Data Transmission With Laser Frequency Combs

Inside an infrared frequency comb in a quantum cascade laser, the different frequencies of light beat together to generate microwave radiation. (Source: J. Sisler, Harvard U.)

Wi-Fi and cellular data traffic are increasing expo­nentially but, unless the capacity of wireless links can be increased, all that traffic is bound to lead to unac­ceptable bottle­necks. Upcoming 5G networks are a temporary fix but not a long-term solution. For that, researchers have focused on terahertz frequencies, the submilli­meter wave­lengths of the electro­magnetic spectrum. Data traveling at tera­hertz frequencies could move hundreds of times faster than today’s wireless.

In 2017, researchers at the Harvard John A. Paulson School of Engi­neering and Applied Sciences SEAS discovered that an infrared frequency comb in a quantum cascade laser could offer a new way to generate tera­hertz frequencies. Now, those researchers have uncovered a new pheno­menon of quantum cascade laser frequency combs, which would allow these devices to act as integrated trans­mitters or receivers that can effi­ciently encode infor­mation.

“This work represents a complete paradigm shift for the way a laser can be operated,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics. “This new pheno­menon transforms a laser into an advanced modu­lator at micro­wave fre­quencies, which has a techno­logical signi­ficance for efficient use of bandwidth in communi­cation systems.” Frequency combs are widely-used, high-precision tools for measuring and detec­ting different fre­quencies of light. Unlike conven­tional lasers, which emit a single frequency, these lasers emit multiple fre­quencies simul­taneously, evenly spaced to resemble the teeth of a comb. Today, optical frequency combs are used for everything from measuring the finger­prints of specific molecules to detecting distant exo­planets.

This research, however, wasn’t interes­ted in the optical output of the laser. “We were interes­ted in what was going on inside the laser, in the laser’s electron skeleton,” said Marco Piccardo, a post­doctoral fellow at SEAS. “We showed, for the first time, a laser at optical wave­lengths operates as a micro­wave device.” Inside the laser, the different fre­quencies of light beat together to generate micro­wave radiation. The researchers discovered that light inside the cavity of the laser causes electrons to oscil­late at micro­wave fre­quencies – which are within the communi­cations spectrum. These oscil­lations can be exter­nally modulated to encode infor­mation onto a carrier signal.

“This func­tionality has never been demon­strated in a laser before,” said Piccardo. “We have shown that the laser can act as a quadrature modu­lator, allowing two different pieces of infor­mation to be sent simul­taneously through a single frequency channel and succes­sively be retrieved at the other end of a communi­cation link.” “Currently, terahertz sources have serious limi­tations due to limited band­width,” said Capasso. “This disco­very opens up an entirely new aspect of frequency combs and could lead, in the near future, to a tera­hertz source for wireless communi­cations.” (Source: Harvard SEAS)

Reference: M. Piccardo et al.: Time-dependent population inversion gratings in laser frequency combs, Optica 5, 475 (2018); DOI: 10.1364/OPTICA.5.000475

Link: Capasso Group, Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, USA

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