Experimental and numerical investigations on asymmetric fused fibre couplers consisting of different single-mode fibres

Summary form only given. Fused fibre couplers (FFCs) are essential components in fibre-based laser and amplifier systems for combining the pump and signal wavelengths into one fibre. Typically, identical fibres are employed in FFCs, which limits the combinable wavelengths to above the cutoff of the fibre. An approach for coupling two widely separated wavelengths is the use of different fibres with single mode guidance for the respective wavelength [1]. Due to the asymmetry introduced by the different fibres a special technique is necessary, for example pre-tapering one fibre before coupler fabrication, in order to achieve a high coupling ratio.To analyse the influence of pre-tapering on the coupling behaviour systematically, we fabricated asymmetric FFCs with different pretaper length. A 630HP (core diameter: 3.5 μm, NA=0.13) and a 1310BHP (core diameter: 8.5 μm, NA=0.13) were used at a pump wavelength of 633 nm and signal wavelength of 1556 nm. Figure 1a) shows the experimentally recorded intensity of the pump and signal power during the fabrication process without pretapering. Only a fraction of the light, approximately 20% of the power, coupled from one fibre to the other. This maximum coupling ratio can be substantially increased by pre-tapering the signal fibre. A pretaper of 6 mm was applied realising a nearly complete power transfer between the fibres, as depicted in Figure 1b). At an extension of 16.4 mm the signal is maximum in the signal fibre and the pump wavelength showed a minimum in the pump fibre, which means that if the fabrication process was stopped at this extension, both wavelengths could be combined in the signal fibre with an efficiency of 85%. In order to find the optimum value several pretaper lengths were tested experimentally.