Optical waveguides – Optical transmission cable – Loose tube type
Reexamination Certificate
2002-01-28
2003-11-11
Nguyen, Khiem (Department: 2839)
Optical waveguides
Optical transmission cable
Loose tube type
Reexamination Certificate
active
06647187
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a submarine optical cable comprising a central polymeric buffer tube, said cable being particularly suitable for long-haul repeatered systems.
Submarine cables for long-haul repeatered systems are typically required to contain a relatively small number of optical fibers, the maximum number of optical fibers being determined by the capacity of optical amplifiers-repeaters. Accordingly, the fiber count for submarine optical cable for long-haul repeatered systems is typically from a minimum of 4 fibers up to a maximum of 48 fibers.
The total length of long-haul submarine cables (also called “segments”) is typically of about 1500-2000 km. These lengths are generally formed by a number of optical cable spans (50-80 Km length), generally separated from each other by amplifiers. The cable-spans are preferably joined together with the amplifiers in the manufacturing plant, in order to manufacture the final 1000-2000 Km length segments which are then loaded on the cable laying ship. The total length of repeatered submarine systems may vary from about 1000 to about 10000 Km, joining two or more of the above segments, if necessary.
2. Description of the Related Art
In view of the low fiber-count capacity requirements and of their relatively long length, these types of cables are thus typically of reduced dimensions, comprising a single central buffer tube carrying the optical fibers. The inner diameter of said fiber-containing buffer tube is generally lower than about 5 mm.
On the other side, fiber-count requirement for submarine cables for non-repeatered systems is generally higher (e.g. up to about 96 fibers) and thus the dimensions of said cable can be, if necessary, increased accordingly. The length of said cables is generally of about 50-150 Km, up to a maximum length of 400-500 km.
A number of cables designs for submarine installation is known in the art.
For instance, U.S. Pat. No. 5,125,062 discloses an undersea cable comprising a central metallic tube, filled with a sealing compound, e.g. silica gel, and containing optical fibers embedded therein, said tube being surrounded by a helical lay of metallic (preferably steel) wires. Interstices between wires and between the helical lay and the central tube are filled with a sealing material (preferably polyurethane resin) which opposes longitudinal propagation of water along the cable. Alternatively, the central tube can be made of plastic and in this case the helical lay also presents the characteristics of an arch for withstanding pressure.
U.S. Pat. No. 4,684,213 relates to a submarine cable comprising a pressure resistant steel tube containing optical fibers, surrounded by two layers of steel wires and by an outer metal tube made of copper or aluminum. Dams of a sticky compound and/or of a jelly of plastic material are disposed at regular intervals inside the central tube and in the gaps between the lay of wires disposed between the central tube and the outer tube.
U.S. Pat. No. 5,463,711 discloses an underwater cable for shallow-water, comprising a central tube made of metal, optical fibers arranged within said tube and surrounded by a water blocking material and six steel wires wound in a helical lay around the central tube.
In the article of D. Felicio, “A non-repeatered achievement”, Telcom report International 19 (1996), pp. 22-25, a submarine cable for non-repeatered systems is disclosed (MINISUB CT®), where the fibers are accommodated into an hermetically sealed central copper tube having an outer diameter of about 5 mm. Said cable is presented in this document as an evolution of a previous cable design (MINISUB 16C®) comprising a central plastic buffer tube surrounded in turn by a hermetic copper tube (6.1 mm diameter).
In the article of J. F. Libert et al., “A New Undersea Cable For The Next Millenium”, Conference Proceedings Suboptic '97, pp. 120-128, a deep-water light weight cable specifically designed for long haul repeatered systems is disclosed. Said cable comprises Large Effective Area (LEA) fibers housed within a jelly filled welded steel tube, which acts as a pressure-resistant component in the cable. The manufacturing process of the tube is controlled in order to provide a small excess length fiber (0.1%). As disclosed in said article, the combination of the slight excess fiber, jelly filling and pressure resistant tube provides a very low stress environment for the fibers, which minimize loss increments in the more bend-sensitive fibres that may be used in wavelength-division-multiplexing (WDM) and high bandwidth systems. As shown in said article, LEA fibers are much more sensitive to bending-induced loss than standard dispersion-shifted fibers (SDS fibers) currently in use.
In view of the above, it is thus apparent that current trends in the production of optical buffer tubes having relatively small dimensions, for use in submarine cables, are in the direction of using metal tubes.
Applicant has however observed that the production of metal buffer tubes of small dimensions (e.g. less than about 6 mm of outer diameter) may be somehow difficult, due to a series of drawbacks.
For instance, optical fibers can be subjected to an undesired over-heating during the welding of the metal tube, with the possibility of being damaged. For avoiding or at least limiting the effects of fiber overheating, a number of technical solutions are provided in the prior art, which solutions however introduce further operative steps or control techniques into the production process. For instance, U.S. Pat. No. 4,852,790 discloses to focalize a laser light above the surface of the tube, in order to weld and hermetically seal the abutted edges of the metal strip without excessive overheating of the underlying structure. U.S. Pat. No. 5,380,977 discloses the use of an optical fiber guide for guiding the optical fibers inside the metal tube, said guide being elastically urged on an inner wall opposite to the welded portion of the metal tube. U.S. Pat. No. 5,760,364 discloses a thermal diffuser interposed between the auxiliary tube for inserting optical fibers into the metal tube and the closure zone that is defined on said metal tube.
In addition, specific techniques and devices should be applied in order to achieve an optimal control on the excess fiber length value inside metallic tubes. For instance, U.S. Pat. No. 4,852,790 discloses the use of a gas flow inside the metal tube for blowing the optical fibers against the outer circumference of the metal tube as the metal tube wraps around the wheel. U.S. Pat. No. 5,231,260 discloses means for the control of the fiber excess length inside a metal tube, which comprises tension adjusting means for optical fiber and for the metal strip forming the metal tube and traction means, including tension variable means, for the metal tube. U.S. Pat. No. 5,143,274 discloses a method for manufacturing a metal tube containing a predetermined excess length of optical fibers, which comprises controlling the fiber speed, the temperature of the metal tube and the pull out tension of the tube.
Applicant has now observed that it would be advantageous to use polymeric materials for manufacturing such buffer tubes. However, Applicant has also observed that polymeric buffer tubes having reduced dimensions may pose additional problems, with respect to metallic tubes, as explained in the following.
In order to reduce the stresses on the optical fibers, it is preferable to lay the optical fibers inside the tube with substantial no excess length or with a slight excess (less than about 0.1%) with respect to the length of the buffer tube. If the fiber is disposed inside a buffer tube with an excess length, its path within the buffer tube will not be linear but the fiber will be disposed on a sinusoidal/helical path. The higher the fiber excess length, the more apparent the sinusoidal/helical path.
In this connection, Applicant has however observed that excess lengths higher than about 0.1% are incompatible with small inner di
Brandi Giovanni
Cecchi Feliciano
Consonni Enrico
Ginocchio Alessandro
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Nguyen Khiem
Pirelli Cavi e Sistemi S.p.A.
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