Data Center Multimode Fiber Technology and Application Scenarios

Multimode fibers allow multiple modes (paths) of light to be transmitted simultaneously within the same core. Different lights enter the core at different angles of incidence, and are then continuously reflected between the core and the cladding for transmission. Compared to light at the center of the fiber, reflected light travels a longer specific path, possibly causing a slight increase in the time it takes for them to reach the receiving end of the fiber.

The big difference from single-mode fiber is that multi-mode fiber has a larger diameter. The larger core diameter means that multi-mode fiber can be used for several transmission methods. Although this makes its price higher than that of single-mode fiber, solid-state laser diodes are mostly used as light sources for single-mode fibers, while LEDs are mostly used for multi-mode fibers. For the light source, it is obvious that the former type of equipment is more expensive than the latter type of equipment, resulting in the cost of applying multi-mode fiber is much lower than the cost of applying single-mode fiber. Especially in the local area network wiring environment, the operation status of multi-mode fiber and single-mode fiber is equally good. Therefore, under the promotion of cost advantages, multi-mode fiber is more suitable for the infrastructure construction of large data centers.

Multimode fiber technology and application fields

The development trend of cloud computing technology has promoted the development trend of integrated circuit technology big data center, which has resulted in a different development trend from traditional corporate big data centers. Whether in China or around the world, the evolution of server port speeds for customers of integrated circuit technology big data centers dominated by cloud computing technology business processes is significantly faster than that of traditional corporate big data centers. Traditional companies will stably apply multiple sets of module OM4 fibers, and about 90% of the system software external links are less than 100m long.

The integrated circuit technology big data center customers choose a large number of single-mode fibers, and 70% of the system software external links exceed 100m in length.

The development trend of integrated circuit technology big data center has improved the utilization rate of single-mode fiber, but multi-mode fiber still has its unique advantages. This advantage includes: lower cost optical modules can be applied, lower power, and the transmission distance can cover most of the external links in the large data center, so according to the multi-mode fiber and multi-module optical module solutions Still have a strong temptation to customers.

Network bandwidth of 850 nm multimode fiber

Different from the multi-mode system software, the transmission distance and speed of the multi-group module system software are limited by the network bandwidth of the multi-mode fiber. In order to use high-speed system software to transmit long distances, the network bandwidth of multimode fiber must be improved. The design scheme of multimode fiber generally uses the gradient color index model to reduce the mode group delay and maintain high bandwidth:

Among them, r0 is the semi-longitude of the core, which is the highest value of the relative refractive index transition of the core, which can be expressed as follows:

Among them, n0 is the central refractive index of the core, and n1 is the refractive index of the insulating layer.

By choosing a suitable value, the network bandwidth of multimode fiber can be improved within the necessary optical wavelength range. The network bandwidth of 50 µm multimode fiber at 850 nm wavelength value shift by 1% is all over, and when the value of fiber is in the best position, the network bandwidth value exceeds 13 GHz.km. It also shows that the network bandwidth of multimode fiber is very sensitive to the value. If the network bandwidth is kept large, the value (core refractive index) must be carefully manipulated, otherwise the core model will be different in the whole process of manufacturing. Disadvantages affect the specific network bandwidth of multimode fiber.

With the development of optical fiber design scheme and manufacturing and processing technology, the network bandwidth of multimode optical fiber has been greatly improved. The 62.5 µm multimode fiber has a high numerical aperture and a large core, which can couple LED light sources (LEDs) into the fiber, and is suitable for data transmission of 2 km at speeds of 10 Mbit/s or even 100 Mbit/s . Following the development trend of Ethernet interface specifications and low-cost 850 nm VCSELs, multimode fibers with a core diameter of 50 µm are more popular in the sales market. The fiber has lower mode color refraction and higher network bandwidth, and the spot size and numerical aperture of VCSEL are smaller than that of LED, which can easily couple the laser into 50 µm fiber. According to the improvement of optical fiber production and processing technology, and the selection of excellent refractive index control system, the development trend of 50 µm multimode fiber is from OM2 (500 MHz.km) to OM3 (2 000 MHz.km), and now the development trend is OM4 (4 700 MHz.km). MHz.km).

For the multi-group module system software applying 850 nm VCSEL, comprehensively improving the network bandwidth of OM4 multimode fiber does not allow the optical module to transmit a long distance, because the network bandwidth of the system software depends on the reasonable method of the fiber network bandwidth and color index ( The comprehensive effect is related to the spectral line width of the VCSEL laser generator and the wavelength of the fiber light). If it is necessary to increase the network bandwidth of the system software, in addition to the reasonable network bandwidth of the optical fiber, the color index value must also be increased. This can compensate for a portion of the chromatic fold in accordance with the differential signaling mode delay (DMD) multimode fiber, and can also be used in 850 nm VCSELs with narrower pattern boundaries or in the long-wave region where the chromatic fold is lower in operation.

The larger relative index of refraction of the fiber core also has an effect on the larger network bandwidth. Since the network bandwidth is proportional to , when the core is reduced from 1% to 0.75%, the network bandwidth will double. However, reducing the core will increase the bending loss, and the bending characteristics must be improved by improving the overall design of the fiber.

Bend Insensitive Multimode Fiber

In the application of large data centers, the application of bend-insensitive multimode optical fibers is becoming more and more common. It can reliably design optical cables, hardware configurations and equipment to save a lot of indoor space, with stronger water cooling and high efficiency. Its easier connection and cable management method. Figure 5 shows the design scheme of the refractive index model of a bending-insensitive multimode fiber. The core is graded-index, and the insulating layer has a low-refractive-index trench.

The pipe groove reduces the optical power in the insulating layer, can prevent the leakage of the optical signal, and further improves the bending characteristics of the optical fiber. The fiber core and groove specifications are optimized in the optical fiber design scheme, and the bending characteristics and the compatible mode with the standard multimode fiber are balanced. According to the effective design of the core and the trench, the multimode fiber can maintain the high bandwidth and low bending loss of the OM4 level. Figure 6 shows a comparison of the bending loss measured at 850 nm. The macrobending loss of a multimode fiber that is less sensitive to bending is about 10 times lower than that of the basic standard multimode fiber.

Long Wave Multimode Fiber

A long-wave boosted high-bandwidth multimode fiber (980 nm/1 060 nm or 1 310 nm) combined with a light source (such as a long-wave VCSEL) is a viable solution for maintaining high-speed transmission over longer distances. The long-wave multimode fiber system software retains the advantages of low coupling loss and easy pointing of the basic 850 nm multimode fiber, while the color index and attenuation coefficient of the fiber are lower. The chromatic index and loss of optical fiber change with the wavelength of light. The chromatic index and loss at 1060 nm light wavelength are reduced by more than half compared with 850 nm. At 1310 nm, the color index is basically 0, and the loss is only 20% of that at 850 nm. .

The low-loss and low-color-refractive multimode fiber system software operating in the long-wave region can maintain higher speeds and longer transmission distances. A series of test results in recent years have also certified this basis: 1310 nm multimode The optical fiber is integrated with a 1310 nm silicon optical module to maintain a transmission distance of more than 820 m, and the fusion of a 1060 nm multimode fiber and a 1060 nm VCSEL laser generator maintains a transmission distance of more than 500 m (100G speed for both experiments).

ZR Cable focuses on the research and development and sales of optical fiber communication products. Optical communication products include standard product optical cables, optical fiber jumpers Cables, mpo/mtp data centers, optical fiber distribution frames, etc., at ZR Cable you can buy all optical communication products in one stop.