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In this paper, a 4-channel demultiplexer for WDM systems is designed by using 2-dimensional photonic crystals with square structure. Various wavelengths can be conducted in the certain paths of crystals using defects that look like ring resonator. The characteristic of this demultiplexer can be analyzed by obtaining the pass band spectrum. The property of this designation relative to previous works can be stated as size reduction, minimum crosstalk in output channels, minimum distance (about nm) for distinct wavelengths in output bands and bandwidth increasing in consequence. These improvements have been verified by simulation results.

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A new microstrip lowpass filter with compact size and ultra-wide stopband using T-shaped resonator is proposed. To make smaller the circuit size of the filter, cursive transmission line is also suggested. Moreover, exact equation of most important transmission zero is calculated based on LC equivalent circuit and transformation function. A demonstration filter with 3 dB cut-off frequency at 1.02 has been designed, fabricated, and measured. Results show that a relative stopband band width of 166.3% (referred to suppression degree of 20 dB) is achieved while obtaining high figure of merit of 46 274.

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