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In recent years, the use of light waves has gained importance, since it can provide a higher speed in optical communication networks and can transfer big data with fast speed up to large distances in optical fiber.

In wide cities that have millions of users, it is important to associate an optical fiber for each user, this results in a massive quantity of cables in order to cover all customers in a specific area, which will be very costly. One of the solutions for this complication is to provide one single fiber for many customers, since it is possible to transfer many wavelengths together inside signal optical fiber with the help of a wavelength-division multiplexing (WDM) technique.[1] Once wavelengths arrive to the optical network end, a device that can separate and send wavelengths for each corresponding user is needed. This device is known as optical demultiplexer. Many scientists have concentrated to design optical device suitable for optical integrated circuits. To realize these devices, the light waves in the waveguide in all-optical systems should be controlled. With the help of photonic crystals (PC) and the photonic band gap (PGB) contained therein, the light waves inside waveguides can be controlled efficiently [2, 3]. Many optical devices based on photonic crystal with different applications have been investigated and used, such as optical filters [4, 5], circulators [6], power splitters [7], switches [8-10], sensors [11] and demultiplexers which is the subject of this work [12-20]. The most important performances that can characterize an optical demultiplexer are a quality factor, crosstalk, channel spacing, number of output channels and size of device. Many …