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 flat spectral response of a 2 X 2 coupler. We propose the technique of the local decreasing of a fiber outer diameter by chemical polishing. The singlemode wideband 3 dB couplers were produced with the split ratio tolerances of 5% for the whole region between 2nd and 3rd telecommunication windows.