Interconnections in powerful computers and linking "blades" in data centers will increasingly rely on optical communication links. Currently, this still requires an individual laser with individual control circuitry for each channel. Now researchers at Purdue University have developed a new technology that allows a single laser to transmit data over a number of individually controlled channels, at different frequencies, simultaneously. They published this research online in the 10 August edition of the journal Nature Photonics.
The key component of this technology is a tiny microresonator. It’s a 100-micrometer-wide optical waveguide loop or microring made from silicon nitride. Because it is as thin as a sheet of paper, it can easily be integrated on silicon chips. The microresonator replaces a whole tabletop studded with the complement of optical components and resonators that are now required to create a mode-locked laser.
In the experimental setup, a pump laser is connected to the resonator. The researchers pump the resonator with a continuous-wave laser at one frequency, explains Minghao Qi, an associate professor of electrical and computer engineering at Purdue. The resonator, though small, can hold a huge amount of power, which leads to non-linear interaction. “Normally, if we pump anything into the resonator, and the interaction is linear, the input and output frequencies are the same,” says Qi. “When the interaction is non-linear, it basically generates higher-order harmonics—new frequencies.”
Qi adds that, because the spacing between the different frequency peaks are the same, the resonator is called a frequency comb. The frequencies can be tuned by changing the resonance frequency of the resonator. This is achieved by an electric heater, a tiny gold wire overlaying the resonator. Changing the temperature changes the resonator’s refractive index, which in turn changes the resonance frequency.
While the experimental setup works well with discrete light pulses, the researchers also noted the presence of “dark pulses,” or very short intervals where no light is transmitted. These intervals can occur every one or two picoseconds, which is a hundred times faster than the switching speed of the most advanced microprocessors. “The advantage of a dark pulse is that this can be repeatedly generated and that means it is very reliable and we can control it. If you want a bright pulse, then it is a very tricky process,” says Qi.
According to the Purdue researchers, they showed that dark pulses can be converted into bright pulses. “So by creating a dark pulse first, you have a process that is robust and controllable,” says Qi.
Besides facilitating high-volume optical communications in computers, microresonators could also be used in optical sensors and in spectroscopy. If you want to probe a compound at many different wavelengths you can use a tunable laser to excite the molecule at those different wavelengths. With conventional lasers, you have to tune the laser to a different frequency for every measurement, which takes time. What’s more, tunable lasers are expensive, explains Qi. But with the Purdue team’s improved laser, “If your probe light itself has many, many frequencies, you are basically doing a spectral scanning, with all the frequencies in one shot,” Qi adds.
For the moment, the scientists have yet to put microresonator on a chip with all the other components. “This will be our next step,” says Qi.