Integrated optics involves the use of thin transparent dielectric layers on planar substrates as optical waveguides. Various methods can be used to couple light into and out of the guiding layers, including butt and transverse coupling, prisms and diffraction gratings. Optical components are defined by photolithography, and may be passive (such as strip waveguides with bends, junctions and directional couplers) or active (such as switches, modulators and beam deflectors). The substrate and film materials may be glass for passive circuits, or electro-optic or semiconductor single crystal for active devices. Hybrid integration of active and passive devices may be used to produce complex integrated circuits. Alternatively, the use of semiconductor materials offers the possibility of monolithic integration of all types of component on a single substrate.