Three decades of steady increases in fiber-optic transmission capacity have powered the growth of the Internet and information-based technology. But sustaining that growth has required increasingly complex and costly new technology. Now, a new experiment has shown that an elegant bit of laser physics called a frequency comb—which earned Theodor Hänsch and John Hall the 2005 Nobel Prize in Physics—could come to the rescue by greatly improving optical transmitters.
In a Nature paper published today, researchers at the Karlsruhe Institute of Technology in Germany and the Swiss Federal Institute of Technology in Lausanne report using a pair of silicon-nitride microresonator frequency combs to transmit 50 terabits of data through 75 kilometers of single-mode fiber using 179 separate carrier wavelengths. They also showed that microresonator frequency combs could serve as local oscillators in receivers, which could improve transmission quality, and lead to petabit-per-second (1000 terabit) data rates inside data centers.
The German and Swiss researchers’ transmission scheme is similar to many of today's fiber-optic networks in that it uses wavelength-division multiplexing, wherein light signals are sent at many separate wavelengths through the same glass fiber. But heretofore, that called for a separate high-precision laser for each of some 100 wavelengths, each transmitting at about 100 gigabits per second. That adds up to an impressive 10 terabits per fiber core, but requires a complex system to keep the separate lasers at the proper frequency spacing.
The experimental frequency comb source described in the Nature paper is based on a monolithic microresonator chip linked to a planar waveguide. It generated 179 separate signals, phase-locked to each other at uniform frequency intervals. It also has other attractions, said Christian Koos, a Karlsruhe researcher who coauthored the paper, in an email. “Wavelength control is much simpler, since only two parameters, center wavelength and line spacing, must be adjusted. The lines have narrow bandwidth, and power consumption can be lower than with arrays of lasers.”
A frequency comb turns the output of a single pump laser into dozens or hundreds of wavelengths by locking the modes oscillating in the pump laser together so it emits a series of short, intense pulses spaced equally in time. Viewed in the frequency domain, that series of light pulses becomes a series of evenly spaced narrow frequency bands – just what's needed for simplified wavelength-division multiplexing.
The first frequency combs, produced in the 1990s, required costly, complex and delicate lasers producing pulses lasting less than a trillionth of a second. But 10 years ago, Tobias Kippenberg of the Swiss Federal Institute of Technology in Lausanne produced a frequency comb by passing laser light through a monolithic microresonator linked to a waveguide. Since then, microresonator frequency combs have become such sensitive measurement tools that they can measure the motion of exoplanets, Alex Gaeta of Columbia University said last month at the Conference on Lasers and Electro-Optics.
Microresonators had been tested for fiber-optic communications, but had not approached the data rates of systems using many discrete lasers. Now Kippenberg has teamed with Koos to test a new design that generates special pulses called solitons in the microresonator. First observed in canals in the 1830s, solitons or solitary waves maintain their shape as they travel because nonlinear effects that might shrink the waves offset dispersion that might spread them.
Gaeta is impressed, and sees the demonstration as an important step toward developing microresonator frequency combs for high-capacity wavelength-division multiplexing. “Soliton mode locking is crucial to ensure stability of each comb line [and] precise frequency spacing of the comb, and to allow for for stable coherent detection of the phases at the receiver end,” Gaeta wrote in an email. “In principle, such a system will be able to be fully integrated on a chip, which will provide energy savings and robustness.”