Design and fabrication of liquid crystal (LC) based electro-optical waveguide device

Abstract

Liquid crystal (LC) technology has a major impact on many areas of science and engineering. It is a promising material for planar lightwave circuit (PLC) devices in optical fiber communication systems due to its mature and inexpensive technology. It can provide low power consumption, and low drive voltage because of its attractive anisotropic optical properties and large electro-optic response. However, it is also very difficult to deposit the LC film and to pattern the film using typical photolithography for fabricating PLC based waveguide devices. Therefore, up to now, only a few number of LC based waveguide devices were proposed. They were fabricated on typical inorganic materials, rather than polymer ones. Moreover, the major shortcomings in the LC based integrated optics are excessive insertion loss and relatively high operating voltage. This project will target at the use of LC in planar optical waveguide devices to replace the typical thermo-optic effect; and to solve the issues in the fabrication of LC based device by novel design. There is therefore a strong need for doing research on the design and fabrication in order to evaluate the characteristics of the LC based electro-optic waveguide device. In this study, we apply LC in different ways to demonstrate integrated variable optical attenuators (VOA), which is widely used for monitoring and active controlling of optical channel power in modern high-speed optical wavelength division multiplexed (WDM) networks. LC is used to change the optical confinement of a waveguide core based on its electro-optic effect. We initially demonstrate a VOA adopting the LCD technologies and applying LC’s anisotropic property to fabricate a waveguide using LC as both core and cladding. A 20dB extinction ratio can be achieved by applying a 10Vpeak at 1550nm. We further improve the design by employing LC as an active cladding layer in order to minimize the propagation loss due to LCs’ optical absorption and scattering. We demonstrate the devices on polymer materials rather than on inorganic materials because polymers are very attractive for optical devices due to their advantages, such as, simple fabrication processes for a large variety of geometries, easy formation of multi-layer structure by suitable micro-fabrication technologies (for example, embossing or UV techniques) and low material cost for high volume fabrication using embossing or UV techniques. The single mode polymer inverted channel waveguide having a nematic LC upper cladding operates at a wavelength of 1550nm. The novel inverted structure is proposed here to provide more homogeneous LC molecular alignment, since the structure can eliminate the LC alignment defects at the edge of a normal rib structure. Besides, it protects the channel from damage during the preparation of the alignment layer by mechanical rubbing. The fabricated device exhibits a 24dB of attenuation range with a tuning range of 10 Vpeak at 1550nm. This device’s asymmetric attenuation characteristics of the two polarization states enable a 16dB dynamic range of polarization dependant loss compensation. Furthermore, we have simulated the device with different core sizes to reduce the controlling applied voltage. The theoretical results are also verified with experiments. The device’s response time is measured by applying the modulated signal. Finally, lithography technique is proposed in making the alignment layer to align with the LC that can potentially replace the traditional rubbing one for improved performance. This study can also be used as a guide in design and fabrication of other LC based waveguide devices.