At present, the mode bandwidth of 850 nm multi-mode fiber is the highest in OM4 fiber, which can support 100 meters transmission of 100 g system. If the mode bandwidth is further increased, the refractive index distribution needs to be controlled more carefully, which puts forward higher requirements on the production process and has a great impact on the yield of the product. On the other hand, the total bandwidth of the system is limited by fiber mode bandwidth and fiber dispersion. Because of the influence of line width of current VCSEL, multi-mode fiber dispersion becomes the most important limiting factor affecting speed and link distance. If you want to increase the transmission rate or transmission distance of the system, you can usually use two methods: using single-mode fiber and single-mode laser. Or the multi-mode fiber is still used, but a narrower line-width laser is used to limit the incidence mode of the multi-mode fiber. The disadvantages of these two methods are that more expensive lasers are required and the fiber coupling process requires higher alignment accuracy, which will result in higher cost and connection cost of the optical module. Therefore, multimode optical fiber technology needs to be improved to realize higher capacity and longer distance transmission. The research on new multimode fiber is mainly focused on the following directions.
1. Long wave multimode fiber
Long-wave optimized high-bandwidth multi-mode fiber (980nm/1060nm or 1310nm) combined with light source (such as long-wave VCSEL) is a feasible scheme to realize long-distance and high-speed transmission. The long wave multimode fiber system retains the advantages of low coupling loss and easy alignment of conventional 850 nm multimode fiber, and the dispersion and attenuation values of the fiber are lower. Work in long wave region of low loss, low dispersion of multimode optical fiber system can achieve a higher rate and longer transmission distance, in recent years, a series of experimental results also prove the conclusion: the combination of 1310nm and 1310nm of multimode fiber silicon optics module, realizes the transmission distance of more than 820m, 1060 nm multimode optical fiber with 1060nm VCSEL laser combination has realized the
Conventional OM3/OM4 fiber bandwidth is generally only optimized for 850nm. In order to support the working mode of SWDM optical module, the performance of the fiber at 940nm needs to be quantified. Therefore, the association of the telecommunications industry (TIA) created a working group in 2014 to develop guidelines for broadband transmission of more than 500m (experiment above is 100 g rate).
2. Broadband multimode fiber
Based on the standard of 40G/100G formulated by ieee802.3ba, the transmission rate of 40G of multi-mode fiber is 4*10Gbp=40Gbps for each pair of fiber, 4*10Gbp=40Gbps for each pair of fiber, 4*25Gbps=100G for each pair of fiber, 4*25Gbps=100G for each pair of fiber. The transmission rate of 400G modules needs 16 pairs of 32 core fiber, which occupies a lot of fiber resources. The industry is exploring ways to use multi wavelength multiplexing to reduce the amount of fiber used.
There are two kinds of multi wavelength multiplexing products on the market. One is BIDI (bi-direction) technology, as shown in the figure below (taking 40G as an example). The optical module has two two-way channels of 20Gbps, and each fiber is capable of sending and receiving (the multi-mode fiber supports 850nm and 900nm wavelengths). Finally, 40G transmission is realized on two fibers, and no additional installation of MPT connector is required. It should be noted that since each fiber in the BIDI transceiver both transmits and receives signals, port branching is not supported. Another technique is short wavelength division multiplexing (SWDM). Similar to BIDI, SWDM only needs a two-core LC duplex connection, but SWDM needs to work at four different wavelengths between 850nm and 940nm, with one fiber for signal transmission and the other for signal reception.
multimode optical fiber (WB MMF) to support SWDM transmission. The WB MMF tia-492aaae standard was released in June 2016. Broadband multimode fiber is actually a kind of OM4 fiber with extended performance, because the broadband multimode fiber still has to meet the requirement of OM4 fiber EMB ≥ 4700 MHz at the wavelength of 850 nm. The bandwidth of km is required, and EMB at 953 nm wavelength is required to meet the requirement of ≥ 2470MHz*Km in October 2016, the international standards organization named broadband multimode fiber OM5 fiber.
BODI and SWDM using OM4 fiber can transmit 150m and 350m respectively at 40G, and the 100G module OM5 can support the transmission of BIDI and SWDM optical module 150m, in contrast, the transmission distance of OM3 and OM4 is 70m and 100m, but this distance is sufficient for most multi-mode scenario. OM4 can support various optical module solutions from 40G to 400G (such as 100G SR4, 100GBiDi, 400gsr4.2, 400GSR8, etc.). In practice, should be combined with the application scenario to select suitable multimode fiber, for example, the need to use the optical module SR4 / eSR4 port branch, OM5 OM4 and consistent performance, thus OM4 is more cost-effective solutions, and in more than 100 g or rate transmission distance of more than 100 m of the link, OM5 / SWDM combination can reflect the advantage of the long distance transport.