They may be simple splitters or 2 x 2 couplers, or up to 64 X 64 ports star couplers. Most are made by fusing fibers under high temperatures while monitoring light transmission through the fibers to get the proper coupling ratios. The fusing of the fibers causes light to split or combine in appropriate ratios. Others have been fabricated by diffusing light paths into a glass substrate and attaching fibers to it. Relevant specifications for couplers are the coupling ratios of each port or the consistency across all the ports, crosstalk and the excess loss caused by the fusing.
Excess loss is the difference between the sum of all the outputs and the sum of all the inputs.
When used in laser based systems, couplers may need testing for optical return loss and wavelength dependence also. Thus, testing couplers involves coupling a test source to each input port in turn and measuring all the outputs for consistency, then summing all the output powers and subtracting that number from the input power to calculate excess loss. Connectorized couplers are tested like connectorized cables, using a launch cable, while couplers with bare fibers must use a cutback method or a pigtail and temporary splice to couple the launch source. Most couplers are optimized at one wavelength unless they are specially designed for both 1300 and 1550 nm operation. Some are even built to be wavelength division multiplexers, coupling light from 1300 and 1550 nm lasers into separate output ports.