Directional coupler

(electronics) A device that couples a secondary system only to a wave traveling in a particular direction in a primary transmission system, while completely ignoring a wave traveling in the opposite direction. Also known as directive feed.

A four-port waveguide device (see illustration) in which an incoming wave at any one port (for example, A) appears at two others (C and D) but not at the fourth (B). This device finds numerous applications in waveguide networks such as microwave waveguides, integrated optics, and optical fibers. It is the equivalent of the hybrid induction coil used in conventional wire circuit telephony to provide side-tone balance.

Four-port directional waveguide coupler. P_A is the input power, and P_B, P_C, and P_D are the powers of the output waves at ports B, C, and D respectively.

Directional couplers are usually made by introducing some form of lumped or continuous coupling among several guides so that the coupled waves interfere constructively in some directions while canceling one another in other directions. A simple two-hole coupler consists of two identical microwave waveguides running adjacent to each other with two small holes of the same size and shape coupling between them. It is limited by a narrow-frequency band over which the directivity is high, and a small coupling factor which is on the order of 10–30 dB. To alleviate these problems, multipath couplers are used, where the backward cancellation is obtained through destructive interference among waves excited by three or more coupling elements (such as holes), usually spaced a quarter-wavelength apart. The coupling holes are not necessarily equal. A natural extension to the multipath coupler is the slot coupler, where the coupling element is a continuous slot that is several wavelengths long and often tapered, and the coupling is continuously distributed along the length of the interaction.

Holes and slots are appropriate coupling elements for closed metallic waveguides, where the propagating field is completely enclosed within the walls of the guide. Other design alternatives exist for open waveguides such as microwave striplines, diffused dielectric optical guides used in integrated optics, and optical fibers. In these guides, the propagating field extends beyond the physical dimensions of the guiding structure (for example, the metallic plates of the stripline or the core of the fiber), and proximity coupling becomes possible. For example, the electromagnetic field of a single-mode optical fiber has an exponentially decreasing, evanescent, nonradiating tail in the first few micrometers of the cladding. Fusion, etching, and mechanical polishing techniques may be used to bring two fiber cores close enough to each other to make possible the transfer of power from one core to the other through the interaction of the corresponding evanescent fields.

Weak directional couplers can selectively monitor either the forward or backward power flow in waveguides, and medium-to-strong couplers serve as amplitude combiners and splitters in various resonators and sensors. All of these applications require high directivity, low insertion loss, and negligible back reflections. Many other useful junctions are also based on the same physical principles. See also Integrated optics; Microwave measurements; Optical communications; Optical fibers; Waveguide.