Today, rapid data transfer via optical fiber is accomplished by transmitting light signals which code data by modulating the light intensity. Because of the physical limitations, the data transfer based on a modulation of light intensity, without utilizing the complex modulation formats, can only reach frequencies of around 40 to 50 gigahertz. Further, in order to achieve this speed, high electrical currents are necessary.
With a novel approach to data transfer, scientists from Ruhr-Universität Bochum (RUB) used a semiconductor spin laser to enable room-temperature modulation frequencies above 200 GHz. According to the researchers, the frequency level is nearly an order of magnitude faster than the best conventional semiconductor lasers. The modulation of light polarization rather than light intensity, is the basis of the new system.
The spin lasers, which are just a few micrometers in size, were used to generate a lightwave whose oscillation direction changed periodically in a specific way. A circularly polarized light was formed when two linear, perpendicularly polarized lightwaves overlapped.
In linear polarization, the vector describing the lightwave’s electric field oscillates in a fixed plane. In circular polarization, the vector rotates around the direction of propagation. When two linearly polarized lightwaves have different frequencies, the result is oscillating circular polarization, where the oscillation direction reverses periodically at a user-defined frequency of over 200 GHz.
“We have experimentally demonstrated that oscillation at 200 GHz is possible,” said professor Martin Hofmann. “But we don’t know how much faster it can become, as we haven’t found a theoretical limit yet.”
The laser light was generated in a semiconductor crystal, which was injected with electrons and electron holes. When the electrons and the holes met, light particles were released. To align the spin successfully, the researchers injected the electrons as close as possible to the point within the laser where the emission of the light particles would occur.
The frequency difference in the two lightwaves was generated using a semiconductor for birefringence. The refractive indices in the two perpendicularly polarized lightwaves emitted by the semiconductor differed slightly, and as a result, the waves had different frequencies. By bending the semiconductor crystal, the researchers were able to adjust the difference between the refractive indices and, consequently, the frequency difference. That frequency difference was what determined the oscillation speed that could someday become the benchmark for accelerated data transfer.
The results of the research suggest how speed limitations of conventional, directly modulated lasers could be overcome and outline an option for the next generation of low-energy, ultrafast optical communication. “The system is not ready for application yet,” Hofmann said. “The technology has still to be optimized. By demonstrating the potential of spin lasers, we wish to open up a new area of research.”
The RUB team implemented the system in collaboration with colleagues from Ulm University and the University at Buffalo.
The study by Hofmann et al. demonstrates that spin lasers have the potential to work 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 could potentially work at room temperature and does not require any external magnetic fields.