This simulator splits one input pulse into a single-mode fiber and a step-index multimode fiber. Change fiber length, wavelength, input pulse width, source spectral width, chromatic coefficient and multimode launch filling, then compare the ray arrival offsets and the RMS pulse widths at the two outputs.
• A 3D scene with a pulse splitter and launch heads, a small-core single-mode fiber with its field envelope, a large-core step-index multimode fiber with representative ray paths, an expanded relative-arrival display and a two-channel pulse monitor. • Eight controls for length, wavelength, pulse duration, spectral width, dispersion coefficient, launch filling and two marker toggles. • Eight readouts including each preset's V, axial transit time, five-ray arrival span, single-mode and multimode output RMS width, chromatic contribution and cutoff wavelength. • Four guided experiments, a verification bench, Curves & measurements and a Learn & assess tab.
A fiber supports a single spatial mode when V = 2πa·NA/λ is below 2.405. The small-core preset (a = 4.1 µm, NA = 0.12) does, and the large-core preset (a = 25 µm, NA = 0.20) supports many. In the multimode fiber, rays at different angles travel different path lengths, τj = nL/(c cosαj), and the spread of arrival times widens the pulse: σmulti = √(σsingle² + variance of offsets).
Chromatic broadening is separate and affects both fibers: σchrom = |D| L σλ / 1000 in ns. A single spatial mode removes modal spreading but the output pulse can still broaden, and because the pulse is normalized to unit area, broadening lowers its peak while conserving energy.
Both fibers are ideal weak-guidance step-index presets. The five equal-weight meridional ray groups are an illustrative delay ensemble rather than solved guided modes, and the common effective index is 1.468 with no attenuation, mode coupling or polarization effects. The dispersion coefficient is a user-specified constant that does not change automatically with wavelength, and phase and coherent interference are not modeled. Cross sections are enlarged and length compressed.
No. A single spatial mode removes modal dispersion, but chromatic dispersion still broadens the pulse when the source has a spectral width. The simulator lets you turn the chromatic term off to see the difference.
Rays at steeper angles travel longer paths through the core than axial rays, so they arrive later. This spread of arrival times widens the output pulse, and it grows with fiber length.
It is the normalized-frequency threshold below which a step-index fiber guides only its fundamental spatial mode. The small-core preset stays below it in the wavelength range of the simulator.
The launch excites only shallower rays, so the arrival span narrows. The large core still supports many modes; launch population is not the same as mode capacity.