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Laser-Driven Radio Transmitter Has A Vintage Sound
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new type of microwave radio transmitter built by physicists in the U.S. could lead to practical devices that operate at terahertz frequencies – something that could boost the capacity of wireless data systems. The transmitter converts light from a quantum cascade laser into microwave signals and the researchers say that in about five years it could be modified to work in the terahertz range.
LASER INTERFERENCE
Terahertz radiation occupies the notch between microwaves and infrared radiation – at frequencies 100 times higher than those used in today’s wireless data systems. While the effective range of terahertz signals is shorter than microwaves, its higher frequency means that it can carry more information – making it a good candidate for short-range data exchange. The problem, however, is that simple and low-cost ways of creating terahertz radiation are lacking.
Systems developed to date have tended to use two relatively large lasers and combine their light in a third device called a photomixer. But in the latest work, Federico Capasso of Harvard University and colleagues have shown how to pull off the same trick using a much more compact set-up centred around a single quantum cascade laser with an optical cavity just a few millimetres long.
Terahertz lasers, for example, typically have to be cooled down to about 80 K and are therefore bulky. Pushing the operational frequency of transistors as high as possible is another option. However, the best performing devices today reach about 0.5 THz, and it remains to be seen how much that limit can be raised.
A more practical approach could be to combine optics and electronics by converting light waves into terahertz signals. The idea is to interfere multiple laser beams with slightly different frequencies to generate an electric current oscillating at the beat frequency that can then feed an antenna. The beat frequency is equal to the difference in frequency of the laser beams.
Unlike standard semiconductor lasers, which generate photons when electrons and holes combine inside a material with a given band gap, quantum cascade lasers instead comprise a sandwich of dozens of thin layers of semiconductor whose structure determines the output frequency. Each electron that travels through the device “cascades” through a series of quantum wells, emitting
Tuning in: interfering laser light could soon be used to generate terahertz signals. (Courtesy: iStock/arttanja)
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