NZ756294B2 - Blown optical fibre unit and method of manufacturing - Google Patents

Abstract

The present invention relates to optical fibre units for air-blown installations and solves the problem of balancing the need of having a material suitable to be peeled-off from the optical fibres with no or limited fibre damage, and at the same time, capable of facing the stress of the installation. In particular, the present invention relates to an optical fibre unit for air-blown installations comprising: a number of optical fibres, an inner layer substantially completely embedding said optical fibres, and an outer layer radially external to the inner layer, wherein said inner layer has a tensile strength of from 0.1 MPa to 1 MPa, and an elongation at break of from 20 to 35%. . In particular, the present invention relates to an optical fibre unit for air-blown installations comprising: a number of optical fibres, an inner layer substantially completely embedding said optical fibres, and an outer layer radially external to the inner layer, wherein said inner layer has a tensile strength of from 0.1 MPa to 1 MPa, and an elongation at break of from 20 to 35%.

Description

BLOWN OPTICAL FIBRE UNIT AND METHOD OF MANUFACTURING BACKGROUND The present invention relates to optical fibre units for air-blown installations. In particular, the present invention relates to a blown optical fibre unit providing high performance in terms of accessibility of the optical fibre(s). The present invention also relates to a method of manufacturing such an optical fibre unit.

PRIOR ART Fibre optic cables have been commonly deployed by installing them in ducts by blowing or pulling, burying them in the ground, or suspending them between above-ground poles.

Optical microcabling technology has been introduced for the deployment of fibre optic cables to increase use of the conduit space and to enhance profitability of the current (and/or future) telecommunications infrastructure. This technology involves the use of standard inner ducts in which microducts are jetted, followed by the jetting of microduct cables or microcables into the microducts when required. Although originally intended for business access networks (FTTB) and fibre-to-the-home (FTTH), this technology has been used successfully in long-haul applications as well.

Microducts are empty tubes that can be blown or pushed into empty or partially filled standard ducts. Optical fibre units, specifically designed for this kind of application, are then installed as needed inside the microduct tubes by blown installation techniques.

In some known blown optical fibre units, a number of coated optical fibres (for example, four, in bundles or ribbon, but also a single optical fibre) are contained within an inner layer enclosed in an outer layer having greater hardness. In the outer layer particulate material (typically hollow or solid glass beads) can be embedded.

EP 0 521 710 A1 discloses an optical fibre package suitable for blown installation and a method of making an optical fibre package for blown installation in a continuous process. The fibres are held in a soft buffer layer. About this buffer layer there is a further resin layer which is a tough layer. For 4-fibre package units, a buffer layer is exemplified made of a material having a tensile strength of 1.3 MPa, a Shore D hardness of 49 and a 115% elongation.

US 2009/0087154 discloses cable designs for indoor installations wherein the cable comprises a dual-layer optical fibre buffer encasement of acrylate resin. The buffer encasement has an acrylate compliant inner layer that protects the fibre and minimizes stress transfer to the fibre; and a hard, tough acrylate outer layer that provides crush resistance. The dual-layer optical fibre buffer encasement is wrapped with reinforcing yarn and encased in an outer protective jacket.

EP 0 296 836 A1 discloses an optical fibre cable comprising an inner sheath containing at least one optical fibre member, and an outer sheath containing the inner sheath. The inner sheath is of a material which is soft and has a low modulus of elasticity. The outer sheath of a material has bulk and surface properties such that the cable can be propelled along a duct by a flow of air travelling along the duct. An intermediate sheath may be provided between the inner and outer sheaths.

EP 1 600 801 A2 discloses a fibre optic cable including a core of coated optical fibres embedded in an inner layer of acrylate material, having sufficient tensile strength when cured to lock at least the outermost fibres in place and still allow the fibres to be easily broken out of the assembly for termination and splicing purposes. Suitable materials for the inner layer have tensile strength greater than 10 MPa.

US 5,533,164 discloses an optical fibre assembly for blown installation, comprising a fibre unit having at least one optical fibre. The unit has a coating comprising an external layer of a material containing hollow glass beads at least some of which project from the outer surface of the external layer. The coating also has an inner, buffer layer of a material having a lower modulus of elasticity than that of the material of the external layer and an intermediate layer of material disposed between the external and inner layers.

