LIQUID CRYSTAL BASED INTEGRATED OPTIC COMPONENTS AND DEVICES

Abstract

Integrated optical devices contain one or more optical components that are combined to fulfill more than one functions on a single substrate, such as splitter, optical filters, and optical switches. Once the device is fabricated, it is difficult to tune or reconfigure the functionality in a wide wavelength range. On the other hand, the key requirements of the next-generation integrated optical devices are that the devices should be dynamically reconfigurable, inexpensive, and power-efficient. These flexibilities in integrated optic devices can be achieved in a very small dynamic range using thermo-optic/ electro-optic material. The characteristics of liquid crystals (LCs) such as high birefringence, large electro-optic effect, and low driving power make them a promising material for realization of electrically tunable integrated optic devices for wide dynamic range. This motivated us to work towards the realization of the electrically tunable integrated optic devices using LC material in this thesis. Waveguide is the basic building block of integrated optic devices, and dynamically reconfigurable/tunable waveguide can be achieved simply by applying electric field in LC waveguide using LC as a core or cladding medium. If the refractive index difference between the core and the cladding layer is small or the core layers is very thin then considerable fraction of the light is carried in the cladding region also then propagation characteristic of waveguide can also be controlled using LC as a cladding material. In this thesis, we have chosen the rib waveguide configuration for the realization of electrically controlled LC-based devices because of the flexibility in the waveguide parameters, easy fabrication process, and easier implementation of the grating due to the presence of slab modes. We demonstrate electrically re-configurable wavelength filters using a long period waveguide grating (LPWG) in a rib waveguide. The rib structures have been fabricated by laser ablation method on various substrates such as BK7 glass, and LiNbO3 substrates. The corrugated LPWG structure has been inscribed on the air clad rib waveguide for realizing temperature-insensitive wavelength filter. Generally, the LPWG based filters work at a fixed central wavelength, and tuning is achieved by varying the refractive index of core/cladding material by the thermal adjustment in the narrow band of wavelength. To electrically tune the filter response, we have used nematic LC as a cladding layer on the LPWG structure in a convention LC cell mode. We have studied and numerically simulated the realization of electrically tunable optical filter over a wide range of wavelengths on the variety of substrates such as ion exchange BK7 glass, proton exchanged LiNbO3, silicon-on-insulator, and silicon nitride substrates. These electrically re-configurable wavelength filters cover most of the S, C, and L band of an optical communication system. We have extended our study on LC core rib waveguide on the BK7 glass for achieving the electrically controlled mode profile over a wide range of wavelengths. Single side anchored in-plane switching (IPS) mode has been proposed for the realization of electrically controlled low threshold voltage LC core waveguide on the fused silica substrate. We have experimentally demonstrated the transition from single mode to dual mode operation by externally applied voltage. In single mode regime the externally applied voltage has been used to tune the mode field diameter of the modes from 11.6 μm to 9.2 μm along the horizontal direction and 7.8 μm to 6.4 μm along the vertical direction. The effect of applied external electric field on index gradation and guiding behavior of the LC waveguide for coupling to/from various types of optical fibers has also been studied. Additionally, we have also studied an electrically controlled wavelength filter and mode convertor using corrugated LPWG structure in LC core rib waveguide on the glass substrate. We have further extended our study for the possibility of tunable large mode area LC rib waveguide for the development of compact and efficient high power integrated optic devices. Vertical anchoring of LC director in IPS mode in rib waveguide structure has been proposed for realization of large mode area single mode waveguide.