Optical waveguides – Optical transmission cable – Loose tube type
Reexamination Certificate
2001-04-06
2003-06-10
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical transmission cable
Loose tube type
C385S103000, C385S110000, C385S109000
Reexamination Certificate
active
06577796
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a cable, in particular an optical fibre cable, which is resistant to the radial permeation and to the longitudinal propagation of water.
The present invention also relates to a method for maintaining a high resistance to the passage of water, both in the liquid state and in the vapour state, inside cables, in particular optical fibre cables.
BACKGROUND OF THE INVENTION
Cables, and in particular optical fibre cables, are used in ambient conditions which include contact with water, both in liquid form and in vapour form.
The presence of water molecules inside optical cables, and in particular close to optical fibres, results in reduction of the transmission capacities of the fibres.
The said reduction in the transmission capacities of the fibres is due in particular to the diffusion of water vapour across ink secondary and primary coatings on the optical fibre and subsequent condensation of water at the ink-secondary coating and glass-primary coating interface. This condensation can lead to local detachment between the ink and the secondary coating or between the glass and the primary coating, giving rise to irregular mechanical stresses (“microbending”), which can cause attenuation of the signal transmitted.
Contact of optical fibres with liquid water can occur either following penetration of water from a poorly wrapped end (during storage or laying of the cable) or following accidental damage to this sheath.
The presence of water, in particular of water in the liquid state, and the possibility of its longitudinal propagation inside cables is also a possible cause of damage to the apparatus to which the cables are connected. In view of the above observations, it is therefore advantageous to block the propagation of water and to limit as much as possible the length of cable which, after contact with water, will have to be decontaminated.
Contact of the optical fibres inside a cable with water in the vapour state occurs when this water permeates through the layers which make up the optical cable, thus being able to get inside to where the optical fibres are located. Up to quite high relative humidity values (typically of about 75-80%), the optical fibres are not adversely affected by the presence of water vapour and can even remain under such conditions for years. Above this threshold, the high humidity in contact with the surface of the optical fibres can lead to drawbacks similar to those caused by contact with liquid water (for example delamination, local detachment between glass and coating and/or detachment between the various coating layers, microbending phenomena) which can result in increases in attenuation. Lastly, prolonged contact of water (either liquid water or water in the vapour state) with the surface of the fibre, such as that which occurs after glass-primary coating delamination, can lead to a reduction in the mechanical strength of the glass part of the fibre.
A range of solutions for limiting or preventing the entry of water into cables is disclosed in the prior art.
For example, to limit the penetration of liquid water into optical fibre cables, it is known practice to introduce a fluid blocking filling material, typically a grease or a thickened oil, into the structure of the cable in order to establish a physical barrier to the passage of water into the cable. These filling materials, since they not have any particular physicochemical interactions with the water, are also known as “inert blocking materials”.
Examples of these inert blocking filling materials are disclosed in patents EP 811,864, U.S. Pat. No. 5,285,513, U.S. Pat. No. 5,187,763 and EP 541,007.
The introduction of the said inert blocking filling materials into the structure of the optical cable during production is often laborious, such as processing of the ends (“heads”) of these cables, which need to be wrapped so as to prevent any loss of the filling material. In addition, during installation and/or maintenance of the cable, in order to be able to make junctions between the different pieces of cable, it is necessary to wash off the blocking filling material from all of the components of the optical cable, which can result in damage to the optical fibres due to the action of the solvents and friction.
Another known solution for limiting the ingress of water into optical cables envisages the use of water-swellable materials, i.e. substances capable of absorbing a certain amount of water, thereby increasing their volume. In contrast with the materials described above, these materials are also known as “active blocking materials”.
Typically, these water-swellable materials are distributed on supports made of fibrous plastic material, for example tapes or yarns, which are placed close to the cable structures which it is desired to protect against contact with water.
For example, U.S. Pat. No. 4,867,526 describes a cable comprising a tape made of nonwoven material (in particular polyester) impregnated with a solution of water-absorbing material, in particular polyacrylic acid made insoluble by crosslinking, which is capable of expanding on contact with water.
U.S. Pat. No. 5,138,685 describes a cable comprising a laminated tape consisting of two superimposed layers of nonwoven polymer material, between which is placed a water-swellable material in powder form.
U.S. Pat. No. 5,642,452 describes a cable comprising a yarn impregnated with water-swellable material, in particular polyacrylic acid. This yarn is wound around a central reinforcing element together with tubular elements containing the optical fibres which are filled with a conventional “inert” blocking material. According to the disclosure given in that patent, this configuration is capable of preventing the longitudinal passage of water in the star-shaped areas created by the helical winding of the tubular elements around the central element.
U.S. Pat. No. 4,767,184 describes an optical cable with a grooved core, in which grooves are placed several strips of superimposed optical fibres, each coated with a film of resin containing a water-swellable or swelling material. In combination with the strips of optical fibres with a coating containing water-swellable material, a coating of the same material applied to the grooved core can be used, whereas in the grooves in which no strips of optical fibres are present it is necessary to use a powder made of water-absorbing material.
German Patent DE 1,765,647, relating to conventional metal-conductor telecommunication cables, discloses a 4-wire cable wherein the cable core and the wires are lapped up with a 100 &mgr;m thickness foil made of a low saponification highly polymerised polyvinylalcohol.
The Applicant has observed that if water-swellable fibrous tapes are used, during manufacture of the cable it is necessary to include an additional wrapping operation. Moreover, the problem of release of the water-swellable powders often arises, with the result, on the one hand, that the water-blocking effect is lower where it is needed and, on the other hand, during installation/maintenance, the structure of the cable needs to be free of the presence of these powders.
Moreover, once the water-absorbing effect is complete, the known water-swellable materials behave like inert filling materials, by establishing a simple physical barrier to the passage of water. It is thus necessary to provide a sufficient amount of these materials in the structure of the cable which it is desired to protect. However, in certain optical cable structures such as, for example, in plastic tubular elements containing optical fibres arranged loosely, the amount of material to be used is excessive and uneconomical and in these cases use is therefore made of conventional inert blocking materials. The possibility also exists that an undesired swelling of this water-swellable mass (even in the presence of small percentages of relative humidity) might give rise to attenuation phenomena of the signal transmitted, on account of the irregularly distributed pressures on the surface o
Anelli Pietro
Colombo Gianfranco
Knauss Scott A
Pirelli Cavi e Sistemi S.p.A.
Sanghavi Hemang
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