1、 TIA TELECOMMUNICATIONS SYSTEMS BULLETIN Enhanced Bandwidth Performance over Laser-Based, Multimode Fiber Local Area Networks TSB62-20 November 2007 TELECOMMUNICATIONS INDUSTRY ASSOCIATION The Telecommunications Industry Association represents the communications sector of NOTICE TIA Engineering Stan
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21、CT, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES (INCLUDING DAMAGES FOR LOSS OF BUSINESS, LOSS OF PROFITS, LITIGATION, OR THE LIKE), WHETHER BASED UPON BREACH OF CONTRACT, BREACH OF WARRANTY, TORT (INCLUDING NEGLIGENCE), PRODUCT LIABILITY OR OTHERWISE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMA
22、GES. THE FOREGOING NEGATION OF DAMAGES IS A FUNDAMENTAL ELEMENT OF THE USE OF THE CONTENTS HEREOF, AND THESE CONTENTS WOULD NOT BE PUBLISHED BY TIA WITHOUT SUCH LIMITATIONS. TSB-62-20 ITM-20 Enhanced Bandwidth Performance over Laser.Based. Multimode Fiber Local Area Networks Contents . Foreword III
23、1 Introduction 1 2 Background and History . 2 2.1 Launch Conditioning 2 2.2 Source Near Field 2 2.2.1 Mode Power Distribution 2 2.2.2 Encircled Flux . 3 2.3 Encircled Flux Round Robins . 4 2.4 Encircled Flux and Link Performance 5 2.5 The Validation Experiment . 6 2.6 Restricted Mode Launch Fiber Ba
24、ndwidth . 8 2.7 Enhanced Bandwidth Criteria . 9 2.8 Risk Assessment 10 3 Recommendation . 11 4 Normative references . 12 Annex A (informative) 13 Annex B (informative) 19 i TSB-62-20 This page left blank. ii TSB-62-20 ITM-20 Enhanced Bandwidth Performance over Laser-Based, Multimode Fiber Local Area
25、 Networks Foreword From TIA Project No. 4892 formulated under the cognizance of TIA FO-2.2.1, Subcommittee on Modal Dependence of Bandwidth. This bulletin is part of the series of test methods included within TINEIA TSB-62. There are two annexes, which are informative. Key words: bandwidth, effectiv
26、e modal bandwidth, encircled flux, launched power distribution, restricted mode launch, restricted mode launch bandwidth iii TSB-62-20 This page left blank. iv TSB-62-20 1 Introduction 1.1 Intent This bulletin describes and gives background information for the laser source and fiber selection criter
27、ia required to achieve enhanced bandwidth performance over local area networks (LANs) based on multimode fiber links and laser sources. Such a link is shown in Figure 1. Figure 1 - Basic link addressed by this bulletin. These criteria come from developments in the TIA Working Group on the Modal Depe
28、ndence of Bandwidth (FO-2.2.1) (the Working Group) from 1997 to 2000. Without appropriate source and fiber selection criteria, fiber link lengths for LANs must be set very conservatively to minimize risk of link failure. The Working Group has developed and tested new test procedures that characteriz
29、e the laser source near field and the multimode fibers bandwidth performance under restricted launch conditions. Limiting the allowable range of these source and fiber parameters assures that a link will meet enhanced bandwidth performance conditions with an acceptable risk of link failure (below 1%
30、) for maximum link lengths. Shorter links will have lower risks of link failure. 1.2 Scope Although the methodology described here to characterize both sources and fiber may be broadly applicable to different source and fiber types, the specific selection criteria addressed in this bulletin are only
31、 applicable to LANs based on short-wavelength (830 - 860 nm) laser sources and graded-index 62.5/125 m optical fibers. Short-wavelength networks based on graded-index 50/125 m fiber alread perform at gigabit-per-second data rates over link lengths of up to 550 meters , criteria for enhanced network
32、performance is therefore less urgent for these networks. Nevertheless, enhanced network performance for 50/125 m fiber systems (e.g. 1 O gigabit-per-second systems) is currently (as of September 2000) under consideration in the Working Group. Y. 1 TSB-62-20 2 Background and History The move toward h
33、igh-speed (2 1 gigabit-per-second) LANs requires the greater modulation capabilities of laser sources like the Vertical Cavity Surface Emitting Laser (VCSEL) or the edge-emitting semiconductor laser diode. Unlike the less coherent light emitting diode (LED), laser sources typically excite only a sub
34、set of the fiber modes available. This change in source property together with the inter- modal dispersion properties of the fiber requires that both source and multimode fiber be characterized and/or modified especially for high-speed LAN applications. 2.1 Launch Conditioning The Working Group bega
