Developing leading-edge fiber-optic network link standards

Hewlett-Packard Journal, Dec, 1997 by David G. Cunningham, Delon C. Hanson, Mark C. Nowell, C. Steven Joiner

However, with some fibers, particularly fibers with centerline refractive index defects, small connector offsets of the order of 5 micrometers result in very low bandwidth compared to overfilled launch. The use of restricted mode launch results in links much shorter than those obtained with overfilled launch for these fibers.

Experimental: Offset Restricted Mode Launch. A surprising prediction of the theoretical model (see Figure 13) is the occurrence of three bandwidth gain peaks. Experimental measurements of the bandwidth gain versus offset for a single-mode fiber-to-62MMF launch are plotted in Figure 17. The wavelength of the Fabry-Perot laser diode used was 1300 nm. The measured refractive index profile of the 62MMF had no dips or peaks and was approximately square-law. Three gain peaks were observed near the theoretically predicted offset values.

Although offset single-mode launches are restricted, for 62MMF and offsets in the range of 15[Mu]m to 25[Mu]m, the output intensity distribution is only slightly restricted compared to overfilled launch. This favorable excitation of the multimode fiber results in bandwidths equal to or greater than the overfilled launch bandwidth. We have observed that at least overfilled launch bandwidth can be achieved with offset launch on all fibers tested to date.

We have also investigated the modal noise penalties, connector effects, and mechanical stability of offset launches. No problems have been encountered in these areas. However, it is not clear that offset launches could be easily integrated into HP's manufacturing processes. HPL continues to investigate both center launch and offset launch to determine which type of launch is best for various LAN and interconnect systems.

Gbit/s IEEE 802.3 Link Specification Status

The July 1996 IEEE 803.3z Gbit/s Ethernet plenary meeting agreed to adopt two optical interface specifications. One interface is defined for 850-run lasers to achieve link lengths of [double tilde] 250 m with 62MMF and [double tilde] 500 m with 50MMF. This gives up some extended link length performance to achieve the lowest possible cost The second interface is defined to meet the installed 62MMF building backbone link lengths with 1300-nm transceivers, which achieve 3 km with single-mode fiber and [double tilde] 500 m with both 62MMF and 50MMF. Because the bandwidth achieved with center launched transceivers may be less than the overfilled launch bandwidth, IEEE 802.3z has defined its link lengths using a worst-case modal bandwidth. If conditioned launches that guarantee overfilled launch bandwidth are used, the IEEE 802.3z draft standard allows longer link lengths.

Conclusion

CSSD's and HPL's involvement in embryonic standards provides valuable insight that allows us to adapt our standards and research strategy to increase the probability of it being accepted. Our involvement in the ATM Forum OC-12 specification led us to refocus our VCSEL development to operation at wavelengths near 850 rim rather than 980 nm. This was because multimode fiber suppliers were opposed to specifying the higher multimode fiber modal bandwidth available at 980 nm and because all other laser transceiver suppliers were 850-nm focused. Our longer-term research was directed towards long-wavelength VCSELs, which have multiple uses with both multimode fiber and single-mode fiber.