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

The HP Communications Semiconductor Solutions Division is a leading supplier of optical-fiber optoelectronic components used to communicate over both premise backbones using primarily multimode fiber and public networks using single-mode fiber. Because of link performance and cost trade-offs, multimode fiber transceivers are developed using 650-nm, 850-nm, and 1300-nm technology. Visible 650-nm LEDs match the transmission window of large-core (980-[Mu]m diameter) plastic optical fiber, which has high attenuation but yields the lowest-cost transceivers and optical connectors as a result of relaxed mechanical tolerances. Infrared 850-mn and 1300-nm technology matches the transmission characteristics of glass multimode fiber having smaller core diameters, that is 62MMF and 50MMF (50/125km core/cladding diameter). These fibers have lower attenuation and higher bandwidth at wavelengths near 1300 run compared to 850-nm operation but yield more expensive systems compared to plastic optical fiber. Single-mode fiber transceiver technology operating at 1300-nm and 1550-nm wavelengths supports the 10-to-50-km distance requirements of telecommunications single-mode fiber links and is still more expensive. Nevertheless, 1300-nm single-mode fiber links have extended transmission capabilities and are being deployed on the campus to extend beyond the distance and data rate limits of multimode fiber.

Fiber Optic LAN Standards Development

The initial fiber-optic backbone link standards developed in the mid-1980s support a 2-km campus backbone length using 62MMF. This requirement influenced the subsequent ISO/IEC 11801 campus backbone link length. The 10-Mbit/s, 2-km IEEE 802.3 Ethernet standard uses 850-mu LEDs while the 100-Mbit/s, 2-km ANSI X3T12 Fiber Distributed Data Interface (FDDI) standard requires 1300-nm LEDs because of the impact of fiber spectral dispersion at this higher data rate. Subsequently, based on the FDDI backbone link standard, a 2-km 62MMF link length specification using 1300-nm LEDs was developed for transmitting Asynchronous Transfer Mode (ATM) cells over Synchronous Optical Network (SONET) links at 155.5 Mbits/s, also referred to as optical carrier level 3 (OC-3). This OC-3 rate standard, initiated in the ATM Forum, was formalized in the TIE1.2 T1.646 broadband ISDN customer interface standard.

Long-Wavelength LED Specification

It was generally assumed that low-cost 1300-nm LEDs would be too slow for operation at 622 Mbits/s (OC-12). However, exploratory work at HPL Bristol and other manufacturers of 1300-mu LEDs indicated that the necessary 1-ns optical response time was achievable with low-cost designs. This resulted in a development program at CSSD yielding the necessary data to support an OC-12 specification for a 500-m 62MMF link length in both the ATM Forum and T1E1.2 T1.646 specifications. This is the highest data rate at which 1300-nm LEDs can reasonably be specified in multimode fiber link applications.

It was obvious to HPL researchers that a new low-cost, LED-like laser technology was required for multimode fiber Gbit/s LANs and computer interconnects. This realization was key to the initiation of vertical-cavity surface emitting laser (VCSEL) development within Hewlett-Packard Laboratories during the early 1990s.[1,2] Since 1300-rim LEDs reach their limit at 622 Mbits/s, Hewlett-Packard developed a link length and data rate extension to Gbit/s ATM based on VCSELs operating at wavelengths near 980 nm.[3] HPL demonstrated that 980-nm VCSELs could support building backbone link lengths at Gbit/s data rates with 62MMF. CSSD and HPL felt that this proposal was very suitable for Gbit/s LAN standards since it was in harmony with ISO/IEC 11801. By comparison, VCSELs operating at 850 mu were felt to be an inferior choice since they cannot support building backbone link lengths at Gbit/s data rates with 62MMF (see Figure 1) based on the standard overfilled launch (OFL) modal bandwidth for 62MMF.