Tutorial.pdf

Tutorials
OptiSys_Design
Optical Communication System Design Software
Version 1.0
for Windows® 98/Me/2000 and Windows NT TM
Optiwave Corporation
7 Capella Court
Ottawa, Ontario, Canada
K2E 7X1
tel.: (613) 224-4700
fax.: (613) 224-4706
e-mail: info@optiwave.com
web: http://www.optiwave.com

Notice
All OptiSys_Design documents, including this one, and the information contained
therein, is copyright material and may not be duplicated or reproduced, in whole or in
part, without the prior written approval of Optiwave Corporation.
Copyright © 2001 Optiwave Corporation
All rights reserved. No part of this document may be reproduced, stored in a retrieval
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Disclaimer
Optiwave Corporation makes no representation or warranty with respect to the adequacy
of this documentation or the programs which it describes for any particular purpose or
with respect to its adequacy to produce any particular result. In no event shall Optiwave
Corporation, its employees, its contractors or the authors of this documentation be liable
for special, direct, indirect or consequential damages, losses, costs, charges, claims,
demands, or claim for lost profits, fees or expenses of any nature or kind.

TABLE OF CONTENTS
INTRODUCTION............................................................................................................. 1
LESSON 1: TRANSMITTER - EXTERNAL MODULATED LASER............................................. 1
Starting OptiSys_Design......................................................................................... 1
Interface Overview.................................................................................................. 2
Using the Component Library................................................................................. 2
Connecting Components ......................................................................................... 4
Visualization of results............................................................................................ 5
Connecting Visualizers ........................................................................................... 6
Visualizers and Data Monitors................................................................................ 7
Component parameters............................................................................................ 8
Visualizers parameters .......................................................................................... 11
Running the simulation ......................................................................................... 11
Displaying results from visualizers....................................................................... 12
Saving the simulation results................................................................................. 16
LESSON 2: SUBSYSTEMS – HIERARCHICAL SIMULATION ................................................ 17
Loading a sample file ............................................................................................ 17
Creating a Subsystem............................................................................................ 17
Looking inside of the subsystem........................................................................... 18
Creating output ports............................................................................................. 19
Subsystem properties: name and icon ................................................................... 21
Adding global parameters to the a subsystem....................................................... 22
Accessing global parameters................................................................................. 23
Adding the component to the Component Library................................................ 25
Creating additional ports ....................................................................................... 27
Running the simulation ......................................................................................... 29
LESSON 3: OPTICAL SYSTEMS - WDM DESIGN ............................................................. 30
Global parameters ................................................................................................. 30
Transmitters........................................................................................................... 30
Parameter groups................................................................................................... 32
Testing the transmitter........................................................................................... 34
EDFA + Fiber spans.............................................................................................. 35
Getting results after the demultiplexer.................................................................. 38
Adding a receiver .................................................................................................. 38
BER Analyzer ....................................................................................................... 39
LESSON 4: PARAMETER SWEEPS - BER X INPUT POWER................................................ 44
Selecting the parameter to be iterated using the Sweep mode .............................. 45
Changing the number of sweep iterations............................................................. 46
Changing the values of sweep iterations............................................................... 47
Running the simulation ......................................................................................... 48
Getting results using the Graph builder................................................................. 49

Graphs and Views ................................................................................................. 50
Browsing parameter sweep iterations ................................................................... 51
Combining graphs from sweep iterations.............................................................. 51
LESSON 5: DESIGN VERSIONS – EDFA CHARACTERIZATION......................................... 53
LESSON 6: OPTIMIZATIONS – SYSTEM MARGIN .............................................................. 55
LESSON 7: OPTIMIZATIONS – EDFA FIBER LENGTH ...................................................... 58
LESSON 8: DISPERSION COMPENSATION USING SUBSYSTEMS AND SCRIPTING ................ 60
LESSON 9: DESIGN OF A BROADBAND RAMAN AMPLIFIER USING SUBSYSTEMS .............. 65
Overview ............................................................................................................... 65
Definition of Multi-line source ............................................................................. 65
Definition of Multi-line source subsystem............................................................ 66
Designing a broadband gain-flattened Raman amplifier....................................... 69
Using the interactive 3D graphics ......................................................................... 71
LESSON 10: OPTIMIZATION OF THE GAIN FLATNESS OF BROADBAND RAMAN AMPLIFIERS
....................................................................................................................................... 73
Optimizing the length of the fiber......................................................................... 73
The influence of fiber losses ................................................................................. 75

Introduction
The most efficient way to become familiar with OptiSys_Design is to complete the lessons located
in this document, where you will learn how to use the software by solving problems. Some of the
information described here is also described in the User’s Manual, in the Quick Start section.
Lesson 1: Transmitter - External modulated laser
This lesson will show you how to create a transmitter using an external modulated laser. You will
become familiar with the component library, component parameters, layout editor and visualizers.
Starting OptiSys_Design
To start OptiSys_Design, in the Taskbar use
Start -> Programs -> OptiSys_Design 1.0 -> OptiSys_Design
Once loading has finished, the OptiSys_Design graphical user interface will appear:
Figure 1 - OptiSys_Design graphical user interface
1

Interface Overview
The main parts of the OptiSys_Design interface consist of the Layout Editor, Project Browser and
the Component Library. The Layout Editor is used to place different components, edit them, and
create connections between them. The Project Browser is used to navigate through the current
project, and to organize a project so that results can be achieved more efficiently. The
Component Library gives access to the components available to create the design.
The Project Overview is used for zooming operations on the layout. Parameter Groups organizes
the parameters of components in a way so that the user does not need to change parameters
such as Laser Wavelength in each component, accessing all of them in the same place. The
Output Window displays the messages generated during the simulations.
Project Overview
Component Library
Layout Editor
Project Browser
Parameter Groups
Output Window
Figure 2 - Main parts of the graphical user interface
Using the Component Library
In the following example we will design the external modulated transmitter. We lay down all the
required components by selecting the components from the Component library. OptiSys_Design
provides the user with a set of built in default components
• Start a new project, from the main menu, go to File > New
• From the component library, go to Default > Transmitters > Optical Sources.
2

• Select CW Laser 1.0 and place it onto the workspace by dragging the icon from the
library, and dropping it into the workspace.
Figure 3 - Adding a CW Laser to the layout
• From the component library, go to Default > Transmitters > Optical Modulators.
• Select Mach-Zehnder Modulator 1.0 and place it onto the workspace by dragging the
icon from the library.
Figure 4 - Adding a Mach-Zehnder Modulator to the layout
3

