Lightguide fiber is quickly supplanting copper wire as the medium of choice for carrying communications signals. As compared to copper wire, lightguide fiber has a much higher bandwidth. Thus, lightguide fiber can carry more voice conversations than copper wire. Further, lightguide fiber exhibits lower losses so that fewer regenerators are required for a long-haul lightguide fiber cable than for one comprised of copper wire. An additional advantage of lightguide fiber is that unlike copper wire, lightguide fiber is immune to radio frequency interference.
While terrestrial, long-haul fiber cables have been in commercial use for several years, development of an undersea lightguide fiber cable is still under way. In fact, only recently was installation completed of the first experimental deep water, undersea lightguide fiber cable linking the islands Gran Canaria and Tenerife off the coast of Morocco. Extensive tests have been carried out on this undersea lightguide fiber cable to determine whether trans-oceanic undersea lightguide fiber cables are feasible and commercially practical.
The experimental undersea lightguide fiber cable linking the islands of Gran Canaria and Tenerife is approximately 120 kilometers in length and is comprised of six individual lightguide fibers, each surrounded by a helically-wound steel strength member. The steel strength members and the fibers surrounded thereby are mounted in an elastomer jacket. A copper sheath, covered with plastic, surrounds the elastomeric jacket containing the steel strength members and the fibers. Typically, each of the six individual fibers within the undersea cable is not of a unitary construction. Present day manufacturing practices make it difficult to produce a single fiber in excess of 15 km in length, whereas the length of the cable between regenerators was selected to be 60-70 km to minimize costs. Thus, each fiber within the cable was comprised of individual pieces (lengths) of fiber spliced end to end.
A problem encountered in fabricating the experimental undersea lightguide cable was determining which individual pieces within an inventory thereof should be selected in order to obtain a fiber having a prescribed length and transmission characteristics. When selecting individual pieces of fiber from the inventory, several criteria had to be satisfied. The cumulative loss of the selected pieces had to be less than a predetermined maximum value to minimize the loss of the fiber fabricated therefrom and thereby minimize the number of regenerators required for the undersea cable span. Also, the cumulative loss of the selected pieces had to be greater than a certain minimum value to assure that the overall loss of the fiber was sufficient to prevent the signals traveling therealong from overloading a regenerator. Overloading of a regenerator causes a condition known as "ringing" which will lead to distortion of the signals amplified thereby.
In terrestrial cables containing one or more lightguide fibers, comprised of individual fiber pieces spliced end to end, ringing is avoided by installing attenuators at selected lengths along the cable. Since the terrestrial cable can be readily attenuated, it is unnecessary to impose the criterion that the cumulative loss of the individual pieces comprising each fiber be above a certain minimum value during selection. Thus, the only criterion that must be satisfied in selecting the individual pieces needed to fabricate a fiber for a terrestrial cable is that the cumulative loss of the pieces be below a predetermined maximum value.
While installing attenuators at various lengths along a terrestrial lightguide cable is feasible, installation of attenuators at various lengths along the undersea lightguide cable is not. Therefore, to assure that each fiber within the undersea cable was sufficiently lossy to avoid ringing, the pieces comprising the fiber were selected such that the cumulative loss thereof was above a predetermined minimum value. Selecting the pieces comprising each fiber for the undersea cable such that the cumulative loss thereof was above a predetermined minimum value (and below a predetermined maximum value) also assured a relatively uniform loss from fiber to fiber. Loss uniformity from fiber to fiber within the undersea cable eliminates the need to know the exact losses of each individual piece comprising each fiber. The loss of any particular section of fiber can be determined, with relatively high accuracy, as a proportion of the total fiber loss on a length basis. Replacement of a damaged section of fiber can easily be accomplished simply by substituting an equivalent length piece of fiber having approximately the same overall loss per unit length. Repair of the cable at sea is thus facilitated.
Another consideration in selecting fibers from the inventory was maintaining the median loss per unit length of pieces substantially constant after selection. In practice, the loss per unit length of each piece is measured at 1290 nm, 1310 nm and 1330 nm in order to gauge the loss thereof within the desired wavelength range of 1290-1330 nm for the fiber to be fabricated therefrom. Typically, the loss of each piece at 1310 nm is lower than the loss at either 1290 nm or 1330 nm. Thus, for each piece, there will be some variation in loss per unit length among the wavelengths. If the selection of the pieces from the inventory was undertaken without regard to the distribution thereof, then the percentage of higher loss pieces would increase. The higher loss pieces are those having either a loss per unit length higher than the median loss per unit length of the pieces in the inventory or a variation in loss per unit length above a preset value. These higher loss pieces within the inventory are less versatile in that they are less usable in fabricating a fiber. Unless an effort is made to use the higher loss pieces in the inventory, they will accumulate and will ultimately have to be scrapped, thereby increasing fiber fabrication costs.
Thus, a problem exists in how to fabricate a fiber, having pre-specified loss characteristics, from individual fiber pieces which are selected from an inventory thereof such that the median loss per unit length of the pieces in the inventory remains generally constant.