An optical wavelength demultiplexer using an asymmetricYjunction is numerically analyzed and the experimental results are reported. This demultiplexer utilizes both the mode splitting characteristic of an asymmetricYjunction and the waveguide dispersion of channel waveguides. Analysis shows that the device with step‐index profile has more than 20 dB isolation for wavelengths with 6.5% separation from the center wavelength. The fabricated device composed of two‐step ion exchanged soda‐lime glass waveguides coated asymmetrically with Corning 7059 glass thin film separated the lights of 0.63 and 0.84 &mgr;m wavelengths.