The fiber-manufacturing process impacts the medium`s information-carrying capabilities.
Preston Buck
Corning Inc.
The Institute of Electrical and Electronics Engineers (IEEE--New York City) standardized Gigabit Ethernet, the latest high-speed solution to network overload, with the adoption of IEEE 802.3z in June 1998, marking a transition to laser-based networks. Gigabit Ethernet responds to two fundamental requirements in fiber-to-the-desk networking: industry-accepted standards and low costs. For example, Gigabit Ethernet can operate with vertical-cavity surface-emitting lasers (VCSELs), which cost about the same as light-emitting diodes (LEDs) but offer superior performance and power. Because VCSELs operate at the 850-nanometer wavelength, they are ideally suited for multimode fiber; singlemode fiber is not designed to operate at this wavelength. At the 1310-nm wavelength, singlemode fiber represents the highest performance solution, but the necessary electronics come at a significant system price premium.
Therefore, multimode fiber is the appropriate, low-cost choice for Gigabit Ethernet systems. However, during the development of the IEEE 802.3z standard, interaction between lasers and multimode fibers produced unexpected bit-error rates because lasers concentrate power in only a small percentage of a fiber`s core, where even small centerline defects can dramatically reduce laser performance, resulting in bandwidth-limiting effects. Moreover, because fibers were specified according to overfilled launch bandwidth (OFLBW)--a good measure of relative fiber-performance capability when used with LEDs, which distribute power in 100% of the fiber core--a significant number of fibers demonstrated high OFLBW but actually experienced decreased performance with lasers. As a result, the IEEE 802.3z standard derated multimode fiber link lengths from 300 to 275 meters in the 850-nm operating window.
Technical difficulties surrounding laser/fiber interaction must be resolved to ensure the successful application of multimode fiber in high-speed, laser-based local area networks (LANs). Ongoing developments in standards and fiber manufacturing, including new multimode fibers that offer guaranteed performance in Gigabit Ethernet-compliant systems using a variety of lasers, are providing solutions to current market needs.
Manufacturing`s effect on laser/fiber interaction
When lasers are used to launch light into a fiber core, optical power is focused at the center, distributed in less than 5% of the fiber modes. Modal dispersion--the spreading of light pulses as modes travel different paths, each a different length--is low, and the potential for high bandwidth is great. In contrast, LEDs distribute optical power in 100% of the fiber core, and hundreds of modes are used, increasing the potential for modal dispersion. But even small imperfections at or near the fiber`s center can dramatically reduce laser performance, resulting in bandwidth-limiting effects. These defects--centerline dips and peaks in some fibers--are created during fiber manufacture.
Optical fibers are produced using either inside vapor deposition (IVD) or outside vapor deposition (OVD). Corning researchers invented both processes, but today Corning uses the OVD process exclusively. The IVD process requires a pre-made tube to form the outside of the fiber. A problem lies in that this tube potentially can be a source of flaws because it is impossible to remove all impurities during the tube`s forming states, which include melting. In the OVD process, no pre-made tube is required; the fiber is made entirely of ultra-high-purity vapor-deposited glass.
The OVD process results in smooth transition in the changing index of refraction from the center of the fiber outward. It eliminates profile roughness and results in a uniform and consistent centerline area. The IVD manufacturing process, on the other hand, produces fibers with an increased probability of centerline dips and peaks in the refractive-index profile. These defects are responsible for the reductions in laser performance in some fibers in Gigabit Ethernet systems.
Compounding this problem has been the use of a bandwidth specification that can be considered robust but, with respect to lasers, is misleading. To indicate information-carrying capacity, multimode fibers have been specified according to a minimum OFLBW standard at 850 and 1300 nm. With the overfilled launch, light is distributed throughout the fiber`s core. However, most of the power is carried by the intermediate light modes, which propagate down the fiber in the area away from the center and the core/clad interface. In other words, power is not concentrated at the core`s center, which is the site of potentially bandwidth-limiting defects.
OFLBW is a good measure of relative fiber-performance capability when used with LEDs, where optical power is distributed in 100% of the fiber core and hundreds of modes are used. But OFLBW alone is not a good indicator of functional performance in laser-based systems. In fact, it is not in itself highly correlated to performance in standard laser-based LAN systems because centerline defects responsible for poor performance do not necessarily degrade the OFLBW measurement.
Fiber that is specified according to elevated OFLBW alone may not meet expected performance requirements when used with lasers, which is especially true for performance targets much higher than the given standard. A significant number of fibers demonstrate high OFLBW but actually suffer decreased performance when used with lasers.
Avoid or eliminate the problem
One way to solve the problems associated with laser/fiber interaction is to direct the laser light source to enter the core of the fiber several microns from the center, thus avoiding the defects present in some multimode fibers. You can accomplish this using mode-conditioning patch cords, sometimes referred to as offset-launch patch cords, which are required by the IEEE 802.3z standard for use at 1300 nm. A mode-conditioning patch cord`s connectors, which couple the laser source to the cable, launch the light from a singlemode fiber into a multimode fiber through a controlled offset. While mode-conditioning patch cords are a solution, they come at a significant cost, may be considered cumbersome, and add complexity to a structured cabling system.
Another solution for successful use in Gigabit Ethernet systems is a multimode fiber manufactured to eliminate centerline dips and peaks and specified using measurement techniques that actually characterize the use of multimode fiber with lasers. Of course, network designers could install singlemode fiber. But singlemode systems also come at a significant expense due to the high cost of singlemode transceivers.
Corning recently introduced a series of multimode fibers called InfiniCor CL, which perform in Gigabit Ethernet-compliant systems using a variety of lasers, including 850-nm VCSELs or 1300-nm lasers. The product line currently includes two fibers: InfiniCor CL 1000, a 62.5-micron multimode fiber with link lengths of 500 meters at 850 nm and 1000 meters at 1300 nm; and InfiniCor CL 2000, a 50- micron multimode fiber with link lengths of 600 meters at 850 nm and 2000 meters at 1300 nm. Both fibers are manufactured using the OVD process, eliminating the need for mode-conditioning patch cords and allowing for laser-based center launches.
In addition, the manufacturer employs direct measurement of the fibers` laser bandwidth using laser sources. These precise measurements provide the guarantee of network speeds at specified link lengths.
Although the Gigabit Ethernet standard was finalized more than a year ago, efforts to improve system performance over multimode fiber continue. The Telecommunications Industry Association`s (Arlington, VA) Task Group on Modal Dependence of Bandwidth (TIA FO-2.2) currently is working on launch criteria for laser sources and new fiber-bandwidth measurements. The objective is to ensure consistent, optimum performance by characterizing multimode fibers according to restricted transceiver launch and by developing practical test methods.
These efforts, along with refinements in fiber manufacturing, are key to the continued success of low-cost, laser-based, high-speed network systems.
An overfill launch distributes hundreds of modes in 100% of the fiber core, resulting in high modal dispersion and bandwidth limitations. A laser launch concentrates fewer modes into 3% of the fiber core, reducing modal dispersion. Laser launches, however, are susceptible to dips and peaks in the fiber`s refractive index.
The relative complexity of the inside vapor deposition fiber-manufacturing process makes index-profile control more difficult to achieve than when the outside vapor deposition process is used.
The overfilled launch light-emitting diode distributes hundreds of modes in 100% of the fiber core, resulting in high modal dispersion and lower bandwidth. Laser launch concentrates fewer modes in 3% of the fiber core, resulting in low modal dispersion and higher bandwidth.
Preston Buck is a premises market manager with Corning Inc. (Corning, NY).