• From the component library, go to Default > Transmitters > Bit Sequence Generators.
• Select Pseudo-Random Bit Sequence Generator 1.0 and place it onto the workspace
by dragging the icon from the library.
• From the component library, go to Default > Transmitters > Pulse Generators >
Electrical.
• Select NRZ Pulse Generator 1.0 and place it onto the workspace by dragging the icon
from the library.
Figure 5 - Adding a Pseudo-Random Bit Sequence Generator and a NRZ Pulse Generator to
the layout
Connecting Components
In order to send the signal from one component to another we must connect the component
output port to the next component input port.
When connecting components, you cannot connect more then one input port to the same
output port. This means only
• Connect the components by clicking on the port of the first component and dragging it to
the port of the next component:
• The Pseudo-Random Bit Sequence Generator to the NRZ pulse generator “Bit
Sequence” input port,
• The NRZ pulse generator output to the Mach-Zehnder “Modulation” input port,
4

• The CW laser output to the Mach-Zehnder “Carrier” input port
1
2
3
Figure 6 - Connecting components.
Visualization of results
There are many ways to visualize results when using OptiSys_Design, the Visualizers folder
in the Component Library allows the user to post process and display results from the
simulation. According to the input signal type the visualizer is categorized in electrical or
optical visualizer.
In order to visualize the electrical signal generated by the NRZ pulse generator in time
domain we can use an Oscilloscope Visualizer:
• From the component library, go to Default > Visualizers > Electrical
• Select Oscilloscope Visualizer 1.0 and place it onto the workspace by dragging the icon
from the library.
Figure 7 - Adding an Oscilloscope Visualizer to the layout
5

The optical signal can be also displayed by selecting visualizers from the library. In order to
visualize the modulated optical signal in time domain we will use an Optical Spectrum
Analyzer and a Optical Time Domain Visualizer:
• From the component library, go to Default > Visualizers > Optical
• Select Optical Spectrum Analyzer 1.0 and place it onto the workspace by dragging the
icon from the library.
• Select Optical Time Domain Visualizer 1.0 and place it onto the workspace by dragging
the icon from the library.
Figure 8 - Adding optical Visualizers to the layout
Connecting Visualizers
In order to visualize the signal from one component we must connect the component output
port to the visualizer input port.
When connecting components to visualizers, you can connect more than one visualizer to
one component output port. This means you can have multiple visualizers attached to the
same component output port.
• Connect the component and visualizers by clicking on the output port of the component
and dragging it to the input port of the visualizer:
• The NRZ pulse generator output to the Oscilloscope Visualizer input port.
• The Mach-Zehnder output to the Optical Spectrum Analyzer input port and to the
Optical Time Domain Visualizer input port.
6

Visualizers and Data Monitors
Figure 9 - Connecting visualizers
When connecting a visualizer to a component output port, OptiSys_Design will first insert a
default Data Monitor in the component output port, and then the visualizer will be connected.
Visualizers are always connected to monitors; this is why you can have multiple visualizers
attached to the same port, because actually they are attached to the same monitor. Data
monitors are represented by a rectangle around the component output port.
Electrical
Signal
Data
Monitor
Optical
Signal
Data
Monitor
Figure 10 - Monitors and Visualizers
Visualizers post process the data saved by the data monitor, you can also insert a monitor to
a port without connecting to a visualizer, and the monitor will always save the data after the
simulation is finished. This means that if you insert a monitor in a port before the simulation
starts, after the simulation has finished you can connect visualizer to this monitor without
running the simulation again.
• From the Layout tools toolbar select the Monitor Tool, the cursor will change to the
Monitor mode,
• Click in the output port of the CW Laser component.
• Click on the Layout tool to disable the Monitor mode.
7

Monitor
Tool
Monitor
Layout
Tool
Figure 11 -Creating a monitor
The monitor will save the signals at the CW Laser output, there is no needs for recalculate
the system if you want to visualize the signal and you forgot to add a visualizer to that port.
However you must remember to add the monitor to that port.
Component parameters
Double clicking on any of the components will bring up a dialog box for editing the properties
of that component.
• In the layout editor, double click in the CW Laser component icon.
Component parameters are organized by categories. There are five parameter categories in
this component: Main, Polarization, Simulation, Noise and Random numbers. The category
“Main” include the parameters often accessed by the user when using a laser, such as
Frequency and Power.
Figure 12 -Component parameters
8

For each category tab, there is a list of parameters. Parameters have the following properties:
Disp, Name, Value, Units and Mode.
Displaying parameters in the layout
The first parameter property is “Disp”. When this property is checked the parameter name,
value and unit will be displayed in the layout. Notice that Frequency and Power have Disp
checked and these parameters are displayed in the layout.
Figure 13 -Displaying parameters in the layout
Changing parameter units
Some parameters can have multiple units; this is the case of the parameters Frequency and
Power. Frequency could be entered in Hz, THz or nm, or Power could be entered in W, mW
or dBm, the conversion will be automatic.
You must press Enter or click in another cell to update the values.
Frequency
Units
Power
Units
Figure 14 -Changing parameter units
9

Changing parameter modes: arithmetic expressions
Each parameter can have three modes: Normal, Sweep and Script. Script mode allows the
user to enter arithmetic expressions and also access parameters defined globally.
• In the CW Laser, click on the property Mode.
• From the drop down menu, select Script mode.
• In the parameter Value, enter the expression ‘193.1+0.1’
• Click on Verify Script button
You will see the value of the script expression in the message list in the dialog box and the
results will be 193.2.
Figure 15 -Using arithmetic expressions
10

Visualizers parameters
In order to access the parameter of the Visualizer you must select the icon and right click on
it, since double clicking on any of the visualizers will bring up a dialog box for visualization of the
graphs and results generated during the simulation instead the parameters.
• In the layout editor, select the Optical Spectrum Analyzer icon.
• Right click on it, it will appear a pop up menu.
• From the pop up menu, select “Component properties”.
Running the simulation
Figure 16 -Accessing visualizers parameters
OptiSys_Design allows the user to control the calculation in different ways:
• Calculate the whole project: multiple design versions will all sweep iterations.
• Calculate the current design version: all sweep iterations.
• Calculate current iteration: current sweep iteration in the current design version.
By default we will calculate the whole project, since we do not have multiple design versions
and no sweep iterations.
• File Menu select Calculate :
11

