In current systems, a laser transmits light signals through the cables and information is coded in the modulation of light intensity. The system, a semiconductor spin laser, is based on a modulation of light polarisation instead.
The study demonstrates that spin lasers have the capacity of working at least five times as fast as traditional systems, while consuming only a fraction of the energy. Unlike other spin-based semiconductor systems, the technology potentially works at room temperature and doesn't require any external magnetic fields.
Due to physical limitations, data transfer that is based on a modulation of light intensity without utilising complex modulation formats can only reach frequencies of around 40-50GHz. In order to achieve this speed, high electrical currents are necessary.
Provided by Ulm University, the lasers, which are just a few micrometres 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 polarised light that is formed when two linear perpendicularly polarised light waves overlap.
When two linearly polarised light waves have different frequencies, the process results in oscillating circular polarisation where the oscillation direction reverses periodically - at a user-defined frequency of over 200GHz.
Despite the researchers having demonstrated that oscillation at 200GHz is possible, they admitted that they don't know how much faster it can get - a theoretical limit has not been discovered.
The oscillation alone does not transport any information; for this purpose, the polarisation has to be modulated, for example by eliminating individual peaks. The researchers claim that they have verified in experiments that this can be done in principle.
Two factors are decisive in order to generate a modulated circular polarisation degree: the laser has to be operated in a way that it emits two perpendicular linearly polarised light waves simultaneously, the overlap of which results in circular polarisation. 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 polarisation of light. Only if the electron spin is aligned in a certain way, the emitted light has the required polarisation - 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.
Accordingly, the refractive indices in the two perpendicularly polarised light waves emitted by the crystal differ slightly. As a result, the waves have different frequencies. By bending the semiconductor crystal, the researchers claim they're 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.
