Rare-Earth-Doped Fibers for Telecommunications Applications
- PDF / 851,638 Bytes
- 7 Pages / 604.8 x 806.4 pts Page_size
- 46 Downloads / 204 Views
Background Fiber optics have revolutionized the telecommunications industry, providing more Information capacity and greater distances between signal boosters than copper wire and coaxial cable. The attenuation in coaxial Systems increases exponentially with signal frequency, making high-speed transmission over long dis tances impractical. The best copper Sys
MRS BULLETIN/SEPTEMBER 1999
tems have a bandwidth of about 10 Mbit/s and are limited to lengths of less than 200 m at high data rates. In contrast, the attenuation of SiC>2 optical fibers is low and independent of signal frequency, thus optical fiber can easily Support 100 Gbit/s (10,000 times the capacity of copper) over 80 km and is currently only limited by the speed of the transmission and receiving electronics, with capacities in excess of 50 Tbit/s theoretically possible. 1 For links in excess of 80 km, signal amplifi cation is necessary to prevent total loss of the signal. In the 1980s, amplification was done with electronic devices called repeaters that detected the light, converted it to an electronic signal, amplified, retimed, and then retransmitted it as an optical pulse. The field of optical telecommunications has itself undergone a revolution. In the late 1980s, the invention of the all-optical amplifier allowed for simultaneous am plification of multiple Channels in a Single optical fiber each at a different wavelength or color of light. Si0 2 fibers have a minimum in attenuation in the infrared (IR) portion of the optical spectrum near 1550 nm, as shown in Figure 1. The EDFA fortuitously provides high gain and low noise in the 1530-1560-nm spectral win dow. This technology now enables simul taneous amplification of 32 Channels in a Single fiber without the need for opticalto-electronic conversion. Thus singlef iber capacities of 320 Gbit/s are currently being deployed today. To perform this electronically, each Channel would have to be separated (demultiplexed), ampli fied by its own costly repeater, and then recombined (multiplexed) in the fiber. Researchers are now perfecting 100-channel EDFAs in the lab.
How Does a Fiber Amplifier Work? The energy levels of the trivalent rareearth (RE) ions are shown in Figure 2. Consider light shining on a medium, such as glass, doped with Er 3+ ions. When a photon of 980-nm light encounters an Er 3+ ion, it has a high probability of be ing absorbed and exciting a ground-state 4 Ii5/2 ion to the 4 In / 2 level. The ion then quickly relaxes to the ^I iy2 level, giving its energy up to vibrational energy (phonons) of the medium. This level is metastable (the lifetime is usually around 10 ms in silica glass), and the ion will eventually emit a photon of light at —1550 n m , r e t u r n i n g it to the ground State. This spontaneous emission of light is called , fluorescence. The 4113/2 level actually consists of seven sublevels, and the 4I i5/2 consists of eight sublevels, so there are 56 possible transitions between the metastable and ground State of Er 3+ . Thus the fluorescence has a spectral line
Data Loading...
