1. WHITE PAPER DIFFERENTIAL MODE DELAY. 1.0 Introduction. The advent of
.... IEC 60793-1-49 mirrors TIA's DMD test procedure, FOTP-220. IEC 60793-2- ...
Differential Mode Delay Tests Ensure Robust Multimode Operation for Laser-Based Applications
May 2007
WHITE PAPER DIFFERENTIAL MODE DELAY
Contents 1.0
INTRODUCTION
1
2.0
BANDWIDTH MEASUREMENT EVOLUTION
1
3.0
10 GB/S PERFORMANCE SPECIFICATIONS
4
4.0
INDUSTRY ACCEPTANCE
4
5.0
CONCLUSION
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WHITE PAPER DIFFERENTIAL MODE DELAY
1.0 Introduction 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 their use precipitated new performance measurements that better qualify fiber to support these laser-based systems. Early attempts evolved into sophisticated measurements that reveal a detailed picture of the modal propagation properties of fiber and expand its application capability.
2.0 Bandwidth Measurement Evolution 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 application’s 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 ‘80s 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 than 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. Figure 1 depicts MPDs of two LEDs and seventeen 1 Gb/s 850 nm lasers used in a Telecommunication Industry Association (TIA) study. While both LEDs closely approximate an overfilled MPD shown by the diagonal green lines, the large variation amongst laser sources is easily seen. Thus finding a single launch condition representative of such sources proves elusive.
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TIA Round Robin Sources
Figure 1: Mode Power Distribution of 1 Gb/s sources
The restricted mode launch bandwidth (RMLBW) measurement developed for 62.5 µm fiber in the late ‘90s was an attempt to emulate these sources. The specific launch condition, prescribed in TIA FOTP-204, was found to correlate on 62.5 µm 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. Part of the results from that study is shown in Figure 2. The minimum effective modal bandwidth (EMB) requirement for fibers to support the 300 m link length for 10 Gigabit Ethernet is 2000 MHz·km. Plotted on the left are the EMBs of twenty one 10 Gb/s VCSELs when launched into one of the fibers in the study. Five of these sources produced EMB below the minimum requirement indicating the fiber should not be considered compliant. On the right are the bandwidth predictions from DMD and four other launch conditions. Only the DMD method predicted performance below the requirement and rejected this fiber. FO2.2 concluded that DMD provided the most reliable results and standardized the method in TIA FOTP-220.
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Figure 2: Predictive Capability of Various TIA Proposals
The DMD technique is depicted in Figure 3. A singlemode 850 nm probe is scanned at small radial increments across the core of the multimode fiber under test. At each position the time response to a short impulse is recorded. The singlemode launch condition enables resolution of mode behavior critically important for 10 Gb/s VCSELs. The DMD temporal width is determined at the 25% threshold level between the first leading edge and the last trailing edge of all traces encompassed between specified radial scan positions. The DMD of compliant fibers must not exceed limits specified in fiber detailed specification standards.
Figure 3: DMD Measurement Process
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3.0 10 Gb/s Performance Specifications 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. One proposal for a 10 Gb/s fiber specification used 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 arrive 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 above proposal and documented in the detailed specification, TIA-492AAAC for laser-optimized 50µm fiber.
4.0 Industry Acceptance 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 emerged within the International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO). IEC 60793-1-49 mirrors TIA’s DMD test procedure, FOTP-220. 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. Figure 4 diagrams 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.
Figure 4: Standards Interdependence
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Today, CommScope sells the new high-performance 50 µm fiber within its SYSTIMAX® LazrSPEED® and Uniprise ® LaserCore™ cables that is verified using a DMD test procedure superior to the requirements of the standard. And 850 nm VCSEL-based transceivers are the most popular of the multimode alternatives for 10 Gb/s Ethernet according to a recent presentation to IEEE 802.3. 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. Table 1 lists the distances supported by the LazrSPEED 300 Solution, based on worst-case analysis using the IEEE 802.3 link model. With two other LazrSPEED performance grades to choose from (LazrSPEED 150 and LazrSPEED 550), the optimal solution for a variety of customer requirements is readily available.
Table 1. Supportable Distance (meters) Comparison Application
LazrSPEED 300 Fiber
Standard 62.5 µm
Gigabit Ethernet
1000
275
10 Gig Ethernet
300
33
1 Gig Fibre Channel
900
300
2 Gig Fibre Channel
550
150
4 Gig Fibre Channel
300
70
10 Gig Fibre Channel
300
33
10 Gig OIF SONET
300
25
2.5 Gig InfiniBand™
400
125
Supporting a 300-m minimum distance across all applications listed in Table 1 addresses over 92% of customer’s in-building backbone needs according to a survey conducted by the IEEE, and positions the fiber as a universal media.
5.0 Conclusion 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 µm fiber is a wise choice for datacom services.
© 2007 CommScope, Inc. All rights reserved. Visit our Web site at www.commscope.com All trademarks identified by ® or ™ are registered trademarks or trademarks, respectively, of CommScope. 05/07 E-WP-C-1