SUMMARY OF THE INVENTION A user might have the need to access the optical fibre(s) of a blown optical fibre unit for instance for termination purposes. When a user accesses the optical fibre(s) within a blown optical fibre unit, damages to the optical fibre(s) should be avoided.

In order to prevent damages to the optical fibre(s) in the optical fibre unit, measures should be taken for safely removing the inner layer from around the optical fibres. As “safely removing” it is here meant that the material of the inner layer should be removed without leaving tenacious residue on the optical fibres and/or without detaching portions of the optical fibres coating and/or causing any damage to the optical fibres in general.

In the present description and claims, the term “fibre breakout failure” or “breakout failure” will refer to any optical fibre damage occurring while removing the optical fibre unit inner layer from the optical fibre(s) and caused by the bond between optical fibres and inner layer.

According to the Applicant, the primary reason for breakout failure lies in the materials used in the unit. Typically, an optical fibre comprises a glass core surrounded by one or more polymer coating layer. The polymer used for the fibre coating layer/s and that of the inner layer of the optical fibre unit are both acrylate based. When the polymers of optical fibre coatings are not adequately cured, the polymer of the inner layer of the optical fibre unit could cross link and bond to the fibre. In addition, some of the proprietary available polymers include components which promote bonding between optical fibres coating and optical fibre unit inner layer. Use of such components may be beneficial in optical fibre unit manufacture, but their presence is likely to be detrimental for “breakout failure” of the optical fibre.

The Applicant has noted that the materials of the inner layer of known optical fibre units, such as those described in the above-mentioned documents, have mechanical features possibly resulting in breakout failure when the optical fibres are to be accessed.

The selection of a material for the inner layer of an optical fibre unit is generally made on the basis of mechanical properties suitable for cooperating with the harder outer layer in providing the optical fibres of the unit with due protection to pressure and/or bending during deployment.

The prior art – see, for example, the already mentioned EP 1 600 801 A2 – recognized the importance of an easy and soft removal of the optical fibre unit layers from the optical fibre. A disadvantage of soft materials, however, is that they are more easily damaged during installation.

The Applicant has tackled the problem of balancing the need of having a material suitable to be peeled-off from the optical fibres of the unit with no or limited fibre breakout failure, and, at the same time, capable of facing the stress of the installation without substantial damage to the optical fibres contained therein.

The Applicant considered as inner layer of an optical fibre unit a polymer material having relatively low tensile strength and elongation at break such as to make it a sacrificial layer when accessing the optical fibres.

The Applicant surprisingly found that an optical fibre unit with an inner layer having such relatively low mechanical properties is suitable for being safely removed from the optical fibres, while still providing the optical fibre unit with sufficient stress resistance to be efficiently deployed by blowing without impairing the attenuation of the optical fibres.

According to one aspect, the present invention provides an optical fibre unit for air-blown installations comprising: a number of optical fibres, an inner layer substantially completely embedding said optical fibres, and an outer layer radially external to the inner layer, wherein said inner layer has a tensile strength of from 0.1 MPa to 1 MPa, and an elongation at break of from 20% to 35%; wherein said outer layer has a tensile strength from 10 MPa to 60 MPa; and a 2.5% secant modulus of from 500 MPa to 1000 MPa.

According to another aspect, the present invention relates to a method of manufacturing an optical fibre unit for air-blown installations, the method comprising: providing a number of optical fibres, applying an inner layer on said number of optical fibres, preferably at a temperature of from 15 °C to 30 °C, applying an outer layer; curing the inner layer to provide a tensile strength of from 0.1 MPa to 1 MPa and an elongation at break of from 20% to 35%; and curing the outer layer to provide a tensile strength of from 10 MPa to 60 MPa and a 2.5% secant modulus of from 500 MPa to 1000 MPa.

In a preferred embodiment, the method of manufacturing according to the invention comprises curing the inner layer before applying the outer layer (wet-on-dry application). Advantageously, the inner layer is cured at least 90% before applying the outer layer.

Alternatively, the inner layer is uncured at the application of the outer layer (wet-on-wet application). After the application of the outer layer, the inner and outer layers are simultaneously cured.

In a preferred embodiment, the curing of the inner and/or the outer layer is carried out by UV or IR irradiation.

The optical fibre unit of the invention may comprise a number of optical fibres of from 1 to 24, preferably from 4 to 12.

