We are studying Photonic Integrated Circuits.　　　▷Japanese

In the optical fiber communication system, are the people connected all optical ? The answer is no. In the exchange stage, optical data is transfered to electric data, and this electric data is exchanged by the electronic automatic exchange system. Then the electric data is transfered optical data again, and transmit through the optical fiber. Why these complicated scheme are used ? Because the optical switch for the key device in the optical exchange system is under development. In the all optical communication system of next generation, optical switch is absolutely necessary for the high speed, large capasity and low consumption exchange system. Lightwave have unique features such as "high speed", "parallelism" and "wavelength multiplexity", and these features will expect the new optical communication system.
Optical switch are classified into Machinery, MEMS, and Waveguide. In the machinery system, optical fiber connection is exchange by the electromagnetic actuator, or optical beam is exchanged by the prism or mirror. MEMS(Micro Electro Mechanical Systems) switch are attraced attention in these days. MEMS switch are fabricated by the micro machine technology, and optical path is exchanged by the optical mirror or schutter of micro size. Waveguide switch are fabricated by the Planar Lightwave Circuits (PLC) or semiconductor process, and the optical path is exchanged by the refractive index change through the thermo-optic effect, nonlinear effect or elctro-optic effect.
Each merits and demerits are shown the following.

	Merits	Demerits
Machinery	Low cost, Low insertion loss High extinction ratio	Large size, Low power consumption Large matrix size
MEMS	Si process Free space optics	Large optical path Alignment error
Waveguide	Small size Connection between fiber Large index change High speed	Insertion loss Extinction ratio Polarization dependence

Waveguide switch is studied various material system. Silica PLC switch or polymer switch are operated by the thermo-optic effect. Dielectric switch such as LiNbO3 or semiconductor switch are operated by the electro-optic effect.
Waveguide structure and operational principle are classified in the following.

	Directional Coupler	Mach-Zehnder	Digital	Intersectinal
Operational principle	Coupling waveguide	Phase change	Index guide	Total internal reflection
Structure