The data center has become the engine of modern life, and the growing network information is transmitted and stored at high speed through the data center. Most of the connection distances inside the data center are short, ranging from a few meters to a few hundred meters. In these short-distance high-speed data communications, multimode optical fibers and optical modules with vertical cavity surface emitting lasers (VCSEL) as the core components have been widely used. Compared with the single-mode transmission scheme, the multi-mode scheme uses a low-cost, low-power laser to achieve fast and efficient coupling between the fiber and the laser. Multi-mode fiber can achieve higher transmission rate or longer transmission distance than copper cable, and lower cost than single-mode fiber system.At present, the internal connection rate of the data center has reached 100 Gbit/s, and 400 Gbit/s is just around the corner. The industry has been developing new types of multimode optical fibers to improve its performance, including broadband multimode optical fiber technology that realizes wavelength division multiplexing in a single optical fiber; and long-wave multimode optical fiber that supports longer transmission distances. In addition, in order to support high-density, miniaturized connections, and improve data center space utilization, heat dissipation efficiency, and cable management efficiency, multimode optical fibers with bending resistance have also been rapidly developed and deployed. This article will combine the technical principles of multimode fiber and the evolution of optical module technology to discuss the development trend of multimode fiber supporting high-speed optical transceivers.
1. The Features and Applications of Multimode Fiber Cables
The development of cloud computing has promoted the development of ultra-large-scale data centers, resulting in a development trend different from traditional enterprise data centers. Whether it is domestic or international, the evolution of server port rates for ultra-large-scale data center users based on cloud computing services is significantly faster than that of traditional enterprise data centers. Traditional enterprises will stably use OM4 Multimode Fiber Cables, and more than 90% of the system link length is less than 100m.
However, ultra-large-scale data center users choose more singlemode fiber cables, and 70% of the system link length exceeds 100m.
The development of ultra-large-scale data centers has increased the utilization rate of single mode fiber cable, but multimode fiber cable still has its unique advantages. These advantages are that the use of lower-cost optical transceiver modules, lower power consumption, and the transmission distance can cover most of the links in the data center, so solutions based on Multimode Fiber Cables and multimode optical modules are still very attractive to customers.
2. The Bandwidth of 850nm Multimode Fiber Cable
Unlike the single mode fiber optic system, the transmission distance and speed of the multimode fiber optic system are limited by the bandwidth of the multimode fiber cables. In order to support the higher transmission distance of the high-speed system, the mode bandwidth of the multimode fiber cable needs to be increased. The design of multimode fiber cable usually adopts a-profile of graded index to reduce the mode group delay and achieve high bandwidth.
Among them, rO is the core radius, ∆0 is the maximum value of the relative refractive index change of the core, which can be expressed as follows.
Among them, nO is the central refractive index of the core and n1 is the refractive index of the cladding.
Choosing an appropriate value of a, the mode bandwidth of the multimode fiber optic cables can be optimized within a certain wavelength range. Figure 3 shows the bandwidth distribution of a 50 µm multimode fiber cable when there’s 1% change in the a value of the 850 nm wavelength. When the a value of the fiber is at the optimal position, the bandwidth exceeds 13 GHz. km. The figure also reflects that the bandwidth of a multimode fiber cable is very sensitive to the A value. To achieve the maximum bandwidth, A value (core refractive index) needs to be very finely controlled, otherwise various defects in the core profile during the manufacturing process will affect the actual bandwidth of the multimode fiber optic cable.
With the advancement of optical fiber cable design and manufacturing technology, the bandwidth of multimode optical fiber cable has been greatly improved. Table 1 shows different types of standard multi-mode fiber cables. The 62.5 µm multimode fiber optic cable has a higher numerical aperture and a larger core, which can couple the light-emitting diode light source (LED) into the fiber, and supports 2 km at a rate of 10 Mbit/s or even 100 Mbit/s. data transmission. With the development of Ethernet standards and low-cost 850 nm VCSELs, multimode fiber cables with a core diameter of 50µm are more popular in the market. The fiber has lower modal dispersion and higher bandwidth, and the spot size and numerical aperture of the VCSEL is smaller than that of the LED, and the laser can be easily coupled into the 50 µm fiber. By optimizing the fiber manufacturing process and adopting advanced refractive index control technology, the 50 µm multimode fiber cable has developed from OM2 (500 MHz. km) to OM3 (2 000 MHz. km), and now it has developed to OM4 (4 700 MHz. km). ).
For the multimode fiber system using 850 nm VCSEL, further increasing the bandwidth of the OM4 multimode fiber optic cable will not enable the optical module to transmit longer distances, because the system bandwidth depends on the effective mode bandwidth and dispersion of the fiber optic cables (related to the line width of the VCSEL laser and the fiber wavelength). If the system bandwidth needs to be increased, in addition to the effective mode bandwidth of the optical fiber cable, the dispersion value needs to be optimized. Partial dispersion can be compensated by differential mode delay (DMD) multimode fiber optic cable, or 850 nm VCSEL with a narrower line width or working in the long-wave region with lower dispersion