The optical fibre of the unit of the invention comprises glass core surrounded by one, preferably two polymer coating layers. In particular a first coating layer surrounds and is in direct contact with the glass core; a second coating layer surrounds and is in direct contact with first coating layer. The fibres may have a coloured secondary coating layer or a third layer can surround and directly contact the second coating layer, this third layer being coloured or having indicia for identification purposes.

In a preferred embodiment, the optical fibre unit of the invention comprises an inner layer having a tensile strength of from 0.5 MPa to 0.9 MPa.

In a preferred embodiment, the optical fibre unit of the invention comprises an inner layer having an elongation at break of from 20% to 35%, more preferably of from 30% to 35%.

In a preferred embodiment, the optical fibre unit of the invention comprises an inner layer having a Shore A hardness of from 10 to 40, more preferably of from 20 to 38, even more preferably of from 25 to 35.

In a preferred embodiment, the optical fibre unit of the invention comprises an inner layer having a 2.5% secant modulus of from 1 MPa to 10 MPa, preferably of from 3 MPa to 6 MPa.

In the optical fibre unit of the invention, the outer layer is provided in a radial external position with respect to the inner layer and in direct contact thereto.

In the optical fibre unit of the invention, the outer layer has a hardness greater than that of the inner layer. In a preferred embodiment, the outer layer has a Shore D hardness of from 30 to 80, more preferably of from 40 to 70.

In a preferred embodiment, the optical fibre unit of the invention comprises an outer layer having a tensile strength of from 10 MPa to 60 MPa, more preferably from 30 to 40 MPa.

In a preferred embodiment, the optical fibre unit of the invention comprises an outer layer having a 2.5% secant modulus of from 500 MPa to 1000 MPa, preferably of from 600 MPa to 750 MPa.

The optical fibre unit of the invention may further comprise an ink layer in radially outer position with respect to the outer layer and in direct contact thereto.

In the present description and claims as “ink layer” is meant a layer of relatively small thickness, coloured and/or bearing indicia.

The ink layer optionally present may have a thickness of from 5 µm to 50 µm, preferably of from 10 µm to 15 µm.

In a preferred embodiment, the ink layer optionally present has a hardness greater than that of the underlying outer layer, preferably a Shore D hardness of from 40 to 90, more preferably of from 50 to 80.

The ink layer optionally present may have a tensile strength of from 10 MPa to 60 MPa, more preferably from 25 to 35 MPa.

In a preferred embodiment, the inner and/or outer layer of the optical fibre unit of the invention is made of a material based on acrylate material.

More preferably, the inner and outer layer of the optical fibre unit of the invention is made of a material based on acrylate material.

The optical fibre unit of the invention may further comprise beads partially embedded into the outer layer, these beads being hollow or solid, preferably solid. When the optical fibre unit of the invention comprises these beads, it does not comprise the ink layer in radially outer position with respect to the outer layer.

The beads optionally present in the optical fibre unit of the invention are applied to the outer layer when this latter is still uncured. For example, beads can be applied as described in WO2014/194949.

The optical fibre unit of the invention may have an advantageously reduced diameter with respect to known optical fibre unit containing the same number of optical fibres having substantially the same diameter.

For example, when the optical fibre unit comprises one optical fibre having a diameter of 250 µm, its outer diameter can be of 680 µm at most; when the optical fibre unit comprises four optical fibres having a diameter of 250 µm, its outer diameter can be of 890 µm at most; when the optical fibre unit comprises twelve optical fibres having a diameter of 250 µm, its outer diameter can be of 1290 µm at most; when the optical fibre unit comprises twenty-four optical fibres having a diameter of 250 µm, its outer diameter can be of 1750 µm at most. These outer diameter values are referred to optical fibre units with no ink layer or beads embedded in the outer layer.

The outer layer or, if present, the ink layer is the outermost portion of the optical fibre unit of the invention.

In the present description and claims: - the term "radial" is used to indicate a direction perpendicular to a reference longitudinal axis of the cable; - the expressions "radially inner" and "radially outer" are used to indicate a position along a radial direction with respect to the above- mentioned longitudinal axis; - a size along the radial direction is termed "thickness"; and - the verb “to embed” means to enclose closely in or as if in a matrix.

The Shore hardness has been evaluated according to ISO 868_2003- 03-01. The tensile strength, the elongation at break and the secant modulus have been evaluated according to ISO 5272012.

For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.