Figure 17 -Running the simulation
• From the calculation dialog box, select PLAY to start the simulation .
Displaying results from visualizers
Double clicking on any of the visualizers will bring up a dialog box for visualization of the
graphs and results generated during the simulation. Double clicking again will close the
dialog box.
Oscilloscope
Electrical signals are visualized in time domain by using an oscilloscope.
• Double click in the Oscilloscope Visualizer
Since OptiSys_Design can propagate the signal and noise separated, the user can also visualize
the results separated. In the oscilloscope the user can access the signal without noise, only
noise, signal and noise added, or all of them in the same graph.
Signal
without
noise
Multiple
Units
Noise
Signal
and
Noise
added
Figure 18 -Oscilloscope
12

Optical Spectrum Analyzer
Optical signals are visualized in frequency domain by using an OSA.
• Double click in the Optical Spectrum Analyzer
Since OptiSys_Design use a mixed signal representation the user can visualize the signal
according to the their representation. Each left tabs refers to one of the representations: Sampled
signal, Parameterized signals and Noise bins, or display all of them in the same graph.
The optical signal polarization can be accessed using the bottom tabs: Total power, Power from
polarization X and Power from polarization Y.
Multiple
Frequency
Units
Sampled
Signals
Parameterized
Signals
Multiple
Power
Units
Noise bins
Internal
Resolution
Total Power Polarization X Polarization Y
Optical Time Domain Visualizer
Figure 19 -OSA.
Optical signals are visualized in time domain by using an optical time domain visualizer.
• Double click in the Optical Spectrum Analyzer
In time domain OptiSys_Design will translate the optical signal and the power spectral density of
the noise to numerical noise in time domain.
The optical signal polarization can be accessed using the bottom tabs: Total power, Power from
polarization X and Power from polarization Y. When selecting polarization X or Y the user can
also select to display the phase or chirp of the signal in that particular polarization.
13

Signal
without
noise
Noise
Multiple
Time
Units
Signal
and
Noise
added
Multiple
Power
Units
Analysis of
Chirp or Phase
Figure 20 -Optical Time Domain Visualizer
Zooming and Tracing
There are many tools to help you to analyze and extract information from the graphs.
• If the Optical Spectrum Analyzer display is closed, double click on it.
• Right click with the mouse over the graph then select the Zoom icon, or click in the Menu
button in the upper left corner and select Zoom tool.
The zoom tool allows you to zoom in on a specific area of the graph.
Figure 21 -Zoom tool
14

You can also trace the to obtain the values for each point in the graph.
• Click in the Menu button in the upper left corner and select InfoWindow.
• Click in the Menu button in the upper left corner and select Tracer.
The tracer tool allows you to visualize the values for each point of the graph in the information
window.
Figure 22 -Tracer tool
Saving graphs
You can also obtain the table of points with the values for each point in the graph and then save
this as an test file, copy the graph to the clipboard as a bitmap or you can export the graph in
different file formats, e.g. meta file or bitmap.
• Click in the Menu button in the upper left corner and select Table of Points.
• In order to save the data as a text file, in the table of points dialog box click in the Export
to file button.
15

Saving the simulation results
Figure 23 -Table of points
OptiSys_Design allows the user to save the data from the monitors. This means you can save the
project file with the signals from the monitors, next time you load the file the visualizers will
recalculate the graphs and results from the monitors.
• After the calculation has finished, go to the file menu and select “Save As…”
• In the Save As dialog box select the Save monitor data check box.
Now all the results will be saved with the project file.
Save the
simulation
results
Figure 24 -Saving the project and simulation results
16

Lesson 2: Subsystems – Hierarchical simulation
A Subsystem is like a component, it also have an icon, parameters, input and output ports,
however they are built using a group of components or another subsystems. You can easily
create a subsystem grouping selected components in the layout.
Subsystems will help you to create your own components based on the component library without
programming, or to organize the layout in different hierarchical levels when there are a large
number of components in different levels.
Figure 25 -Hierarchical simulation.
This lesson will show you how to create a subsystem using the previous external modulated laser
from the first lesson. You will become familiar with the subsystems and the component library.
Loading a sample file
• Select the menu File/Load,
• In the samples folder, load “Lesson 1.osd”
Or you can use the previous project from the fist lesson.
Creating a Subsystem
• Select all the components in the layout,
• Right click over the selection,
• From the pop up menu, select Create subsystem
17

A subsystem icon will replace the components.
Observe that the visualizers that are not included in the selection will be disconnected. This is
because the subsystem will not add additional ports to connect the visualizers.
Figure 26 -Creating a subsystem.
Looking inside of the subsystem
• Select the subsystem icon,
• Right click over the selection,
• From the pop up menu, select Look inside
Observe that a new tab was added to the layout tab, now you can change parameters and create
ports in order to access the signals from the subsystem.
Figure 27 -Looking inside of the subsystem
To go back to the main system, you can close the subsystem by right clicking in the layout
background and then selecting Close Subsystem, or you can click in the Main layout tab in and
keep the subsystem opened.
18

Closing the subsystem
Main layout
Subsystem
Figure 28 -Closing subsystems
Creating output ports
• In the layout tools toolbar, select Draw Output Port Tool,
• Position the cursor near to the document border, left click on it,
• Connect the Mach-Zehnder modulator output to the subsystem port.
The port was created, if you go back to the Main layout there will be an output port in the
subsystem icon. In order to create input ports use the same procedure. Since this is a transmitter,
in this example we will not create input ports.
19

Draw
Output
Port
Draw
Input
Port
Figure 29 -Creating ports in the subsystem
You can change the port position and location by double clicking on the port in the subsystem
icon in the main layout, or by double clicking in the port inside of the subsystem.
Figure 30 -Port properties
20

Subsystem properties: name and icon
You can change the subsystem name and icon by changing the parameters of it.
• In the main layout, double click in the subsystem icon
• In the Subsystem properties dialog box, change the Label to “External Modulated
Transmitter”
• Press OK button.
The component name and the tab will be updated with the new name.
Figure 31 -Renaming the component
To change the subsystem Icon you can load a user defined image from a file.
• In the main layout, double click in the subsystem icon,
• In the Subsystem properties dialog box, set the value of the parameter “Use Image”
to checked.
• Click in the value of the parameter “Filename”, it will open a dialog box to define the
image file name.
•
You can use the image “External.bmp” in the samples directory.
21

