|What is it? |
DMD is a measurement of multimode fiber bandwidth. It's a measurement done in the factory. There is no field test equipment capable of making DMD measurements. Users should contact the fiber or cable manufacturer who will be able to provide the DMD for the series of fiber he/she has.
How is it measured?
- The output spot from a pulsed laser is aligned to the center of the core of the fiber under test.
- A high-speed detector and sampling oscilloscope receives the laser pulses exiting the fiber, and the output pulse amplitude as a function of time is recorded.
- The laser spot is then stepped across the fiber core radius and at each measurement position the same amplitude and time information is recorded.
- The relative delay of the radial pulses is determined using some form of pulse location technique (i.e., peak amplitude, center of gravity, x% of maximum power on leading or trailing edge, etc.).
In this differential mode delay measurement result, the fastest leading edge is at radial launch position is r = 0 m and the slowest edge is at r = 15 m.
The results of a DMD measurement on a well-compensated (low DMD) fiber would look something like figure above. Note that in this example, circles and dotted lines mark the fastest leading edge and the slowest trailing edge in the DMD data set.