A novel notch-based directional coupler for use in a bidirectional plastic optical fiber system
Document Type Theses, Ph.D
This thesis proposes a novel notch-based directional coupler designed specifically for use in a bidirectional plastic optical fiber digital transmission system. The aim of this thesis is to propose a high performance coupler with a low total coupler insertion loss and a high directivity. The proposed coupler has a theoretical low total coupler insertion loss of 2.3 dB and a high directivity of a more than 60 dB. A prototype coupler demonstrated a total coupler insertion loss of 4.05 dB and a directivity of 37.3 dB, which is significantly larger than the steady-state Rayleigh isolation of 28.5dB. This thesis proposes for the first time a hybrid directional coupler using a silica fiber and a plastic optical fiber. A notch is fabricated on the plastic optical fiber surface to allow coupling from the silica fiber into the plastic optical fiber. A new analysis is presented for the first time for calculation of the coupling efficiency between two angularly misaligned step-index multimode fiber jointing with differential numerical apertures. This analysis is used for the calculation of the insertion loss from the silica fiber into the plastic optical fiber and also the directivity of the proposed coupler. Another analysis is presented for the first time for the characteristics of the numerical aperture of an angle-ended step-index multimode fiber. The numerical aperture and also the deflection angle varies with the tilt angle of an angle-ended step-index multimode fiber. This analysis is also used for calculation of the directivity of the proposed coupler. Finally, an analysis is presented for the calculation of the coupling efficiency of the proposed coupler in the downlink direction. An approximation is used when estimating the re-coupling efficiency from the coupling facet into the plastic optical fiber. Subsequent measurements verify this approximation.