Multimode fibers normally refer to 50/125μm fiber and 62.5/125μm fiber. The 50μm and 62.5μm represent the diameters of the glass or plastic core inside the fiber optical cable. The fiber core carries the light which encodes your data. And the 125μm stands for the diameter of the cladding, which confines the light to the core. Here is the cross-section of the two types of multimode fiber optic cables. The difference in the core size will result in different bandwidths. The smaller core size will cause higher bandwidth.
Normally, multimode fiber can be divided into OM1 fiber, OM2 fiber, OM3 fiber, OM4 fiber and OM5 fiber determined by the ISO 11801 standard. Among these fibers, only OM1 fiber is 62.5/125μm fiber, the other four are 50/125μm fibers. Multimode fiber enables multiple light modes to be propagated in the fiber core at a given time. With different core size or bandwidths, the five types of multimode fiber differ in data rate, transmission distance, color, etc. The smaller a core size is, the higher data rates and transmission distance a fiber cable can reach.
Normally, light-emitting diode (LED) light sources and 62.5μm fiber are used in 10/100Mbps Ethernet. With the need of network upgrading for higher rates, the light source of new technology, vertical-cavity surface-emitting laser (VCSEL), is required. VCSELs can switch more rapidly than LEDs, making them better for higher data rates. And for longer transmission distances, 50μm fibers are more adopted than 62.5μm fibers. Therefore, most premises installations are now using laser-optimized 50/125μm fiber for 1/10/100 Gigabit Ethernet instead of the legacy 62.5/125μm fiber. However, existing fiber cable systems still use 62.5µm fibers in many applications, resulting in numerous needs in mixing the 62.5μm and 50μm fibers.
There are two occasions for mixing a 62.5/125µm multimode fiber and a 50/125µm multimode fiber. One is that the light needs to go into the 62.5/125µm fiber from the 50/125µm fiber, and the other is that the light goes into the 50/125µm fiber from 62.5/125µm fiber.
For the first case, the smaller core of the 50/125µm fiber can easily couple to the 62.5/125µm fiber and is very insensitive to offset and angular misalignment. Therefore, nearly no problem will happen. However, one big issue, link failing, may occur when upgrading to a 50/125µm multimode fiber from a 62.5/125µm one. When a 62.5/125µm multimode fiber is being mixed with a 50/125µm multimode fiber, the light in the 62.5/125µm fiber will escape into the cladding of the 50/125µm multimode fiber, which can cause a coupling loss. If the loss is relatively huge, it is not acceptable in mixing the 62.5μm and 50μm fibers.
Then, how to tell the feasibility of the mixed link with a lower coupling loss? Actually, the traditional range of mismatch coupling losses has been covered in several documents such as the Fiber Optic Technicians Manual published by Delmar in August, 2005, stipulating the range from 0.9 dB to 1.6 dB when mixing the 62.5/125µm multimode fiber and 50/125µm multimode fiber. If the actual loss exceeds the range, it is not recommended to mix 62.5μm fibers to 50μm fibers.
Although there is an acceptable range of mismatch coupling losses, one can not distinguish how the real losses will be without actual practices. Given such situations, many tests have been set up to prove whether the mismatch is reliable in most of the cases.
Several groups of tests connecting the 62.5μm fibers to 50μm fibers from FOA indicate that losses are higher with LEDs than VCSELs and the VCSEL has slightly lower excess loss at 20 meters than at either 1 meter or 520 meters. The tests with LED as the light source were failed because the coupling losses exceeded the acceptable range during 0.9 dB to 1.6 dB. The tests with VCSEL as the light source all passed that standard.
In addition to FOA, Corning also made contributions to the possibility and reliability of the mixing between 50µm and 62.5µm multimode fibers. Unlike the FOA, Corning has made several thousands of tests to make the reports more practical and meaningful. The Corning tests found no significant coupling losses both in laser sources and 800nm/1300nm LED sources.
Although 50μm and 62.5μm fibers are fully compatible with laser sources with thousands of tests, industry standards, leading media as well as equipment manufacturers recommend that you should not mix different types of fiber in a single link. They recommend you to plan for the worst and assume you will see a huge loss in one direction. Of course, if you can tolerate that loss, you can mix the 62.5µm fiber with the 50µm fiber directly without hesitation.
Regardless of the fiber compatibility in mixing the 62.5μm and 50μm fibers, multimode optical fiber compatibility of different bandwidths or from various suppliers also matters. For example, if you want to use the legacy 62.5µm fiber and increase the whole network bandwidth without mixing the 50µm fiber, fiber compatibility of different bandwidths is essential. In those applications where mixing fiber bandwidth types are unavoidable, it is technically feasible to do so and has been proved by companies like Corning. And the fibers from different manufactures are compatible with each other as long as the fibers and links are standards-compliant.