Figure 32 -Renaming the component
Adding global parameters to the a subsystem
In this example we have a subsystem called External Modulated Transmitter, if we need to
access any parameter inside of the Transmitter we must look inside of the subsystem.
In order to change the component parameter values of the subsystem without looking inside each
time you need to access them you can create global parameters.
In the next steps we will export the parameters Power and Frequency to be accessed by the user
without looking inside of the External Modulated Transmitter.
• In the Main layout, double click on the External Modulated Transmitter,
• In the properties dialog box click on the Add Param… button,
• In the Add parameter dialog box enter the following values:
o Name: Frequency
o Type: floating-point
o Category: Main
o Minimum value: 100
o Maximum value: 200
o Current value: 193.1
o Units: THz
• Click on Add button
The parameter Frequency will be added to the External Modulated Transmitter.
• In the properties dialog box click on the Add Param… button,
22

• In the Add parameter dialog box enter the following values:
o Name: Power
o Type: floating-point
o Category: Main
o Minimum value: -100
o Maximum value: 30
o Current value: 0
o Units: dBm
• Click on Add button
Accessing global parameters
Figure 33 -Creating parameters
In the last section we added two parameters to the subsystem, now we must create a reference
between the components inside of the subsystem and the global parameters.
• Select the External Modulated Transmitter and look inside,
• Double click in the CW Laser component,
• In the properties dialog box, select the parameter Frequency,
• Change the parameter mode to Script,
• Set the unit to THz
• In the parameter value enter “Frequency”.
Repeat the same procedure for the parameter Power, enter “Power” as the parameter value, and
set the unit to dBm.
23

Figure 34 -Using script parameters
You can also select the parameter value and right click on it, the pop up menu will show the
global parameters available for the subsystem.
Figure 35 -Accessing global parameters
• Click on OK button,
• Close the subsystem.
With these steps we set the value of the parameter Frequency and Power of the CW Laser to be
read from the global parameter of the subsystem “Frequency” and “Power”.
24

Global parameter
frequency
Global parameter Power
Figure 36 -Global and local parameters
From now on, when you change the values of the parameters Frequency or Power of the External
Modulated Transmitter, you are changing the values of the Laser CW, inside of the Transmitter.
Adding the component to the Component Library
The component library of OptiSys_Design has two main folders: Default and Custom. The Default
component library is read-only, that means you cannot add or change the parameters of this
components. The Custom library allows the user to expand the library adding new components.
This new components can be based on components from the Default library, however the
component name must be different from the one on the Default library. The user can also include
subsystems in the Custom library.
Read only
components
User defined
components
and
subsystems
Figure 37 -Component library
25

Creating sub folders in the Custom Library
In order to add the External Modulated Transmitter to the Custom library we will create a new
folder named “Transmitters”.
• Select the Custom folder in the component library,
• Right click in the background of the custom library,
• In the pop up menu, select “Add Folder…”
• In the New Folder dialog box, enter “Transmitters” in the edit box,
• Press the OK button.
In the Custom folder in the Component library a new folder named “Transmitters” will appear.
Figure 38 -Creating folders in the component library
Adding the component to the library
You can drag and drop components to any folder in the Custom library. We will add the External
Modulated Transmitter to the Custom/Transmitters library.
• Select the Transmitters folder from the custom library,
• In the layout editor, select the External Modulated Transmitter component,
• Drag the External Modulated Transmitter and drop it in the Transmitters folder.
The subsystem icon will be created with the component name: External Modulated Transmitter.
26

Figure 39 -Adding components to the component library
Creating additional ports
In order to access the electrical signal from the NRZ Pulse Generator inside of the External
Modulated Transmitter we can create an additional output port in the subsystem.
• Look inside of the External Modulated Transmitter,
• Create a new output port in the subsystem,
Figure 40 -Adding a new output port to the subsystem
However we cannot connect the Pulse Generator output to this port because it’s already
connected to the Mach-Zehnder Modulator input. To solve this problem we can duplicate the
signal from the Pulse Generator and connect it to both ports: Mach-Zehnder Modulator input and
Subsystem output.
27

• From the component library, go to Default > Tools.
• Select Fork 1x2 1.0 and place it onto the workspace by dragging the icon from the
library, and dropping it into the workspace.
It will be easier to accommodate the new component if you change the layout size. You can
change the layout size by pressing Ctrl+Shift and dragging in the layout background to change
the size.
Figure 41 -Adding a Fork to the subsystem
• Select and delete the blue connection between the Pulse Generator and the Modulator,
• Connect the Pulse Generator output to the Fork input,
• Connect the first Fork output to the subsystem output port,
• Connect the second Fork output to the Modulator input.
You can also delete the internal data monitors by selecting monitor tool and clicking in each one
of the monitors in the component output ports.
Figure 42 -External Modulated Transmitter with two output ports
• Close the subsystem.
If you want to reflect the changes you made in the subsystem you must delete the External
Modulated Transmitter from the Custom library and update the library by dragging and dropping
the External Modulated Transmitter with two ports from the layout.
28

Running the simulation
You can connect the External Modulated Transmitter output port to another components or
visualizers.
• Connect the first Transmitter output to the Oscilloscope Visualizer,
• Connect the second output to the Optical Spectrum Analyzer and to the Optical Time
Domain Visualizer,
• Run the simulation
The visualizers will show the results and graphs from the output ports. Observe that the center
frequency and power of the transmitter are the ones defined by the values of the parameters
Frequency and Power.
Figure 43 -Results from External Modulated Transmitter
29

Lesson 3: Optical Systems - WDM Design
This lesson will show you how to simulate a WDM system with 8 channels. You will become
familiar with the component library, parameter groups and visualizers such as Eye/BER analyzer.
Global parameters
For this simulation we will use default parameters for the Bit rate, Bit sequence length and
Sample rate.
• Select the menu File/New,
• Double click on the layout background.
Observe the global parameters used for this simulation,
Bit rate = 2.5 GB/s
Sequence length = 128 bits
Time window = 5.12e-08
Transmitters
Figure 44 -Global parameters
• Create an external modulated laser following the steps of the first lesson,
• Select the components of the external modulated laser,
30

• Resize the layout by pressing Ctrl+Shift, click in the layout background and drag the
cursor over the layout.
• Copy/paste the components in order to create 8 transmitters.
Figure 45 -Creating 8 external modulated lasers
• From the component library, go to Default > WDM Multiplexers Library > Multiplexers
• Select WDM Mux 8x1 1.0 and place it onto the workspace by dragging the icon from the
library.
• Connect the Mach-Zehnder Modulator outputs to the WDM Mux 8x1 inputs.
You can also select the values for the Mux internal filters to be displayed in the layout by
selecting the Disp option in the Channels tab of the Mux properties dialog box.
31

