| Summary: | 博士 === 國立中山大學 === 光電工程學系研究所 === 103 === Photonic crystal fiber (PCF) is a new class of optical fiber whose cladding region comprises the periodic air holes, photonic crystal, along its length. The refractive index of the core of the solid-core PCF is larger than that of PCF cladding. It guides light by modified total internal reflection (mTIR). The specially optical characteristic is that it can confine light using photonic bandgap (PBG) effect when the effective index of cladding exceeds that of core. In this dissertation, we filled high tunability liquid crystal (LC) into the cladding holes of solid-core PCF which is called photonic liquid crystal fiber (PLCF) to transform its guided method from mTIR to PBG. We modulated LC in PCF, changed its refractive indices, and varied the forbidden photonic band. At first, we studied the basic transmission property of PLCF under the influence of photo and electrical fields. Besides, we simulated the transmittance frequency of PLCF by finite-difference frequency-domain (FDFD) method. Then, we manipulated the blue phase (BP) LC with large thermal hysteresis and LC gel to achieve the bi- and multi- stable PLCFs.
Firstly, we studied photo alignment and electrical tuning effects in PLCF. The short and long transmission edges shift about 45 nm and 74 nm toward longer wavelengths, respectively, when the voltages of 0~130 V and 250~400 V are applied on the sample. Appling voltage of 140~240 V, the range of tunability of the notch filter is around 180 nm in the PLCF without the use of gratings. A permanently tilted LC structure in PCF reduces the threshold voltage and can be further modulated by electric fields. We also presented the polarization dependent spectrum and response time of photo-aligned PLCF. The finite-difference frequency-domain method is adapted to analyze the shift of the transmission band and the numerical simulation results are agrees with the experimental data.
Next, we presented a bistable valve in a PLCF using the thermal hysteresis effect of the phase transition between the cholesteric phase and the BP. Both cholesteric and BPs can stably exist at room temperature (RT) and can also be switched to each other using temperature-control processes. The transmission spectrum and intensity can be controlled with various extents of scattering loss. The loss is due to various scattering attenuations in different phases. For application of optical communications, this device can be manipulated over a loss difference of 10 dB at RT and insensitive to the polarization of light.
Finally, we demonstrated a multi-stable variable optical attenuator based on a LC gel infiltrated PCF. By means of varying the cooling rate or biasing during gelation, different extents of scattering can be obtained. Hence, various transmittances of LC gel-filled PCFs also can be realized. At a wavelength of 1550 nm, an attenuation rate of –33.4 dB/cm is achieved at a cooling rate of 30 °C/min and a voltage of 400 V during gelation. The proposed all-in-fiber variable optical attenuator possesses a tunable attenuation rate and multiple stable states at RT.
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