Lasers in LANs: the effect on multimode-fiber bandwidth

TIA FO-2.2.1 specifications provide the means for optimizing the performance of high-speed network links.

BY PETER PONDILLO

Light-emitting diodes (LEDs) have long been the preferred mode of powering laser-based local area networks (LANs). They are cost-effective and perfectly suited to multimode fibers with their wide numerical apertures and core sizes. For many applications, LEDs still offer users the ability to meet their requirements for both network speed and link distances. But as users move to higher-speed protocols, low-cost 850-nm VCSELs are emerging, allowing users to obtain the power and link distances they desire, making it more critical than ever to understand multimode-fiber bandwidth and the delicate balance between speed and link distances.

The combination of lasers and multimode fiber in high-speed applications introduced several technical challenges, which had to be overcome to achieve optimum system performance.

  • Restrictions on the launched power distribution of the laser source.
  • Restrictions on the bandwidth properties of multimode fiber under a restricted mode launch.

Bandwidth refers to information-carrying capacity. Fiber bandwidth can be measured in several ways, but typically it is expressed in megahertz-millions of cycles per second-normalized over a kilometer: For example, 500 MHz-km @ 850 nm, or 500 million cycles/sec at the 850-nm wavelength of infrared light.

Optical-fiber band width is limited by pulse dispersion-how much the pulses of light spread as they travel down the length of a fiber. Too much dispersion causes the discrete pulses to merge, causing bit errors. The lower the dispersion, the higher the bandwidth of the fiber.

Multimode fibers are primarily affected by modal dispersion, which occurs because light in a multimode fiber travels in many modes, or paths. Due to the way light travels down the fiber core, each path is a different length, meaning light pulses that originate at the same instant arrive at the fiber end at different times, causing pulse spreading.

To reduce modal dispersion, multimode fibers have a core with a graded refractive-index profile. The refractive index of the core is highest at the center and gradually decreases as we approach the cladding glass. This effectively slows down the pulses of light that travel through the center of the core (the most direct and therefore shortest pathway) so that all the pulses arrive simultaneously at the end of the fiber link. Although this method does not eliminate pulse spreading altogether-dispersion is an inherent characteristic of multimode fiber-the better the index profile, the lower the modal dispersion and the higher the bandwidth.

For years, multimode-fiber specifications were set according to a minimum over-filled launch bandwidth (OFL BW). This method measures a fiber's information-carrying capacity when light is distributed uniformly throughout its core. It is very reliable when using LEDs as the transmission source, as LEDs distribute optical power throughout the fiber's core using hundreds of modes.

When lasers are used, however, we cannot rely on the same method to provide a true measure of system bandwidth. Lasers concentrate the optical power in a much smaller percentage of the fiber's core than LEDs, which can change the effective modal bandwidth of the fiber. In fact, some fibers that demonstrate a very high OFL BW may not offer the same bandwidth performance when a laser is used. That occurs when there are small defects at or near the centerline of the core. When lasers such as 1,300-nm singlemode lasers are used, the optical power is so tightly focused that any defects in the core have a large impact on the fiber's bandwidth.

Laser launch conditions are an important factor in optimizing bandwidth. Higher bandwidth is typically achieved when we excite fewer modes, effectively decreasing modal dispersion. That is theoretically anticipated when fiber optimized for one wavelength such as FDDI grade 62.5-micron, which is optimized for peak performance near 1,300 nm, is operated at a different wavelength (e.g., 850 nm). Also, it's important to use fiber specified for laser bandwidth under restricted launch conditions. Without appropriate source and fiber selection criteria, we cannot accurately determine the link lengths that can be supported by a given data rate.

In response to the growing interest of using lasers in high-speed LANs, the Telecommunications Industry Association (TIA) formed FO-2.2.1 Task Group on Modal Dependance of Bandwidth in 1996. The group's mission has been to develop a recommendation for a laser-based system that transmits 1 Gbit/sec up to 500 m using 62.5-micron multimode fiber and 850-nm sources (vertical-cavity surface-emitting lasers-VCSELs).

To achieve this goal, the task group experimented with restrictions on the launch power distribution of the laser source and the bandwidth properties of the multimode fiber under a standard restricted-mode launch (RML). TIA FO-2.2.1 concluded that the following two criteria need to be met for successful deployment:

  • Transceivers need to meet source encircled flux (EF) criteria.
  • Fiber needs to meet a new requirement for RML bandwidth.

(EF) is the percentage of power within a given radius when a transmitter launches light into a multimode fiber. The EF transmitter launch measurement characterizes the launch spot size carried by a multimode fiber, specifies transmitter launch conditions, and ensures restricted mode launch. Short-wavelength sources such as VCSELs have a smaller spot size, which increases modal bandwidth in multimode fiber. Since variations in laser-source launch conditions among manufacturers are potentially great, specifying the launch is a crucial step in guaranteeing the consistent performance of fiber links in multigigabit systems. To standardize this process, the task group developed Fiber Optic Test Procedure (FOTP)-203, a standard procedure for measuring launched power distribution for multimode-fiber transmitters.

Once the laser launch condition is specified, the next step is to ensure the optical fiber meets bandwidth requirements under restricted launch conditions. The RML bandwidth-measurement launch condition is created by filtering the overfilled launch condition with a special 23.5-micron fiber, also referred to as the RML fiber. The RML fiber has a 23.5-micron ± 0.1-micron core diameter, 0.208 ± 0.01 numerical aperture, and a graded-index profile with an alpha of approximately 2 and an OFL bandwidth greater than 700 MHz-km at both 850 and 1,300 nm. The RML fiber must be at least 1.5 m long to eliminate leaky modes and less than 5 m long to avoid transient loss effects. The launch exiting the RML fiber is then coupled into the fiber under test. The task group developed FOTP-204, which describes two methods for determining and measuring information-carrying capacity of multimode fibers using either of two launches: OFL or RML.

TIA FO-2.2.1 confirmed its specifications by measuring system effective modal bandwidth (EMB), the information-carrying capacity of a system, taking into account both fiber modal delays and transmitter launch condition. EMB measures the effects of multimode-fiber and transceiver interaction to accurately evaluate overall system performance. TIA FO-2.2.1 is in the process of documenting its findings on EMB in a technical service bulletin, or TSB, which should be available shortly.

The use of low-cost lasers offer users an excellent way to extend the reach and speed of their LANs. The work done by TIA FO-2.2.1 provides the tools for optimizing the performance of high-speed network links: restricted laser launch criteria and the new multimode-fiber bandwidth measurements. These characteristics are key to the success of low-cost, laser-based high-speed networks. As a result, end users now can take advantage of the unique qualities of each to extend the reach and increase the speed of multigigabit local area networks.

Peter Pondillo is a senior market-development engineer at Corning Inc. (Corning, NY). He wrote this article on behalf of the TIA Fiber Optics LAN Section (FOLS). Member companies include 3M/Volition, Allied Telesyn International, AMP Netconnect, Belden Wire & Cable, Berk-Tek, CommScope, Corning, LANcast, Leviton Voice and Data Div., Lucent Technologies, Micro Linear, Leviton Voice and Data Div., Ortronics, Panduit, the Siemon Co., Siecor, Sumitomo Electric Lightwave, and Transition Networks. The FOLS Website is www.fols.org.

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