Tunable photonic devices based on liquid crystals and composites

Tunable photonic switches and filters employing liquid crystals (LC) or LC-composites can be used in several fields of application such as optical communications, sensors and imaging systems. Their excellent electro-optic, thermo-optic and nonlinear optical responses can be exploited for producing components in guided-wave microstructures operating at low optical and electric powers. This review deals with various integrated optics structures, some of them already experimentally demonstrated for optical processing, including routers, Bragg filters and all-optical switches. A compact (160. m long) two-way router in a nematic liquid crystal (NLC) waveguide was designed and demonstrated operating in the near infrared with voltage modulation as low as 0.21 V. Wavelength-tunable voltage-controlled Bragg reflectors were analyzed in different geometries: one has a reflectivity above 80% in a 14 nm range (1530. 1550 nm) with bias voltages from 2.5 to 3.0 V; another one exploits coplanar comb electrodes to achieve an extended tuning range of about 104 nm (1521. 1625 nm) with reflection above 50% for voltages from 2.9 to 10.2 V. Tunable gratings made with microslices of polymers and NLC on glass waveguides were also characterized in the 1.55 mu m spectral window, demonstrating electro-optic filters adjustable over 4 nm for bias fields of about 3 V/ mu m. An alloptically tunable filter was also demonstrated in dye-doped NLC with tuning range over 6.6 nm when illuminated with a green laser beam of a few mW.