An optical coupler is a passive (unpowered) device that diverges (1:N) or converges (N:1) optical signals from one fiber or optical signal path to more than one (or vice versa.)

Configurations

Splitters

Typically, in a 1:N optical coupler or 'splitter', the entire aggregated multiwavelength optical signal from one fiber is redirected to 2 or more output fibers or signal paths. The splitting ratio is usually evenly distributed, meaning each of N output fibers receives 1/N of the initial signal power. So, a 1:2 optical splitter sends the same incident signal to 2 destinations, each receiving 1/2 the incident power, or a 3 dB nominal power penalty.

An exception to this rule is a 1:2 device, often used in testing or monitoring, that splits off a small fraction of the incident optical power and leaves the majority of the signal intact to continue propagating. This is often called a 'tap'.

Combiners

Similarly, a N:1 optical combiner takes the input signals from N incident optical signals and redirects them to be coincident at the output signal. Generally speaking, the signals to be combined are usually orthogonal (here, non-overlapping) with respect to wavelength, so that the signals being combined do not interfere with each other. In some special cases, however, the two (or more) signals being combined may be at the same wavelength, and the output will be a superposition of the electromagnetic modes presented (plus any nonlinear effects that might ensue.) This is useful in interferometric switching, for example, or for some types of time-division multiplexing.

Directional Couplers

Another type of coupler is the directional coupler. In this configuration, a 1:2 coupler (for example) is constructed to connect one input port and one output port to a bidirectional fiber channel. The key to this approach is to maximize the isolation between the outgoing and incoming signals, which essentially means that the amount of power in the 'forward' direction is maximized, while that in the 'reverse' direction is minimized.

General Coupler Characteristics

Many couplers take the form of fibers that are fused together. This has the advantage of providing identical input and output waveguide characteristics. The actual zone of interaction, however, introduces discontinuities that may affect propagation, and hence must be carefully monitored. Each coupler will have excess insertion loss, and potential modal distortion, dispersion, polarization-affecting or other bandwidth limiting effects.

Couplers may also be fabricated using 'printed-circuit' techniques, in which optical waveguides are patterned onto a substrate much like a circuit etch trace. These etch traces propagate the optical signal much as an optical fiber would, and couplers can be implemented as simple geometrical 'wye' configurations.[PIRI, ...]

Specific Coupler Types and their Characteristics