Fig. 3(i) & 3(ii) shows sectional view of a single channel path of an eight channel CWDM demultiplexer. Circular ring resonator which is clearly depicts the same kinds of rod in the structure. Fig. 4 shows the device design which consists of eight circular ring resonators, where all the eight ring resonators are positioned serially one by one and right side of the linear bus waveguide. The right side of the circular ring resonators are having linear waveguide to drop the channel. This waveguide is also known as dropping waveguide. The left side of the structure has seven T shaped waveguides for λ1 to λ7 and eighth path is having L bend waveguide.
The linear bus waveguide is formed by introducing line defects i.e removing rods in a single column which is used for passing the Gaussian light signal with better linearity. The width of linear waveguide is about 820nm. The width of the linear bus waveguide is calculated from the radius of the dielectric rod and lattice constant of a single periodic row or column of the proposed structure. The other important parts of the devices are circular ring resonator with centre rod, channel selector rod and scatterer rods.
Circular Ring Resonator
This is made by circular shaped resonator with centre rod. Each resonator contains one centre rod which is named as circulator rod. The radius of the each ring resonator is about 870nm and radius of the centre rod is 360nm with relative refractive index is 1. The circulator rod linearly circulates and boosts the light signal inside the resonator. The circular ring resonator is designed by introducing both line and point defects.
Channel Selector Rod
The small sized channel selector rod is placed at the starting end of the T shaped and L shaped bend waveguides. The radius of the rod is varying from 80nm to 10nm with decreased by the factor of 10nm for the channels λ1 to λ8. The channel selectivity and wavelength is based on the size of a rod is used in the device. The radius of this rod tunes the wavelength of the channel for the CWDM applications.
Scatterer Rod
The scatterer rod is incorporated at each corner of the circular ring resonator. Generally, the scattering rod is employed to reduce the counter propagations modes and back reflection of the incoming light signal inside the ring resonator. According to the channels λ1 to λ8, the radius of the rods is varying from 127nm to 120nm decreasing by the factor of 1nm. This process is enhancing the performance of the demultiplexer.
Table 1
Channel selector rod radius, Scatterer rod radius and resonant wavelength of the proposed demultiplexer
Channels
|
Channel Selector Rod
Radius
(nm)
|
Scatterer Rod Radius
(nm)
|
Resonant Wavelength[λo] (nm)
|
λ1
|
80
|
127
|
1460.3
|
λ2
|
70
|
126
|
1450.1
|
λ3
|
60
|
125
|
1445.0
|
λ4
|
50
|
124
|
1440.0
|
λ5
|
40
|
123
|
1435.3
|
λ6
|
30
|
122
|
1430.2
|
λ7
|
20
|
121
|
1425.2
|
λ8
|
10
|
120
|
1420.3
|
Table 1 enumerates the resonant wavelength of the channels λ1 to λ8 which are tuned by channel selector rods and the scattered light rays which are reducing by scatterer rods. According to the size of channel selector rod, circular ring resonator with effective refractive index of centre rod and scatterer rod positioned at the four corner each ring is the active role to select and drop the channels like 1460.3nm, 1450.3nm, 1445.0nm, 1440.0nm, 1435.3nm, 1430.2nm, 1425.2nm and 1420.3nm wavelengths (λ1-λ8) respectively.
The Gaussian light signal with 1µW power is launched into the linear waveguide. The field distribution of proposed demultiplexer is attained by linear waveguides, T shape and L bend waveguides for the channel λ1 (1460.3nm) and channel λ8 (1420.3nm) is shown in Fig. 5 (a) and 5(b), respectively.
On resonance input signals are reached at λ1 = 1460.3nm with higher signal strength at port 1 and λ8 = 1420.3nm is reached at port 8. The off resonance signal is reflected to the source. However, the size of the structure is very small, the crosstalk among the channel little bit occurred in the proposed device.