Spin Lasers Facilitate Rapid Data Transfer

Spin lasers whose oscillation frequency can be mechanically controlled via the mount. Electrical contact can be made via an adjustable needle.

Engineers at Ruhr-Universität Bochum have developed a novel concept for rapid data transfer via optical fibre cables. In current systems, a laser transmits light signals through the cables and information is coded in the modulation of light intensity. The new system, a semiconductor spin laser, is based on a modulation of light polarization instead. Published in the journal Nature, the study demonstrates that spin lasers have the capacity of working at least five times as fast as the best traditional systems, while consuming only a fraction of energy. Unlike other spin-based semiconductor systems, the technology potentially works at room temperature and doesn’t require any external magnetic fields. The Bochum team at the Chair of Photonics and Terahertz Technology implemented the system in collaboration with colleagues from Ulm University and the University at Buffalo.

Rapid data transfer is currently an energy guzzler

Due to physical limitations, data transfer that is based on a modulation of light intensity without utilizing complex modulation formats can only reach frequencies of around 40 to 50 gigahertz. In order to achieve this speed, high electrical currents are necessary. “It’s a bit like a Porsche where fuel consumption dramatically increases if the car is driven fast,” compares Professor Martin Hofmann, one of the engineers from Bochum. “Unless we upgrade the technology soon, data transfer and the Internet are going to consume more energy than we are currently producing on Earth.” Together with Dr. Nils Gerhardt and PhD student Markus Lindemann, Martin Hofmann is therefore researching into alternative technologies.

Markus Lindemann is working on the development of ultrafast spin lasers as part of his doctoral thesis.

Circularly polarised light as information carrier

Provided by Ulm University, the lasers, which are just a few micrometers in size, were used by the researchers to generate a light wave whose oscillation direction changes periodically in a specific way. The result is circularly polarized light that is formed when two linear perpendicularly polarized light waves overlap.

In linear polarization, the vector describing the light wave’s electric field oscillates in a fixed plane. In circular polarization, the vector rotates around the direction of propagation. The trick: when two linearly polarized light waves have different frequencies, the process results in oscillating circular polarization where the oscillation direction reverses periodically – at a user-defined frequency of over 200 gigahertz.

Speed limit as yet undetermined

“We have experimentally demonstrated that oscillation at 200 gigahertz is possible,” describes Hofmann. “But we don’t know how much faster it can become, as we haven’t found a theoretical limit yet.”

The oscillation alone does not transport any information; for this purpose, the polarization has to be modulated, for example by eliminating individual peaks. Hofmann, Gerhardt and Lindemann have verified in experiments that this can be done in principle. In collaboration with the team of Professor Igor Zutic and PhD student Gaofeng Xu from the University at Buffalo, they used numerical simulations to demonstrate that it is theoretically possible to modulate the polarization and, consequently, the data transfer at a frequency of more than 200 gigahertz.

The polarization describes a light wave’s oscillation direction. Linear polarization (red, blue): the vector describing the light wave’s electric field oscillates in a fixed plane. Circular polarization can be described by a superposition of two linear perpendicularly polarized light waves. The electric field vector rotates around the propagation direction. If the frequencies of the overlapping fields are different, oscillating circular polarization is the result (black). The circular polarization degree (green) is modulated depending on the frequency difference. T is the cycle duration of this modulation.

The generation of a modulated circular polarization

Two factors are decisive in order to generate a modulated circular polarization degree: the laser has to be operated in a way that it emits two perpendicular linearly polarized light waves simultaneously, the overlap of which results in circular polarization. Moreover, the frequencies of the two emitted light waves have to differ enough to facilitate high-speed oscillation.

The laser light is generated in a semiconductor crystal, which is injected with electrons and electron holes. When they meet, light particles are released. The spin – an intrinsic form of angular momentum – of the injected electrons is indispensable in order to ensure the correct polarization of light. Only if the electron spin is aligned in a certain way, the emitted light has the required polarization – a challenge for the researchers, as spin alignment changes rapidly. This is why the researchers have to inject the electrons as closely as possible to the spot within the laser where the light particle is to be emitted. Hofmann’s team has already applied for a patent with their idea of how this can be accomplished using a ferromagnetic material.

Frequency difference through double refraction

The frequency difference in the two emitted light waves that is required for oscillation is generated using a technology provided by the Ulm-based team headed by Professor Rainer Michalzik. The semiconductor crystal used for this purpose is birefringent. Accordingly, the refractive indices in the two perpendicularly polarized light waves emitted by the crystal differ slightly. As a result, the waves have different frequencies. By bending the semiconductor crystal, the researchers are able to adjust the difference between the refractive indices and, consequently, the frequency difference. That difference determines the oscillation speed, which may eventually become the foundation of accelerated data transfer.

“The system is not ready for application yet,” concludes Martin Hofmann. “The technology has still to be optimized. By demonstrating the potential of spin lasers, we wish to open up a new area of research.”

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