Multichannel systems add another level of complexity: each channel should be analyzed not only separately, but also as part of the whole system. In the Optiwave environment, the main tool for such analyses is OptiSystem. The manufacturer describes it as an environment for designing and simulating optical links from the component level to a complete system, covering, among others, LAN, SAN, and MAN networks as well as ultra-long-haul systems. OptiSystem supports, among other things, CWDM, DWDM, PON, single-mode and multimode transmission, OFDM, optical amplifiers, and both direct and coherent receivers.
What does "broadband fiber-optic network" mean?
The notion of broadband fiber-optic transmission can refer either to a high data rate of a single channel or to sending many optical channels at the same time. In WDM (Wavelength Division Multiplexing) systems, a single fiber carries several or many signals at different wavelengths. In DWDM (Dense Wavelength Division Multiplexing), the spacing between channels is smaller, which allows the available optical bandwidth to be used more efficiently.
The system diagram can be simplified to: transmitters → multiplexer → fiber → amplifiers / compensation elements → demultiplexer → receivers. However, as the data rate and the number of channels grow, the phenomena that occur during propagation become increasingly important.
OptiSystem: modeling the entire link
OptiSystem is an environment intended for designing optical communication systems. The manufacturer's current page points to a library of about 600 components that can be combined into complete system models. A project can include, among others, optical sources, modulators, signal generators, multiplexers and demultiplexers, fibers, amplifiers, filters, photodetectors, DSP blocks, coherent receivers, BER analyzers, and spectrum and WDM analyzers. This lets you build a model much closer to a complete telecommunications path than the analysis of a single component.
From transmitter to receiver
The first stage of modeling is defining the transmission architecture. A typical model can contain three basic parts: transmitter, optical link, and receiver. The same division is used in Optiwave training materials on designing WDM systems.
Each of these blocks can then be expanded. The transmitter can include a laser source, a data generator, coding, and a modulator. The link can consist of many fiber sections, amplifiers, and compensation elements. The receiver can contain a demultiplexer, a photodetector, filters, regeneration circuits, and signal quality analyzers. The ability to study the mutual influence of these elements is one of the most important reasons for using system-level simulation.
WDM: many channels in one fiber
One of the basic uses of OptiSystem is designing WDM and DWDM systems. Optiwave provides an official tutorial showing how to build a WDM system with eight channels. The model uses the component library, parameter grouping, and analyzers, including the BER Analyzer.
In a multichannel model you can analyze not only the transmission itself, but also the parameters of individual channels. OptiSystem has WDM tools that make it possible to analyze, among other things, signal power, gain, noise figure, and OSNR for individual channels. This matters because a system that looks correct at the level of the total spectrum does not necessarily provide the same transmission conditions for every channel.
Chromatic dispersion: one of the basic link parameters
An optical pulse sent through a fiber does not keep its ideal shape. One reason is chromatic dispersion: if different spectral components of the signal propagate at different group velocities, the pulse can broaden. As the link length grows, this effect can lead to successive symbols overlapping and to a degradation of the received signal quality. That is why a simulation has to take into account not only fiber attenuation, but also its dispersion characteristic. In its OptiSystem training materials, Optiwave provides a separate exercise on dispersion compensation using dispersion-compensating fiber.
OptiFiber: where do the fiber parameters come from?
OptiSystem analyzes the behavior of the whole link, but in a more advanced project we may first want to study the fiber itself. OptiFiber is intended for this purpose. It uses numerical modal solvers and models specialized for optical fibers to calculate, among other things, dispersion, losses, birefringence, and PMD. These parameters depend on the geometry of the fiber cross-section, the materials, and the refractive index profile. OptiFiber also makes it possible to analyze fundamental and higher-order modes, cutoff length, and birefringence effects.
A natural division therefore emerges: OptiFiber is responsible for the properties of the fiber, and OptiSystem for the behavior of the whole link.
