014

A research group at the California Institute of Technology has developed a laser with a spectral purity that enables a range of frequencies 20 times narrower than possible with the S-DFB workhorse laser—thus potentially increasing by orders of magnitude the rate of data transmission in long-distance, Internet-based optical-fiber networks.

Light is capable of carrying approximately 10,000 times more bandwidth than microwaves. Laser light needs to be as spectrally pure—as close to a single frequency—as possible. The purer the tone, the more information it can carry, and for decades researchers have been trying to develop a laser that comes as close as possible to emitting just one frequency.

Today's worldwide optical-fiber network is still powered by the distributed-feedback semiconductor (S-DFB) laser, developed in the mid-1970s. The S-DFB laser's longevity stems from its unparalleled spectral purity—for that time.

As the result of a five-year effort, lead researcher and Caltech electrical engineering professor Amnon Yariv and his team have developed a high-coherence laser that, in a fundamental departure from the S-DFB laser, stores the light in a layer of silicon, which does not absorb light.

Spatial patterning of this silicon layer causes the silicon to act as a light concentrator, pulling the newly generated light away from the light-absorbing III-V material and into the virtually absorption-free silicon.

Although the old S-DFB laser had a successful 40-year run in optical communications—and was cited as the main reason for Yariv's receipt of the 2010 National Medal of Science—the spectral purity, or coherence, of the laser no longer satisfies the ever-increasing demand for bandwidth.

"What became the prime motivator for our project was that the present-day laser designs—even our S-DFB laser—have an internal architecture which is unfavorable for high-spectral-purity operation. This is because they allow a large and theoretically unavoidable optical noise to commingle with the coherent laser and thus degrade its spectral purity," explained Yariv.

Yariv and his team have tried to come as close to absolute purity as possible to meet today's coherent-phase communication methods, in which the data resides in small delays in the arrival time of the waves—and which are fundamentally limited by the degree of spectral purity of the laser beam.

The findings are published in the online edition of the Proceedings of the National Academy of Science.

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