redOrbit Staff & Wire Reports – Your Universe Online
Caltech researchers have developed a new laser that could send data over the Internet backbone up to 20 times faster.
The breakthrough culminates five years of work by researchers working under Amnon Yariv, professor of applied physics and electrical engineering at Caltech. The current study was led by postdoctoral scholar Christos Santis and graduate student Scott Steger.
While laser light is capable of carrying vast amounts of information, it needs to be as spectrally pure, or as close to a single frequency, as possible. The greater the spectral purity, or coherence, the more information the light can carry.
Researchers have worked for decades to develop a laser that comes as close as possible to emitting a single frequency, but today’s global optical-fiber networks are still powered by a laser known as the distributed-feedback semiconductor (S-DFB) laser, which was developed in the mid 1970s in Yariv’s research group.
That laser’s unusual longevity stemmed from what, at the time, was considered unparalleled spectral purity. The laser’s increased spectral purity directly translated into a larger information bandwidth of the laser beam and longer possible transmission distances in the optical fiber. The result was that more information could be carried farther and faster than ever before.
That unprecedented spectral purity was a direct consequence of the incorporation of a nanoscale corrugation within the multilayered structure of the laser, which acted as an internal filter that discriminated against “noisy” waves contaminating the ideal wave frequency.
Despite the successful 40-year run of the old S-DFB laser, its spectral purity no longer satisfies today’s fast-growing 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 comingle with the coherent laser and thus degrade its spectral purity,” the researchers told Caltech’s Jessica Stoller-Conrad.
The old S-DFB laser consists of continuous crystalline layers of materials known as III-V semiconductors, which convert into light the applied electrical current flowing through the structure. Once generated, the light is stored within the same material.
Since III-V semiconductors are also strong light absorbers, which leads to a degradation of spectral purity, the researchers wanted to find a different solution for their new laser.
So they developed a new, high-coherence laser that still converts current to light using the III-V material, but 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 near absorption-free silicon.
This higher spectral purity offers a 20 times narrower range of frequencies than possible with the S-DFB laser, something that could be particularly important for the future of fiber-optic communications.
Originally, laser beams in optic fibers carried information in pulses of light – data signals were impressed on the beam by rapidly turning the laser on and off, and the resulting light pulses were carried through the optic fibers. However, to meet the ever-increasing demand for bandwidth, communications system engineers are now adopting a new method of impressing the data on laser beams known as coherent phase communication, which no longer requires this “on-off” technique.
In coherent phase communications, the data resides in small delays of just a tiny fraction (10-16) of a second in the arrival time of the waves. These delays can accurately relay the information even over thousands of miles, the researchers said.
The digital electronic bits carrying video, data, or other information are converted at the laser into tiny delays in the otherwise steady light wave. However, the number of possible delays, and thus the data-carrying capacity of the channel, is fundamentally limited by the degree of spectral purity of the laser beam.
Yariv and his team concluded that while absolute purity can never be achieved due to limitations set forth by the laws of physics, they have nevertheless tried to come as close as possible to achieving this goal.
A paper about the work, entitled “High-coherence semiconductor lasers based on integral high-Q resonators in hybrid Si/III-V platforms,” was published last week in the journal Proceedings of the National Academy of Sciences.
In addition to Yariv, Santis, and Steger, other coauthors include graduate student Yaakov Vilenchik, and former graduate student Arseny Vasilyev (PhD, ’13).