Figure 46 -Displaying multiplexer frequency channels
• From the component library, go to Default > Amplifiers Library > Optical > EDFA
• Select EDFA Ideal 1.0 and place it onto the workspace by dragging the icon from the
library.
• Connect the WDM Mux 8x1 outputs to the EDFA Ideal input.
Figure 47 – Adding a amplifier to the Transmitter
Parameter groups
In order to enter the frequency values for each channel you can double-click on each CW Laser
and enter the frequency value. To simplify this process of entering parameter values for each
component you can user Parameter groups.
32

Parameter groups table gives you access to relevant parameters for the WDM simulation such as
Frequency and Power in only one place. This means you don’t need to change the parameter by
going to each component and entering the values for these parameters.
• From the main menu, select View/Parameter groups, or press Ctrl+5,
Figure 47 -Parameter groups
In the Parameter group table, you can access the following parameters: Bit rate, Frequency,
Iterations, Output signal type, Power and Sample rate. You can also enter the units of the group
and edit the parameter values by entering new values or using the tools from the context menu.
• In the parameter group table, select the column ‘Value’,
• Right-click over the selection,
• From the popup menu, select ‘Spread…’
• From the Parameter Group Spread dialog box, enter the first frequency channel
frequency and the spacing: 193.1 and 0.1.
• Press OK.
The values of the frequencies will be updated showing 8 frequency values:, from 193.1 THz to
193.8 THz.
33

Figure 48 -Entering frequency values
Testing the transmitter
In order to verify the system setting for this design we will use an OSA and a WDM analyzer to
obtain the signal spectrum and the total power for each channel.
• From the component library, go to Default > Visualizers > Optical
• Select Optical Spectrum Analyzer 1.0 and place it onto the workspace by dragging the
icon from the library.
• Select WDM Analyzer 1.0 and place it onto the workspace by dragging the icon from the
library.
• Connect these visualizers to the Ideal EDFA output
• Run the simulation.
• Double-click in the visualizers to display the results and graphs.
You should obtain the signal spectrum showing 8 channels equally spaced, the WDM Analyzer
will also show the 8 channels, the average signal power for each channels is around 17 dBm for a
resolution bandwidth of 0.1 nm.
34

Figure 49 -Simulation results from the visualizers
EDFA + Fiber spans
To create a fiber connected to an EDFA follow these steps:
• From the component library, go to Default > Optical Fibers Library
• Select Nonlinear Dispersive Fiber 1.0 and place it onto the workspace by dragging the
icon from the library.
• In the Nonlinear Dispersive Fiber properties, change the parameter Length to 80 km.
• From the component library, go to Default > Amplifiers Library > Optical > EDFA
• Select EDFA Ideal 1.0 and place it onto the workspace by dragging the icon from the
library.
• Connect the Nonlinear Dispersive fiber output to the EDFA Ideal input.
Figure 50 -Fiber + EDFA span
35

In order to calculate the system performance based on the number of Fibers and EDFA spans
you can use the Loop Control.
The Loop control allows you to set the number of times the signal will propagate in the
components connected between the Loop Input and Loop Output ports.
• From the component library, go to Default > Tools
• Select Loop Control 1.0 and place it onto the workspace by dragging the icon from the
library.
• Connect the EDFA Ideal (the one connected to the Multiplexer) to the Loop Control input,
• Connect the Loop output port to the Nonlinear Fiber input,
• Connect the EDFA Ideal (the one connected to the Fiber) to the Loop input port.
Figure 51 -Loop control
The parameter Number of loops in the Loop control defines the number of round trips for the
signal in loop.
• In the Loop control properties, set the value of the Number of loops to 3.
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Figure 52 -Loop control parameters
This means we will propagate the signal in 3x80km = 240 km.
• From the component library, go to Default > WDM Multiplexers Library >
Demultiplexers
• Select WDM Demux 1x8 1.0 and place it onto the workspace by dragging the icon from
the library.
• Connect the Loop control output to the WDM Demux 1x8 input.
You can also display the channel frequency values in the layout.
Figure 53 –Demultiplexer
37

Getting results after the demultiplexer
In order to verify the system setting for this design, we will use an OSA, a WDM analyzer, and a
Time Domain visualizer to obtain the signal in time and frequency and the total power for each
channel after the demultiplexer.
• From the component library, go to Default > Visualizers > Optical
• Select Optical Time Domain Visualizer 1.0 and place it onto the workspace by dragging
the icon from the library.
• Duplicate the Optical Spectrum Analyzer and the WDM Analyzer,
• Connect these visualizers to the first demultiplexer output (193.1 THz)
• Run the simulation – this simulation will take some time.
• Double-click in the visualizers to display the results and graphs.
Adding a receiver
Figure 54 -Simulation results from the visualizers
After the demultiplexer we will include a photodetector, an electrical amplifiers and a Bessel
filter.
• From the component library, go to Default > Receivers Library > Photodetectors
• Select Photodetector PIN 1.0 and place it onto the workspace by dragging the icon from
the library.
• From the component library, go to Default > Amplifiers Library > Electrical
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• Select Electrical Amplifier 1.0 and place it onto the workspace by dragging the icon
from the library.
• From the component library, go to Default > Filters Library > Electrical
• Select Low Pass Bessel Filter 1.0 and place it onto the workspace by dragging the icon
from the library.
• Connect the first Demultiplexer output port to the Photodetector PIN,
• Connect the Photodetector output to the Electrical amplifier input,
• Connect the Electrical amplifier output to the Filter input.
• Set the electrical amplifier gain to 0 dB.
Figure 55 -Adding the receiver to the WDM system
BER Analyzer
In order to calculate the system performance you can use the BER Analyzer. This component can
predict the BER, Q-factor, threshold and Eye aperture. You can also obtain the BER patterns
and the BER value in each point of the Eye diagram using 3D graphs.
• From the component library, go to Default > Visualizers > Electrical
• Select BER Analyzer 1.0 and place it onto the workspace by dragging the icon from the
library.
The first input port of the BER Analyzer receives the binary signal.
• Connect the output of the first Pseudo-Random Bit Sequence Generator to the first input
port of the BER Analyzer.
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The second input port receives the original sampled signal, used to compensate the delay
between the signals transmitted and the received.
• Connect the output of the first NRZ Pulse Generator to the second input port of the BER
Analyzer.
• Connect the output port of the Electrical amplifier to third port of the BER Analyzer.
Figure 56 -Connecting the BER Analyzer
• Run the simulation – this simulation will take some time.
• Double-click in BER analyzer to display the results and graphs.
Graphs and Results
• In the BER Analyzer, select the Show Eye Diagram option
When opening the BER analyzer you will obtain the following graphs together with the Eye
diagram:
Q-Factor: this is the max value for the Q-Factor versus decision instant.
Min BER: this is the min value for the BER versus decision instant.
Threshold: this is the threshold value versus decision instant that gives the max Q-Factor
and the min BER.
Height: This is the Eye height for versus decision instant.
BER Pattern: when enabled, shows the regions were the BER in the region is less than
the user-defined values.
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Figure 57 -BER Analyzer graphs
You can also visualize the main results from these graphs in the Analysis frame: Maximum Q-
Factor value, minimum BER value, maximum eye aperture, threshold and decision instant at the
Max Q-Factor/ Min BER.
Figure 58 -BER Analyzer results
BER Patterns
In order to calculate the BER patterns you must select the check box in the BER Analyzer
display, or you can select the properties from the analyzer in the component.
• Select the BER Patterns check box – the visualizer will recalculate the graphs and
results
• In the BER analyzer display, select the BER patterns tab.
41

