Multimode fiber：
Transmission of multi-mode fiber at a given wavelength. According to its refractive index distribution, it can be divided into mutation type and gradual change type. The numerical aperture of common multimode fiber is 0.2 ± 0.02, core diameter / outer diameter is 50 μ m / 125 μ m, and its transmission parameters are bandwidth and loss. As there are hundreds of transmission modes in multimode optical fiber, the propagation constant and group rate of each mode are different, so the optical fiber has narrow bandwidth, large dispersion and large loss, which is only suitable for medium short distance and small capacity optical fiber communication system.
The core diameter of multimode fiber is 62.5 μ m / 125 μ m. or 50 μ m / 125 μ m. There are 2, 4, 6, 8, 12, 16, 20, 24, 36, 48, 60, 72, 84, 96 cores. The cable outer sheath materials include common type, common flame retardant type, low smoke halogen-free type and low smoke halogen-free flame retardant type.
Core diameter: an optical fiber consists of a protective layer, a cladding layer and a core, all of which are cylindrical. Core diameter refers to the diameter of the section circle of the cylindrical core. Multimode fiber refers to the fiber that allows multiple modes of light propagation.
50 / 125 μ m and 62.5/125 μ m multimode fibers are the common fiber types in optical transmission networks, in which 50 μ m and 62.5 μ m represent the core diameter used to transmit optical signals in the fiber, while 125 μ m represents the cladding diameter, which can protect the core and limit the propagation of light in the core. Although the cladding sizes of the two kinds of multimode fibers are the same, the different core diameters make their bandwidth different. So, can these two kinds of multimode fibers be mixed? What is the impact on the transmission performance of optical fiber after mixing?
According to ISO 11801 standard, multimode fiber can be divided into om1, om2, OM3, OM4 and om5. Among them, the core diameter of om1 multimode fiber is 62.5 μ m, and that of the remaining four kinds of multimode fiber is 50 μ m. These five types of multimode fibers are different in transmission rate, transmission distance and sheath color. The smaller the core diameter is, the higher the transmission rate can be achieved and the longer the transmission distance is.
Why need to mix multimode fiber?
The 62.5 μ m multimode optical fiber uses light emitting diode (LED) as light source and is usually used for 10 / 100Mbps Ethernet.
With the continuous upgrading of network speed, multimode fiber with LED as light source can’t meet the transmission requirements of high-speed network. Thus, 50 μ m multimode fiber with vertical cavity surface emitting laser (VCSEL) as light source appears. Compared with LED light source, 50 μ m multimode fiber with VCSEL light source has higher power and higher quality laser output. Therefore, 50 μ m multimode fiber is widely used. Although many large-scale networks are installed with 50 μ m multimode fiber, there are still many applications that need to use 62.5 μ m multimode fiber, so the demand for mixing 50 μ m and 62.5 μ m multimode fiber is also increasing.
What are the problems of hybrid multimode fiber?
There are two cases of hybrid multimode fiber, one is the light from 62.5/125 μ m multimode fiber into 50 / 125 μ m multimode fiber, and the other is the light from 50 / 125 μ m multimode fiber into 62.5/125 μ m multimode fiber. As shown in the figure below:
For the first case, the 50 / 125 μ m multimode fiber has a small core diameter and can be easily coupled with 62.5/125 μ m multimode fiber. In this case, the offset and coupling angle difference will not affect the transmission of the fiber. However, when 62.5/125 μ m multimode fiber is mixed with 50 / 125 μ m multimode fiber, due to the larger core diameter of the former, the light in 62.5/125 μ m multimode fiber will disperse from the core to the cladding of 50 / 125 μ m multimode fiber, resulting in some loss. If the fiber loss is large, it is not recommended to mix 62.5/125 μ m and 50 / 125 μ m multimode fiber.
So, how to judge whether it is feasible to mix these two types of multimode fibers while ensuring low coupling loss? The acceptable coupling loss range of 62.5/125 μ m and 50 / 125 μ m multimode fiber is 0.9db ~ 1.6db. If the actual loss exceeds this range, it is recommended not to mix 62.5 μ m multimode fiber with 50 μ m multimode fiber.
Although the acceptable coupling loss range of 50 μ m and 62.5 μ m multimode fibers has been determined, we can’t know the specific value of the coupling loss of these two multimode fibers without testing according to the actual link conditions.
The test shows that although the laser sources of 50 μ m and 62.5 μ m multimode fibers are different, they are fully compatible. However, it is recommended that you do not mix different types of fibers in a single link. If the loss is within your acceptable range, you can mix 50 μ m and 62.5 μ m multimode fibers according to the situation.
Compatibility of multimode fibers with different bandwidths / different fiber manufacturers
Not only the compatibility of 62.5 μ m and 50 μ m multimode fibers is very important, but also the compatibility of multimode fibers with different bandwidths or from different suppliers should be paid attention to. If you want to use the traditional 62.5μ m multimode fiber to increase the bandwidth of the whole network instead of mixing with 50 μ m multimode fiber, you need to consider the compatibility of different bandwidth fibers.
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