Optical encoders are pivotal elements in optical communication applications. There is much need for ultra-compact and high-speed novel designs. This work proposes two new designs of fast, compact 4 x 2 optical encoders using two dimensional photonic crystals. The proposed structures consist of square lattice silicon rods embedded in an air background. The operation of these encoders is based on the wave interference technique. The encoders are designed to help in achieving better performance through increasing the contrast ratio and decreasing the power loss and the return loss. The PWE method is used to analyze the photonic band gap. We used FDTD simulation to obtain the electric field distribution inside each structure and the normalized output power. We prove that the scattering rods improve the directivity of the light toward the desired paths and decrease the backward reflection. The proposed encoders have small footprint areas of 204.8 and 160.4 mu m(2) and operate at wavelength 1550 nm. They achieve low response time (254 and 163 fs) and high contrast ratio (6.69 and 12.9 dB). Simplicity and compactness of the designs make them suitable for optical signal processors and photonic integrated circuits. Another advantage of these designs is that low input power is enough for the encoders' operation, because there is no non-linear materials included. Our designs compete with the published works in the last few years especially in their footprint and response time.
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