Reducing the 1,383-nm water absorption peak in singlemode fibers holds the promise of making another 100 nm of spectrum available for dense wavelength-division multiplexing. Both the International Telecommunication Union's Working Party 4/15 and the International Electrotechnical Com mission's SC86A WG1 agreed on standards for this fiber type at their meetings in April.
Because fibers are particularly sensitive to hydrogen-induced loss near 1,383 nm, a hydrogen test is expected to be adopted as a qualification test for fibers used in this spectral region. Depending on application, Telcordia (formerly Bellcore) requires in its GR-20-CORE, Issue 2, July 1998, requirement R6-82, that the amount of hydrogen generated by cable materials be measured. Hydrogen generated by cable materials, hydrogen can also originate from external sources, and hydrogen partial pressures of 4x10-4 atmospheres have been measured in duct cables several years after installation. Although this partial pressure is so low that its contribution to the added loss in the usual 1,310-nm and 1,550-nm windows is insignificant, the effect near 1,383 nm can be more pronounced. To ensure that the loss increases near 1,383 nm under typical in-service conditions remains stable with time, a new test is needed.
The low observed in-service exposure of 4x10-4 atmosphere is sufficient to induce significant reactive added loss at wavelengths near 1,383 nm. It is believed that this loss increase is due to hydrogen reacting with silicon (Si) defects to form SiOH. Exposing a fiber to this level of hydrogen for 250 days produces about the same loss increase at 1,383 nm as a 10-day exposure at 0.01 atmosphere at room temperature. Consequently, a low-pressure hydrogen-aging test at room temperature can serve as a good indicator of the expected loss increase for in-service fibers.
Low pressure/temperature tests on numerous nonhermetic fibers were re ported by Dan Fletcher of Alcatel at the Telecommunications Industry Associa tion FO-6.6.5 meeting in January. These tests showed that the 1,383-nm loss increase reaches saturation in less than 10 days when subjected to 0.01 atmosphere of hydrogen at room temperature. The magnitude of this saturated loss depends on the method used to make the fiber, and occurs at room temperature when the product of hydrogen partial pressure and exposure time exceeds 0.1 atmosphere-day.
British Telecom developed a high-pressure/temperature test conducted at 70°C at 1 atmosphere for 1,000 hours (42 days). Whereas the previous test detects Si defects, the BT test also exposes germanium (Ge) defects. Although the loss increases in fibers subjected to the BT test can be high, they are almost irrelevant to attenuation increases at 1,383 nm under normal field conditions. When extrapolated to the lower temperatures and pressures experienced under typical field conditions, the loss increases at 1,310 nm, 1,383 nm, and 1,550 nm are only about several thousandths of a dB/km over 25 years.
This amount is negligible compared to the rapid SiOH reactions that can increase the 1,383-nm loss by as much as 0.21 dB/km after only 0.1 atmosphere-day of exposure to hydrogen at room temperature.
James J. Refi is with Lucent Technologies (Norcross, GA) and participates in TIA and IEC standards groups on optical fiber. He can be reached at (770) 798-2737 or e-mail: firstname.lastname@example.org.