Hydrogen in optical cables

There is considerable experimental evidence relating increases in fibre optical loss at 1.3 ¿m to both the hydrogen concentration encountered by the fibres and elapsed time. However, there are much less data available on the levels of hydrogen found in cables and their dependence on construction, temperature etc. The present paper describes measurements of hydrogen evolution from materials commonly employed in cable manufacture together with results from measurements of the levels of hydrogen found adjacent to the fibre in actual cable structures exposed to various temperatures. Comparisons have been made between the cable values and ones estimated from a knowledge of the cable structure, published data on the permeability and other parameters of the materials employed. The results of such comparisons indicate an encouraging measure of agreement between measured and estimated values. Levels of hydrogen balance obtained in the cable designs examined are generally found to be of the order of 500/1000 parts in 106at normal ambient temperatures, depending on cable construction and materials. For longer-term service, the fall in hydrogen evolution found from experiments on materials suggests that a lowering of hydrogen levels may be expected. Gas purging techniques have been shown to considerably decrease hydrogen concentration and indicate that, in unfilled cables, e.g. those employing loosely sleeved fibre units, hydrogen can be controlled to low levels, if necessary, by gas purging techniques.