How to interpret the energy distribution map of multimode optical fiber

An energy distribution map of a multimode fiber shows how optical power is distributed among low- and high-order modes, revealing mode mixing, attenuation, and launch conditions that affect fiber performance.

Understanding Modal Distribution

In a multimode fiber, light propagates along multiple paths called modes. The low-order modes travel near the fiber axis, while high-order modes bounce more frequently off the core-cladding boundary and travel longer paths. The energy distribution map visualizes the relative power in these modes across the fiber core.

Step-index fibers: High-order modes experience more dispersion and attenuation, so the energy map often shows reduced intensity at the edges of the core for long fibers. Graded-index fibers: The refractive index gradually decreases toward the cladding, causing higher-order modes to travel faster and reducing modal dispersion. The energy map tends to be more uniform across the core.

Key Features to Interpret

Core Filling: Overfilled launch: Most modes are excited, producing a broad, uniform energy distribution. Underfilled launch: Only low-order modes are excited, resulting in a concentrated central peak in the map. Mode Mixing: Over distance, light redistributes among modes due to imperfections, bends, or scattering.

A well-mixed fiber shows a smoother, more symmetric energy map. Attenuation Effects: High-order modes are more susceptible to scattering and absorption. In long fibers, the energy map may show diminished intensity at the periphery, indicating equilibrium modal distribution (EMD).

Speckle Patterns: For coherent sources, interference between modes can create speckle in the energy map. This is normal and varies with fiber position and time.

Quantitative Measures

Modal Power Distribution (MPD): Represents the fraction of total power in each mode. Encircled Flux (EF): Measures the cumulative power within a given radius from the fiber axis, often used to standardize launch conditions. Core Power Ratio (CPR): Compares power in the multimode core to a reference single-mode core, indicating overfilled or underfilled conditions.

Practical Interpretation

A centralized peak indicates underfilled launch or dominant low-order modes. A uniform distribution suggests overfilled launch and good mode mixing. Peripheral attenuation highlights high-order mode loss, which can affect bandwidth and signal quality.

Temporal or spatial variations in the map may indicate modal noise or fiber perturbations. By analyzing these features, engineers can assess launch efficiency, fiber health, and system performance, and adjust coupling, source type, or fiber handling to optimize multimode fiber operation.