How is 50/125um or 62.5/125um fiber core
50/125um fiber core called OM1 too, 62.5/125um fiber core called OM2, they are the name of different muti-mode fiber core.
The first set of numbers - 50 and 62.5 refer to the diameter of the fiber cable's core.
The second set of numbers - 125 refer to the diameter of the outside of the fiber cable's cladding.
The cladding is a special coating that keeps the light from escaping the glass core.