BRIEF DESCRIPTION OF DRAWINGS The present invention will be further described in the following detailed description, given by way of example and not of limitation, with reference to the following figures, wherein: - Figure 1 shows a cross-section of a blown optical fibre unit according to a first example of the present invention; and - Figure 2 shows a cross-section of a blown optical fibre unit according to a second example of the present invention.

DESCRIPTION OF EXAMPLES Figure 1 shows a cross-section of a blown optical fibre unit 1 according to a first example of the present invention. The unit 1 comprises four optical fibres 2, an inner layer 10 and an outer layer 12.

It should be noticed that the number of optical fibres 2 is not relevant for the present invention and the number of optical fibres could be any number. Also an optical core with a single optical fibre is deemed within the scope of the present invention.

Each optical fibre 2 comprises a glass core comprising an optical waveguide 3, for example a single mode optical waveguide, and a cladding 4 surrounding the waveguide 3. A first polymeric coating 5 surrounds the cladding 4 and a second polymeric coating 6 surrounds the first polymeric coating 5. According to embodiments, each optical fibre 2 may further comprise a third polymeric coating 7, typically an ink layer, surrounding the second polymeric coating 6.

Preferably, the optical fibres 2 of the optical core are arranged in a bundle.

Each optical fibre 2 can have a fibre outer diameter of from 150 µm to 300 µm, preferably from 200 µm to 245 µm, when the third polymeric coating 7 is absent.

The third polymeric coating 7 could have a thickness of 5 µm.

The blown optical fibre unit 1 according to the present invention further comprises an inner layer 10 embedding the optical fibres 2.

Preferably, the inner layer 10 of the embodiment of Figure 1 (relating to an optical fibre unit 1 comprising four optical fibres 2 having an outer diameter of 255 µm) has a diameter of 780 µm.

The blown optical fibre unit 1 according to the present invention further comprises an outer layer 12 surrounding and in direct contact with the inner layer 10. The outer layer 12 of the embodiment of Figure 1 (relating to an optical fibre unit 1 wherein the inner layer 10 has a diameter of 780 µm) has an outer diameter 12 of 920 µm.

The blown optical fibre unit 1 according to the present invention further preferably comprises an ink layer 13 surrounding the outer layer 12.

The ink layer 13 has a thickness of 10 µm.

The Applicant has tested several materials for the inner buffer, with the object to reduce fibre breakout failure. A suitable material is Herkula® Series 830/801, produced by Herkula Farben GmbH, Willich, Germany.

Figure 2 is a cross-section of a second example of the present invention. In Figure 2, the same reference numbers of Figure 1 apply to the same cable parts. The blown optical fibre unit 1 of Figure 2 comprises twelve optical fibres 2. The optical fibre unit 1 of Figure 2 has beads 14 partially embedded in the outer layer 12. Beads 14 can be of glass or the like.

TEST 1 The fibre breakout performance in an optical fibre unit according to the invention was tested as follows.

A first blown optical fibre unit according to the example of Figure 1 was manufactured. The novel unit was made using Herkula® Series 830/801 for the inner layer, so that this layer had a Shore A hardness of 28, a tensile strength of 0.6 MPa and an elongation at break of 30%.

The optical core comprised four optical fibres manufactured by different manufacturers. The four fibres were embedded in the inner layer made of Herkula® Series 830/801 at a temperature of 27 °C. An outer buffer made of DSM Cablelite® 328775 was applied over the inner layer.

A second blown optical fibre unit according to the example of Figure 1 was manufactured. This second unit is a comparative one. The optical core comprised the same four optical fibres of the first unit but embedded in an inner layer made of DSM 328739A having a tensile strength of 1.3 MPa and an elongation at break of 135% and DSM Cablelite® 3287- 9-75 for the outer layer. The application of the inner layer was carried out at a temperature of 40 °C.

Different temperatures for the application of the inner layers of the first and of the second optical fibre unit were necessary for having the two materials at substantially the same viscosity.

The fibre breakout performance was evaluated according to the British Telecom standard CW1574, Issue 13 (1993), section 3.4.

Table 1 shows the results of Test 1.

Table 1 Fibre Second unit First unit Breakout Breakout Grey F P Violet P P White F P Orange P P P = positive F = failed The first unit according to the invention reached a Positive grade, while only two fibres of the comparative second unit were considered positive in the test. Without being necessarily limited to any one particular explanatory theory, the Applicant considers that this extremely positive result has been obtained owing to the mechanical features of the inner layer according to the invention, especially in terms of tensile strength and elongation at break.

