Quantum Cascade Laser Frequency Combs Could Be The Future of WiFi

  • Scientists developed a technique to generate terahertz frequencies through an infrared frequency comb in a quantum cascade laser.
  • Data traveling at this band can move more than hundred times as fast as existing wireless networks. 
  • This is the first time someone has demonstrated that a laser is capable of acting as quadrature modulator. 

Cellular data and WiFi traffic are increasing at an enormous rate. By 2020, there will be more than 50 billion WiFi-connected devices worldwide. But there speed will be limited by the capacity of wireless networks, and the traffic generated by these devices may lead to intolerable bottlenecks.

Upcoming 5th generation wireless system is a temporary solution that will be deployed in 2018 and later. Its millimeter wave bands can handle up to 20 gigabits of data per second (Gbit/s). However, this doesn’t seem like a long-term solution.

Therefore, scientists have focused on submillimeter band of the electromagnetic spectrum, called terahertz frequencies. Wavelengths in the terahertz band range from 1 millimeter to 0.1 millimeter; data traveling at this band can move more than hundred times as fast as existing wireless networks.

Infrared Frequency Comb In A Quantum Cascade

In 2017, researchers at the Harvard University developed a technique of generating terahertz frequencies through a frequency comb [infrared] in a quantum cascade laser. Now they have developed a new mechanism of quantum cascade laser frequency combs that enables equipment to act as an integrated receiver or transmitter, to effectively encode data.

This technique converts equipment functioning at optical wavelength into advanced modulators at microwave wavelengths. This allows the device to use network bandwidth efficiently. It completely transforms the way a laser is operated.

What’s frequency combs? 

An optical frequency comb is a source of laser whose spectrum contains a series of discrete [evenly spaced] frequency lines. It’s widely used to precisely measure and detect different frequencies of light. Unlike traditional laser — that emits a light of single wavelength — this one emits light at multiple wavelengths, concurrently.

It’s known as a frequency comb because these lights of multiple frequencies are equally spaced and they resemble a comb’s teeth. At present, we use these optical frequency combs for almost everything, from finding distant exoplanets to analyzing fingerprints of certain molecules.

However, the research is not about the laser’s optical output. Scientists were interested in what’s going on inside the laser’s electron structure. They have demonstrated that an optical laser can function as a microwave instrument.

Reference: OSAPublishing | doi:10.1364/OPTICA.5.000475 | Harvard 

How Does It work?

Image credit: Jared Sisler / Harvard University

The multiple wavelengths of laser light beat together to produce microwave radiation. The light present in the laser cavity triggers the electrons to oscillate at different microwave wavelengths. These wavelengths fall under the same spectrum that is used for communications. In order to encode data onto a carrier signal, one could externally modulate these oscillations.

According to the researchers, no one has done this before. This is the first time someone has demonstrated that a laser is capable of acting as quadrature modulator, which enables 2 different data to be transferred at the same time using one frequency channel.

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Furthermore, wireless signal could be outcoupled into free space by integrating antennas in the laser. This would make a quantum cascade laser a unibody modulator and transmitter.

At present, the sources of terahertz radiation have critical limitations because of limited bandwidth. This research opens up a potential route for a new type of quadrature mixers, which could be easily integrated into next-generation wireless communication architectures.

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