3D BER Graph
Figure 59 -Calculating BER patterns
In order to calculate the 3D BER, you must enable the calculation of the BER patterns and the 3D
graph.
• Select the BER Analyzer icon in the layout,
• Right click on it, from the popup menu select “Component properties”
• Select the BER Patterns tab,
• Enable the parameters Calculate patterns and Calculate 3D graph.
• Press OK – the visualizer will recalculate the graphs and results.
Figure 60 -Calculating 3D graph
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The 3D graph is available in the graphs tab of the project browser:
• In the Main menu, select View > Project Browser, or press Ctrl+2,
• In the project browser, select the Graphs tab,
• In the tree with components, select the BER Analyzer,
• Expand the tree and select the BER Pattern 3D Graph,
• Right click on it and from the pop up menu select “Quick View”
Figure 61 -3D graphs from the project browser
43

Lesson 4: Parameter Sweeps - BER x Input power
The following procedure will show you how to combine the results from the BER/Eye
Analyzer versus the signal input power using parameter sweeps. You will become familiar
with parameter sweeps, graph builder, results, graphs and views.
In this example we iterate the parameter Power at the CW Laser component. The file
“BER_InputPowerSweep.osd” has the setup to generate the results; we will show the steps
needed to follow in order to obtain these results in different designs.
Figure 62 -Sample file that calculates BER x Input power
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Selecting the parameter to be iterated using the Sweep mode
First, we will select the parameter Power to be iterated using the parameter sweep feature. We
want to be able to calculate the design using different values of the input power.
• Double click over the CW Laser component,
• In the Parameters dialog box, click on the Mode for the parameter Power
• Select the Sweep mode
These steps are described in Figure 63:
Figure 63 -Changing the parameter mode to Sweep
First, we will select the parameter Power to be iterated using the parameter sweep feature. We
want to be able to calculate the design using different values of the input power.
45

Changing the number of sweep iterations.
In order to define the number of iterations sweep you must use the following procedure:
• Go to the Parameter Seeps Tab,
• In the Design Version menu, select “Set Total Sweep Iterations”
• Set the Total Iterations value to 10.
These steps are described in Figure 64:
Figure 64 -Adding iterations
46

Changing the values of sweep iterations.
In the Parameter Sweeps tab you can enter the values directly in the table, or you can set the
fist and the last value and use the Sweep iteration spread tool:
• In the Parameter Sweeps tab, enter the value for the Power in the first cell, -50 dBm,
• Enter the value for the Power in the last cell, 0 dBm,
• Select the entire table column,
• In the Tools menu, select “Sweep Iterations Spread->Linear”
These steps are described in Figure 65:
Figure 65 -Entering values for all iterations
47

Running the simulation
From the File Menu select Calculate :
Figure 66 - Running the simulation
48

Getting results using the Graph builder
In order to create a graph showing the BER versus the power, you need to access the results
generated by the BER Analyzer.
• In the Tools menu, select “Graph Buider”
• In the Graph builder tool, select the parameter “Power” from the “CW Laser”
component in the X coordinate source area.
• Now select the results “Min. BER” from the BER Analyzer in the Y coordinate source
area.
• Click on “Add” button, the graph will be added to the Graphs tab in the project
browser.
These steps are described in the Figure 68:
Figure 67 -Building the graph
49

Graphs and Views
After the simulation we built the graph to visualize the BER versus input power, this graph was
added to the Graphs tab in the project browser, you can access this graph following this
procedures.
• Open the Project Browser (Ctrl+2),
• In the Project Browser select the Graphs tab,
• In the graph tab, click over the “Power vs. Min. BER”,
• By right clicking over the graph name you can choose to show the graph using quick
view or adding the graph to the Views tab.
These steps are described in Figure 68:
Ctrl+2
Graphs
Figure 68 - Quick view and Adding to the views tab
50

Browsing parameter sweep iterations
You can also open the BER visualizer and obtain the Eye diagrams and graphs such as Q
factor and BER for different iterations changing the current sweep iteration.
• Double-click in the BER Analyzer,
• Select ‘”Show Eye Diagram,
• In the toolbar, change the Sweep iteration,
Observe that for each iteration you will obtain different results, one for each value of input
power.
These steps are described in the Figure 70:
Figure 69 -Browsing through the parameter sweep iterations, the Eye diagram, and Q-Factor
for each value of laser power
Combining graphs from sweep iterations
Visualizer can generate graphs for all sweep iterations, however the visualizer will display only
the current sweep iteration when the user accesses the graphs by double clinking on it and
browse for different iterations.
It’s possible to access the graphs for all sweep iterations by using the Graph tab in the Project
Browser.
51

• Go to the Project Browser (Ctrl+2),
• Select the Graphs tab,
• In the Graphs tab, select the BER Analyzer 1.0 in the tree,
• Select the Q Factor graph,
• Right click on it, then select Add to view or Quick view
Observe that you will obtain a display with the graphs for all sweep iterations, one for each
value of input power.
These steps are described in Figure 70:
Figure 70 -Combining graphs from different sweep iterations in one graph
52

Lesson 5: Design Versions – EDFA Characterization
The sample file “EDFA Characterization.osd” shows how to use OptiSys_Design with multiple
design versions. In this example we characterize the Erbium Doped Fiber Amplifier by using
sweeps for the input parameters of the amplifier.
Figure 71 -EDFA Characterization.osd
This example use different parameter sweeps for each design version:
• Length: Iterations of EDFA fiber length from 1 to 20 meters.
• Input Signal Power: Iterations of CW laser signal power from -40 to 0 dBm
• Pump Power: Iterations of Pump laser power from 10 to 200 mW
• Wavelength: Iterations of CW laser signal wavelength from 1525 to 1600 nm
Figure 72 -EDFA Multiple design versions and parameter sweeps
53

