Optical couplers are passive devices that couple light through waveguides or fibers. They play a very important role in the applications of photonic devices and systems. Optical couplers are used in many different ways. They can be the interface between devices in a system or can be important devices themselves. The most straightforward, yet important, application is to route optical waves around for coupling different devices. Sophisticated applications include devices such as polarization converters, mode converters, guided-wave beam splitters, beam combiners, directional couplers, branch couplers, wavelength filters, wavelength multiplexers, and so on. In this chapter, we discuss the waveguide couplers based on mode coupling. Input and output couplers, which couple light between free space and waveguides, are also discussed. Coupling due to active modulation, such as electro-optic switches, and coupling characteristics specific to a particular device are discussed in later chapters.

Grating waveguide couplers

Grating waveguide couplers have many useful applications and are one of the most important kinds of waveguide couplers. They consist of periodic fine structures that form gratings in waveguides. The grating in a waveguide can be either periodic index modulation or periodic structural corrugation. Periodic index modulation can be permanently written in a waveguide by periodically modulating the doping concentration in thewaveguide medium, for example, or it can be created by an electro-optic, acoustooptic, or nonlinear optical effect. In the latter case, the grating can be time dependent if the modulation is time varying. It can also be a moving grating if the modulation signal is a traveling wave. In the case of periodic structural corrugation, the corrugation is a permanent structure of a waveguide.