Gigabit Ethernet standard addresses multimode bandwidth
By A. JOHN RITGER
The Gigabit Ethernet standard, IEEE Std 802.3z-1998, was officially approved on June 25, 1998 (see Lightwave, August 1998, page 3). Two wavelength alternatives are specified: The 1000Base-SX specification defines operation at short wavelengths near 850 nm, and the 1000Base-LX specification defines long-wavelength operation near 1300 nm. Both use laser transmitters.
Multimode fiber bandwidth depends on both the relative delay times of the individual modes (which depend strongly on the detailed shape of the refractive index profile) and on the distribution of optical power among the modes. The conventional bandwidth measurement uses a mode scrambler to launch equal power into all the modes of the fiber. This measurement is not a good performance indicator for systems that use direct launch from laser sources and lengths less than 550 m.
Direct launch is particularly problematic for 1000Base-LX because the standard calls for the same transmitter to be used with either singlemode or multimode fiber. Efficient singlemode operation dictates a concentrated on-axis launch. The conventional bandwidth measurement launches very little power into the modes excited by an on-axis laser launch, so they have almost no influence on the resulting bandwidth value. Yet the behavior of these on-axis modes is important to system performance, as significant differences in propagation delay can result in undesirable pulse splitting.
The industry mounted a strong effort to understand and solve these problems. The work began with the TIA FO-2.2 Task Group on the Modal Dependence of Bandwidth and was continued by several ad hoc groups sponsored by the IEEE 802.3z Task Force.
The solution for the problem of fiber/launch sensitivity was two-fold. In the case of LX (1300-nm) transmitters, the IEEE determined by extensive measurement and modeling that a launch from a singlemode fiber into a multimode fiber, offset at approximately half the multimode core radius, results in an effective bandwidth at least as high as the conventional bandwidth. The offset is implemented in a special patch cord. This technique ensures that a large fraction of the modes are excited and that the on-axis modes are avoided.
In the case of SX transmitters (850 nm), the required bandwidths are already set close to the worst possible values for the multimode fiber, as the system operates far from the fiber`s wavelength of peak bandwidth. In addition, it is common for SX transmitters to excite a large fraction of the modes, resulting in greatly reduced problems with pulse splitting.
Work continues in the TIA. Both empirical studies and model simulations suggest that if the modal power distribution launched into a fiber is properly controlled, the lower limit of effective bandwidth can be increased from the conservative values used in the SX standard. The current focus of the work is to create a standard for determining the modal power distribution excited in a multimode fiber by a given source.
With such a procedure, it will be possible to verify that particular transmitters conform to a mask of allowed modal power distributions, which may ensure a high enough effective bandwidth to significantly extend the range of SX transceivers over installed fiber. o
A. John Ritger is a member of the technical staff at Lucent Technologies--Bell Labs (Norcross, GA), working in the area of fiber development and characterization. He may be reached at 770-798-2784, or email: [email protected].