The need for fiber optic couplers
We use electronic couplers all the time: such as a telephone coupler which lets you connect both a telephone and a fax machine to the same telephone line. Or a CATV coupler which lets you connect several TV sets to a single cable from Comcast. Basically you can buy these couplers from Home Depot or other electronic retailers.
Optical couplers have the same functionality as electronic couplers: They split the signal to multiple points(devices). Fiber optic couplers are needed for tapping(monitoring the signal quality) or more complex telecommunication systems which require more than simple point-to-point connections, such as ring architectures, bus architectures and star architectures.
:: Passive couplers and active couplers
Fiber optic couplers can be either active or passive devices. The difference between active and passive couplers is that a passive coupler redistributes the optical signal without optical-to-electrical conversion. Active couplers are electronic devices that split or combine the signal electrically and use fiber optic detectors and sources for input and output.
:: The difference between electronic couplers and fiber optic couplers
Electronic couplers are easy to make because electric current flows as long as you have physical contact between conductors. But optical signal is in a completely different domain. You have to align the tiny optical fiber cores precisely (9um for single mode and 50um or 62.5um for multimode fibers), so there won’t be a huge power loss when you split the signal.