Communication fibres can be categorized as single-mode or multimode, depending on the number of transmission modes they possess at the operating wavelength. Due to its large core diameter, multimode fibre can be used with low-cost light sources, making it widely used for short-range transmission scenarios, such as data centers and local area networks (LAN). The rapid development of data centers over recent years, together with multimode fibre's popularity in data center and LAN applications, has led to its widespread popularity. This article outlines the development of multimode fibre.
The ISO/ IEC 11801 standard divides multimode fibres into five categories: OM1, OM2, OM3, OM4, and OM5, whose correspondence with IEC 60792-2-10 standard specifications is given in Table 1. Categories OM1 and OM2 refer to conventional 62.5/125 μm and 50/125 μm multimode fibre; OM3, OM4 and OM5 refer to novel 50/125 μm 10 Gbit/s multimode fibres.
| Category by ISO/IEC 11801 | OM1 | OM2 | OM3 | OM4 | OM5 | ||
|---|---|---|---|---|---|---|---|
| Core Diameter/μm | 50 | 62.5 | 50 | 62.5 | 50 | 50 | 50 |
| Correspondence of Category by IEC 60793-2-10 |
A1a.1 | A1b | A1a.1 | A1b | A1a.2 | A1a.3 | A1a.4 |
When R&D on multimode fibre began in the 1970s and 1980s, many sizes of fibre were investigated. Four sizes were initially included in the International Electrotechnical Commission (IEC) standard, with core/cladding diameters of 50/125 μm, 62.5/125 μm, 85/125 μm, and 100/140 μm. But larger core/cladding sizes increase manufacturing costs, decrease bending resistance, increase transmission modes, and lower bandwidth, thus, the two larger sizes were eliminated, leaving two main core/cladding sizes remaining in use: 50/125 μm and 62.5/125 μm.
In early LANs, low-cost LED light sources were generally used in order to minimize system costs. These generate light with low output power and a large divergence angle, but the core diameter and numerical aperture of 50/125 μm multimode fibre is small, making it ill-suited to efficient coupling with LED sources. The larger core diameter and numerical aperture of 62.5/125 μm multimode fibre allows more optical power in to the fibre link. Thus, prior to the mid-1990s, this size of multimode fibre was more popular than 50/125 μm multimode fibre.
As transmission rates have increased since the end of the 20th century, LANs have achieved 1 Gbit/s and higher, which exceeds the bandwidth capabilities of 62.5/125 μm multimode fibre with LED light sources, whose maximum bandwidth lies in the hundreds of megabits per second range. But the small numerical aperture, small core diameter, and low number of conduction modes of 50/125 μm multimode fibre effectively reduce its modal dispersion, greatly increasing its bandwidth capacity, and the small core diameter of this fibre has also lowered its manufacturing cost, aiding its return to popularity.
According to the IEEE 802.3z Gigabit Ethernet standard, both 50/125 μm and 62.5/125 μm multimode fibres can be used as transmission media, but for new networks, 50/125 μm multimode fibre is generally preferred.
Technological development eventually gave rise to the 850 nm Vertical Cavity Surface Emitting Laser (VCSEL). Thanks to its low price compared with long-wavelength lasers, and fast networking speed, VCSEL has become a popular light source for optical networking, but one to which optical fibre required adaption.
To meet the requirements of VCSEL, the International Organization for Standardization /International Electrotechnical Commission (ISO/IEC) and the American Telecommunications Industry Alliance (TIA) jointly drafted a standard for the next generation 50 μm core diameter multimode fibre, denoting this class of laser-optimized multimode fibre as "OM3" category (IEC standard A1a.2) .
