Model Identifies Limits on Filters

Daniel Woolls

Fiber optic cables carrying multiple channels of light rely on a number of precision devices, including tunable narrow-bandwidth filters. But the process of manufacturing such filters is tricky and prone to defects. Now researchers have developed a model that simulates these glitches and pinpoints their cause.

Researchers have developed a model that simulates defects in tunable narrow-bandwidth filters. The uniformity of lithium-niobate crystal limits to 0.6 nm the linewidth of a spectral filter operating around 1550 nm.

A team led by Franck Chollet at the Institut des Microtechniques, part of the Université de Franche-Comté in Besançon, France, focused on electro-optic filters in lithium-niobate crystals, but the technique can be applied to any instrument based on co-directional mode coupling.

An integrated electro-optic filter works by converting light from one polarization to another for one wavelength, leaving all other wavelengths in their initial polarization. Chollet said that most research on the properties of such filters has centered on the coupling coefficient, which quantifies the distance that is needed to convert light from one polarization to another. But this seems to underestimate the influence of defects in phase birefringence, the difference in the speeds at which two polarized light beams propagate inside the filter's lithium-niobate crystal. The difference is critical because it keeps the two waves from blending.

The defects are so tiny that they are hard to measure directly, but together they affect a filter's transmittance, Chollet said. They fall into three classes: random deviations, symmetrical deviations with the symmetry center in the middle of the filter, and monotone deviations.

Chollet said that the cause of the last two, which are the most significant, seems to lie in the manufacturing process, probably in the deposition and patterning of the titanium that is used to assemble the filters. The uniformity of the lithium-niobate crystal is the ultimate factor, the team found, limiting to 0.6 nm the linewidth of a spectral filter operating around 1550 nm.

Chollet currently works as an assistant professor at Nanyang Technological University in Singapore under a cooperative agreement with the Institut des Microtechniques. Jean-Pierre Goedgebuer and Gadang Ramantoko were the other researchers on the project.