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New designs for a 1 times 4 and a 1 times 8 CWDM multiplexers based on cascaded groups of series coupled ring resonators are presented. Compared to other integrated optical alternatives such as MMI phasars, cascaded Mach-Zehnder interferometers and cascaded AWG, the proposed circuits offer superior performance in their very sharp roll-off factor that exceeds 0.75, their reduced crosstalk level that lies below -80 dB and their negligible insertion loss for the 1 times 4 design. For the 1 times 8 design, the worst case insertion loss is 4 dB. However, the performances obtained exhibit passband ripples in the order of 5 dB, and besides, they are not very tolerant to fabrication errors. Being designed for SOI technology, the proposed circuits are compact as the circuit areas are 130 times 130 mum2 and 90 times 150 mum2 for the 1 times 4 and 1 times 8 designs, respectively. They also have a high potential for MEMS tunability.