An optical device for dividing optical power into two or more is called as an optical divider. A coupling device, which combines two or more optical powers into one, is called as an optical coupler. The optical divider is typically used as an optical coupler, when light is introduced into the device from the rear side. Recently, an optical divider is also called as an optical coupler.

An optical coupler is categorized by its structure into a bulk-type, waveguide-type, and fiber-type (optical fiber coupler). Figure summarizes the schematic of these three types of optical coupler.



Fig. Sorts of optical couplers. (a) Spatial type, (b) waveguiding type, (c) fiber type.



A bulk-type optical coupler is composed of small optical devices such as a half-mirror, beam-splitter, and filter. The division ratio can be changed by tuning the reflectivity at a plane of dielectric multilayer. This is advantageous in producing a wideband coupler. The bulk-type optical coupler has a larger insertion and is more expensive to produce loss than a fiber-type.



A fiber-type optical coupler is manufactured by melting and extending a number of optical fibers, and is called as an optical fiber coupler. When a couple of optical fibers come close at the core, a mode coupling occurs, then, an optical power is divided into two, one of which is trasmitted to the other. By controlling the mode coupling length, an optical coupler with an expected division rate at specific wavelength can be produced, while a wideband coupler, and WDM coupler with a narrow wavelength interval  is difficult to produce. Compared with the bulk-type optical coupler, the insertion loss and manufacturing cost are higher. The fiber-type optical coupler is used not only for the optical fiber communication, but also for a light source, monitoring optical fiber, exciting a fiber laser, an optical fiber amplifier, and a wavelength multiplexing system. The optical coupler with the division ratio of 1:1 is, in particular, called as the 3 dB coupler, because it reduces the optical power into a half (-3 dB).



A waveguide-type optical coupler manufactured by forming an optical circuit on a quartz substrate. Because a many-to-many optical coupler (waveguide-type coupler) can be collectively manufactured by the photolithographic technology, the waveguide-type coupler is advantageous in a mass-production and production cost.