Hydrogen in fibers and cables
William B. Gardner, AT&T
Hydrogen-induced increases in the attenuation of installed fibers have been studied since the problem was discovered in 1983. Papers on the subject have been presented at the annual International Wire and Cable Symposia from 1989 to the present.
Though the optical absorption peak emerges around 1240 nanometers, the absorption spreads into both the 1310- and 1550-nm fiber transmission windows. The problem has occurred frequently enough to attract the attention of fiber and cable standards bodies.
The International Electromechanical Commission SC86A Working Group 3 on Cables is letter-balloting Method 794-1-F8, titled "Testing for Hydrogen Effects in Optical Cables." This test method shows how to accelerate the hydrogen attenuation in crease and measure the resulting absorption peak at 1244 nm.
In the United States, the Telecommunications Industry Association created the Hydrogen Effects on Cables Task Group FO-6.7.14, which is chaired by Casey Wieczorek of Bell Communications Research in Morristown, NJ. This group is working on Test Method FOTP-183, "Completed Fiber Optic Cable Hydrogen Evolution Test." This test method, which is similar to the IEC method, is almost ready for letter balloting.
The TIA Task Group FO-6.6.7 on Fiber Coatings, chaired by Eric Urruti at Corning Inc. in Wilmington, NC, generated Informative Test Method ITM-2, TSB 62-2, titled "Method for Measurement of Hydrogen Evolved from Coated Optical Fiber."
Instead of measuring the increase in attenuation, this test method measures hydrogen concentration. Hydrogen released by heating the coating is captured in a vial, and the parts-per-million concentration is measured with a gas chromatograph, for example. An annex provides guidance on how to use this data to estimate what the partial pressure of hydrogen would be in a given cable design.
Pirelli Cables in the United Kingdom has proposed that the International Telecommunication Union Study Group 15 (Transmission Systems and Equipment) insert into its singlemode fiber recommendations a maximum allowable loss increase resulting from hydrogen. Rosamund Neat at Pirelli in Harlow, England, is using a questionnaire to gather information on this subject for the February 1995 meeting of Study Group 15.
In the meantime, ITU Study Group 6 (Outside Plant) is considering a draft recommendation, "Method for Estimating the Quantity of Hydrogen Generated in Optical Fiber Cables," which was prepared by Tadashi Haibara at Nippon Telegraph and Telephone in Ibaraki, Japan.
This document lists five mechanisms by which hydrogen can be generated in the cable. It also presents steps that can be taken in designing the structure and choosing the materials of the cable to minimize hydrogen generation. To evaluate different cable designs, this draft recommendation proposes a test method for measuring the hydrogen released from components of the cable when heated. Arrhenius Relations, which determine the temperature and time dependencies of the rate of hydrogen release, are given for projecting the results to other temperatures and times.
Most standards efforts address hydrogen generation and the resulting reversible molecular hydrogen absorption loss. A better understanding of the permanent loss increases caused by hydrogen reaction with atomic defects in silica will also be needed to accurately predict the long-term hydrogen aging loss increases under service conditions.
Multimode fibers are more susceptible to this permanent loss increase, but they are less likely to be used in environments where hydrogen might be present. q