A method of tuning wavelength division multiplexers (WDM) for two arbitrary wavelengths has been developed. The resulting fused single mode WDM's show low insertion loss, <0.5dB, and high isolation, >20dB.
H. A. Roberts, H. A. Roberts,
"Single Mode Fused Wavelength Division Multiplexer", Proc. SPIE 0574, Fiber Optic Couplers, Connectors, and Splice Technology II, (15 January 1986); doi: 10.1117/12.950706; https://doi.org/10.1117/12.950706