Single mode fused wavelength division multiplexerH. A. RobertsADC Fiber Optics8 Walkup Drive, Westborough, Massachusetts 01581AbstractA method of tuning wavelength division multiplexers (WDM) for two arbitrary wavelengthshas been developed. The resulting fused single mode WDM's show low insertion loss, 20dB.IntroductionSingle Mode fused biconnical taper (FBT) couplers are inherently wavelength dependent de-vices. This is a result of the variation in the strength of the coupling coefficient (c)with wavelength. In general the coupling coefficient increases with wavelength, although ithas been showni that in some special cases of evanescent wave polished couplers this coeffi-cient may decrease with wavelength.We may take advantage of the wavelength dependence of the coupling coefficient in a FBTcoupler to fabricate WDM's. In the simplest case a coupler is fabricated in much the same2manner as has been reported previously in the manufacture of single mode FBT beamsplitters