Graded-index Multimode Fiber (MMF) Optimisation and Transmitter/Receiver Coupling for Optical Muli-Input/Output Transmission System

Multi-mode fibers provide interesting features for long distance transmissions theoretically because of the high tolerance for the non-linear effects, its relative ease for installation and high coupling efficiency due to the large core diameter. But there are problems that limit the use of multi-mode fiber: as the name indicates itself, multi-mode fibers induce more than one mode unlike single mode fibers. This modes carrying the information propagate in the fiber with different propagation property and they will arrive at the receiver end at different time, the delay between the modes, DMD, is not zero. This will cause the broadening of the signal and inter symbol interference. And as the length of the fiber increases, the broadening also increases. As a result, the application of MMF's has been limited to short range communication. The other problem is chromatic dispersion or group velocity dispersion which caused due to the non-monochromatic nature of the light coupled to the MMF. The different frequency components of the modes will propagate at different speed which also contributes to the degradation of the signal. The third and important effect is the mode coupling due to micro and macro bending along the fiber.

In this thesis, graded index optical fibers are studied as a way to minimize DMD. The refractive index profile of GI NMF can have different structure; among the many the power-law profile function structure is studied. The propagating modes are approached both analytically and numerically to have consistent analytical equation for the modes of different profile structures. The profile of the fiber is optimized by finding a profile which gives minimum DMD. The effect of the profile optimization on the GVD is simulated as well. Mode coupling effect is not included here.

Finally MIMO system is discussed briefly. First the channel matrix is calculated with DMD and GVD, then the effect on the pulse is simulated from MIMO perspective. Then the effect of the profile optimization on the length of the fiber is simulated and discussed along with the signal processing capacity of the MIMO system.