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Currently, rapid data transfer via optical fiber is accomplished by transmitting light signals that code data by modulating the light intensity. 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. Further, in order to achieve this speed, high electrical currents are necessary.
In a novel approach to data transfer, researchers from Ruhr-Universität Bochum (RUB) used a semiconductor spin laser to enable room-temperature modulation frequencies above 200 GHz. According to the researchers, this 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.
Spin lasers whose oscillation frequency can be mechanically controlled via the mount. Electrical contact can be made via an adjustable needle. Courtesy of RUB, Kramer.
“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 oscillation alone did not transport any information. For data transfer, the polarization had to be modulated, by eliminating individual peaks for example. The researchers used numerical simulations to demonstrate that it was theoretically possible to modulate the polarization and, consequently, the data transfer at a frequency of more than 200 GHz.
The researchers said that two things were necessary to generate a modulated circular polarization degree. First, the laser had to be operated so that it emitted two perpendicular, linearly polarized lightwaves simultaneously. The overlap of these waves was what caused the circular polarization. Second, the frequencies of the two emitted lightwaves had to be different enough to facilitate high-speed oscillation.
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 polarization describes a lightwave’s oscillation direction. Linear polarization (red, blue): The vector describing the lightwave’s electric field oscillates in a fixed plane. Circular polarization can be described by a superposition of two linear perpendicularly polarized lightwaves. 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. Courtesy of RUB, Chair of Photonics and Terahertz Technology.
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.
Research team from Bochum. (l) to (r): Martin Hofmann, Markus Lindemann, and Nils Gerhardt. Courtesy of RUB, Kramer.
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.
The research was published in Nature (https://doi.org/10.1038/s41586-019-1073-y).READ MORE