Multimode Fiber Modal Dispersion Calculator
Premises networking & data center optics: Compute multimode fiber (MMF) modal dispersion delay spread, parabolic graded-index (GRIN) optimization, and OM1–OM5 bandwidth-distance limits.
Fiber Type & Core Specifications
Modal Dispersion & Bandwidth Limits
Numerical Aperture (NA)
--
Light gathering cone
Relative Index $\Delta$
--
% ($(n_1 - n_2)/n_1$)
Modal Delay Spread $\Delta \tau_{mod}$
--
ps (over link length $L$)
Modal Bandwidth $\times$ Dist.
--
MHz·km (EMB)
Max Reach $L_{max}$ (@ $B$)
--
meters (ISI limit)
Guided Mode Count $M$
--
Spatial bound modes (@ 850 nm)
Ray Path Trajectory (Step vs Graded Index)
In graded-index (GRIN) fibers, rays traversing longer outer paths travel faster through the lower-index periphery, equalizing transit times across spatial modes.
Intermodal Dispersion Mechanics
In step-index multimode fibers, the time delay difference between the slowest ray (at critical angle $\theta_c$) and the fastest axial ray is severe:
$$\Delta \tau_{\text{step}} = \frac{L \, n_1}{c} \left( \frac{n_1 - n_2}{n_2} \right) \approx \frac{L \, n_1 \Delta}{c} \quad (\sim 50 \text{ ns/km})$$
$$\Delta \tau_{\text{GRIN}} \approx \frac{L \, n_1 \Delta^2}{8 c} \quad (\sim 0.05 \text{ ns/km for } \alpha = 2)$$
A parabolic graded index ($\alpha \approx 2 - 2\Delta$) reduces intermodal dispersion by a factor of over 500 to 1,000, enabling modern laser-optimized OM3/OM4/OM5 fibers to support 10G, 40G, and 100G Ethernet links across modern enterprise data centers.
Recommended Tools & Equipment
Tested hardware and components for high reliability
100% Free Tool
Zero Sign-Up