After design the PhC structure, we are going to simulate the proposed all optical encoder and study its optical behavior. Excitation of light from the four inputs and their optical behaviors has been performed using finite difference time domain (FDTD) method. To simulate the optical waveguiding in the proposed encoder, a coherent Gaussian light source at a wavelength of 1.14µm with TM polarization is used at the input ports. The working states of the designed 4×2 encoder has 4 approaches. In approach 1 only input I0 is ON and other inputs are OFF (I1 = I2 = I3=0). As can be seen from Fig. 3, when input I0 is excited, the light should not been received at any of the outputs 1 and 2 because I0 input has no connection with the outputs. To do this, we first excited the input I0 with a coherent light source at a wavelength of 1.14um and the result is illustrated in Fig. 5(a). When input port 0 is exited the logic state (0, 0) should be at the outputs. It is shown from the transmission spectrum that the optical power to the outputs is very low. As mentioned above, using gold rods at the boundary between the line defects and the main structure lead to the plasmonic effect. So the confinement of the light beam in the waveguide is so much and the light will not leak to the photonic crystal structure. This claim is proved by observing the light behavior in the first input waveguide (I0) and transmission spectrum in Fig. 5. It is shown from Fig. 5 (b) that the value of the optical transmission at the outputs is minimum and in the order of 0.0001 at the wavelength of 1.14µm. As a result, the use of gold rods with the mentioned radius and lattice constant will increase efficiency and reduces the output loss.
As shown in Fig. 6, in designing the logic circuits if the output signal is higher than 45%, it is equal to logic 1, and if it is less than 5%, it will be logic 0.
As it was mentioned for approach 1, the value of transmission spectrum from the two outputs of encoder are less than %5, so the logic state in the output ports is (00). In approach 2, I1 = 1 and I0 = I2 = I3=0, and the encoder will generates the logic state (01) at the output ports. By exciting port I1, the maximum output optical power is expected to be received at the output 1. As can be seen from Fig. 7, we have 82% of the input light at the output 1, and the rest has entered into the ring resonator. The point defect in the ring resonator prevents the optical transmission to the other inputs and outputs. So the maximum optical power is transferred to the output 1. As it is represented from the transmission spectrum (Fig. 6.b) the output port 2 has very little optical signal at the order of 0.01.
As a result of the output transmission spectrum, the output port 1 has the signal more than %45 and the output port 2 has the signal less than %5, so the logic state (01) is at the output ports for input I1. In approach 3, the input port I2 is active (logic 1) and the other inputs (I0 = I1 = I3) are in logic state 0. So the encoder generates the logic state (10) at the output ports. As expected, by exciting port I2, we receive 96% of the light at the output 2 and very less or no optical signal at the output 1. The transmission curve of the outputs is represented in Fig. 8.
In approach 4, I3 = 1 and I0 = I1 = I2=0 and the two outputs of the encoder will be in logic 1. By exciting I3, the light couples to both ring resonators and enters to the output waveguides. Figure 9 shows the light behavior and transmission diagram when port I3 is excited. It is shown that the light is splitted in two parts as follows: the upper ring resonator couples the light in to the output 1 and the lower ring resonator couples the light in to the output 2. It can be seen from transmission spectrum that at the wavelength of 1.14µm, the maximum amount of signal is received at the outputs 1 and 2. From the normalized value of transmission spectrum, 50% and 48% of the input power enters to the output 1 and output 2 respectively. Since the value of both signals is higher than 45%, the logic state of the output will be (11). From the light behavior of the encoder (Fig. 9.a), the coupled light to the output doesn’t return back to the input waveguides and doesn’t leak to the photonic crystal structure due to gold rods.
The results of previous works have shown that the use of plasmonic in photonic crystal structures leads to more light trapping in the line defects and less dimension of device. Considering that these circuits should be used in optical integrated circuits, it is important to reduce their dimensions.