Excitation and separation of spatial modes in a few mode fiber

Abstract

Space division multiplexing is the key of the new paradigm that overcoming the maximum capacitance of the wavelength division multiplexing system using a single strand of optical fiber. Multi core fiber and few mode fiber approaches for the space division multiplexing rise for a considerable increasing the optical transmission capacitance. In this dissertation, we have studied the higher order mode coupling in the few mode fiber using mode selective couplers. Mode selective coupler is directional coupler which provide mode coupling between fundamental mode in a single mode fiber and higher order mode in a few mode fiber. In order to convert between the $LP_{01}$ mode and the higher order mode in a pair of optical fibers, effective refractive index of the guided mode in single mode fiber matches that of a mode in few mode fiber. The few mode fiber we used in this experiment has 4.5 of V value and this optical fiber guides $LP_{01}$, $LP_{11}$, $LP_{21}$ and $LP_{02}$ modes. Effective refractive index of radiated modes in the few mode fiber and the pair of proper single mode fiber is measured using prism output coupling method. In order to satisfy the phase matching condition for the $LP_{01}$ mode in single mode fiber and target mode in few mode fiber, one of the pair of optical fiber which has higher effective refractive index of the particular mode should be tapered, since tapering progress reduces the effective refractive index as optical fiber diameter decreases. Optimum taper diameter is obtained by numerical calculation and after experimental fine tuning. In this experiment, we use side-polished type mode selective coupler. Unlike the fused type mode selective coupler, side-polished type is easy to fabricate especially in a cascade structure for the mode division multiplexing systems. Tapered optical fiber with optimum diameter and the pair of optical fibers are attached to the quartz blocks and polished until cladding remains few microns. These polished coupler halves are mated for achieving the mode coupling between the two modes by evanescent field coupling. In this study, we compare the performance of the higher order mode coupling between from the standard single mode fiber and from the effective refractive matched single mode fibers. The few mode fiber with highly Ge-doped in a core was used for the experiment since effective refractive index of the guided modes are well separated. Due to the high N.A. value of the few mode fiber, wavelength dependence of the effective refractive index ( $dn_{eff}/d\lambda$ ) of higher order modes have bigger value than that of single mode fiber. From the phase mismatch condition over the wide wavelength range, higher order mode coupling using standard single mode fiber shows big degradation of the performances. On the other hand, higher order mode coupling for the broadband operations achieved when we use the highly Ge-doped single mode fiber, since wavelength dependence of the effective refractive index of high N.A. single mode fiber has almost same value with that of a higher order mode in few mode fiber. Further, we have demonstrated the mode division multiplexing system using one strand of few mode fiber based on cascaded mode selective couplers. We confirmed that this mode division multiplexer covers wide operation wavelength range with high coupling efficiencies.