The later OM4 category is actually an upgrade of OM3, from which it differs only by an improved fibre bandwidth. That is, compared to OM3 fibre, OM4 fibre has improved effective modal and overfilled modal bandwidths (EMB & OFL) at the 850 nm wavelength, as Table 2, below, shows.
| Fibre Type | Overfilled Modal Bandwidth (MHz·km) |
Effective Modal Bandwidth (MHz·km) |
|---|---|---|
| 850 nm | 850 nm | |
| OM3 | ≥1500 | ≥2000 |
| OM4 | ≥3500 | ≥4700 |
The enormous transmission modes of a multimode fibre limit its bending resistance: when fibres are bent, their high-order modes are prone to leakage, causing signal loss, i.e. bending loss, of the fibre. And as indoor application scenarios have become increasingly common, cramped indoor environments have imposed increased bending resistance requirements on multimode fibres.
While a simple refractive index profile is sufficient for a single-mode fibre, multimode fibre must be designed and manufactured with an extremely precise refractive index profile. Of the four mainstream techniques for manufacturing fibre preforms existing worldwide, the plasma chemical vapor deposition (PCVD) technique offers the most precision, and is typically adopted by YOFC. Unlike the other techniques, several thousand layers are deposited and each layer thickness is only about 1 micrometer, permitting the ultra-fine control of the fibre's refractive index curve necessary to achieve high bandwidth.
Optimization of the refractive index profile can also greatly improve bending resistance, as macro-bending loss for bending insensitive multimode fibre, presented in Figure 1, below, shows.
The OM3 and OM4 fibres are multimode fibres used mainly in the 850 nm band. However, as transmission rates increase, exclusive use of a single-channel over a fibre will lead to increased cabling costs and high management and maintenance costs. In order to address this, wavelength division multiplexing (WDM) has been introduced into the multimode transmission system. Via permitting multiple wavelengths transmission within a single fibre, this can greatly decrease numbers of parallel fibres and cabling and maintenance costs. WDM provided the context in which OM5 fibre was introduced.
Based on OM4 fibre, OM5 multimode fibre widens the high-bandwidth operating window supporting transmission in the 850 nm - 950 nm. Current mainstream applications adopt either SWDM4 or SR4.2 designs. SWDM4 uses WDM of 4 short waves, of 850 nm, 880 nm, 910 nm and 940 nm, permitting a single optical fibre to transmit services previously carried over four. SR4.2 employs two-wavelength division multiplexing, and is mainly applied in single-fibre bidirectional technology. OM5 fibre is compatible with high-performance, cost-effective VCSEL light sources, to better fulfill data centers' short-distance communication needs. Table 3, below, compares the main bandwidth specifications of OM4 and OM5 fibres.
| Fibre Type | Overfilled Modal Bandwidth （MHz·km） |
Effective Modal Bandwidth （MHz·km） |
||
|---|---|---|---|---|
| 850 nm | 953 nm | 850 nm | 953 nm | |
| OM4 | ≥3500 | Not required | ≥4700 | Not required |
| OM5 | ≥3500 | ≥1850 | ≥4700 | ≥2470 |
The novel high-endmultimode OM5 fibre has already found broad application. Its greatest business case is the commercial use of YOFC OM5 fibre in China Railway Corporation's main data center, which leverages the strengths of OM5 fibre in an SR4.2 WDM system, providing maximum communication capacity at the lowest cost, while remaining prepared for increased 100 Gbit/s - 400 Gbit/s transmission rates in future with dramatically reduced upgrade costs, since already-installed fibre will not need to be replaced.
Summary: As application demands have increased, the trend has shifted towards low bending loss, high bandwidth, and multi-wavelength multiplexing compatibility for multimode fibres. Of the existing multimode fibres, OM5 fibre offers optimal performance while enjoying the broadest potential application, and provides a powerful fibre solution for future 100 Gbit/s and 400 Gbit/s networking. Novel multimode fibres, such as single-mode/multimode universal optical fibre, are also under development to meet data centers' requirements in terms of speed, bandwidth and cost of data transmission. Together with industry partners, YOFC will continue to launch breakthrough novel multimode fibre solutions, further lowering costs for data centers and fibre interconnections.
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