PMD: when polarization affects transmission
In high-speed transmission systems, Polarization Mode Dispersion (PMD) can also matter. In a real fiber, the two orthogonal polarization components do not have to propagate under exactly the same conditions, and these differences can lead to differences in propagation time. OptiFiber has tools for analyzing birefringence and PMD, including stochastic models. The program documentation covers first- and second-order PMD analysis and simulation of wavelength dependence.
In a broadband network simulation, this lets you separate two questions: what properties the designed fiber has, and how those properties affect the transmission of the whole system.
Attenuation is not the only loss in a system
The simplest power budget takes into account the transmitter power, the fiber attenuation, and the receiver sensitivity. In a real system, however, additional elements appear: connectors, splitters, multiplexers, demultiplexers, filters, amplifiers, and compensation elements. Each of them affects the system balance. A simulation therefore lets you analyze the power budget as a property of the complete path, not merely as the product of fiber attenuation and its length.
EDFA amplifiers
In long-haul systems, transmission losses can be compensated by optical amplifiers. One of the basic solutions is the EDFA (Erbium-Doped Fiber Amplifier). Introducing an amplifier does not, however, simply mean increasing the signal power: in a system-level analysis, gain, noise figure, wavelength range, input power, output power, and the behavior of many channels also matter.
OptiSystem includes models of EDFAs, SOAs, Raman amplifiers, and hybrid configurations, as well as functions related to amplifier design and optimization. In WDM, it is also important whether the individual channels are amplified sufficiently uniformly.
OSNR: why power alone is not enough
High optical power at the receiver input does not automatically mean high transmission quality, because noise accompanies the signal in the system. That is why one of the basic parameters analyzed in optical systems is OSNR (Optical Signal-to-Noise Ratio). OptiSystem provides a WDM Analyzer for evaluating the parameters of individual channels, including their power and OSNR. In practice, this lets you check whether the system still maintains adequate signal quality after successive fiber spans and amplifiers.
BER and Q-factor: from optical parameters to transmission quality
The ultimate goal of a telecommunications system is not to obtain a nice optical spectrum but to transmit information correctly. That is why BER (Bit Error Rate) is an important parameter. OptiSystem has tools for analyzing BER and Q-factor. The manufacturer states that these parameters can be determined using numerical or semi-analytical methods for systems limited by intersymbol interference and noise.
The analysis can also use the eye diagram, constellation diagram, optical spectrum, signal chirp, polarization state, signal power, and OSNR. This lets you observe the whole chain: physical phenomenon → signal degradation → change of reception parameters → BER.
Eye diagram: a quick assessment of the signal
In direct-detection systems, one of the most intuitive diagnostic tools is the eye diagram. By overlaying successive segments of the signal waveform, you can observe the eye opening and the influence of noise and intersymbol interference. OptiSystem includes a BER Analyzer with results that cover, among other things, the eye diagram, minimum BER, and Q-factor. Optiwave tutorial materials show the use of this analyzer in WDM projects as well. It is worth remembering, however, that the eye diagram does not replace an analysis of the whole system; it is one of the diagnostic tools.
Coherent transmission
Modern high-capacity systems also use coherent detection. OptiSystem supports the design of transmitter and receiver subsystems with both direct and coherent detection. Among the supported modulation formats, the manufacturer lists, among others, DPSK, QPSK, DP-QPSK, PM-QPSK, 16-QAM, and 64-QAM.
In such systems, analysis of the constellation diagram becomes very useful. The ideal constellation points can be blurred and shifted by noise and transmission distortions. A simulation lets you observe these changes before building a physical transmission testbed.
OFDM in optical networks
OptiSystem also supports OFDM (Orthogonal Frequency Division Multiplexing), in both direct and coherent variants. In OFDM, the data stream is divided among many orthogonal subcarriers. In optical systems, this technique can be analyzed, among other things, in the context of the influence of chromatic dispersion, PMD, and other transmission limitations. Optiwave provides examples of Coherent Optical OFDM, including applications in high-speed DWDM systems.
