A star coupler is a device that takes in an input signal and splits it into several output signals.
In fiber optics, and especially in telecommunications, a star coupler is a passive optical device, used in network applications. An optical signal introduced into any input port is distributed to all output ports. Because of the way a passive star coupler is constructed, the number of ports is usually a power of 2; i.e., two input ports and two output ports (a "two-port" coupler, customarily called a directional coupler, or splitter ); four input ports and four output ports (a "four-port" coupler); eight input ports and eight output ports (an "eight-port" coupler), etc.
A star coupler was also the name of a device sold by Digital Equipment Corporation (now part of Hewlett-Packard) of Maynard, Massachusetts. In this case, the star coupler interconnected links to computers via coaxial cable rather than optical fibres, but the function was essentially the same. The signal that was distributed was 70 Mb/s computer interconnect (CI) data and the star coupler provided two redundant paths of either 8 or 16 ports each. Digital's star coupler was developed for use with the VAX- and later Alpha-based computers running Digital's VMS operating system, to provide a passive, highly reliable interconnect for Digital's cluster technology.