Wavelength-dependent transmission of monomode optical fiber tapers.

The transmission of light as a function of wavelength through biconical tapers formed in sections of monomode and low-mode number optical fibers is considered. It is found that in the monomode propagation regime the transmission varies sinusoidally with wavelength with a 50% peak loss and a 50-nm period, for tapers typical of those used in the manufacture of fused-biconical directional couplers. The observed dependence of transmission on wavelength can be understood using a coupled-mode formalism and leads to new insights into the operation of monomode fused-biconical directional couplers.