Liquid crystal gel-filled photonic crystal fiber based multi-stable variable optical attenuator
Chih-Wei Wu1*, Chun-Hong Lee1, Chun-Wei Chen1, Hung-Chang Jau1, Tsung-Hsien Lin1
1Department of Photonics, National Sun Yat-sen University, Kaohsiung, Taiwan
* presenting author:吳致維,
Photonic crystal fibers (PCFs) have a core that is surrounded by cladding with air holes periodically located along its length. The light propogation in PCFs is by modified total internal reflection; however, when filling some of the air holes with liquid crystals, a preliminary index-guiding PCF can be converted into a bandgap-guiding. A liquid-crystalline physical gel typically is composed of mesogenic molecules and a fibrous polymer network of gelators. Owing to its reversible polymerization, the devices based on gelators could achieve tunable and stable applications.In this work, we present a multi-stable variable optical attenuator (VOA) using a liquid-crystalline physical gel-filled PCF.
The mixture used was formulated by doping 3 wt. % gelator additive–12-hydrox ystearic acid into a nematic LC, E44. The clear point of ∼75°C is higher than the solgel transition temperature of ∼55°C to form fibrous networks of gelators that were aligned along the long axis of the rod-like LCs. The temperature for gel formation was reduced from 85°C to 25°C at a specific cooling rate. To manufacture an all-in-fiber device, the LC infusion methods were combined with the fiber splicing technique. The transmission power can be controlled by varying the cooling rate and applying the bias. The cooling rate of 1, 5, 10, and 30°C/min was presented. To increase the attenuation rate, a voltage of 0, 100, 200, or 400 V was applied during the polymerization of the LC gel.
The attenuation rate could be modulated by varying the cooling rate or applying an electric field during the self-assembly of the gelators, and every state was stabilized by polymer stabilization. When the PLCF was cooled at 30°C/min, attenuation was approximately −10.5 dB/cm. At an applied bias of 400 V, the attenuation rate increased to −33.4 dB/cm. Based on these findings, then LC gel-infiltrated PCF can be used as an all-in-fiber, multi-stable, and tunable VOA that has potential application in optical communications.

Keywords: Fiber optics, Photonic crystal fibers, Liquid-crystal devices