Wideband tapered directional coupler

The tapered biconical directional couplers are typically narrowband devices. The oscillations of the output signals is the reason of this feature. Two fibers with different core diameters can be used for wideband nat spectral response of a 2x2 coupler. We propose the technique of the local decreasing of a fiber outer diameter by chemical polishing. The singlemode wideband 3dB couplers were produced with the split ratio tolerances of 5% for the whole region between 2-nd and 3-rd telecommunication windows.