Analysis of Differential Excitation and Mode Coupling for Optical Fiber Characterization and Communication

• Main Author: Kobrinski, Haim
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1983
• Subjects: Optics
• ISBN: 9798205220231

The differential excitation technique is employed for studying propagation characteristics of multimode optical fibers and as a means of optical multiplexing in these fibers. First, the refractive-index profile of a given fiber is determined by recording the far-field intensity distribution of a short sample of that fiber and by relating it to the corresponding modal power distribution resulting from the differential excitation of that fiber. Further analysis of the far-field recordings under different excitation conditions enables the determination of the time delay associated with the various propagation modes in the fiber and to estimate the impulse response of the fiber for any input distribution for any length. Mode coupling, resulting from the non-ideal properties of actual optical fibers, manifests itself in the spatial redistribution of the optical power propagating along the fiber. Consequently, these properties cause changes in the temporal behavior of pulse propagating in that fiber. These effects are analyzed quantitatively through the solutions of the time-independent and time-dependent power flow equations under a specific angular dependent mode coupling mechanism. This analysis yields calculated results that correlate to the corresponding measurements with higher degree than previous models. Yet, the solutions for the power angular distributions are expressed analytically and physical insight may be drawn without tedious computation. Analysis of the temporal response of step-index fibers indicates that plane-wave excitation may increase fiber bandwidth by up to two orders of magnitude over full excitation bandwidths, as long as the mode coupling is not excessive. The Angular Division Multiplexing (ADM) technique is based upon the simultaneous differential excitation of a number of modal groups, or channels, in a single step-index fiber. The performance of the ADM concept in practical fibers, namely, the number of multiplexed channels, the fiber length, the crosstalk among the various channels, the temporal bandwidth of each channel and the interrelations among these parameters, is determined according to the preceeding analysis of mode coupling effects on the angular and temporal power distributions. In typical, currently available, fibers ADM enables up to five channels to be multiplexed to approximately one kilometer distance with about -40db electrical power crosstalk (-20db optical) and with a total bandwidth of up to about 2.5GHz.