Fiber specifications list the core and cladding diameters as a ratio. The top example shows the ratio of core to cladding as 62.5/125 microns. Below, the ratio is 9/125 microns. Multimode fiber is commonly 62.5/125 or 50/125 micron, single mode fiber is commonly 9/125 micron.
In Single mode applications the amount of back reflection on assemblies can be critical. L-com offers both PC and APC Single mode assemblies off the shelf. UPC finishes are available by custom order.
Stepped Index Fiber relies on the reflection of light whereas in Graded Index Fiber, the properties of the glass are changed in layers to "bend" the light as it travels down the cable. This reduces modal dispersion within the cable allowing for higher transmission rates.