For analysing the properties of a fiber being designed, Optiwave offers OptiFiber, which uses numerical mode solvers and models dedicated to fiber analysis. Among other things, the program computes loss, dispersion, birefringence and PMD.
Fiber attenuation
If the input power is P0 and the fiber has an attenuation coefficient α, the power decreases with distance. In fiber-optic engineering it's convenient to express loss in decibels. For a span of length L, the total loss can be written, in a simple model, as:
A = αL
where A is the span's attenuation in dB, α is the attenuation coefficient in dB/km, and L is the length in km. This, however, is a system-level description — it doesn't yet say what physical phenomena give rise to the value of α.
Material loss
One source of attenuation is absorption in the material. Its significance depends on the type of glass, the dopants, the wavelength, and the purity of the material. That's why, when designing a fiber, the material properties should be analysed across the wavelength range in which the device is meant to operate.
Scattering
Light can also lose energy propagating in the intended direction as a result of scattering. In classical telecom fibers, a very important mechanism is Rayleigh scattering, associated with microscopic inhomogeneities in the material. In other constructions, additional mechanisms arising from the fiber's geometry or structure may appear.
Bend loss
A perfectly straight fiber and a fiber laid on a small bend radius are not equivalent structures. Bending can increase loss, because part of the field stops being effectively confined within the guiding structure. This matters a great deal in industrial designs — a real device's fiber may be routed inside a tight enclosure, wound, bent repeatedly, or mounted close to mechanical elements. The design therefore has to account for more than ideal propagation conditions.
Loss and the mode distribution
The basis of any loss analysis is finding the modes of the fiber being designed. If the field is very strongly confined in the core, the structure can behave differently than a fiber in which a significant part of the field sits near the core boundary or outside it. That's why loss analysis is closely tied to mode analysis. OptiFiber can compute the fundamental mode, higher-order modes, and selected groups of modes.
Loss of higher-order modes
For multimode or specialty fibers, not all modes necessarily have the same loss. This can lead to selective attenuation of specific modes. In some constructions this is an unwanted effect; in others it can be exploited as part of the design. That's why, instead of analysing only the total power, it's worth observing the parameters of individual mode solutions.
Wavelength dependence
A fiber's loss is generally a function of wavelength: α = α(λ). This dependence directly matters when choosing an operating band — the same fiber won't necessarily behave identically at 850, 1310 and 1550 nm. Specialty fibers can have an even more complex wavelength dependence, which is why analysing a single spectral point may not be enough.
Loss isn't the only criterion
Minimizing loss by itself shouldn't be the sole goal of fiber optimization. Changing the geometry can simultaneously affect the number of guided modes, dispersion, effective mode area, birefringence, PMD, and bend sensitivity. OptiFiber was designed precisely to analyse these interdependencies between a fiber's construction parameters and its propagation properties.
The refractive-index profile
The design starts with the fiber's cross-section. For a simple fiber this might be a step-index construction, but more complex profiles are also possible. Changing the core diameter, the refractive-index difference, the profile shape, the material, or additional layers changes the mode solution, and, through it, the other parameters as well. That's why loss analysis should be carried out together with profile and mode analysis.
Optimizing the design
A typical process might look like this:
- define the geometry;
- define the materials;
- compute the modes;
- check the propagation parameters;
- change the geometry;
- repeat the calculations;
- compare the results.
You shouldn't, however, pick a geometry simply because it gives the lowest loss at a single point. A construction that is nominally somewhat worse can be a better choice if it's more robust to manufacturing tolerances.
OptiFiber versus a measurement of the real fiber
A numerical model should ultimately be checked against a measurement. In a real fiber, effects can arise from profile inaccuracies, dimensional deviations, contamination, stress, micro- and macro-bends, and splices and connections. OptiFiber can describe the fiber being designed, but the model's correctness has to be verified against experimental data. Optiwave also points to the option of using OptiFiber together with real fiber analyzers, citing the NR-9200 Optical Fiber Analyzer as an example.
OptiFiber versus OptiSystem
This is another important distinction. OptiFiber mainly answers the question "what properties does the fiber being designed have?". OptiSystem answers "what do those properties mean for the complete link?". In OptiSystem, the fiber becomes one element of the path: transmitter → fiber → amplifier → receiver. You can then observe the effect of loss on received power, OSNR, the eye diagram, Q-factor and BER — a natural transition from the component level to the system level.
Coupling loss is a different problem
It's also important to distinguish two kinds of loss. The first arises during propagation inside the fiber. The second arises when light is launched into, or collected from, the fiber. If a laser or a lens system isn't well matched to the fiber's mode, part of the available power simply won't be coupled in at all. This kind of problem calls for Lambda Research tools. TracePro describes its use for optimizing coupling between fibers and between a fiber and elements such as lenses, collimators or other optical components. OSLO, in turn, includes tools for computing fiber-coupling efficiency using the overlap integral. A complete loss budget may therefore require two kinds of analysis: propagation loss (OptiFiber) and coupling loss (OSLO/TracePro).
Analysing the complete power budget
In a practical device, the budget can be broken down into input-coupler loss, fiber propagation loss, connector loss, splice loss, passive-component loss, and output coupling loss. Only the sum of these components lets you predict the actual power at the detector. This also shows why a fiber-design tool doesn't replace a whole-system simulator.
Validation
Loss analysis should end with a check that the numerical results make physical sense. It's worth running a numerical convergence test, comparing several mesh resolutions, sanity-checking the results for a simple case, and comparing against material data and against measurement. If a result changes significantly after changing the solver's numerical parameters, it shouldn't yet be treated as a stable physical characteristic.
Summary
Loss in an optical fiber is not a single, arbitrarily assigned number — it results from the construction, the materials, the wavelength, the character of the guided mode, and the conditions the fiber operates in. OptiFiber lets you start the analysis at the level of the fiber's physical design and study loss, dispersion, birefringence, PMD and mode properties together. If you then want to check the effect of these parameters on the whole link, you move on to OptiSystem. And when the problem isn't propagation inside the fiber but the efficiency of launching light into it, tools such as OSLO and TracePro become useful. The complete loss analysis therefore looks like this: source → coupling → propagation → connectors and components → receiver.
Manufacturer materials and illustrations
The OptiFiber product page with a sample program screen and charts: Optiwave – OptiFiber
A set of official tutorials covering the fiber profile and mode calculation: Optiwave – OptiFiber Tutorials
Lambda Research materials on loss arising from coupling light into a fiber: Lambda Research – Optimizing Fiber Optic Coupling Efficiency with TracePro
The TracePro page with sample optical- and fiber-path models: Lambda Research – TracePro