Modal Dispersion

Fiber optic cables work essentially by flashing light through a transparent tube at a specific frequency and then measuring the frequency of those flashes on the other end (its slightly more complicated than that but only slightly and that’s a story for a different day.)

In thicker fiber optic cables, there’s a phenomenon called modal dispersion. Basically, some of the light in the fiber is taking a relatively straight path down the fiber while some of the light is bouncing back and forth off the sides of the fiber taking a longer route.

This means that some of the light from each flash gets delayed and the crisp flashes of light you started with tend to “smear” into one another if you transmit them over long distances. This limits how quickly you can flash the lights which ultimately limits the bandwidth of the cable.

Incidentally, limiting modal dispersion is a major reason why the glass core on some fiber optic cable is as little as 5 microns in diameter. For comparison, a typical human hair is 75 microns wide.

The reason I wrote this post wasn’t to explain how fiber optic data transmission works (although that’s pretty cool too). The reason I wrote this post is to express my dismay that the relatively obscure and very technical term “modal dispersion” also goes by all of the following names:

Multimode distortion

Multimode dispersion

Modal distortion

Intermodal distortion

Intermodal dispersion

Intermodal delay distortion

and Optical fiber dispersion

That’s far too many names for one very precise concept. Note to fiber optics engineers: sort your shit out... you’re embarrassing yourselves.