High-radix transparent optical switches is one of the promising and applicable techniques to deal with the rapidly increasing bandwidth requirement of data centers in optical interconnected networks. Based on the configuration of routing pattern, the optical networks can be divided into two major categories. One is the pre-defined, fix routing pattern wavelength-selective passive network and the other one is the switching network which dynamically set the routing pattern based on electrical control. The optical switching network can support higher aggregate bandwidth by cooperating with WDM technology. In this section, several optical-switch network examples are given, the auto-data extraction technique will also be introduced.
Theory
In order to create a nested optical switch network, multiple 2 x 2 switch elements are interconnected in stages. For a N-input, N-output switching system, \( 2\left(\log _{2} N\right)-1 \) stages with \( N / 2 \) switches at each stage are required. The figure below gives the structures of the 4 x 4 switch and 8 x 8 switch systems. Please see the Modeling Instruction section for detailed information on how to create the systems.
Overview
Modeling Instruction
There are two methods to generate the optical-switch systems, the first one is to use the provided script files; the other method is to drag and drop elements from the element library and connect them to make a system. In this section, using the 4 x 4 switch system as an example, the step by step instruction of how to build an optical-switch system will be given. The final systems generated by the two methods are the same.
Set Up Model
- Start a new INTERCONNECT project. You can start a new project by pressing Ctrl+N, or by selecting New in the File menu.
- Drag and drop an Optical Switch (Element Library\ Network\ Optical Switch) from the Element Library.
- Duplicate the Optical Switch 5 times by using the duplicate button
in the tool bar or by using copy and paste, and then arrange and connect the 6 optical switches as following:
- Change the switch names according to the following figure:
- Select all the elements, right click and choose "Create compound element", the compound element is shown as below:
- Right click on the compound element and choose "Edit". Change the name of the compound element to "SWITCH". The setting is shown below:
- Click on the "Ports" tab, add 8 ports, set the "type" of the 4 "Left" ports to be "Input" and the "Right" ports to be "Output". Click on the "Arrange" button to auto arrange the location of the ports. The setting is shown below:
- Click on the "Script" tab, enter the following script to the "Setup Script" window then click the "Test" button. The code and the setting after test is shown bellow, after test, click "Ok" to close the Edit window.
routing_control = [0, 0, 1; 1, 1, 0];
for (i = 1 : 3) {
for (j = 1 : 2) {
switch_name = "S_" + num2str(i) + "_" + num2str(j);
setnamed(switch_name, "control", routing_control(j, i));
}
}
- Right click on the compound element and choose "Expand". Connect and arrange the Relays as shown below:
The final 4 x 4 SWITCH element is shown as below, the 8 x 8 switch modeling instruction follows similar procedures as given above.
Routing Instructions
The routing control table for the switches can be set by editing the "Setup Script" of the compound element SWITCH. When the "control" of the switch is set to "0", the input signals are transparent to the output port; when the "control" is set to "1", the signals input are crossing to the output ports. Every time the routing control is set, the "Test" needs to perform again to apply the changes. Another way to do the setting is to expand the compound element and manually change the control of each switch according to the routing control table, however the advantage of massive setting using script will start to show when the number of switches grows.
Auto Data Extraction
The auto data extraction of an element could be done by running the script auto_data_extract.lsf, the script can be applied to every optical element in the element library or optical elements customized by users as well as optical compound elements. It will automatically extract the frequency domain response s-parameter of the element which is connected to the Optical Network Analyzer (ONA) and save the result to a dat file extracted.dat. This example extracted.dat is generated for the 4 x 4 optical switch system.
Auto Data Extraction
After generate the SWITCH element, open and run the script file auto_data_extract.lsf. Please note that the parameter COMP_name may need to be changed according to the element name. When running the script, the input and output ports of the element is auto-detected and connected to the ONA. The ONA will then sweep through the input ports of the element and attach all the information collected at the output ports of the element to the extracted file. The sweeping may take a few seconds or minutes depends on the level of complexity of the element.
Results
The extracted results file can be imported to an Optical N Port S-Parameter element and verified. It should give the same result as directly measuring the SWITCH element and the SWITCH element can be replaced by the Optical N Port S-Parameter element.
The SWITCH can then be fully characterized by the S-Parameter element. The following up measurements carried out by using the S-Parameter element spends less time than using the original SWITCH element since the transfer function of the element is already calculated and does not need to be recalculated in every run. The following figure shows the S-Parameter element, ONA and the SWITCH element.
The Optical N Port S-Parameter element can also be auto measured by the script, one only needs to change the parameter COMP_name to "SPAR_1" and the parameter OUT_filename to another name so that it will not rewrite the original dat file. The two extracted dat files should be exactly the same.
Example
4x4 Optical Switch
This example simulates a frequency-independent 4 x 4 optical-switch system, the system can be generated by using the script given below or can be manually set up following the steps given in Modeling Instruction section.
After generating the 4 x 4 optical-switch system, the compound element should have the following external (compound) and internal (schematic) look. From each input port, by setting the control of the switch elements, the signal could go to the corresponding output port accordingly.
Fig 1. 4 x 4 optical-switch external (compound) look
Fig 2. 4 x 4 optical-switch internal (schematic) view
A simple method to verify the SWITCH is to connect different input signals to the input ports of the SWITCH; by monitoring the routing control of the SWITCH, corresponding outputs are received. In the example file test_4x4_switch.icp, laser sources with different power and central frequency are connected to the 4 input ports of the SWITCH, following figures give the routing map and outputs of the testing system for the routing control \( \left[\begin{array}{lll}{0} & {0} & {0} \\ {0} & {0} & {0}\end{array}\right] \text { and }\left[\begin{array}{lll}{1} & {1} & {1} \\ {1} & {1} & {1}\end{array}\right] \), respectively.To verify the characterization of the SWITCH element, the extracted file could be load to an Optical N Port S-Parameter element, the S-Parameter element should have the same characterization as the SWITCH element.To characterize the switch, the auto_data_extract.lsf script could be used, or user can add the ONA element and verify the response of each input port manually. The extracted data file for the initial setting of the element is 4x4_switch_extracted_initial.dat. The control setting can be done by setting the control routing table and running the setup script of the SWITCH element or by manually setting the control of the switches in the expand view of the SWITCH; different extracted data will be generated for different routing.
Fig 1. 4 x 4 switch test for routing control = [0, 0, 0; 0, 0, 0]
Fig 2. 4 x 4 switch test for routing control = [1, 1, 1; 1, 1, 1]
References
[1] Nikolova, Dessislava, et al. "Scaling silicon photonic switch fabrics for data center interconnection networks." Optics Express 23.2 (2015): 1159-1175.
[2] Ji, Ruiqiang, et al. "Five-port optical router for photonic networks-on-chip." Optics express 19.21 (2011): 20258-20268.