L-com Connectivity Products Explains Fiber Optic Connector Terms - http://www.L-com.com
What do the fiber terms 9/125, 50/125 and 62.5/125 refer to?
They refer to the diameter of the core and cladding.
The first set of numbers - 9, 50 and 62.5 refer to the diameter of the core.
The second set of numbers - 125 refer to the diameter of the outside of the cladding.
Cladding keeps light from escaping the glass core.
9/125 refers to singlemode. 50/125 and 62.5/125 refer to multimode.
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