Multimode fibers are mainly used in data centers. Statistics show the bandwidth requirements for optical interconnections in large data centers have almost doubled every two years. This speed is unmatched by access networks, metropolitan area networks, and backbone networks. The 400G optical transceivers were first used in the data center. The 400 Gigabit Ethernet standards of 400GBASE-SR16, 400GBASE-SR8 and 400GBASE-SR4.2 for multi-mode fibers have been released.
Driven by the increasing transmission rate, the applications of multimode fibers have witnessed three important changes in recent years. Firstly, the single wavelength rate has reached 25G and 50G; secondly, the Parallel Optics Technology using MPO connectors has been widely used; thirdly, the industry chain related to SWDM (Short Wavelength Division Multiplexing) has become mature gradually. All of these changes have put forward new requirements for multimode fibers, especially the geometry and bandwidth performance of multimode fibers, and the consistency and uniformity of products.
Progress of multimode fibers and related Ethernet standards
Over the past decades, the multimode fiber industry has been highly associated with VCSEL (Vertical Cavity Surface Emitting Laser) industry. The IEEE 802.3 series Ethernet standards have consolidated multimode fibers and VCSELs to constitute a variety of transmission solutions.
From OM1 to OM5, multimode fibers have evolved to the fifth generation. The specifications are as shown in Table 1. The OM3 to OM5 fibers are designed with VCSELs as the light source and add effective modal bandwidth as a characteristic. The international standard for OM5 fibers was released by ISO/IEC in 2017, with an additional bandwidth of 953nm, laying a foundation for SWDM.
|Category||Core Diameter (μm)||Min.Effective Modal Bandwidth (MHz.km)||Min.Overfilled Launch (MHz.km)|
Table 1 Main Characteristics of Multimode Fibers
According to the IEEE 802.3 series Ethernet standards, the transmission distances of OM3 to OM5 fibers in the 10G to 400G systems are shown in Table 2. The parallel optics technology and SWDM technology are adopted in the 40G, 100G, and 400G systems.
|Rate||Standard||Wavelength (nm)||Bidirectional Communication Fiber||Max. Transmission Distance|
Development trend of multimode fibers
The development of multimode fibers has showed three major tendencies in terms of performance: high bandwidth, bending resistance, and multi-wavelength.
High bandwidth is mainly achieved by optimizing the preparation technology, fiber refractive index profile and material composition. The optical fiber preform manufactured by the PCVD process, which is also the best choice for manufacturing high bandwidth multimode fibers. Preform is composed of thousands of deposited layers and presents a precise refractive index distribution.
Bending resistance is achieved by designing the refractive index profile of the depressed inner cladding, as shown in Figure 2. The comparison of the macro-bending performance between bend-resistant multimode fibers and traditional multimode fibers is shown in Figure 3. The significant improvement in bending resistance provides higher reliability for complex dense cabling in data centers.
The OM5 fiber is the first standardized multimode fiber that focuses on multi-wavelength applications. There were already many reports on transmission tests of OM3 to OM5 fibers at wavelengths other than 850nm. Some companies and scientific research institutions have mainly carried out transmission tests at 880nm, 910nm, 940nm, 980nm, and 1060nm wavelengths. The results show that in the 880 to 1060nm wavelength range, the transmission distance of the OM5 fiber is obviously longer than that of the OM4 fiber. The development of multimode fibers for multi-wavelength applications and related VCSELs is still in an initial stage. In the future, we will go deep into the development in this field.
Applications of OM5 fibers and the future
OM5 fiber was originally designed to meet the needs of WDM in a multimode transmission system, and its most valuable application is in the field of short-wavelength WDM. At present, most multi-wavelength optical transceivers, with 50G per wavelength, for multimode fibers remain in the R&D stage, and are only available from a limited number of optical transceiver suppliers in small quantities, as samples for internal experimental use. PAM4 modulation can achieve a single-wavelength rate of 50G using present 25G VCSEL. The two-wavelength bidirectional (BiDi) and SWDM4 techniques reduce fiber usage by half and three quarters, respectively, for high-speed Ethernet links above 100G.
Investigators have discovered that a fluorine-doped fiber core layer reduces the dependence of the optimal alpha values on various wavelengths, thus increasing the bandwidth of “ultra-wideband multimode fiber” across the entire 850nm – 1050nm range, and proving that “ultra-wideband multimode fiber” is capable of supporting eight WDM channels at 30-nm intervals within the 850nm – 1050nm window.
Over the past three years, experimental results for data transmission using OM5 fiber and “ultra-wideband multimode fiber”, under the application of PAM4 modulation and WDM, have been reported by various optical fiber suppliers and optical transceiver suppliers. As indicated in these experimental results, reported in Table 2, OM5 fiber is sufficient to support 100G, 200G and 400G multi-wavelength transmission systems with ranges in excess of 150 meters.
Apart from this, after design optimizations, 50μm core diameter multimode fiber can achieve lower differential mode group delay (DMGD) compared with few-mode fiber in the 1550nm window, and can thus increase fiber capacity several-fold when applied in multi-input multi-output (MIMO) MDM system ,demonstrating the future potential of multimode fiber in MDM.
Multimode optical fiber has always been regarded as an efficient, flexible transmission medium, and its potential applications to higher network transmission speeds have been continuously explored. The scheme with multimode fiber and VCSEL has advantages in terms of link cost, power consumption and availability, making it the most cost-effective data center solution for most enterprise customers. With the continuous steady growth of traffic demands in both cloud and enterprise local data centers, the cost-effective multimode fiber solutions have a broad potential market. The OM5 fiber solutions defined by new industry standards are optimized for multi-wavelength SWDM and BiDi transceivers, and provide longer transmission link distances and network upgrade margins for high-speed transmission networks exceeding 100G.