RESEARCH Science Physics

All-optical frequency shifter is fast and accurate

James E. Kloeppel, Physical Sciences Editor
(217) 244-1073; Kloeppel@uiuc.edu

3/11/03

AUSTIN, Texas — Researchers at the University of Illinois at Urbana-Champaign have demonstrated an all-optical frequency shifter that could remove a bottleneck in optical communications networks. The device can rapidly and accurately shift the frequency of optical signals without the time-consuming tasks of detection, storage and rebroadcast.

To jam more information onto a single optical fiber, engineers would like to use a technique called wavelength division multiplexing. In this technique, different colors of light correspond to individual channels in much the same way that radio frequencies delineate stations. Each color can be independently modulated and then recombined for transmission through the fiber.

"Traffic on a complex fiber system involves many nodes, and inevitably a signal will arrive at a node on the same channel as that occupied by another signal," said James Eckstein, a professor of physics at Illinois. "Since both signals can’t be transmitted, one must be shifted to a different, open channel. The electrooptic frequency shifter does this by automatically shifting the energy of the photons a little, so they change color."

At the heart of the frequency shifter is a standard optical phase modulator, Eckstein said. The device has an optical waveguide made of lithium niobate and a gold electrode deposited on the substrate. By applying a microwave signal to the electrode, Eckstein and doctoral student Dario Farias can control the properties of the optical waveguide.

"The strength and direction of the microwave electric field changes the refractive index of the lithium niobate material," Eckstein said. "This causes the optical signal to either compress or stretch slightly as it propagates through the waveguide, effectively shifting the light to a different wavelength and frequency. By changing the phase and amplitude of the applied microwave signal, we can shift the optical frequency as required."

Unlike semiconductor optical amplifiers, which use the data encoded in one channel to modulate a similar pattern in a second channel, the electrooptic frequency shifter requires no additional optical source.

"In existing networks, a signal must first be detected, then processed, and finally
re-emitted at some other wavelength from another source," Farias said. "This takes time and limits the system’s data capacity. The electrooptic frequency shifter is faster because it is just a matter of controlling the number of microwave photons that are added to each optical photon as the light pulse passes through the device."

In their experimental setup, the researchers used a microwave source that was phase-locked to the optical pulse repetition rate. "In commercial applications, however, a clock recovery circuit would sample the incoming optical pulses to lock the oscillator to the pulse rate," Eckstein said. "A microwave circuit would then quickly stabilize a microwave source at the clock frequency to the power level required to accomplish the desired frequency shifting."

Farias described the electrooptic frequency shifter at the American Physical Society meeting in Austin. His talk took place March 7 in the Austin Convention Center.

The National Science Foundation funded the work.