Photonic filter to clean up fiber optic communications

Researchers have developed a fiber optic filter that they say goes a long way toward eliminating electrical components from optical transmission links and offering virtually flawless data reception to end users of the Internet and other fiber-based network systems.

Researchers at the US Department of Energy’s Ames Laboratory say the technology, a three-dimensional photonic crystal add-drop filter, promises greatly enhanced transmission of multiple wavelength channels traveling along the same optical fiber by letting the system rapidly add or drop wavelengths as needed.

Researchers explained that there are up to 160 wavelength channels traveling through an optical fiber at the same time and as information is transported over these multiple channels, it’s necessary to drop off individual wavelength channels at different points on the fiber. At the same time, it’s essential to be able to add data streams into unfilled wavelength channels, researchers said on their Web site.

“When the data being transported in multiple frequency channels over an optical fiber comes to a receiving station, you want to be able to pick off just one of those frequencies and send it to an individual end user,” Rana Biswas, an Ames Laboratory physicist and one of the developers of the new add-drop filter. “That’s where these 3-D photonic crystals come into play.”

To prove their concept, the researchers used a three-dimensional, microwave- scale photonic crystal constructed from layered alumina rods and containing a wavelength range in which electromagnetic waves cannot transmit known as a full bandgap. Just as electronic bandgaps prevent electrons within a certain energy range from passing through a semiconductor, photonic crystals create photonic bandgaps that confine light of certain wavelengths, researchers said.

The add-drop filter created by the Ames Laboratory contains an entrance waveguide and an exit waveguide created by removing rod segments from the layered photonic crystal. A one-rod segment separates the two waveguides, which is a system or material that can confine and direct electromagnetic waves.

A defect cavity is located one unit cell above the waveguide layer. The waveguides can communicate through the cavity, allowing a specific wavelength frequency to be selected from the input waveguide and transmitted to the output waveguide, excluding other input frequencies and resulting in near 100% efficiency for the drop frequencies, researchers said.

The idea of using photonic crystals for add-drop filters is not new, the researchers noted. Since the mid 1990s, many groups worldwide have been working to develop the technology with two-dimensional photonic crystals.

“There is loss of some intensity to the end user because 2-D photonic crystals don’t confine the light completely. For example, in a phone conversation, the voices would dim out. But with 3-D photonic crystal add-drop filters, the communication would be clear,” Biswas said.

While the researchers have shown that 3-D photonic crystals would make highly efficient add-drop filters, there are still problems to address. Getting the size of the photonic crystals down to work at the wavelengths used for Internet communications - 1.5 microns - is the big challenge. The Ames Lab group now has some of these photonic crystals working in that range, but to make these controlled structures with one input, another output and a defect will take some work.

A future direction is to simplify the design of the add-drop filter by reducing the layers in the photonic crystal - perhaps having all the action happen in one layer, the researchers said. 

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