| Summary: | 碩士 === 國立暨南國際大學 === 應用材料及光電工程學系 === 103 === The fundamental mode of a small-core step-index few-mode fiber (FMF), originally designed for single-mode operation at 1310/1550 nm, was selectively excited at visible wavelengths 633nm from a single-mode fiber (SMF) designed at 633nm by adiabatically tapering the fusion splice.
The SMF was first excited using end-fire coupling. Light propagated as the core mode through the SMF and was adiabatically transferred into cladding mode at the tapered region. The cladding mode remained unchanged at the splice point where the waists of two tapered fibers were uniformly controlled. After entering the tapered region of the FMF, the cladding mode remained and was gradually transferred into the fundamental guided core mode of the FMF.
The fundamental mode profile of the FMF was thus excited, and it can be observed with a charge-coupled-device (CCD) camera. First- and second-order spatial derivatives of the fundamental guided mode profiles were measured using the modified end-fire coupling method, and the refractive index profile (RIP) of the FMF can then be reconstructed by applying the differential inverse calculation algorithm, which applies only for single-mode guiding.
Good accordance can be seen from the reconstructed RIP and the known parameters of the fiber. A nearly step-index profile can be seen, with approximate index difference of 0.006. The core size of the FMF is estimated to be 8.2 m. Compared with the parameters provided by the manufacturer, with expected refractive index difference of 0.36%, these results show good agreement.
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