Fiber optic couplers either split optical signals into multiple paths or combine multiple signals on one path. Optical signals are more complex than electrical signals, making optical couplers trickier to design than their electrical counterparts. Like electrical currents, a flow of signal carriers, in this case photons, comprise the optical signal. However, an optical signal does not flow through the receiver to the ground. Rather, at the receiver, a detector absorbs the signal flow. Multiple receivers, connected in a series, would receive no signal past the first receiver which would absorb the entire signal. Thus, multiple parallel optical output ports must divide the signal between the ports, reducing its magnitude. The number of input and output ports, expressed as an N x M configuration, characterizes a coupler. The letter N represents the number of input fibers, and M represents the number of output fibers. Fused couplers can be made in any configuration, but they commonly use multiples of two (2 x 2, 4 x 4, 8 x 8, etc.).

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.

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