Guided wave optical switches using liquid crystals for high capacity communication systems

Wavelength division multiplexing (WDM) optical networks, able to operate at Tb/s data speed, to satisfy the continuous growth of the world wide web, require signal format transparent all-optical switches, routers, filters, add and drop multi/demultiplexers. A successful technology to make such devices must allow to get high performance and large integration scale at low fabrication costs. A new class of materials to make photonic devices are liquid crystals (LC), mostly developed so far for flat panel display applications. The novel interest for LC photonic devices is mainly due to their high birefringence (0.1-0.2) and high electro-optic effect, which allows low driving voltages with relatively short switching times in the range of microseconds, as in the case of ferroelectric LC (FLC). Furthermore LC are characterized by refractive indices close to that of glass and silica used to make optical fibers and optical waveguides, therefore low LC-waveguide coupling losses can be expected. By presenting a couple of device examples we intend to demonstrate that liquid crystals can be effectively used to create a novel generation of low cost and high performing integrated optical devices. Glass waveguides and LC can be combined to make integrated optic circuits to be employed both in sensor and telecommunication applications. The fabrication of integrated optic devices using LC and optical glass waveguides is not too much different from the well-established flat panel display fabrication process. Furthermore optical transparency of glass substrates allows to observe liquid crystal orientation. We experimentally demonstrated an optical bistable switch using FLC and ion-exchanged glass optical waveguides. The switch, based on a vertical directional coupler made of an FLC layer embedded between two optical waveguides, showed losses less than 5 dB, short switching time of 270 µm and required low driving voltages of about 20 V to obtain extinction ratios over 15 dB, with a coupling length shorter than 4mm at wavelength of 632.8 nm [1]. A novel design by using diffused channel waveguides obtained by ion exchange operating at the wavelength of 1550 nm will be presented in which coplanar electrodes are used to switch liquid crystals to reduce losses due to light absorption. The optimized device configuration is characterized by a coupling length as short as 140 µm, losses below 0.5 dB and crosstalk, defined as ratio between optical power at the output ports, above 40 dB. Low power and ultrashort optical switches can be obtained also by using a simple nematic liquid crystal layer as a voltage assisted optical waveguide switch. An applied voltage lower than just 1.5 V can provide lateral confinement of the signal, whereas its increase of about 119 mV can switch the light output between two different positions in a device length of just 230 µm. The investigated geometry allows the realization of a versatile and fully electro-optic Y-junction switch, which can be optimized for operation at any wavelength of interest by acting on the bias [2]. References [1] A. d’Alessandro, R. Asquini, F. Menichella, C. Ciminelli, “Realisation and characterisation of a ferroelectric liquid crystal bistable optical switch”, Mol. Cryst. Liq. Cryst., vol 372, pp. 353-363, (2001). [2] R. Asquini, A. d’Alessandro, G. Assanto, “Electro-optic guided-wave router using nematic liquid crystals”, Europ. Conf. on Integrated Optics ECIO 2003, vol. 1, pp. 249-52 (2003).