© 2020 Optical Society of AmericaIn this paper, we present the numerical and experimental demonstration of a wavelength demultiplexer (WDM) based on the photonic crystal (PC), in which the waveguide has a tapered width. Owing to the tapered waveguide, propagating light can be slowed down and be trapped by a local mode gap effect at certain distances from the entrance of the waveguide. The corresponding effect leads to the localization of four different wavelengths at different points inside the waveguide. The drop-channels are introduced at these specified locations to separate selected wavelengths. Here, we utilized an optimization algorithm to enhance the coupling efficiencies of the introduced drop-channels. The presented WDM PC separates the wavelengths of 22.29, 21.63, 20.80, and 19.87 mm (13.46, 13.87, 14.42, and 15.10 GHz, respectively) into different drop-channels with coupling efficiencies at around 80%. Experimental verifications of the numerically presented results are realized at the microwave frequency regime where the coupling efficiencies of each drop-channel are measured as around 75%. The designed WDM PC structure is all dielectric, compact, and efficient, and it exhibits low cross talk between drop-channels. Experimental measurements show a rainbow-trapping phenomenon and verify the simulation results of wavelength demultiplexing design with the margin of error between 0.8% and 1% frequency shifts in peak transmission values.