Nonlinear effects: when increasing power stops helping
Increasing the signal power seems a natural way to compensate for losses, but it cannot be done without limits. Nonlinear effects can occur in the fiber, and their importance grows with system parameters, including the power level, the transmission length, the fiber properties, and the channel configuration.
A compromise therefore has to be sought in the design: too little power means problems with the power budget and noise, while too much power means a greater influence of nonlinearities. OptiSystem makes it possible to analyze both linear and nonlinear transmission penalties at the system level. This is one of the reasons why transmitter parameters should not be chosen in isolation from the rest of the link.
Parametric sweep: instead of a single simulation
A single simulation answers only the question of how the system works for one set of parameters. A much more useful answer is to the question of how transmission quality changes when I change a design parameter. OptiSystem has automatic parameter sweep and optimization functions. You can analyze, for example, the following dependencies: BER as a function of input power, BER as a function of link length, Q-factor as a function of transmitter power, OSNR as a function of the number of spans, and BER as a function of amplifier parameters. The official Optiwave tutorials include, among others, the exercise Parameter Sweeps: BER x Input Power. It is exactly such analyses that let you determine the region of parameters in which the system meets the requirements.
OptiGrating: spectral elements of the network
Some fiber-optic systems use Bragg gratings and other periodic structures. To design them, Optiwave offers OptiGrating. The program can model integrated and fiber-based devices containing optical gratings. The manufacturer lists applications that include, among others, dispersion compensation. In the Single Fiber module, it is possible to model coupling between forward- and backward-propagating modes, and the manufacturer lists applications including selective WDM filters, add/drop filters, dispersion compensators, and long-period fiber grating filters used to flatten the EDFA characteristic. You can also run grating parameter scans and analyze, among other things, bandwidth, sidelobes, peak position, and dispersion.
In a more advanced workflow, you can therefore combine the design of a spectral element in OptiGrating with an analysis of its influence on the whole transmission system in OptiSystem.
Example workflow for designing a broadband link
1. Define the transmission requirements
First, determine the data rate, the number of channels, the wavelength range, the transmission distance, the network architecture, the detection method, the modulation format, and the required quality parameters.
2. Build the transmitter model
The model should correspond to the real way the signal is generated and modulated. Depending on the system, it can include a laser, a modulator, a data generator, and appropriate coding circuits.
3. Build the WDM model
For each channel, a suitable wavelength or optical frequency is defined. The channels are then combined by a multiplexer. OptiSystem has official examples of both 8-channel systems and more extensive WDM projects.
4. Define the fiber
Specify the parameters relevant to the given simulation, including attenuation and dispersion properties. If a detailed analysis of the fiber itself is needed, you can use OptiFiber.
5. Add amplifiers and compensation elements
For longer routes you can introduce suitable amplifier stages and elements related to dispersion management.
6. Build the receiver
On the receiving side you can use a demultiplexer, a detector or a coherent receiver, and suitable signal-processing elements.
7. Add analyzers
Depending on the architecture, it is worth observing the spectrum, channel power, OSNR, eye diagram, BER, Q-factor, and constellation diagram.
8. Run a parametric sweep
Instead of manually testing individual configurations, you can vary selected parameters automatically.
9. Find the system limitation
The most important result of a simulation is not merely the statement that the system "works". You have to determine what limits its operation and how large the design margin is.
Which Optiwave tool for which task?
| Problem | Tool |
|---|---|
| Design of a complete optical link | OptiSystem |
| WDM / DWDM | OptiSystem |
| PON | OptiSystem |
| Coherent transmission | OptiSystem |
| OFDM | OptiSystem |
| EDFA / SOA / Raman | OptiSystem |
| BER and Q-factor | OptiSystem |
| OSNR and WDM channel analysis | OptiSystem |
| Eye / constellation diagram | OptiSystem |
| Parameter sweeps | OptiSystem |
| Detailed fiber parameters | OptiFiber |
| Fiber dispersion | OptiFiber |
| Fiber birefringence and PMD | OptiFiber |
| Bragg gratings and periodic elements | OptiGrating |
| Design of filters and compensators using gratings | OptiGrating |
And where does Lambda Research fit in?
