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A new approach for eliminating aberration effects through introduction of elliptical grating facets in optical comb filters is presented. Simulation results show remarkable improvements of comb filter performance for aberration-free elliptical facets compared to traditional flat facet designs. The proposed theory was validated through design and fabrication of 100-GHz-period planar silica-on-silicon comb filters for use in multiwavelength optical sources. Multimode interference structures were used to achieve both flat-top and Gaussian spectral comb filter responses. All devices showed high extinction ratio, low nonuniformity in insertion loss, and high degree of passband shape control.