The proposed structure of an optical 4 × 2 encoder with and without enable control input is simulated using RSoft FullWAVE tool shown in Figure 4 and Figure 5 based on FDTD method. The Table 2 shows the Truth table of 4x2 Encoder with practical results with and without enable input power. Also we have analyzed the output power variation for different wavelengths around centre wavelength λ=1550nm and results are shown in the Table 3. The contrast ratio is defined as the ratio of logic 0 and logic 1 power levels of the output.
Case 1: Encoder with Enable=0
When we apply valid inputs for the proposed encoder with control input as ZERO power, we will not receive enough power at the encoder output and is assumed to be logic ZERO power for different cases of input as shown in figure 5. This validates the actual truth table of logic encoder. The threshold is 0.4 arbitrary units (a.u.) above that value will be treated as logic HIGH and below that will be logic LOW output as shown in Figure 5.
Case 2: Encoder with Enable=1
When the enable port is applied with input power 1 a.u., the proposed 4x2 optical encoder will work like logic encoder with a maximum power confined at the output ports monitor A and B is 0.94 a.u. and 0.76 and minimum power confined at the output ports is 0.13 a.u. and 0.18 a.u. respectively. These power confined values are used in the computation of power ratios as a contrast ratio in the optical device designing as measuring factors in dB. The proposed optical encoder is having a highest contrast ratio 8.59 dB achieved at the monitor output A. The designed encoder is tested for various operating wavelengths for different applications and at 1550nm wavelength. We have obtained a maximum confinement of the power at the two output ports. To describe the operation of optical encoder, we consider the optical signal power for enable input port to be equal to 1 a.u, and a sequence of input lights (I0, I1, I2, I3) as following cases:
Case 2(a): At I0 = 1, I1 = 0, I2 = 0, and I3 = 0 as shown in Figure 6(a), the optical signal of input I0 (High) optical signal passing through the T splitter waveguide and reaching the output port A = 0 (0.13 a.u), and the output port B = 0(0.2 a.u).
Case 2(b): At I0 = 0, I1 = 1, I2 = 0, and I3 = 0 as shown in Figure 6(b), the optical signal of input I1 (High) optical signal passing through the L waveguide is divide through T splitter waveguide and reaching the output port A = 0 (0.18 a.u), and the output port B = 1(0.76 a.u).
Case 2(c): At I0 = 0, I1 = 0, I2 = 1, and I3 = 0 as shown in Figure 6(c), the optical signal of input I2 (High) optical signal passing through the L waveguide is divide through T splitter waveguide and reaching the output port A = 1 (0.76 a.u), and the output port B = 0(0.18 a.u)
Case 2(d): At I0 = 0, I1 = 0, I2 = 0, and I1 = 0 as shown in Figure 6(d), the optical signal of input I3 (High) optical signal passing through the T waveguide and reaching the output port A = 1 (0.94 a.u), and the output port B = 1(0.74 a.u).
The response time for the proposed work is determined from the figure 4 as,
From the figure 4, for 10% of output CT=29µm and the time
T1=29µ/3E8=0.096ps and for 90% of output obtained time
CT=47µm which is T2=47µ/3E8=0.156ps then,
Response time= (T2-T1)*4=0.24 ps
Further the contrast ratio of the proposed structure is calculated as,
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As per the above equation the obtained contrast ratio at the output A and B is,
Contrast Ratio (A) = 10 log (0.94/0.13) = 8.59 dB
Contrast Ratio (B) = 10 log (0.76/0.18) = 6.25 dB
Table 2
Truth table of 4x2 Encoder with practical results
Inputs
|
Theoretical Outputs
|
Practical Outputs at λ=1550 nm
|
Enable
|
I3
|
I2
|
I1
|
I0
|
A
|
B
|
A
|
B
|
0
|
0
|
0
|
0
|
1
|
0
|
0
|
0.13
|
0.39
|
0
|
0
|
1
|
0
|
0
|
0
|
0.12
|
0.18
|
0
|
1
|
0
|
0
|
0
|
0
|
0.26
|
0.08
|
1
|
0
|
0
|
0
|
0
|
0
|
0.13
|
0.11
|
1
|
0
|
0
|
0
|
1
|
0
|
0
|
0.13
|
0.2
|
0
|
0
|
1
|
0
|
0
|
1
|
0.18
|
0.76
|
0
|
1
|
0
|
0
|
1
|
0
|
0.76
|
0.18
|
1
|
0
|
0
|
0
|
1
|
1
|
0.94
|
0.74
|
Table 3
Output power variation for different wavelengths around centre wavelength λ=1550nm
Wavelength in μm
|
Inputs I[3:0]
|
Outputs
|
Inputs I[3:0]
|
Outputs
|
Inputs I[3:0]
|
Outputs
|
Inputs I[3:0]
|
Outputs
|
0001
|
A
|
B
|
0010
|
A
|
B
|
0100
|
A
|
B
|
1000
|
A
|
B
|
1.50
|
0.09
|
0.03
|
0.02
|
0.01
|
0.01
|
0.05
|
0.19
|
0.15
|
1.51
|
0.07
|
0.05
|
0.18
|
0.26
|
0.31
|
0.44
|
0.32
|
0.29
|
1.52
|
0.06
|
0.08
|
0.07
|
0.28
|
0.29
|
0.14
|
0.44
|
0.45
|
1.53
|
0.04
|
0.12
|
0.04
|
0.31
|
0.39
|
0.05
|
0.29
|
0.55
|
1.54
|
0.08
|
0.19
|
0.02
|
0.36
|
0.54
|
0.04
|
0.42
|
0.72
|
1.55
|
0.13
|
0.2
|
0.18
|
0.76
|
0.76
|
0.18
|
0.94
|
0.74
|
1.56
|
0.08
|
0.13
|
0.02
|
0.61
|
0.68
|
0.15
|
0.92
|
0.43
|
1.57
|
0.06
|
0.07
|
0.13
|
0.74
|
0.59
|
0.19
|
0.91
|
0.46
|
1.58
|
0.06
|
0.06
|
0.16
|
0.24
|
0.26
|
0.17
|
0.84
|
0.70
|
Table 4
Comparison of 4x2 Encoder reported in the literature and proposed work
Works reported in the literature and Proposed
|
Methods
|
Area (No of Si rods)
|
Contrast Ratio (dB)
|
Response Time (ps)
|
Hassangholizadeh-Kashtiban et. al. (2015) [5]
|
Line defects,
elliptical resonant ring
|
26x24
|
--
|
--
|
Moniem, T. A. (2016) [6]
|
line defect T branch waveguides and photonic crystal ring resonator (PCRR) waveguides
|
35x35
|
--
|
--
|
Ouahab et.al. (2016) [8]
|
Nonlinear ring resonators with L-shape waveguides,
|
24x34
|
--
|
--
|
Gholamnejad et. al. (2017) [9]
|
8 optical waveguides and 2 nonlinear resonant rings.
|
41x47
|
--
|
1
|
Seif-Dargahi, H. (2018). [11]
|
Photonic crystal Ring Resonator
|
35x67
|
9.2
|
1.8
|
Mostafa et. al. (2019) [12]
|
One photonic crystal ring resonator (PCRR) and five arm waveguides
|
21x17
|
7.1
|
0.1
|
Hamedi et. al. (2021) [13]
|
Six optical waveguides and two resonant rings
|
19x33
|
--
|
--
|
Latha et. al. (2021) [14]
|
Triangular lattice,
six waveguides
|
37x35
|
8.1
|
0.28
|
Proposed work
|
Square Lattice,
T and L waveguides and Point defects
|
36x20
|
8. 59
|
0.24
|