For this topic, the right division of competences should be maintained. TracePro and OSLO from Lambda Research are not equivalents of OptiSystem and are not used for system-level simulation of BER, OSNR, or multichannel DWDM. TracePro is primarily an environment for designing and analyzing optical systems using ray tracing, including illumination and stray light analyses. The current version of the program develops both non-sequential ray tracing and tools related to optical design.
In a fiber-optic infrastructure project, Lambda Research tools therefore make sense mainly when the problem goes beyond transmission in the fiber itself. Examples include optical coupling of a transmitter or receiver, collimation, the optical system of a transceiver, or stray light analysis in an optoelectronic module. The project can then be divided: Lambda Research handles the optics of the physical module, and Optiwave handles the transmission and the behavior of the fiber-optic system. There is no justification, however, for artificially introducing TracePro into a classical DWDM or BER analysis.
The most common mistakes in fiber-optic network simulation
Analyzing only the power budget
Adequate power at the receiver does not guarantee correct transmission. Noise, dispersion, nonlinearities, and receiver parameters also matter.
Analyzing only one WDM channel
A multichannel system should also be checked at the level of individual channels.
An ideal amplifier
A real amplifier affects not only power, but also noise and the characteristic of the whole system.
Ignoring fiber properties
Attenuation is only one of the fiber's parameters. Depending on the system, dispersion, PMD, and nonlinear effects can matter.
One configuration instead of a sweep
A design that works for one set of parameters can have a very small safety margin.
Choosing the simulation method by product name instead of by problem
For analyzing a complete WDM system, the natural tool is OptiSystem. OptiFiber is used for more detailed characterization of the fiber, and OptiGrating for designing grating-based elements.
Simulation before building the network
The most important advantage of a system model is the ability to test variants before building the physical path. We can change the fiber length, the transmitter power, the number of channels, the amplifier parameters, the modulation format, and the compensation parameters, and observe the influence of these changes on BER, Q-factor, OSNR, the spectrum, the eye diagram, or the constellation. This lets you not only find a configuration that meets the requirements, but above all understand why a given configuration works and what limits it.
From a single fiber to a complete network
The design of broadband transmission can be divided into several levels. At the fiber level, in OptiFiber, we check dispersion, losses, birefringence, and PMD. At the level of a spectral component, in OptiGrating, we analyze the behavior of a filter or grating. At the system level, in OptiSystem, we check how transmitters, WDM channels, fibers, amplifiers, and receivers behave as a single transmission path. At the level of the module's physical optics, with Lambda Research, we analyze the operation of coupling, collimation, and transceiver optics. This approach lets you use each model where it is most justified physically.
Summary
Simulating broadband fiber-optic networks requires analyzing the entire transmission path, not just a single fiber. As the data rate, the number of channels, and the transmission length grow, you have to take into account the mutual interaction of the transmitter, the fiber, dispersion, amplifiers, filters, noise, and the receiver.
In the Optiwave ecosystem, the central tool for this task is OptiSystem, which supports, among other things, CWDM, DWDM, PON, coherent transmission, OFDM, optical amplifiers, and analysis of BER, Q-factor, OSNR, spectra, eye diagrams, and constellations. OptiFiber complements this process with a more detailed analysis of fiber properties such as dispersion, losses, birefringence, and PMD, while OptiGrating lets you design fiber-based and integrated elements that use optical gratings.
As a result, it becomes possible to move from the question "what parameters does the fiber have?" to a much more important one: how will the whole transmission system work across the real range of operating parameters?
Manufacturer resources
OptiSystem product page with a description of the component library: Optiwave: OptiSystem
Tutorial on building an 8-channel WDM system: Optiwave: Optical Systems: WDM Design
OptiFiber product page: Optiwave: OptiFiber
OptiGrating product page: Optiwave: OptiGrating