35、n looking into short-wavelength (-850 nm) laser source characterization in the spring of 1998. Characterization of the multimode fiber would follow later that same year. This work followed work that the Group performed in association with the I EEE 802.3 Gigabit Ethernet standardization effort. Ther
36、e, it was found that laser launch conditioning led to more predictable worst-case bandwidth performance. In particular, an offset single-mode fiber launch into multimode fiber employed with 1300 nm laser sources and implemented with a special hybrid patchcord gave a bandwidth performance that predic
37、tably met or exceeded the overfilled launch (OFL) performance specification of the 62.5 pm core multimode fibeQ. This result allowed the adoption of an acceptably long link length of 550 meters for the Gigabit Ethernet 1000BASE-LX standard. No such launch conditioning was prescribed for the short-wa
38、velength (-850 nm) 1 000BASE-SX Gigabit Ethernet standard, and the resulting link length for 62.5 m fiber was less than 300 m. The Working Group realized that conditioning the launch of the short-wavelength sources might lead to significant improvement in allowable link length, particularly for 62.5
39、 m fiber. Unlike the offset launch solution employed for the 1300 nm sources, the solution for the short-wavelength sources would ideally involve no external patchcord. This was desired to preserve the low-cost potential of the short-wavelength sources. A simple restriction of allowable source launc
40、h characteristics might yield the desired performance enhancement. The Working Group began by looking at the near-field properties of the laser source. 2.2 Source Near Field 2.2.1 Mode Power Distribution Initially, the Working Group attempted to characterize the near field of the source by applying
41、a measurement of the mode power distribution produced by the laser source in a length of multimode fiber. This measurement is described in EIA/TIA ITM-3. It quickly became apparent, however, that such an approach was not 2 TSB-62-20 satisfactory. ITM-3 assumes that all modes within a mode group have
42、 the same energy. This equipartion-of-energy assumption does not generally hold for multimode fibers under 1 km in length. Energy redistribution amongst modes within a mode group occurs over longer fiber lengths, if it occurs at all. Since lasers initially excite a fraction of the modes excited by a
43、n LED, this failure of energy redistribution becomes particularly important for laser source characterization. The near-field intensity as a function of radius was often observed to be non-symmetrical and non-monotonic. These near-field patterns produced non-physical computed mode power distribution
44、s, i.e. negative mode- group powers. 2.2.2 Encircled Flux The Working Group eventually settled on another measure of the laser near field, the Encircled Flux (EF). Encircled Flux is obtained from the near-field intensity pattern produced by a laser source in a 10 m length of 62.5 m core, graded-inde
45、x multimode fiber. It is the near-field pattern at the end of this fiber (as opposed to the near field at the laser source itself) that is more relevant to system performance. This is because the only relevant part of the laser source energy is that which is coupled into the fiber. In general, such
46、a near field will have a non-uniform shape that includes speckle. By integrating over radius and over all azimuthal angles, the Working Group collapsed the two-dimensional near-field information into a one-dimensional radial function, giving the fractional power launched by a source within a given f
47、iber radius. The complexity in the raw near-field data is avoided by exploiting the axial symmetry of the fiber, and, unlike the measurement of mode power distribution, no assumption on the equipartition of energy within mode groups is required. Typical near-field patterns for an LED and a VCSEL sou
48、rce at the end of a IO-meter, graded-index, 62.5 m fiber jumper are shown in Figure 2. Note that while the near-field intensity and annular flux for the VCSEL are irregular, the encircled flux as a function of radius is smooth and monotonically increasing. Since it is the launch power distribution a
49、nd not the absolute power that is of interest, the normalized encircled flux is plotted and compared. The detailed procedure for obtaining encircled flux is defined in a test procedure, ElMIA FOTP-203. 3 TSB-62-20 Radius (Ikm) Figure 2 - Near Fields, Intensity Profiles, Annular Flux, and Encircled Flux for an LED and a VCSEL 2.3 Encircled-Flux Round Robins To be a useful parameter, Encircled Flux needs to correlate with link performance and be reproducibly measured from lab to lab. The latter was tested with inter-laboratory round robins. The first round-robin comparison involve
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