TEST 2 The attenuation performance of the first blown optical fibre unit as from Test 1 was tested according to ITU-T G.652 (06/2005).

Attenuation results for first unit are indicated in Table 2 below.

Table 2 First Unit 1310 nm 1550 nm 1625 nm Fibre (dB/km) (dB/km) (dB/km) Grey 0.354 0.229 0.330 Violet 0.325 0.214 0.232 White 0.331 0.214 0.228 Orange 0.327 0.208 0.215 The attenuation of the optical fibres in the unit of the invention resulted in conformity with the values requested by of ITU-T G.652 (06/2005) standard, Table 4 (G.652.D). This test showed that, though the optical fibres of the unit of the invention were embedded in a “soft” inner layer, such inner layer was anyway suitable for protecting the fibres against attenuation. The material of the inner layer according to the invention provided improved fibre break-out with no detrimental effect to the optical performance.

TEST 3 The blowing performance of a length of the first optical fibre unit of the invention as from Test 1 was tested according to the British Telecom standard CW1574 Issue 13 (1993), section 7.3.1.

The blow test was carried out under the following conditions/parameters: Blown Parameters Tube Emtelle FC6187624 Bore [mm] 3.5 nominal Tube Outside Diameter [mm] 5 No. of Times Used 14 Route Details Length [m] 500 (Internal ducted) Route Details Delivery drum Airflow @ [l/min] 11 bar Test Details Compressor model 2 x Factair Comp. pressure / capacity 11.0 bar/120 l/min Blowing equipment details Plumettaz Pressure at input [bar] 9.80 Dewpoint [°C] -24.8 Clutch Setting 4.00 Fibre Guide Brass Ambient temperature [°C] 18 Weather conditions overcast, damp Results Distance Blown [m] 500 Time [min] 20.15 Speed [m/min] 24.8 After blowing, the first optical fibre unit complied with the above mentioned standard in terms of optical fibre attenuation showing that the inner layer can provide the optical fibres with suitable protection during deployment.

Claims (12)

1. An optical fibre unit for air-blown installations comprising: a number of optical fibres, an inner layer substantially completely embedding said optical 5 fibres, and an outer layer radially external to the inner layer, wherein said inner layer has a tensile strength of from 0.1 MPa to 1 MPa, and an elongation at break of from 20% to 35%; 10 wherein said outer layer has a tensile strength of from 10 MPa to 60 MPa; and a 2.5% secant modulus of from 500 MPa to 1000 MPa.

2. The optical fibre unit of claim 1 comprising from 1 to 24 optical 15 fibres.

3. The optical fibre unit of either claim 1 or 2, wherein the inner layer has a tensile strength of from 0.5 MPa to 0.9 MPa. 20

4. The optical fibre unit of any one of claims 1 to 3, wherein the inner layer has a Shore A hardness of from 10 to 40.

5. The optical fibre unit of any one of claims 1 to 4, wherein the inner layer has a 2.5% secant modulus of from 1 MPa to 10 MPa.

6. The optical fibre unit of any one of claims 1 to 5, wherein the outer layer has a Shore D hardness of from 30 to 80.

7. The optical fibre unit of any one of claims 1 to 6, comprising an ink 30 layer in radially outer position with respect to the outer layer and in direct contact thereto.

8. The optical fibre unit of claim 7, wherein the ink layer has a Shore D hardness of from 40 to 90.

9. The optical fibre unit of any one of claims 1 to 8, having an outer diameter of 890 µm at most and comprising four optical fibres having a diameter of 250 µm.

10 10. A method of manufacturing an optical fibre unit for air-blown installations, the method comprising: providing a number of optical fibres, applying an inner layer on said number of optical fibres, applying an outer layer; 15 curing the inner layer to provide a tensile strength of from 0.1 MPa to 1 MPa and an elongation at break of from 20% to 35%; and curing the outer layer to provide a tensile strength of from 10 MPa to 60 MPa and a 2.5% secant modulus of from 500 20 MPa to 1000 MPa.

11. The method of claim 10, wherein said applying an inner layer on said number of optical fibres is carried out at a temperature of from 15 °C to 30 °C.

12. The method of either claim 10 or 11, wherein said applying an outer layer is carried out on an uncured inner layer and then the inner and outer layers are simultaneously cured.