We employed the FDTD method to analyze and simulate the light propagation inside the proposed full-adder shown in Fig. 5, which contains three input ports. Therefore, according to the computation principle, we have 23 (2N, N is the number of input ports) different input states. The optical intensity of the input ports is equal to10 mW/µm2. The simulation results are discussed as follows for all states of the input ports.
Case #1
In this state, all the input ports are OFF (i.e., A = 0, B = 0, and C = 0); thus, there is no optical signal in the structure, and both output ports are OFF, and finally, the amounts of Sum and Carry will be zero.
Case #2
When A = 1, B = 0, and C = 0, the optical signal coming from input port A, travels close to RCs1 through W1 and W3. Since the optical intensity is equal to 10 mW/µm2, the optical signal will be dropped into W5 and W7, and it is dropped into W10 using RCs2 and travels toward the full-adder's Sum port, thus, Sum = 1 and Carry = 0. The light propagation inside the proposed full-adder is shown in Fig. 6(a). Figure 6(b) shows that in this case, the normalized powers at Sum and Carry output ports are more than 90% and less than 2%, respectively. Also, the time delay is about 3.5 ps.
Case #3
When A = 0, B = 1, and C = 0, the optical signal coming from input port B, travels close to RCs1 through W2 and W3. Since the optical intensity is equal to 10W/µm2, the optical signal will be dropped into W5 and propagates inside W7, it is dropped into W10 using RCs2 and travels toward the Sum output port. Therefore, we have Sum = 1 and Carry = 0. The light propagation inside the structure is shown in Fig. 7(a). Figure 7(b) shows that in this case, the normalized powers at Sum and Carry are more than 90% and less than 2%, respectively. Also, the time delay is about 3.5 ps.
Case #4
When A = 0, B = 0, and C = 1, the optical signal coming from input port C, travels close to RCs2 through W6 and W7. Since the optical intensity is 10 mW/µm2, the optical signal will be dropped into W10 and travels toward the Sum output port; thus, we have Sum = 1 and Carry = 0. The light propagation inside the structure is shown in Fig. 8(a). Figure 8(b) shows that in this case, the normalized powers at Sum and Carry are more than 88% and less than 1%, respectively. Also, the time delay is about 3 ps
Case #5
When A = 1, B = 1, and C = 0, the optical signals coming from input ports A (in W1), and B (in W2) are combined at W3 and form a resultant signal with an optical intensity of 20 mW/µm2. Therefore, RCs1 drops the optical signal into W4 and it travels toward the Carry output port through W9. The light propagation inside the structure is shown in Fig. 9(a). It demonstrates that there is no optical beam in W10. Thus, this case has Sum = 0 and Carry = 1. Figure 9(b) shows that Sum and Carry's normalized powers are less than 5% and more than 160%, respectively. Also, the rise time and the steady-state time are about 0.3 ps and 3ps, respectively.
Case #6
When A = 1, B = 0, and C = 1, the optical beam coming from input port A (in W1), travels close to RCs1 through W3. since the optical intensity is 10 mW/µm2, a resonant mode occurs, and the optical signal is dropped into W5. The optical beam coming from input port C with an optical intensity of 10 mW/µm2 propagates in W6 and is added to the signal coming from W5 at the input of W7. Then the resultant signal is formed with an optical intensity of 20 mW/µm2. This new signal propagates inside W7. Since the optical intensity in this waveguide is 20 mW/µm2, the RCs2 drops the optical beam from W7 into W8, and it travels toward the Carry output port through W9. Thus, in this case, we will have Sum = 0 and Carry = 1. The light propagation inside the structure is shown in Fig. 10(a). Figure 10(b) shows that for this case, the normalized powers at Sum and Carry are less than 2% and more than 125%, respectively. Also, the steady-state time is about 3 ps.
Case #7
When A = 0, B = 1, and C = 1, the optical beam coming from input port B (in W2), propagates in the vicinity of the RCs1 through W3. Then the optical signal is dropped into W5 because the optical intensity is 10 mW/µm2. Similar to Case #6, the optical beam coming from input port C propagates in W6. It is added to the signal coming from W5 at the input of W7 and forms an optical beam with an intensity of 20 mW/µm2. This new signal propagates inside W7. Since the optical intensity in this waveguide is 20 mW/µm2, the RCs2 drops the optical beam from W7 into W8, and it travels toward the Carry output port through W9. Thus we have Sum = 0 and Carry = 1. The light propagation inside the structure is shown in Fig. 11(a). Figure 11(b) shows that in this case, the normalized powers at Sum and Carry are less than 2% and more than 125%, respectively. Also, the steady-state time is about 3 ps.
Case #8
When A = 1, B = 1, and C = 1, the optical signals coming from input ports A (in W1), and B (in W2) are combined at W3 and form a resultant signal with an optical intensity of 20 mW/µm2. Therefore, RCs1 drops the optical signal into W4, and it travels toward the Carry output port through W9. The optical signal coming from input port C, travels close to RCs2 through W6 and W7. Since the optical intensity is 10 mW/µm2, the optical signal will be dropped into W10 and travels toward the Sum output port; thus, we have Sum = 1 and Carry = 1.
The light propagation inside the structure is shown in Fig. 12(a). Figure 12(b) shows that in this case, the normalized powers at Sum and Carry are about 90% and 160%, respectively. Also, the steady-state time is about 3 ps.
The numerical results of all eight input states are summarized in Table 1, and it shows that the proposed structure is acting as an all-optical full-adder. The results of this study were compared with other published papers in Table 2. It shows the input intensity, the steady-state time, and minimum output powers for logics 0 and 1 and confirms the superiority of our structure's results compared to previously reported works.
Table 1
The outputs of the proposed full-adder for all eight input states.
Case
|
Input
|
Normalized Outputs (%)
|
A B C
|
Sum Carry
|
#1
#2
#3
#4
#5
#6
#7
#8
|
0 0 0
1 0 0
0 1 0
0 0 1
1 1 0
1 0 1
0 1 1
1 1 1
|
0 0
90 2
90 2
90 1
5 160
2 125
2 125
90 160
|
Table 2
Comparison of the results of the proposed structure with previously reported works
Work
|
Input intensity (mw/µm2)
|
Steady-state time (ps)
|
Minimum Transmission (%) logic 0 logic1
|
Ref [42]
Ref [43]
Ref [32]
Ref [6]
This work
|
-
4*105
5*105
100
10
|
1.06
3
2
8
3
|
0.5 26
3 80
< 5 80
3 53
1 90
|