Polymer May Offer Switching Solutions

Perry J. Greenbaum

An organic polymer and liquid crystal waveguide device under development by a team of researchers in Italy and Sweden promises applications in integrated optical circuits and in all-optical switches for telecommunications. If proved commercially feasible, it would complement inorganic materials such as LiNbO₃ or GaAs now used in integrated optics.

The device combines an isotropic polymer waveguide made by photochemical crosslinking with a ferroelectric liquid crystal in an integrated electro-optical modulator. The three-stage waveguide is a sandwich of sorts, with passive core polymers

surrounding a ferroelectric liquid crystal active-core waveguide. Light is coupled into and out of the passive polymer waveguide stages using high-index prisms.

An organic waveguide would offer network operators a cost-effective means to produce compact, complex switches for applications where speed is not paramount. Nevertheless, ferroelectric liquid crystals feature switching times in the microsecond range. Of greater importance, a polymer-based waveguide would enable operators to chemically tune optical parameters such as refractive index and optical loss.

Thus far, the device shows potential in the latter application. "Compared to other polymers, our polymer waveguides have low optical losses and good thermal properties," said David S. Hermann, a member of the research team from Chalmers University of Technology in Göteborg, Sweden.

Boosting efficiency

With regard to transmission efficiency, the device will require optimization. In recent experiments, its output was approximately 1 µW from an input of a few milliwatts, which the researchers attribute to a number of factors.

For example, the refractive index of the liquid crystal film is approximately 1.63 and that of the polymer is 1.58, yielding an index discontinuity of 0.05.

Moreover, the thickness of the film was greater than that of the core polymer, resulting in mode-coupling losses. Edge roughness at the intersection between the liquid crystal and the polymer compounded the losses.

Hermann said the team is working to improve the performance of the device.