Have you ever tested a multimode fiber optic link with light sources from two different equipment vendors? By simply using a different light source you may notice that the loss measurement is different by as much as 50%! Ever wonder why this happens? Without proper controls, multimode light sources will inject light into multimode fiber over variable paths, or “modes”. “Modal power distribution” is a way of explaining how many modes are supported by a multimode fiber; typically, multimode fiber supports hundreds of modes while a singlemode fiber supports one mode. This modal power distribution is commonly referred to as a “launch condition”. When light sources operate under different launch conditions, link-loss measurements will vary proportionately, resulting in different and confusing test results.