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A novel triplexer is introduced for optical communication applications in this letter. The triplexer consists of two cascaded multimode interference (MMI) couplers and can be fabricated by planar lightwave circuit technology. The concept of the extraneous self-imaging phenomenon, which is not generally mentioned in MMI theory, is applied to this triplexer to design a relatively simple and compact device. From the simulation results for wavelengths of 1310, 1490, and 1550 nm, the excess losses were determined to be 1.37, 0.63, and 0.79 dB, respectively, while the extinction ratios were 15.93, 24.28, and 18.44 dB, respectively.