The mode dependency of the coupler ratio for multi-mode fusion couplers can lead to technical problems in some applications. For example, when coupling conditions alter, the coupling ratio of the coupler may change.
50/50 couplers may inadvertently become 75/25 couplers.
A mode independent alternative to the fusion couplers is the taper coupler from ATI. The coupling ratio remains practically unchanged, even with altering coupling conditions. Another important advantage is the absolute wavelength independence.
When manufacturing a taper coupler with a splitting ratio of 50/50, the ends of two fibres are first beveled at a specified angle to half of the fibre diameter and polished. Then, these two fibres are glued together with a third fibre,forming a Y-shape.
The mode field of the input signal is thereby effectively split in the middle. Thus the coupler ratio for all modes is practically the same. Other division ratios can be reached by simply varying the polish section angle of the fibres.
