A new idea for wavelength-insensitive fibre couplers

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Description/Abstract

Wavelength-insensitive single-mode optical fibre couplers are useful components in spectral measurements and will be important in future wavelength-multiplexed optical fibre networks. In a normal single-mode fibre coupler, the power transfer between two fundamental modes of each fibre can be considered as interference between the first two normal modes in the composite waveguide? Since the difference between the propagation constants of the normal modes is wavelength-insensitive, the power division between the coupling arm and the throughput arm is also sensitive to wavelength change.
In our new scheme, the propagation constants of the fundamental modes in each fibre are unequal and the difference between them, as well as the coupling coefficient, varies slowly along the coupler. Only the first mode in the composite waveguide is excited significantly, while the second mode is either negligibly excited or is cut-off.
Due to absence of interference, the power division between the two arms depends only on the power distribution of the first mode in the composite waveguide at the end of the coupler, which is wavelength-insensitive. If at the end of coupler the two coupled-fibres are identical, then a 3dB wavelength-independent coupler results