Long-period liquid-crystal fiber gratings are electronically controllable

Paula Noaker Powell

Researchers at Seoul National University (Seoul, Korea) and Kwangju Institute of Science and Technology (Kwangju, Korea) have demonstrated an electrically controllable liquid-crystal (LC) fiber grating for broadband rejection. Internal control is accomplished by periodically poling a liquid-crystalline core in a hollow-core fiber using an external long-period combed electrode.

According to Yoonchan Jeong and colleagues, long-period LC fiber gratings are possible because the periodically poled LC with a comb-structured electrode serves as an optical grating in the core region for the propagating fiber modes. In other words, the LC produces an index modulation in the core region since its effective index changes based on the direction of poling. This allows the poled grating to function like a fiber grating.

In the Korean researchers` experiments, the liquid-crystalline core in a period of 483 µm coupled the fundamental core mode to the leaky cladding modes. A maximum transmission loss dip of approximately 15-nm bandwidth and 6-dB band rejection was obtained for a nonpolarized light with a combed 250-V external voltage modulation. These experimental results matched well with a numerical expectation, which the scientists analyzed by the discretized coupled-mode theory for an anisotropic perturbation.

Because this was the first demonstration of a long-period LC fiber grating, the scientists plan further study in the areas of the polarization-dependent property due to the material anisotropy, as well as its dynamic property. For instance, they believe a different hollow-core fiber, perhaps one with no silica core, will be required to vary the controllability of the grating. In addition, further work is necessary to simplify the packaging of the LC-core fiber. For more details, contact Byoungho Lee at byoungho@plaza.shu.ac.kr.

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