Transmission and Coupling Characteristics of Ultra-Broadband Cr-Doped Multimode Fibers

We report the results of experimental and theoretical investigations of the transmission and coupling characteristics of an in-line multimode Cr-doped fiber (MMCDF) in a single mode fiber (SMF) optical link. This is the situation involved in the employment of a broadband MMCDF optical amplifier in an optical network. The excitation of several modes in the MMCDF can lead to a severe insertion loss. By selecting a proper core diameter of the MMCDF, it is possible to achieve mode-matching at the junctions between the SMF and the MMCDF. This leads to the excitation of predominantly the fundamental LP₀₁ mode in a step-index MMCDF, result in a minimization of the insertion loss at both ends of the MMCDF. Using an MMCDF with a length of 12.5 cm and an average core diameter of 13.5 mum, we obtained a coupling efficiency of 64.6% ~ 74.1% in the spectral region from 1300 to 1600 nm. The measured coupling efficiencies are in good agreement with our theoretical modeling. The result indicates that the low insertion loss of MMCDF may be used as an inline ultra-broadband optical amplifier in an optical link for the entire 1300 to 1600 nm spectral range of low-loss windows of silica fibers. Our results are presented and discussed.