When we talk about multimode fiber optic cable we are typically refering to either 50/125μm fiber or 62.5/125μm fiber. The 50μm and 62.5μm refer specifically to the diameters of the glass or plastic cores of the cable (μm being the symbol of the micrometer or "micron" as it is typically refered to). The 125μm refers to the diameter of the cladding which surrounds the core of the cable.
Below is a cross section of two types of multimode fibers (50/125μm and 62.5/125μm). The difference shown is primarily the core sizes which determine the available bandwidthfor the fiber. The smaller the core the higher the bandwidth.
Cross-Section of 62.5/125μm and 50/125μm Cables
Multimode fiber is typically available in different categories names OM1, OM2, OM3, OM4 and OM5. Only OM1 fiber is 62.5/125μm and is generally encased in an orange jacket for easy identification. Multimode fiber allows for multiple light modes to be propagated in the core at a given time. Different core sizes allow for variations in data transfer rates and transmission distances. The smaller the core the higher the data rate and the longer the transmission distance the fiber can be used for.
What is the Problem with Mixing Multimode Fibers?
There are two possible ways of mixing 50/125μm and 62.5/125μm multimode fibers. Either from 50/125μm to 62.5/125μm or from 62.5/125μm to 50/125μm.
Mixing Multimode Fibers
It is far more feasable to run from 50/125μm and 62.5/125μm as this combination is far less sensative to ofset and angular misalignment. Think of it as water running from a narrow hose into a slightly thicker one, the only real issue that may occur is what is known as link failure. 62.5/125μm to 50/125μm is not as successful as the light from the 62.5/125μm will bleed into the cladding surrounding the narrower 50/125μm fiber which results in coupling loss. If the loss is great enough the cable will be considered unsuccesful.
Combining 62.5/125μm and 50/125μm Cables
How Feasable is Mixing Multimode 62.5μm and 50μm Fibers?
As shown above you can do it, but should you? The short answer is no. While both 50μm and 62.5μm fibers are compatible with plenty of standard laser sources you should always plan for the worst and assume you will see a huge loss in your fiber when combining different fiber types. If you are in a pinch and feel that your network can handle the loss then feel free. Ideally if you are forced into this situation rather run 50µm to 62.5µm as the loss will be far less than the other way around.