optsim
DESCRIPTION
softwareTRANSCRIPT
OPTSIM
Optical Communications Systems Simulator
MEE13 Photonic Networks
Introduction
This laboratory is an introduction to the commercial software CAD simulator OPTSIM. The packageis accessed from your UNIX account on a SOLARIS workstation. First log on to your UNIX accountand make a note of the full path of the directory where you want to store your optsim files. You cando this by using the pwd command after changing to the directory. Make a note here:
Full path to optsim files:
Solaris machines in 2rd floor research lab:
To set up the correct environment you first type:
source /usr/local/ARTIS/OptSim32/OpSetup.csh
Linux machines in N402:
To set up the correct environment you first type:
source /opt/ARTIS/OptSim32/OpSetup.sh
Then to start the software you type:
optsim
The initial screen gives you three options, select the top button which is to open a new simulation. Thefirst dialog box to appear allows you to set the filename to be used, type new in the filename box. Thenext dialog box asks you to set some simulation parameters which we can leave at their default valuesby clinking on OK. The following GUI should now appear on your screen:
Basic OPTSIM screen.
The buttons on the left hand side contain the model components available for simulations. Forexample the top line of buttons has three arrows coloured black, blue and red. These are used to createconnections between components of a logical, electrical or optical nature. If you position the cursorover one of the component buttons and leave it stationary a tool tip will appear giving you the name ofthe component. To create a simulation you select the appropriate modelling component by clicking onits button and then click on the simulation area.
Create a simple model by connecting an Optical Pulse Generator to an Optical Probe by an OpticalLink. Note that the two components have a question mark imposed on the icon indicating that nospecific parameters have been chosen. In fact the model components all come with default valeswhich can be observed by double clicking on the icons. Do that now for both components and explorethe menus to see what parameters can be changed. When you have finished the question marks on theicons will have disappeared. You should now have a simulation looking like this:
Simple simulation of an optical source connected through a fibre to anoptical probe.
We are now in a position to run the simulation. If you look under the Simulate menu you will find twotypes of simulation as follows:
SPT
This stands for spectral propagation technique and is a simple simulation based on the assumption thatall the components in the system are linear. When this is the case it is only necessary to calculate theoptical power spectrum. Run this simulation first by clicking on SPT and then Start. A window willopen and record the progress of the simulation, when prompted click on the window and press return.If you try and use the Optical Probe outputs (by right clicking on the icon) you will find that they arenot available as the SPT simulation does not take account of measurement components. However, youcan look at the output power spectrum by right clicking on the Optical Link and selecting �ViewPower Spectrum Chart...�. Close this chart and continue with the next type of simulation.
VBS
This stands for variable bandwidth simulation and uses a form of beam propagation to calculate theresponses of the components. This simulation is able to include nonlinear device response and istherefore much more complex and time consuming. Run this simulation by clicking on VBS and thenstart and close the progress window as before.
When the simulation is complete right click on the Optical Probe and select �View Chart�. A newwindow will appear which shows the optical spectrum, now in more detail than before. Notice theseparation of the peaks, you can establish the bit rate of the optical source by measuring theirseparation. To do this, move the markers (red and blue) to line up with two adjacent lines in thespectrum and look on the Markers tab (on the top right of the command pane next to the graph),. Youcan also check this value against the properties of the source where you will find the specification ofthe pulse separation under the Signal Shape button (click on Raised Cosine and select it again fromthe menu of shapes that appears).
You can now investigate the charts available in the Optical Probe in more detail. In this simplesimulation the only other chart of interest is the time domain graph which can be viewed by clickingon the appropriate icon on the icon bar above the chart pane (the tool tip will say InstantaneousOptical Power). At first this looks a bit of a mess as the time display is set to 12.8ns and we have a10Gbit/s source. Right click on the xaxis and select �Axis Properties...�, then change themaximum range to 0.2 (note the units are set in one of the fields on this window) and click OK. Youcan now see the detail of some individual pulses in the optical signal. At present we are onlytransmitting a sequence of ones and to do anything more complex we would need a better transmittermodel which we will look at in the next example. For now we will examine some more modelconstructs using this simple model.
Before moving on to the next part close down the graph windows and exit the data display window.You will usually need to do this when you modify a simulation.
