Can multimode fibers with different core diameters be mixed?

- Nov 20, 2020-

According to the ISO 11801 standard, multimode fiber can be divided into five types: OM1, OM2, OM3, OM4 and OM5. Among them, the core diameter of the OM1 multimode fiber is 62.5μm, and the core diameter of the remaining four multimode fibers is 50μm. These five types of multimode fibers are different in transmission rate, transmission distance, and sheath color. The smaller the core diameter, the higher the transmission rate that the optical fiber can achieve, and the longer the transmission distance of the optical fiber.

Why do we need to mix multimode fibers?

The 62.5μm multimode fiber uses light-emitting diodes (LED) as the light source, and is usually used for 10/100Mbps Ethernet.

With the continuous upgrade of the network rate, the multimode fiber with LED as the light source is far from being able to meet the transmission requirements of high-speed networks. As a result, a 50μm multimode fiber with a vertical cavity surface emitting laser (VCSEL) as the light source appeared. Compared with LED light sources, 50μm multimode fiber with VCSEL as light source has higher power and higher quality laser output. Therefore, the use of 50μm multimode fiber is becoming more and more widespread. Although many large-scale networks (such as data centers, etc.) are installed with 50μm multimode fibers, there are still many applications that require 62.5μm multimode fibers. Therefore, there is also a demand for mixing 50μm and 62.5μm multimode fibers. keep growing.

What are the problems with hybrid multimode fiber?

There are two cases of hybrid multimode fiber. One is that the light enters from the 62.5/125µm multimode fiber to the 50/125µm multimode fiber, and the other is that the light enters from the 50/125µm multimode fiber to the 62.5/125µm multimode fiber. Multimode fiber. As shown below:

For the first case, the 50/125μm multimode fiber has a smaller core diameter and can be easily coupled with the 62.5/125μm multimode fiber. In this case, the offset and coupling angle differences will not affect the fiber. The transmission caused too much impact. However, when the 62.5/125μm multimode fiber is mixed with the 50/125μm multimode fiber, due to the larger core diameter of the former, when the two multimode fibers are coupled, the light in the 62.5/125μm multimode fiber will be removed from the fiber. A part of the loss occurs in the cladding of the 50/125μm multimode fiber dispersed in the core. If the fiber loss is large, it is not recommended to mix 62.5/125μm and 50/125μm multimode fibers.

So, how to judge whether it is feasible to mix these two types of multimode fibers while ensuring low coupling loss? In fact, the traditional coupling loss range has been introduced in some documents. For example, in the "Optical Fiber Technology Handbook" issued by Delmar in August 2005, it specifies the acceptable coupling of 62.5/125μm and 50/125μm multimode fibers. The loss range is 0.9dB~1.6dB. If the actual loss exceeds this range, it is recommended not to mix 62.5μm multimode fiber with 50μm multimode fiber.

Although the acceptable coupling loss range for the mixed use of 50µm and 62.5µm multimode optical fibers has been clarified, we cannot know the specific values of the coupling loss of these two multimode optical fibers unless the actual link conditions are tested. Therefore, optical fiber manufacturers and related research institutions will conduct tests on the mixed use of multimode optical fibers in different situations to prove its feasibility. For example, a number of tests conducted by FOA show that the loss of the fiber with LED as the light source is higher than that of the fiber with VCSEL as the light source, and the loss of the fiber with VCSEL light source at 20 meters is higher than that at 1 meter or 520 meters. All low. The test of the fiber jumper using LED as the light source failed because its coupling loss exceeded the range of 0.9dB to 1.6dB, but the coupling loss of the fiber jumper using VCSEL as the light source was within the normal range.

In addition to FOA, Corning Optical Fiber has also done a lot of tests to prove the feasibility and reliability of mixing 50µm and 62.5µm multimode fibers. Unlike FOA, Corning has conducted thousands of tests, so the report is more practical and instructive. The test found that the mixed use of multi-mode fiber with laser and 800nm/1300nm LED as the light source has no significant coupling loss.

As mentioned above, thousands of tests have proved that although the laser sources of 50µm and 62.5µm multimode fibers are different, they are completely compatible. However, it is recommended that you do not mix different types of fibers in a single link. Otherwise, be prepared for the possibility of a larger link loss. If the loss is within your acceptable range, you can mix 50µm and 62.5µm multimode fibers as appropriate.

Compatibility of multimode fibers of different bandwidths/different fiber manufacturers

Not only is the compatibility of 62.5μm and 50μm multimode fibers important, but the compatibility of multimode fibers with different bandwidths or from different suppliers also requires attention. If you want to use the traditional 62.5μm multimode fiber to increase the bandwidth of the entire network instead of mixing it with the 50μm multimode fiber, you need to consider the compatibility of fibers with different bandwidths. Optical fiber manufacturers such as Corning have proven that as long as optical fibers and links comply with industry standards, they can be used together even with different bandwidths.