Differential Mode Delay and Modal Bandwidth
For the differential mode delay measurement (DMD), an 850 nm probe is scanned at small radial increments across the core of the multimode fiber under test.
At each position the temporal response to a short impulse is recorded. After removal of the reference pulse temporal width, the DMD temporal width is determined at the 25% threshold level between the first leading edge and the last trailing edge of all traces encompassed between specified radial positions.
Encircled Flux
Encircled flux, as the name implies, describes the flux encircled within a circular radius inside the fiber.
Encircled flux is often quantified as the radius from the center of the fiber required to encircle 25% and 75% of the light energy through the fiber.
The power distribution of a fiber described by the encircled flux value is a crucial factor in ensuring the required data transmission rate in Gigabit Ethernet systems.