The devices which are used to distribute light signal from one to many and from many to one optical fiber are called couplers. A few of the types of the couplers are:
1. Biconically Tapered Directional Couplers: In this type, the multimode fibers are made bare by removing the sheath and then these fibers are placed side by side and twisted together. Then the twisted portion of the fiber is heated gently and stretched. If the signal is passing through the first fiber then it will now start passing through the second fiber. Thus the coupler now becomes one input having two outputs.
2. Beam Splitting Couplers:
In this type, light incident from one fiber on a beam splitter inclined at an angle of 45 degree. The beam splitter as the name suggests will split the light ray into two parts, one is reflected part and another is transmitted part. The reflected and transmitted light will incident on two fibers placed at reflected and transmitted sides respectively. Thus this type of coupler will have one input and two outputs.
It can be understood by simple method also. You all know that what a prism does. Suppose a fiber has white light and this light will incident on prism and prism will split the white light into seven parts called VIBGYOR. In front of each part placed a fiber, so the prism will have one input and seven outputs. If we placed the prism oppositely, the prism will convert the VIBGYOR into white light. Thus it will behave as seven inputs and one output.
By same method, beam splitting couplers act.
Note: Try to make the diagram yourself