Many Paths, One Fiber
In a multimode optical fiber, light does not travel along a single path. Hundreds of guided modes propagate simultaneously, each bouncing off the core-cladding interface at a different angle. The axial ray travels the shortest path straight down the center, while high-order modes zigzag at steep angles, covering considerably more distance. This path-length difference is the root cause of modal dispersion.
Pulse Broadening
A sharp laser pulse launched into a multimode fiber arrives at the far end as a broadened blob. The fastest mode (axial) arrives first; the slowest mode (highest order, bouncing at the critical angle) arrives last. The delay spread between them grows linearly with fiber length. When the spread exceeds the pulse spacing, adjacent bits overlap — inter-symbol interference destroys the signal.
Bandwidth-Distance Tradeoff
Modal dispersion imposes a hard ceiling on the bandwidth-distance product. A step-index multimode fiber with a 2% index difference has a BW-L product of only about 20 MHz-km — enough for 100 Mbit/s over 200 metres, but hopeless for long-haul telecom. Graded-index fibers with parabolic profiles equalise mode velocities and push the product above 1 GHz-km.
Visualising Mode Delay
This simulation shows the input pulse shape at the launch end and the broadened output pulse at the far end. As you increase fiber length or index difference, watch how the output pulse widens and flattens. The animated ray paths inside the fiber cross-section show low-order and high-order modes racing each other — a visceral demonstration of why single-mode fiber dominates long-distance communication.