For each design version we have three graphs showing the Output signal power, Gain and Noise
figure versus the sweep parameter. These three graphs were combined into one view, this means
we have four views: Gain, Noise Figure and Output power versus Length, signal power, pump
power and wavelength.
Figure 73 -Graphs and Views
54

Lesson 6: Optimizations – System margin
OptiSys_Design can optimize parameters in order to maximize, minimize or to find the target
value for results.
This means you can optimize the fiber length of the EDFA to obtain the maximum gain, calculate
the attenuation/gain to obtain a target Q-Factor, minimize the BER by optimizing the fiber length
of the link, etc.
The sample file “System Margin.osd” shows how to estimate the System margin; this margin
shows the amount of power penalty that may be added to the system. The target BER is 10 -9 , that
means a target Q-Factor of 6.
Figure 74 -System Margin.osd
In this example the subsystem “System under test” is an empty system. The optimization will
optimize for the parameter attenuation from the Attenuator component to obtain the target
value of the min Q-Factor from the BER Analyzer result. The parameter attenuation will be
the system margin in dB.
55

Figure 75 -Parameter optimizations
You must enable the optimizations in the calculation dialog box in order to run the optimizations.
Figure 76 -Enabling optimizations
After the calculation the system margin value is around 38 dB for a Q-Factor of 6.
56

Figure 77 -Getting results
57

Lesson 7: Optimizations – EDFA Fiber length
The sample file “EDFA Fiber Length Optimization.osd” shows how to maximize the gain
optimizing the EDFA fiber length.
Figure 78 - EDFA Fiber Length Optimization.osd
In the Optimization tab, you can select the Optimization properties to access the parameters used
for the optimization.
In this example we search for the length of the fiber from 1 to 20 meters in order to maximize the
gain measured from the WDM Dual Port Analyzer.
58

Figure 79 -Parameters for fiber length optimization
You must enable the optimizations in the calculation dialog box in order to run the optimizations.
In this example the maximum gain is 36 dB, and the fiber length is 8 meters.
Figure 80 -Results after calculation
59

Lesson 8: Dispersion compensation using subsystems
and scripting
This lesson will illustrate how to simulate a simple dispersion compensated link elaborating
further the concepts of subsystem definition and scripting.
Let us set up the following layout, which includes:
Figure 81 -Dispersion compensated link span
• An abstract source of Gaussian RZ pulses according to a user defined bit
sequence
• Booster amplifier and filter
• SMF and DCF
• Inline amplifier and filter
• Visualizers
Let us examine the signal waveforms at the input, after the SMF and at the end of the span.
One can observe that after the SMF the signal pulses are attenuated, broadened by a factor
of approx. 2 and that there is considerable inter-symbol interference.
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Input
After SMF
Output
Figure 82 -Signal evolution in a dispersion compensated link span
We can now use the powerful scripting feature of OptiSys in order to avoid the necessity to
redefine those of the layout parameters that depend on the signal in case input signals
change. One such parameter is the bandwidth of the two inline filters. Open the filter
parameter dialog box and switch the “Mode” of the “Bandwidth” to “Script”. Right-click In the
value field and select “Bit rate”. Then add ”3 * ” so that the whole field reads “3 * Bit rate”.
Click on the layout workspace and show the “Global parameters” dialog. Increase twice the
bit rate and calculate again. The following figure shows that regardless of the severe
dispersive broadening in the SMF, the signal is again preserved after the whole span, both in
terms of power and pulse shape.
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After SMF
Output
Figure 83 -The results of the scripted calculations of a dispersion compensated link span
Chances are that this single link might be useful for building longer multi-span systems. In that
case it is recommended to encapsulate it into a sub-system, which would later on allow
reproducing the same combination of components quickly and efficiently.
We create a new design version, selecting “Design Version/ Add Design Version” from the main
menu of OptiSys and change its name to “Amplified Dispersion Compensating Subsystem”. After
that we copy and paste all the components of Version 1 in it.
Next, as shown in Lesson 2: Subsystems – Hierarchical simulation, we select all the components,
except the signal source and the booster amplifier (including or skipping the visualizers), and
select “Create subsystem” from the right click context menu. Our layout collapses into the visual
representation of a subsystem. First we create an output port in the subsystem by selecting “Look
Inside” from the right-click menu and using the “Output port” tool. After that we can change the
name of the new subsystem to something informative, for instance “Amplified Dispersion
Compensated Span”, using the “Component Parameters” menu item.
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Figure 84 -The dispersion compensated span represented as a subsystem
Next we can use the newly created subsystem to build longer multi-span links. Let us create
another design version and change its name to “Multi-Span Link”. After that, we copy and
paste all the components of “Amplified Dispersion Compensating Subsystem” in it.
Now we can arrange a three span system by copying and pasting the “Amplified Dispersion
Compensated Span” subsystem component and linking the respective input and output ports.
Figure 85 -Building multi-span system using the user defined subsystem
63

Let us now define a more complex waveform and perform the calculation. After that we can
display and compare the input and output signals in terms of their amplitude and pulse shape.
Input
Output
Figure 86 - Input and output waveforms of the dispersion compensated links
64

