In the last years there is a considerable interest in designing integrated optoelectronic or all-optical circuits based on photonic crystals (PhC). A PhC structure possess photonic band gap in which the light with a certain frequency range cannot propagate. However, the existence of linear defects causes dispersion relations in photonic band gaps. Light that satisfies in the dispersion relations decay except linear defects and can exist only in linear defects. Modifying some scatters it is possible to create a waveguide inside the PC. This waveguide have great potential in application for their ability to control light wave propagation and the possibilities of implementing PhC based optical devices. We propose a PhC diplexer based on a square lattice of silicon rods. The demupltiplexing mode is fed exploiting the different dispersion relation of the light in the three braches of a T-junction. A difficult challenge is to realise active …