The importance of modal bandwidth in Gigabit Ethernet systems

W ith the advent of Gigabit Ethernet Systems utilizing multimode fiber, the limiting factor in the fiber backbone is no longer the attenuation of the passive link, but rather, the bandwidth of the fiber. Due to different signal characteristics of the transmitter, new test procedures and a new understanding of the interaction between the optical fiber and the optical signal are required. Gigabit Ethernet systems are the result of the continual evolution of networks toward higher and higher speeds. The incessant demands of end users to move large amounts of data at gigabit speeds required a new technology to handle the 10ð Faster to Gigabit increase in the transmitting speed used in the network. With the introduction of such speeds, the performance of multimode optical fiber came into question, as distances thought to be achievable over standard 62.5-micron fiber were not possible using existing technology. However, new high-speed laser transmitters at 850 nm offered the necessary characteristics to make Gigabit Ethernet a reality. To this end, the Institute of Electrical and Electronic Engineers (IEEE) 802.3z committee revised the Ethernet protocol standards for transmission for optical networks at one gigabit per second. The revised protocol standards are based upon the Fast Ethernet system protocol developed in 1995.1 These new protocols are 1000Base-SX for 850 nm wavelength using laser-based transceivers over multimode optical fiber and 1000Base-LX for 1300 nm wavelength using laser-based transceivers over multimode or singlemode optical fibers, which are to be considered Gigabit Ethernet (GbE). The device that was developed for high-speed multimode transmission at 850 nm was a Vertical Cavity Surface Emitting Laser or VCSEL, which uses a gallium arsenide substrate for speed.1 Unlike light emitting diodes (LEDs), these devices are fast enough to transmit a gigabit signal. Additionally, they can be manufactured very cost effectively when compared to typical singlemode semiconductor lasers. VCSELs are the first laser devices that transmit signals in more than one mode, making them ideal for use with multimode fiber. Each type of device transmits the optical signal in a different manner; LEDs excite all the different fiber modes, VCSELs excite only a portion and the singlemode lasers excite only the fundamental mode. See Figure 1 for a representation of how LEDs, VCSELs and semiconductor lasers transmit in a multimode fiber. Using this VCSEL source technology in Gigabit Ethernet links proved to be more complicated than merely replacing the existing technology due to the