The evolution from 50 to 62.5 µm multimode fiber happened when the first-generation optical LAN application and standard was put in place. The primary reason for the standardization on 62.5 µm at the time was that more light could be coupled into 62.5 µm fibers (Numerical Aperture, NA = 0.275) using lower cost transceivers, light emitting diodes (LEDs), that produce a uniform, overfilled launch condition. This made more light available to support longer link lengths with a given power budget. Besides that, 62.5 µm had better handling capabilities.
Corning® 62.5 µm multimode fiber was developed and used in long-haul systems such as AT&T and Northeast Corridor in the early 1980s as well as the local telephone interoffice applications. After the commercial introduction of Corning® Single-Mode Fiber in the long-haul projects with MCI and other operators in 1982, the far superior transmission capabilities of single-mode fiber compelled operators to convert these projects to single-mode fiber such that multimode fiber is rarely used in telephony public networks today. The high price of single-mode transceivers helped multimode fiber maintain a value proposition in local, private-area networks. Overseas, applications continued to favor 50 µm fiber, but in the U.S. several users and applications selected 62.5 µm, since it was a Bell System standard. The first was IBM, choosing it for a channel extended product to increase the interconnect distance between computers.
It was then designed into the first optical network standard, the fiber distributed data interface (FDDI), which specifies a 160/500 MHz•km bandwidth for 62.5 µm at the 1300 nm wavelength and later into early IEEE 802.3 ethernet standards.
The overwhelming advantage of single-mode fiber in transmission capability drove 62.5 µm into local area networks (LANs) and out of telephony. Then fiber geometry, connector polishing and alignment as well as LED output power have improved significantly. As a result, numerical aperture is no longer considered as critical as it once was.
Also, data rate requirements have exceeded the capabilities of LEDs (622 Mbps maximum direct modulation speed) due to fiber chromatic dispersion. The advent of low cost gigabit laser technology – 850 nm VCSELs (vertical cavity surface emitting lasers) — is generating more demand for higher bandwidth 50 µm multimode fibers. The low cost 850 nm 1 gigabit and 10 gigabit lasers are characterized by small spot sizes. Light is launched into a small area in the center of the core, with no need for the large acceptance angle to couple the light. This, in combination with the high bandwidth advantage of 50 µm fiber in the 850 nm window, forms a strong value proposition for using 50 µm for high performance data transmission systems.
In addition, the fiber processing and cabling technologies have improved significantly. Bare and cabled 50 µm fibers can have lower attenuation than 62.5 µm fibers.
The major benefit of 50 µm fiber is that it is specifically designed to produce higher bandwidth values than 62.5 µm at 850 nm, which enables the fiber to be used with lower cost 850 nm VCSEL transmitters. Standard 50 µm fiber (500/500 MHz•km bandwidth) has three times the bandwidth of standard 62.5 µm fiber (160/500 MHz•km) in the short wavelength (850 nm) operating window, while some of the newer laser-based 50 µm fiber designs have 10-20 times the bandwidth of standard 62.5 µm fiber (160/500 MHz•km). 50 µm fiber is also specifically measured for 850 nm laser-based system performance. These laser-based bandwidth measurements provide a guarantee of system performance over longer link lengths and/or higher-speed protocols than 62.5 µm.
This is a common question from network managers and installers about both new and installed multimode fiber systems, and it is no wonder since bandwidth performance is such an important factor in ensuring the highest performing, most reliable and cost-effective systems for premises networks. Unfortunately, because of the high cost, physical size, and extreme fragility of the equipment involved in measuring bandwidth, accurate field-testing of multimode fiber is not available.
In fact, fibers with high bandwidth can be difficult to measure even in a laboratory environment. At Corning’s Center for Fiber Optic Testing, we conduct in-house testing on short lengths of high bandwidth Corning® InfiniCor® fibers using a combination of bit-error rate (BER) and eye diagram analysis with a variety of commercially available transceivers. In addition, Corning is able to measure effective modal bandwidth (EMB) in the laboratory in accordance with the TIA 455-204 (FOTP 204) and IEC 60793-1-41 standards. These BER and EMB measurements require access to both ends of the cabled fiber and racks of equipment at a combined cost of over $1 million. This testing, and the historical associated analysis, provided the foundation for laser-based bandwidth measurements standards used today. The equipment for this type of testing is not portable or rugged enough for use in the field.
Therefore, Corning cannot recommend an accurate bandwidth test for the field. Rather, we continue to recommend a high-quality, standards-compliant bandwidth measurement from the fiber manufacturer. Corning has spent several years analyzing the reliability of the different laser bandwidth measurement methods against the current industry trends and technologies. With the publication of the IEEE 10G standard and the move toward high-performing laser-optimized 50-micron fibers for use with low-cost 850 nm transceivers, it is more important than ever to make sure the bandwidth metric used is scalable and can be applied to any bit rate or link length scenario. As a result, Corning has chosen the most reliable of the available laser bandwidth metrics, the minimum calculated effective modal bandwidth (minEMBc) for the InfiniCor® SX family of laser-optimized fibers.
The minEMBc measurement is unlike traditional bandwidth measurements that use averaging and simplifying assumptions about transceiver power distribution. It starts with a differential mode delay measurement (DMD) that captures the temporal, spatial and attenuation performance of a single fiber in extraordinary detail. The fiber’s performance is then characterized across a wide range of standards-compliant VCSELs. The weightings of every possible fiber-laser pairing are combined with the raw DMD data to build an output pulse for each specific combination. The worst-case output pulse calculation is then used to certify the fiber’s EMB in MHz.km. Additionally, by using the range of acceptable VCSELs specified in the IEEE 802.3ae standard, the fiber’s performance can then be accurately predicted against the full range of standards-compliant sources. By knowing the minimum calculated EMB value, you are guaranteed that the specific Corning fiber you receive will perform with any standards-compliant VCSEL in the field.
Some might say differences in measurement methods are subtle and unimportant. However, without a reliable laser bandwidth measurement, the performance of your network will be in question. To ensure optimal field performance, you should research the manufacturing and measurement methodologies of each fiber supplier you consider and understand the available choices. At Corning, every meter of every reel of InfiniCor® fiber is tested with a laser. There is no sampling and no shipping to a third party for measurement. If you are unsure where to begin and what guidelines are important, please reference Corning’s Fiber Selection Guide for Premises Networks, which provides a brief overview of appropriate laser bandwidth metrics and important specifications in fiber selection. Of course, you can always contact Corning’s Premises Fiber team who would be glad to help direct you to the many informational resources about bandwidth metrics and measurement methodologies on the Corning Optical Fiber website.
Costs and equipment complexity prevent the ability to measure bandwidth in the field, which makes the choice of fiber manufacturer even more important for design considerations at 1G, 10G and beyond. Making the right choices at the time of fiber selection will give you confidence in the bandwidth performance of your premises network. The bandwidth of the fiber in your network is far too important to take risks. Backed by high-quality and reliable bandwidth measurements, you can scale your Corning® InfiniCor® fiber network to meet your performance requirements now and into the future.