Development of Multimode Fiber in Data Centers

Most connections within a data center are short distances, ranging from a few meters to hundreds of meters. In these short-distance high-speed data communications, multimode fibers and optical modules with vertical-cavity surface-emitting lasers (VCSELs) as the core devices have been widely used. Compared with the single-mode transmission, the multi-mode uses a low-cost, low-power laser to achieve fast and efficient coupling between the fiber and the laser. Multimode fiber can achieve higher transmission rates or longer transmission distances than copper cables, at a lower cost than single-mode fiber systems. At present, the internal connection rate of the data center has reached 100Gbit/s, 100G optical transceivers have become the mainstream products in the industry, and 400Gbit/s is just around the corner. The industry has been developing new types of multimode fibers to improve their performance, including broadband multimode fiber technology that enables wavelength division multiplexing in a single fiber; and long-wavelength multimode fibers that support longer transmission distances. In addition, to support high-density, miniaturized connections. To improve the space utilization, heat dissipation efficiency, and cable management efficiency of data centers, multimode fibers with bending resistance have also been rapidly developed and deployed. This article will discuss the development trend of multimode fibers supporting high-speed optical modules based on the technical principles of multimode fibers and the evolution of optical module technology.

Multimode Fiber Development of Next-Generation Multimode Fiber

At present, the highest mode bandwidth of 850nm multimode fiber is 0M4 fiber, which can support 100m transmission of 100G system through 100G QSFP28 SR4. If the mode bandwidth is further increased, finer control of the refractive index distribution is required, which puts forward higher requirements on the production process and has a greater impact on the product yield. On the other hand, the total bandwidth of the system is limited by the two factors of fiber mode bandwidth and fiber dispersion, and a single increase in mode bandwidth has limited improvement in system transmission performance. This is because affected by the line width of the VCSEL currently used, the multimode fiber dispersion becomes the most important limiting factor affecting the speed and link distance. If you want to increase the system transmission rate or distance, there are usually two methods: use single-mode fiber and single-mode laser; or still use multi-mode fiber, but use a narrower linewidth laser to limit the incident mode of the multi-mode fiber. . The disadvantage of these two methods is that more expensive lasers are required, and the fiber coupling process requires higher alignment accuracy, which will lead to higher cost of optical modules and connection costs. Therefore, there is a need to improve multimode fiber technology to achieve higher capacity and longer distance transmission. Research on new multimode fibers. Mainly focus on the following directions.

Multimode Fiber Long Wave Multimode Fiber

Long-wave optimized high-bandwidth multimode fiber (980nm/1060nm or 1310nm) combined with a light source (such as long-wave VCSEL) is a feasible solution to achieve long-distance high-speed transmission. The long-wave multimode fiber system retains the advantages of low coupling loss and easy alignment of conventional 850nm multimode fiber, while the fiber has lower dispersion and attenuation values. The dispersion and loss of the optical fiber vary with wavelength. The dispersion and loss at the wavelength of 1060nm are reduced by half compared with those at 850nm, and the dispersion is almost zero at 1310nm, while the loss is only 20% of that at 850nm. Low-loss and low-dispersion multimode fiber systems operating in the long-wave region can achieve higher rates and longer transmission distances.

Multimode Fiber Broadband Multimode Fiber

Based on the 40G/100G standard formulated by 丨EEE802.3ba, the transmission meter of 40G multimode fiber supports the rate of 10Gbps with each pair of fibers 4*10Gbps=40Gbps, and every 4 fibers are required to send and receive, a total of 8 core fibers, 100G each 4 fibers are used to send and receive 4*25Gbps=100G, and a total of 8 fibers are used. The 400Gb/s transmission rate needs to use 16 pairs of 32-core fibers, which takes up a lot of fiber resources. The industry is exploring ways to use multi-wavelength multiplexing to reduce the number of fibers used.

There are currently two products on the market based on multi-wavelength multiplexing technology. One is BiDi (Bi-direction) technology. The optical module has two 20Gbps bidirectional channels, and each fiber has the function of sending and receiving (multimode fiber supports two wavelengths of 850nm and 900nm), and finally realizes 40G transmission on 2 fibers without additional installation of MPT connectors. It is worth noting that each fiber of the BiDi transceiver both transmits and receives signals, so the port branch function is not supported. Therefore, the Telecommunications Industry Association (T1A) created a working group in 2014 to compile guidelines on Broadband Multimode Fiber to support SWDM transmission. The TIA492AAAE standard of WBMMF was released in June 2016. Broadband multimode fiber is actually an OM4 fiber with extended performance because broadband multimode fiber still must meet the bandwidth requirements of OM4 fiber at 850nm wavelength EMB>4700MHz*km, and it is also stipulated that EMB at 953nm wavelength meets>2470MHz *km.

The above table compares the transmission distance of different optical fibers (OM3/4/5) matching different optical modules. It is sufficient for most application scenarios of multi-mode solutions. Figure 10 lists the optical module solutions based on OM4 fiber at 100 meters at various rates. OM4 can support a variety of optical module solutions from 40G to 400G (such as 100G SR4, 100G BiDi, 400G SR4, 400G SR8, etc. ). In practical applications, the appropriate multimode fiber should be selected according to the application scenario. For example, in the scenario where SR4/eSR4 optical modules need to be used for port branching, the performance of OM5 and OM4 are basically the same, so OM4 is a more cost-effective solution, while For links with a transmission distance of 100G or above, the OM5/SWDM combination can demonstrate the advantages of long-distance transmission.