The optical switches discussed in previous sections are wavelength transparent switches, which are not blocking light waves with any wavelength. In this section, the wavelength selective switches will be discussed and designed to support WDM systems. The wavelength selective switches can be realized by using ring resonators or Mach-Zehnder interferometers (MZI).
Modeling Instruction
Referring to [1], the basic 2 x 2 wavelength selective switch consists of two silicon micro ring resonators and one crossing silicon waveguide. The switch has two states, one is the "drop" state, at witch the lights are on resonance and the signals are coupled to the rings and coupled back to the waveguide, then dropped to the outputs. The other one is the "through" state, at witch the lights are off resonance and the signals go straight through to the outputs. The following figures show the design and the two states of the switch [1].
When the rings are on resonance, they follow the following given equations.
$$ L=\frac{\lambda}{n_{e f f}} \cdot m $$
$$ F S R=\frac{\lambda^{2}}{n_{g} \cdot L} $$
$$ \varphi=\frac{2 \pi}{\lambda} \cdot n_{e f f} $$
where L is the perimeter of the ring, neff and ng are the effective index and group index of the waveguide, m is a positive integer, and FSR is the free spectrum range of the resonator.
The switch can be broken down into several segments, includes 4 optical couplers and 4 pieces of waveguides. The following figure shows the break down segments of the switch.
To design the wavelength selective optical switch, the following steps could be used as a guide.
- Start a new INTERCONNECT project. You can start a new project by pressing Ctrl+N, or by selecting New in the File menu.
- From the element library, drag and drop 4 Waveguide Couplers (Element Library\ Waveguides\ Couplers\ Waveguide Coupler) to the schematic editor. Set the Mode 1, coupling coefficient 1 to be 0.01 and arrange them as following:
- From the element library, drag and drop 4 straight waveguides (Element Library\ Waveguides\ Straight Waveguide) to the schematic editor. Set the waveguides' frequency, length, TE mode effective index 1 and group index 1 as shown in the following table, arrange and connect them as below:
|
Property |
Value |
|---|---|
|
frequency |
1.55 μm |
|
length |
10.2423 μm |
|
effective index 1 |
2.27 |
|
group index 1 |
4.26 |
- Select all the elements, right click on the elements and create compound element. Set the compound element name to be "SWITCH".
- Right click on the element and select Edit, in the "Ports" tab, add 4 ports, set two of them on the Right Position and two on the Left Position and click on the "Arrange" button. The compound element "SWITCH" should look like the following:
- Expand the element, connect and rename the relays as following:
Results & Discussion
To examine the wavelength selective optical switch, the Optical Network Analyzer could be used. Set the ONA number of input ports to be 2 and connect it to the SWITCH element as shown below, the "Standard" setting of the ONA is as following:
|
Property |
Value |
|---|---|
|
center frequency |
1.55 μm |
|
frequency range |
90 nm |
|
number of ports |
1000 |
|
plot kind |
wavelength |
Run the simulation and plot the transmission of input 1 and input 2 of the ONA in the same plot, The following figure is the transmission plot of the drop and through ports. If the output port of the ONA is connected to port 2 of the SWITCH element, the drop and through ports transmissions are swapped as shown in the following figure.
As indicated in the transmission plot, only the light wave with 1550 nm wavelength (and all the wavelengths separated from the central wavelength by a integer number of FSR ) can go through the SWITCH to output port 3, lightwaves with different wavelengths will be blocked to the output port 4 of the switch.
To make the switch tunable, we can introduce an altering in the effective index to the straight waveguide by a small value Δneff.The slightly change of effective index will change the rings' resonant frequencies, hence shift the transmission waveform. The following figure shows the transmission of the switch with the outer ring (WGD_2 & WGD_4) effective index tuned according to the table below, all the other parameters are kept the same.
|
Element |
effective index 1 Value 1 |
effective index 1 Value 2 |
|---|---|---|
|
WGD_2 & WGD_4 |
2.27 |
2.29 |
References
[1] Nikolova, Dessislava, et al. "Scaling silicon photonic switch fabrics for data center interconnection networks." Optics Express 23.2 (2015): 1159-1175.