Characteristics study of multimode directional coupler by elliptical point contacts and CMT

The developed directional coupler using polymer optical fiber performance is studied analytically where related theories are integrated and the outcomes are analyzed. Important theories such as simplified coupled mode theory and elliptical point contacts are integrated where the parameters such as coupling length, distance between the two fibers cores and forces are varied. Using simplified coupled mode theory, coupling coefficient and coupling efficiency is obtained based on the parameters of multimode fiber coupler such as the operating wavelength, numerical apertures, coupling length and diameter of the cores. The two fibers are initially tapered at certain length at most 20 mm and attached to geometrical blocks with certain radii and the middle tapered regions of the fibers are brought closed in proximity and they are lapped to each other. Investigation of different radii of the geometrical blocks represents the effect of macro-bending when the fibers are bent when attached to the circular blocks. This concept is used to transfer the modes from first fiber to the second. Then a particular amount of load force is exerted upon one side of the blocks so that the gap is closed and leads to increment of coupling length between the two fibers. The various load force amount will give different coupling lengths and distances between the two cores, thus leading to different coupling efficiencies. Analytically the expression that represents the coupling efficiency with force is an integrated expression from simplified coupled mode theory and Hertz’s Law of elliptical point contacts. Optimized coupling efficiency obtained is as high as 70% for this study.