A laser with a polarization that can be made to oscillate at over 200 GHz has been created by researchers in Germany and the US. The device could be used to boost the data capacity of optical telecommunications server farms, although much work remains to be done before practical devices could be available.
Global internet traffic is growing exponentially with the ever-increasing popularity of bandwidth-hungry activities such as online gaming. As demand grows, telecoms companies are replacing copper wires with optical fibres, which transmit signals at higher speed with lower loss. It has also led to a proliferation of server farms, which contain giant banks of data processors. The energy requirements of these farms are huge: one estimate suggests that, by 2025, they could cause 3.2% of global carbon emissions.
The millions of tiny connections between servers inside a farm also use optical fibre and tiny intensity-modulated lasers. The speed at which a bitstream can be pushed through the fibre depends on how quickly the laser can switch between the intensity corresponding to a “1” and the intensity corresponding to a “0” – currently about 35 GHz. Moreover, explains Markus Lindemann of Ruhr University Bochum in Germany, “The typical technique to increase the modulation bandwidth is to just increase the pumping current. Higher modulation bandwidth is always connected with higher power consumption.”
Fast and efficient
For this reason, researchers are exploring ways to modulate other properties of light. One possibility that has been explored for a decade or so is the use of spin lasers, which produce polarization-modulated light. Researchers have predicted that this could be much faster than intensity modulation, but a clear demonstration had not been provided, and practical considerations such as power consumption remained largely unexplored.
Now, Lindemann, Nils Gerhardt and colleagues a Ruhr University Bochum, Ulm University and the State University of New York at Buffalo have designed a new spin laser based on the birefringence of the laser’s gain medium. Birefringence occurs when the refraction index of a crystalline material depends on the polarization and direction of propagation of light. The effect is usually seen as something to be minimized because it makes lasers unstable.
Instead, the Ruhr team maximized birefringence by bending the crystal that they used as the laser gain medium. To operate the laser, it is first electrically pumped above it lasing threshold and then spin-polarized light from another source is injected into the laser. This causes the laser to undergo resonant oscillations between two orthogonal polarization modes at frequencies up to of 214 GHz. Furthemore, the oscillation rate of the laser is almost independent of its power consumption.
Slow modulation
This may be an impressive proof of principle, but Lindemann acknowledges that oscillations alone cannot transmit data. In a subsequent experiment, therefore, the researchers transmitted a series of data bits by modulating the polarization of the pump light. The maximum data transmission rate they achieved was 25 GHz: “Why is it not at 200 GHz?” asks Lindemann, “The answer is that there are no components on the market that could modulate polarization at that speed.”
Another challenge is that optical spin injection would be impractical in a real server farm. However, the transmission of information using electron spin is at the core of spintronics – one of the most exciting areas in physics research today. If researchers could modulate the polarization of the laser light by modulating the polarization of the electrons used to pump it, they would effectively have a direct interface between spintronics and fibre optics. It is no surprise, therefore, that the US researchers working with Lindemann and Gerhardt are specialists in spintronics.
Ultrafast spin laser could boost optical data transmission
Werner Hofmann of the Technical University of Berlin points out that the concept is neither new nor unexpected and emphasizes the need for electrical modulation: “If you can modulate the polarization of a laser, that has great potential for [transmitting data at] high speeds: that’s known,” he explains. “The big question is how to make a laser move energy from one polarization to another one with an electrical signal, and that’s not addressed here.”
Ortwin Hess of Imperial College London, who did a theoretical investigation of polarization splitting in lasers of this type when he was at the University of Stuttgart, is more enthusiastic: “I don’t see this as a technical device yet, but as a beautiful demonstration of a fundamental physical effect that we were hoping someone would measure even 20 years ago,” he says.
The research is described in Nature.
