i. The concept of a coupler encompasses a variety of functions, including splitting a light signal into two or more streams, combining two or more light streams, tapping off a small portion of optical power for monitoring purposes, or transferring a selective range of optical power from one fiber to another.

ii. Fiber optic couplers can be either active or passive devices.

iii. The difference between active and passive couplers is that a passive coupler redistributes the optical signal without optical-to-electrical conversion.

iv. Active couplers are electronic devices that split or combine the signal electrically and use fiber optic detectors and sources for input and output.

Couplers could be classified as follows:

Basic 2* 2 coupler:

The 2* 2 coupler is a simple fundamental device that we will use here to demonstrate the operational principles of optical couplers. These are known as directional couplers.

A common construction is the fused-fiber coupler illustrated in Fig4.1. This is fabricated by twisting together, melting, and pulling two single-mode fibers so they get fused together over a uniform section of length W.

Each input and output fiber has a long tapered section of length L, since the transverse dimensions arereduced gradually down to that of the coupling region when the fibers are pulled during the fusionprocess. This device is known as a fused biconical tapered coupler.

As shown in Fig. ,

• P0 is the input power on the top fiber (which we will take as the primary fiber in a link),

• P1 is the throughput power, and P2 is the power coupled into the second fiber.

• Parameters P3 and P4 are extremely low optical signal levels (-50 to-70 dB or, equivalently,

factors of 10-5 to 10-7 below the input power level).

These result from backward reflections and scattering due to packaging effects and bending in thedevice.

The figure below shows that the power distributions of any given mode are not confined completely to the fiber core, but instead extend partially into the cladding

Therefore if two fiber cores are brought close together, the tail of the power distribution in one fiber will extend into the adjacent fiber core. Consequently, some of the optical power will transfer to the adjacent fiber through evanescent coupling.

The amount of optical power coupled from one fiber to another can be varied by changing the coupling length W or the distance between the two fiber cores.