So-called spin lasers have the potential to significantly accelerate data transmission in fiber optic cables. And consume much less energy.
A new concept for ultrafast data transfer over fiber optic cables has been developed by engineers at the Ruhr University Bochum. In conventional systems, a laser sends light signals through the cables and the information is encoded in the modulation of light intensity. The new system, a semiconductor spin laser, instead works with changes in light polarization. The study, published on April 3, 2019 in the journal Nature, shows that spin lasers could work at least five times faster than the best conventional systems, using only a fraction of the energy. Unlike other spin-based semiconductor systems, the technique works at room temperature and without external magnetic fields.
Fast data transfer is currently energy guzzler
The data transmission, which is based on a direct modulation of the light intensity, can not be done much faster than with a frequency of 40 to 50 gigahertz without complex modulation concepts due to physical limits. To achieve this speed, high electrical currents are required. "It's like a Porsche, the right gasoline consumed when it should be fast," said the Bochum engineer Prof. Dr. med. Martin Hofmann. "If we do not change the technology soon, data transmission and the Internet will consume more energy than we currently produce on Earth." Nils Gerhardt and doctoral student Markus Lindemann are therefore researching Martin Hofmann on an alternative technology.
Circularly polarized light as information carrier
With a few micrometers of laser provided by the team of the University of Ulm, the researchers produce a light wave whose direction of oscillation changes periodically in a special way. It is circularly polarized light that is created by superimposing two linearly polarized light waves.
In linearly polarized light, the vector describing the electric field of a light wave oscillates constantly in a plane. In circularly polarized light, it rotates around the propagation direction. The trick: If the two linearly polarized light waves differ in their frequency, the result is an oscillating circular polarization, in which the direction of oscillation turns over and over again - with adjustable speed.
Speed limit not yet reached
"We experimentally showed that the oscillation can take place at 200 gigahertz," says Hofmann. "How much faster they can become, we do not know. We have not found a theoretical limit yet. "
The oscillation alone, however, does not carry any information. For this purpose, the polarization must be modulated, for example individual peaks are to be extinguished. Hofmann, Gerhardt and Lindemann have experimentally confirmed that this is possible in principle. With numerical simulations they showed together with the team around Prof. Dr. med. Igor Žutić and PhD student Gaofeng Xu from the University at Buffalo also said that modulation of the polarization and thus the transmission of information at more than 200 gigahertz is theoretically possible.
This is how the modulation is generated
To generate a modulated circular polarization, two factors are crucial: the laser must be operated in such a way that it simultaneously emits two perpendicularly polarized light waves, the superimposition of which results in the circular polarization. In addition, the frequency of the two emitted light waves must be sufficiently different that the fast oscillation arises.
The laser light is generated in a semiconductor crystal into which the researchers inject electrons and electron holes. When they meet, light particles are released. For the light to get the desired polarization, the spin - a kind of intrinsic angular momentum - of the injected electrons is crucial. Only if the spin of the electrons is aligned in a certain way, the emitted light has the right polarization - a challenge, since the spin alignment is lost quickly. The researchers therefore have to bring the electrons into the laser as close as possible to the point at which the light particle is to be formed. An idea on how to do this with the help of a ferromagnetic material has already been filed for a patent by Hofmann's team.
Birefringence ensures frequency difference
The frequency difference in the two emitted light waves required for the oscillation is determined using a technique of the Ulm team headed by Prof. Dr. med. Rainer Michalzik and doctoral student Tobias Pusch generated. The semiconductor crystal used is birefringent. The refractive index is thus slightly different for the two mutually perpendicular polarized light waves emerging from the crystal. As a result, the waves have different frequencies. By bending the semiconductor crystal, researchers can adjust the difference in refractive index and thus the frequency difference. It determines the speed of the oscillation, which could ultimately be the basis for accelerated data transmission.
"The system is not ready to be used," summarizes Martin Hofmann. "There is a lot of technological optimization required. With our work demonstrating the potential of spin lasers, we want to open up a new research field. "
Source: Ruhr-University Bochum (RUB)