Lesson 9: Design of a broadband Raman amplifier using
subsystems
Overview
The number of channels deployed in long-haul DWDM systems is rapidly increasing beyond
hundreds of channels over the C-band (1528-1563 nm) and L-band (1575-1610 nm). This
demand for more bandwidth is driving the search for new and more sophisticated fiber amplifiers
that are flattened over a very wide spectral range for maximum bandwidth transmission over the
long haul.
The specifics of the stimulated Raman scattering effect allow for an interesting technique for gainflattening
of broadband Raman amplifiers (Y. Emori, K. Tanaka and S. Namiki, Electron. Lett. 35,
1355, 1999). It requires only a suitable multiple-pump configuration and does not rely on passive
filters, gratings, etc.
Definition of Multi-line source
At an initial stage of the pump system lets set up a group of twelve pump laser diodes having
“Parameterized” signal representations and the following wavelengths and powers:
LD λ [nm] P [mW] LD λ [nm] P [mW]
1 1405.0 322 7 1450.0 122
2 1412.5 311 8 1457.5 113
3 1420.0 311 9 1465.0 110
4 1427.5 346 10 1480.0 127
5 1435.0 115 11 1495.0 126
6 1442.5 92 12 1510.0 219
After that we multiplex their outputs using, for instance, two multiplexers and a 2x1 power
combiner. The channels of the multiplexers should be accordingly adjusted to the
wavelengths of the pumps, as shown for the case of the 8x1 multiplexer:
Figure 87 -Adjusting multiplexer channels
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We arrive at the following layout and can now calculate and observe the compound output.
Figure 88 - Multi-line source layout
Definition of Multi-line source subsystem
It would be handy to use that combination of elements for defining both the pumps and the
signal sources of the broadband Raman amplifier. The easiest way to achieve this is to hide
the details in a subsystem, which is to be used later for different applications with a minor
tuning of the parameters.
First we open a new design version named “Multi-Line Sources”.
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After creating the subsystem as shown in Creating a Subsystem we copy and paste it two times.
The two new instances of that subsystem will be used as signal sources in the region 1510 –
1630 nm. Let us change the names of the subsystems respectively to “Multi-Line Pump 1400 -
1510 nm“, “Multi-Line Source 1510 - 1570 nm” and “Multi-Line Source 1570 - 1630 nm” by double
clicking their icons and filling the “Label” field in the parameter dialog box.
Now we need to adjust the internal parameters of the two new subsystem instances in order to
use them as signal sources. The following steps are the fastest way to do that:
• Right-click on the icon of “Multi-Line Source 1510 - 1570 nm”
• Select “Look inside” from the context menu. The internal details are displayed in the
layout editor.
• Open the “Parameter groups” by clicking Ctrl-5.
• In the “Group” drop-down list select “Power”, in the “Units” drop-down list select
“dBm”
• Click on “Value” button to select all values in that column
• Right-click on “Value” and select “Assign Multiple” from the context menu. In the
dialog box that pops up enter –10.
Figure 89 - Rapidly adapting the parameters of a multi-line source
67

• Next in the “Group” drop-down list select “Frequency”, in the “Units” drop-down list
select “nm”
• Right-click on “Value” and select “Spread” from the context menu. In the dialog box
that pops up enter 1510 for “Start Value” and 5.2 for “Increment”.
Figure 90 - Rapidly tuning a multi-line source to a new spectral region
• Select “Output Signal Type” from “Group”. As described above “Multiple Assign” 0 to
the “Value” of the “Parameterized” parameter. This means that the signals will be
generated with full sampled spectra, using the “sampled signal” representation
instead of the “parameterized signal” one.
• Close the subsystem.
• Repeat the same steps for the next subsystem setting its source wavelengths from
1572.6 to 1630 nm and the powers to –10 dBm
Now we have the necessary pumps and signals and can start building the Raman amplifier
68

Figure 91 – Raman Amplifier subsystems
Designing a broadband gain-flattened Raman amplifier
First we open a new design version named “Broadband Raman Amplifier”.
Then we copy and paste all the subsystems from the previous design version into the new one.
After adding the Raman amplifier component from the “Amplifiers” component library, 2x1
combiner and several OSA visualizers we arrive at the following layout:
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Figure 92 - Broadband gain-flattened Raman amplifier layout
For the properties of the amplifier fiber we enter the typical values of a SMF-28 fiber and set
its length to 40 km.
Everything is now set and we start the calculation of the current design version only. When it
is finished we can display and analyse the signal spectra at different points of the system for
example the output powers:
70

Figure 93 - Forward output spectrum of the amplified signals and the backscattered pump
It is seen that the gain of the device is relatively flat in a broad spectral region. The next lesson
will show how to improve that feature further by using the optimization tools of OptiSys_Design.
Using the interactive 3D graphics
The Raman amplifier component provides rich visual information in the form of 3D surfaces that
are interactive in real time. These 3D plots show the evolution along the fiber length of the
following important characteristics of the device:
• Signal power spectrum
• Gain spectrum
• Gain coefficient spectrum
• Noise figure spectrum
• Double Rayleigh scattering spectrum
Two 3D graphs of each type are supported – for the forward and backward propagating signals
respectively.
The following picture shows the evolution of the gain experienced by the forward propagating
waves. The high gain of the ASE in the shorter wavelength region (right) is due to the fact that it
experiences both Rayleigh and stimulated Raman scattering in the field of the strong pumps.
71

On the other hand it is clearly seen that the combined Raman amplification almost perfectly
compensates for the fiber losses along the full length of the amplifier.
Figure 94 - The longitudinal evolution of the forward gain spectrum in the broadband gainflattened
Raman amplifier
72

Lesson 10: Optimization of the gain flatness of
broadband Raman amplifiers
The flatness of the gain of inline fiber amplifiers is a matter of primary importance for WDM
systems. This lesson will demonstrate how to optimize further the gain uniformity of the Raman
amplifier described in “Lesson 9: Design of a broadband Raman amplifier using subsystems”.
There are numerous parameters that influence considerably the performance of Raman
amplifiers, but the most easily controlled one is probably the fiber length.
Optimizing the length of the fiber
First we set the mode of the parameter “Fiber Length” to “Sweep”. Next we adjust the number of
iterations to 13 by using the sequence of steps described in Changing the number of sweep
iterations.
Next we define range of values for the fiber length from 10 to 40 km, as shown in Changing the
values of sweep iterations.
Figure 95 - “Sweeping” the length of the broadband gain-flattened Raman amplifier
Next we have to add the “Results” “Maximal Forward Gain [dB]” and “Forward Gain Flatness [dB]”
to the results display table as shown in the following figure:
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Figure 96 - Selecting results
Now we perform the calculations by checking “Calculate all iterations in current design
version”. When finished the “Results” tab window contains the respective values of the two
results as a function of the parametrically swept fiber length.
Figure 97 - The results as a function of the parametrically swept fiber length
74

It is immediately seen that optimal gain flatness of < 1 dB can be achieved using fiber lengths of
approximately 25 km.
The influence of fiber losses
In the case of nonlinear devices like that broadband Raman amplifier it is not always true that less
background fiber losses assure better performance. In order to determine quantitatively the
influence of this parameter on the gain flatness of the amplifier we can perform another
“sweeping” calculation. Let the values of attenuation vary in the range 0.2 –0.3 dB/km.
Figure 98 - “Sweeping” the fiber attenuation
After the calculation we get the following results, which show that the optimal flatness and
small positive gain correspond to attenuation coefficient of 0.22-0.23 dB/km.
Figure 99 - The results as a function of the parametrically swept fiber attenuation
75

It is informative also to display and analyse the superimposed forward output gain spectra
obtained for the different values of the attenuation as shown in the following figure:
Figure 100 - Forward output gain spectra as depending on the attenuation of the fiber
76