New multimode tests ensure robust operation at 10 GB/S
November 1, 2002
The advent of Gigabit Ethernet in the late 1990’s ushered in the age of 850 nm vertical cavity surface emitting lasers (VCSELs) to replace slower light emitting diodes (LEDs) as the transmitter of choice on multimode fiber datacom links.
VCSELs excite fiber very differently than LEDs and precipitated new performance measurements that better qualify fiber to support these laser-based systems. Early attempts have evolved into sophisticated measurements that reveal a detailed picture of the modal propagation properties of fiber and expand its application capability.
Light travels through multimode fiber in multiple modes. Common commercial graded index multimode fiber typically supports hundreds of modes that travel in groups of common velocity.
Depending on the index profile of the fiber and the wavelength of light, modes groups may travel at different velocities. The difference in travel time is called the differential mode delay (DMD). The smaller the DMD, the less the pulse spreads out in time and the higher the minimum bandwidth will be.
There are two domains in which bandwidth can be measured, the time domain and the frequency domain. Both methods give comparable results. The critical feature of either method for extracting meaningful data is the launch condition. To provide bandwidth values that accurately predict performance, the launch condition must be similar to that of the system transmitter.
As fiber-based datacom systems have evolved from 10 Mb/s to 10 Gb/s, the modal excitation of the fiber changed markedly as slower LEDs gave way to lasers. LEDs are used up to 100 Mb/s. Between 100 and 622 Mb/s, systems employ both LEDs and 850 nm VCSELs. Above 622 Mb/s LEDs give way entirely to VCSELs.
Since modal bandwidth is highly dependent on the mode power distribution of the launch condition, the industry developed new bandwidth measurements better suited to the sources.
An overfilled launch condition emulates the modal excitation of LEDs. This launch places equal power into each mode by uniformly illuminating the fiber by a source of higher numerical aperture and larger area than the fiber being measured, essentially flooding the core with light. The so-called overfilled bandwidth (OFLBW) is the original measurement condition standardized in the 1980’s and is useful for LED-based systems at data rates up to 622 Mb/s.
Bandwidth measurements for systems above 622 Mb/s require a launch condition emulating VCSELs. These devices have a lower numerical aperture and smaller area then LEDs. VCSELs launch into only a portion of the modes and their mode power distribution (MPD) can be highly non-uniform and highly variable, leading to large bandwidth variability.
The MPDs of two LEDs and 17 1 G/s 850 nm lasers used in a Telecommunication Industry Association (TIA) study, revealed that while both LEDs closely approximate an overfilled MPD, there was a large variation among laser sources. Finding a single launch condition representative of such sources proved elusive.
The restricted mode launch bandwidth (RMLBW) measurement developed for 62.5 m fiber in the late 1990’s was an attempt to emulate these sources. The specific launch condition, prescribed in TIA FOTP-204, was found to correlate on 62.5 m fiber to a subset of possible VCSEL launches defined by limits on the encircled flux (EF) of the transmitter as measured by TIA FOTP-203. However, these corresponding EF specifications were not adopted by any application standard.
To address the needs of 10 Gb/s VCSEL applications, TIA FO2.2 examined alternate ways to characterize fiber bandwidth. Data on various individual launch conditions were compared to a method of extracting bandwidth from a DMD measurement.
Results of another study revealed that the minimum effective modal bandwidth (EMB) requirement for fibers to support the 300 m link length for 10 Gigabit Ethernet is 2000 MHz*km.
10 GB/S PERFORMANCE SPECS
TIA FO2.2 realized during the work on 1 Gb/s applications that to create the most cost-effective solution the industry would need to agree on mutually compatible specifications for the transmitters and fibers. So proposals included both fiber specifications and transmitter launch conditions.
OFS proposed a 10 Gb/s fiber specification using DMD to constrain the modal delays to values less than the bit period for those modes carrying significant power. This concept is based on the observation that if all modes excited by the DMD measurement over appropriate radii lie within a period of time related to the bit interval, then the fiber should provide sufficient bandwidth no matter what subset of modes is excited within those radii. Transmitters would be required to launch into modes corresponding to the prescribed radii via the EF test method.
FO2.2 extensively modeled and simulated the system using modal theory that accounted for laser-fiber interactions and the effects of mode mixing at offset connections. The fiber DMD and transmitter EF specifications that emerged are consistent with the OFS concept and documented in the new 50 m fiber detailed specification, TIA-492AAAC.
TIA FO2.2 worked closely with application standards bodies during the development of these fiber and transmitter specifications.
This coincided with the development of 10-Gigbit Ethernet by IEEE 802.3, 10-Gigabit Fibre Channel by ANSI NCITS T11.2, and OC-192 VSR-4 by the Optical Internetworking Forum (OIF). Each of these applications adopted both the new fiber specifications and compatible transmitter launch conditions. This suite provides solutions spanning the LAN (Local Area Network), SAN (Storage Area Network), and CO (Central Office) markets.
The cabling industry also embraced the new fiber detailed specifications by referencing it within addenda to structured cabling standards like TIA 568B.
Equivalent international standards are emerging within the International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO). IEC PAS 60793-1-49 mirrors TIA’s DMD test procedure, FOTP-220. Draft IEC 60793-2-10 Edition 2 contains the fiber specifications from TIA 492AAAC. And ISO/IEC 11801 2nd Edition assigns these new fiber specifications to a new multimode fiber level of performance called OM3.
The graph below illustrates the relational interdependence between all of these standards. The measurement standards provide the foundation upon which the fiber specifications rest. Cabling standards reference the fiber specifications, which in turn support the applications standards.
Today, leading fiber manufacturers sell the new high-performance 50 m fiber verified using the DMD test procedure, and several cable manufacturers offer this fiber within new cables. Already 850 nm VCSEL-based transceivers with compatible launch conditions appear in 10 Gb/s Ethernet products, and these are offered at the lowest prices among the transceiver alternatives by wide margin.
End-users find this new fiber solution attractive not only to ensure easy migration to 10 Gb/s applications, but to also support existing applications to extended reaches.
Through refined measurements of fiber modal propagation properties, the fiber industry in cooperation with the transceiver industry has succeeded in providing a robust solution for the cost-sensitive short-reach market that supports low-cost serial transmission on advanced multimode fiber. Broad industry acceptance, standards backing, and support for major LAN, SAN and CO applications assure end-users that 850 nm laser-optimized 50 m fiber is a wise choice for datacom services.
Paul Kolesar’s is a Distinguished Member of the Technical Staff at OFS.