Characterization of polymer dispersed liquid crystal for photonic device applications

Abstract

Applying liquid crystal (LC) based materials as promising electro-optic material in the field of optical communication is becoming the focusing point of many researchers. The fabrication of polymer based planar waveguide devices have great potential for achieving low power consumption compared with conventional silica-based devices. By combining both polymer and liquid crystal/polymer based material more power can be saved by using electro-optic effect instead of thermo-optic effect and this will be considered as more green technology for environment as well. In this research work we characterize and study the electro-optic (EO) properties of nematic liquid crystals (LC) and polymer dispersed liquid crystal (PDLC) for applying as waveguide material. In our study, we observe the non uniform response of nematic LC by applying amplitude modulated (AM) driving signal appears at low modulating signal frequency. A uniform EO response is achieved by adding a holding voltage to the driving waveform or changing the duty cycle %, but in practice controlling these complex driving waveform parameters is undesirable as it is time consuming. Consequently, this leads us to use PDLC as an alternative LC-based electrically tunable optical material. Basically, PDLC is prepared by mixing LC with polymer material at a certain proportion. Several PDLC mixtures with different mixing ratios were prepared and studied, and the PDLC mixture with higher LC content (70 wt%) shows better EO properties as relatively faster response and lower driving voltage compared to other PDLC mixtures. Compared to LC material, PDLC has advantages as; no alignment layer is needed, has polarization independent property, and it exists in solid phase. Besides, PDLC showed more stable and uniform EO response compared to LC even with simple driving waveform. These advantages of PDLC material offers a better solution than LC for applying to conventional polymer planar waveguide devices. As solid material it will add good mechanical property to polymer-based waveguide and will offer an easier fabrication process and lower cost tunable optical waveguide. In order to demonstrate the applicability of PDLC to conventional planar waveguide application, PDLC mixture with optimized EO properties was selected to construct a variable optical attenuator (VOA) by applying the PDLC as an active upper cladding material on the top of the fabricated waveguide. This PDLC-based waveguide device offers lower polarization dependence and higher dynamic range compared to the counterpart LC-based waveguide. Furthermore, aligned and non-aligned PDLC were applied as waveguide upper cladding and the EO performance of such VOA devices is studied. The waveguide with aligned PDLC upper cladding has lower operating voltage compared to that with non-aligned PDLC. The dynamic range at 10V applied voltage is 32.3dB (TM-mode) and 27.4dB (TE-mode), and 39dB (TM-mode) and 41.37dB (TE-mode) for the waveguides with aligned and non-aligned PDLC upper claddings, respectively. However, both waveguides show higher dynamic range than that achieved by LC based VOA (18 dB for TM-mode and 0.586 dB for TE-modes). In summary, the potential of this work is that we applied PDLC as waveguide material to a conventional polymer planar waveguide structure and compared its EO performance with LC-based waveguide. The PDLC-based waveguides show better EO performance as lower polarization dependence, higher dynamic and attenuation ranges compared to LC-based counterparts. But on the other hand they require little higher operating voltage. Based on this work, the fabrication of other PDLC-based waveguide devices with different geometry and structure can be easily implemented with this kind of material using similar fabrication steps.

Date of Award	16 Jul 2012
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Hau Ping Andy CHAN (Supervisor)