Laser Frequency Combs Operating at Terahertz Frequencies May be the Future of Wi-Fi

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 a long-term solution, researchers are exploring terahertz frequencies, the submillimeter wavelengths of the electromagnetic spectrum. Data traveling at terahertz frequencies could move hundreds of times faster than what we have today.

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, as it transforms a laser - a device operating at optical frequencies - 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 - a.k.a. colors - 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 showed, for the first time, a laser at optical wavelengths operates as a microwave device. The scientists, however, were not interested in the optical output of the laser.

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 research has shown that the laser can act as a so-called 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. 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.