GB2087589A - Optic Fibre Cables Containing a Dessiccant - Google Patents

Abstract

A communications cable comprises at least one optical fibre 1 within a casing e.g. a metal water barrier 2. The casing contains a desiccant optionally in a jelly. The cable thus constructed is capable of holding an interior relative humidity ???1% for a period of approximately 20 years. The desiccant is preferably a molecular sieve desiccant e.g. certain alumino silicates. <IMAGE>

Description

SPECIFICATION Improvements in Cables Based on Optical Fibres This invention relates to communication cables based on optical fibres, i.e. elongate glass or other silica based filaments along which light signals can pass. Optical fibres are now being commercially developed and installed in operational communications systems. It is important to ensure that a commercially laid optical fibre cable should have an extended operational life of say 20 years or more.

Without precautionary measures optical fibres are liable to water attack, notably when stressed and strained. For example the stews at the tip of a small crack or stress raiser in an optical fibre is greater than anywhere else in the fibre, but it may not be great enough for the crack to be propagated. If, however water is allowed to attack a crack or raiser tip, the crack will be propagated until it reaches a critical length whereupon the crack propagation becomes supersonic leading to the fracture of the fibre. Other mechanisms of water attack which degrade the fibre and may result in fibre breakage are surface corrosion or ice formation inside the cable.

It has been proposed to protect an optical fibre by including in the cable a protective sheath or water barrier to reduce the ingress of water.

We have appreciated for the first time that the water content in the atmosphere of the fibres within a cable results not only from the slight ingress of water through a water barrier, but also from the release of water originally contained in the materials from which the cable was manufactured, and that the tendency of the fibre to undergo the aforesaid fatigue mechanism or be otherwise attacked by water can be reduced by lowering this atmospheric water content.

According to the present invention there is provided a communications cable based on optical fibres comprising at least one optical fibre extending within a casing characterised by the presence of a desiccant within the casing in the region of the fibres.

It is preferred that the desiccant should be a material whose crystal structure contains cavities or pores into which other molecules may be absorbed. Such desiccants will hereinafter be referred to as molecular sieves. Certain alumino silicates are at present commercially available as molecular sieves.

The moisture content around an optical fibre should be kept at < 10% RH (Relative Humidity) preferably at 1% or less and it is known that molecular sieves can absorb up to 20% of their own dry weight in water before the water content of the air in equilibrium with them reaches 10% RH. If for example the initial water content of a cable =0.3 g/m and 0.1 g/m of water permeates through the water barrier in 20 years the RH in the cable will be < 10% if > 2.5 g/m of desiccant is used.In practice the initial water content of the cable is < 0.3 gum~' and the resulting RH after 20 years is 1 %. In principle however any desiccant which is capable of holding the RH of the cable atmosphere < 10% and preferably at about 1%, for a period of about 20 years can be used in accordance with the present invention. An example of such an alternative desiccant is phosphoric acid in a gel.

Two embodiments of the invention will now be described with reference to Figures 1 and 2 of the accompanying drawings which are sections through cables in accordance with the present invention.

Each cable comprises four fibres 1 positioned with radial clearance within a water barrier 2 which in turn is contained within an outer plastic sheath 3. In accordance with Figure 1 of the drawings the space between the water barrier and the cable is filled with jelly containing a molecular sieve desiccant. In accordance with Figure 2 a discrete layer of molecular sieve dessicant 4 lines the metal casing 2 the space between the fibres 1 and the casing 2 being filled with jelly.

Claims

1. A communications cable based on optical fibres comprising at least one optical fibre extending within a casing characterised by the presence of a desiccant within the casing in the region of the fibres.

2. A communications cable as claimed in Claim 1 which is capable of maintaining a relative humidity 1% for approximately 20 years.

3. A communications cable as claimed in either Claim 1 or Claim 2 wherein the desiccant is a material whose crystal structure contains cavities or pores into which other molecules may be absorbed.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in Cables Based on Optical Fibres This invention relates to communication cables based on optical fibres, i.e. elongate glass or other silica based filaments along which light signals can pass. Optical fibres are now being commercially developed and installed in operational communications systems. It is important to ensure that a commercially laid optical fibre cable should have an extended operational life of say 20 years or more. Without precautionary measures optical fibres are liable to water attack, notably when stressed and strained. For example the stews at the tip of a small crack or stress raiser in an optical fibre is greater than anywhere else in the fibre, but it may not be great enough for the crack to be propagated. If, however water is allowed to attack a crack or raiser tip, the crack will be propagated until it reaches a critical length whereupon the crack propagation becomes supersonic leading to the fracture of the fibre. Other mechanisms of water attack which degrade the fibre and may result in fibre breakage are surface corrosion or ice formation inside the cable. It has been proposed to protect an optical fibre by including in the cable a protective sheath or water barrier to reduce the ingress of water. We have appreciated for the first time that the water content in the atmosphere of the fibres within a cable results not only from the slight ingress of water through a water barrier, but also from the release of water originally contained in the materials from which the cable was manufactured, and that the tendency of the fibre to undergo the aforesaid fatigue mechanism or be otherwise attacked by water can be reduced by lowering this atmospheric water content. According to the present invention there is provided a communications cable based on optical fibres comprising at least one optical fibre extending within a casing characterised by the presence of a desiccant within the casing in the region of the fibres. It is preferred that the desiccant should be a material whose crystal structure contains cavities or pores into which other molecules may be absorbed. Such desiccants will hereinafter be referred to as molecular sieves. Certain alumino silicates are at present commercially available as molecular sieves. The moisture content around an optical fibre should be kept at < 10% RH (Relative Humidity) preferably at 1% or less and it is known that molecular sieves can absorb up to 20% of their own dry weight in water before the water content of the air in equilibrium with them reaches 10% RH. If for example the initial water content of a cable =0.3 g/m and 0.1 g/m of water permeates through the water barrier in 20 years the RH in the cable will be < 10% if > 2.5 g/m of desiccant is used.In practice the initial water content of the cable is < 0.3 gum~' and the resulting RH after 20 years is 1 %. In principle however any desiccant which is capable of holding the RH of the cable atmosphere < 10% and preferably at about 1%, for a period of about 20 years can be used in accordance with the present invention. An example of such an alternative desiccant is phosphoric acid in a gel. Two embodiments of the invention will now be described with reference to Figures 1 and 2 of the accompanying drawings which are sections through cables in accordance with the present invention. Each cable comprises four fibres 1 positioned with radial clearance within a water barrier 2 which in turn is contained within an outer plastic sheath 3. In accordance with Figure 1 of the drawings the space between the water barrier and the cable is filled with jelly containing a molecular sieve desiccant. In accordance with Figure 2 a discrete layer of molecular sieve dessicant 4 lines the metal casing 2 the space between the fibres 1 and the casing 2 being filled with jelly. Claims

1. A communications cable based on optical fibres comprising at least one optical fibre extending within a casing characterised by the presence of a desiccant within the casing in the region of the fibres.

2. A communications cable as claimed in Claim 1 which is capable of maintaining a relative humidity 1% for approximately 20 years.

3. A communications cable as claimed in either Claim 1 or Claim 2 wherein the desiccant is a material whose crystal structure contains cavities or pores into which other molecules may be absorbed.