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Holey fibers get liquid crystal boost

29 Oct 2003

Photonic crystal fibers filled with liquid crystals create a high-performance optical switch.

Liquid-crystal-filled photonic crystal fibers (LC-PCF) that guide light by the photonic bandgap effect make ideal all-optical signal processing devices, according to a team from Denmark and Sweden. The team has made a tunable switch with an extinction ratio of 60 dB and an insertion loss of only 1 dB. (Optics Express 11 2589)

Photonic crystal fiber (PCF), a fiber with a pattern of holes running along its length, is renowned for its exotic light-guiding properties. Liquid crystals (LCs) are organic materials whose phase (molecular configuration) and optical properties are very sensitive to temperature.

Thomas Tanggaard Larsen and colleagues’ work exploits these phase changes to control the wavelength of the PCF’s photonic bandgap.

The researchers filled a 20-mm long section of PCF with a cholestric LC and heated it from 77 °C to 94 °C using a hotplate. As the temperature increased, Larsen and colleagues saw that the color of the guided mode changed from green to yellow, then into an off-state and then blue.

“The off-state is attributed to the highly scattering behaviour of the LC at the phase transition, where the molecules tend to dis-align from their cholestric phase,” say the authors in their paper.

Larsen and co-workers also filled a 10-mm long section of PCF with a liquid crystal that has a phase transition around 26 °C. To heat the fiber, the researchers coated it with a thin conducting layer. This layer essentially acted as a resistive microheater when a voltage was applied across it.

Coupling a 974 nm laser to the LC-PCF, the researchers found that the LC-PCF worked as a tunable switch with an extinction ratio of 60 dB over a temperature range of only 0.4 °C. According to Larsen, the team has just produced a device with an extinction ratio of 80 dB.

“Our plans are to explore the behaviour of other liquid crystal phases, increase response times and work on ways of confining the liquid crystal to a specific section of the PCF,” said Larsen. “These devices will hopefully find applications within tuneable PCF devices for example to tune dispersion in nonlinear processes. We can hopefully find more application when we obtain better control of the liquid crystal.”

The research team was made up of scientists from the Technical University of Denmark, Danish firm Crystal Fibre and the Chalmers University of Technology in Sweden.

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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