Change the measurement time in the Optical Probe to 1ns. Change the signal shape in the OpticalPulse Generator to soliton and repeat the VBS run. Note now that the spectrum has changed as hasthe pulse shape. Actually nothing is really happening in this model as the optical link we have selectedso far is an ideal optical link which has no nonlinearity and no dispersion. Right click on the OpticalLink and select �Properties�. Select the fibre type �Standard_SM� which is standard single modefibre, specify the fibre length to be 10km, click on OK and rerun the VBS simulation.
You should now be able to see that the phase of the pulse and the instantaneous frequency (=d/dt)have some structure. You can save a copy of these graphs for comparison with others by selecting themenu File/Export Curve give the phase plot the name 10mwphi.prn and the frequency plot10mwf.prn. Now change the properties of the soliton pulse source by changing the peak power to60mW. When the simulation has completed, look at the time curve and note the position of the peakof a pulse and then look at the phase graph again and import the curve 10mWphi.prn. Note that the
phase near the pulse peak is flat. (Remember transform limited pulses have at most a constant phase,variations in phase across the pulse indicate a departure from being transform limited.) This flatness isthe signature that the dispersion and nonlinearity have cancelled out giving rise to soliton formation.The parts of the pulse train between the peaks do not have a constant phase. The theory for solitonspredicts that they should have a constant phase across the whole profile of the pulse, why is itdifferent here?
Optsim Sample file
Load the sample file first.opf that can be found in:
/usr/local/ARTIS/OptSim32/examples/Getting_started/First/
or
/opt/ARTIS/OptSim32/examples/Getting_started/First/
depending on which machine you are using (type this path into the box labelled filter). Save a copy inyour file space, this is where the full path noted at the top of the first page is needed, and then closethe previous simulation. Spend some time examining the various components in the model and be surethat you understand what each is doing. (You can get a lot of information from the help button on thecomponent properties window) Before you run the simulation, change the Optical Spectrum Analyzerto an Optical Probe and set the measurement time of the probe to 1ns. Compare the pulse trains for asystem of length 1km, 20km and 50km (To save rerunning the simulations as we go through variousparts it would be better to save these three runs as separate files first1, first20 and first50). You shouldhave a graph like this:
Comparison of output pulse trains for 1km, 20km and 50km lengths in model first.opf.
Note that apart from the pulse amplitudes decreasing due to the fibre loss, the pulse positions change
in time, why is this? (Hint: look at the phase plot of the 1km system) Remember that the time windowof the probe is moving along at the speed of light of the central frequency in the simulation. Since thatis the frequency of the laser source used the pulses should not move in time.
If at any time after you have imported a graph you want to look at another graph you either need torestart the graph system or remove the imported graphs. To remove a graph you first need to select itin the graph description pane (usually it needs expanding) above the graph.
Next look at the electrical characteristics of the model. The Electrical Probe is used in the same wayas we used the Optical Probe but now the graphs available are the eye diagram, time plot, frequencyspectrum and two histograms. There are also some measurements associated with the eye diagramsuch as the Q factor and the jitter. Examine how the Q factor decreases with increasing system length.Try and find out how long the system would have to be to decrease the Q factor to 6? (why 6?) Youcan get a first estimate by extrapolating the Q vs fibre length plot.
Component Iteration
In order to look at the system performance in the previous example we had to keep changing the fibrelength and rerunning the simulation. It would be much easier to get the simulator to do this for us andone way is to use its ability to create blocks of elements that can be iterated. First load the simulationfile first20.opf from your directory and save it as second.opf. Edit the fibre length to be 10km.
In order for a set of components to be iterated they must satisfy the following conditions (these aretaken from the help files):
1.The iterated network must have a single optical input and a single optical output.
2.The iterated network must start with a fiber and end with an optical component (with a single opticaloutput). For example, an optical splitter may not be used as an endpoint because it has multipleoutputs.
3.The iterated network may contain electrical or logical components with unassigned outputconnections. However, all optical components must have all optical connections assigned, except forthe one that acts as an endpoint.
Let us set up an iterated network to allow us to view the propagation of the pulses in the fibre.Connect the fibre to an Optical Splitter and connect the Optical Splitter to an Optical Probe and aSplice. Now select (use the shift key to make multiple selections) the components in the followingorder fibre, everything except the splice, splice and select the menu command Iterate/Multiple Span.When the multiple span window comes up select 5 spans. You should now have a block that lookslike this:
Iterated block of components
Run the VBS simulation and then view the chart on the optical probe within the block. When