BRPI0520166B1 - METHOD FOR MANUFACTURING AN OPTICAL CABLE, AND APPARATUS FOR MAKING AN OPTICAL CABLE - Google Patents

Description

"METHOD FOR MANUFACTURING AN OPTICAL CABLE AND APPARATUS FOR MANUFACTURING AN OPTICAL CABLE" BACKGROUND OF THE INVENTION The present invention relates to a method for manufacturing an optical cable, an apparatus for the manufacture mentioned and a cable optical fiber comprising at least one metal tube containing at least one optical fiber.

The protection of optical fibers, packing in a metal tube, finds application in aerial, underground and underwater cable.

Typically, the metal tube surrounds a single fiber or several optical fibers, preferably joined together in a bundle of several fibers. Metals, such as steel, aluminum, aluminum alloy or copper, can be used to make the pipe. Prior Art A method for providing a metal tube around one or more optical fibers comprises the step of shaping a metal tube with a longitudinal slot, said slot being sealed after positioning the optical fibers in the tube. See, for example, US 6,522,815 (in the name of Nexans Deutschland Industries AG & Co. FG).

US 6,047,586 (in the name of Alcatel) refers to the process where a metal pipe is manufactured by extruding a starting material through a formed nozzle. During the time the metal is extruded, optical fibers are conducted to the center of the formed nozzle through a second opening. The technique is the Conform ™ process for manufacturing non-ferrous metal profiles and tubes (as described, for example, in US 3,765,216). With this technique, wire-shaped metal is introduced into a groove arranged at a circumference of a rotating friction wheel. The wheel drives the metal into a retention space of a retaining block that engages in the groove and seals the groove. Rotation of the friction wheel produces high temperatures and high retention space pressures that plastically deform the metal. The metal may then be extruded through a formed nozzle and solidified to form a profile or a tube.

In the aforementioned types of cables, optical fiber is usually provided to have a higher length than the metal tube, in particular, to take into account the different thermal coefficients of fiber glass and metal. pipe.

Mentioned additional fiber length (EFL) usually varies with a function of cable construction, materials employed and metal tube diameter. A suitable EFL protects the optical fibers from attenuation increases and mechanical failures due to the pressures generated on the cable during their lifetime, such as during installation or caused by ambient conditions such as temperature changes, vetoing, ice during operation.

US 6,047,586 discloses that to produce additional length within the metal tube by gently moving the fibers at a higher feed rate through a channel terminating in the formed nozzle.

The Applicant notes that this method is difficult to perform, and the results in terms of additional fiber length are not reliable.

US 6,522,815 shows that in order to produce the additional length of the optical fibers compared to the metal tube, a corrugation device is arranged downstream of a rotary shaft that imprints a corrugation on the tube wall in a continuous operation. The additional fiber length is a function of the depth and pitch of the corrugation.

According to US 2004/0008956, so that the optical fiber in the metal tube is supplied with an additional length, the metal tube is continuously clamped between extraction clamps whose pairs of clamps lock the metal tube securely and apply bending forces. The metal tube undergoes elastic deformation, ie it is stretched. Accordingly, equal lengths of stretched metal tube and optical fibers are wound into an extraction disk. The state of elastic deformation "relaxes" in the extraction disk and the metal tube is shortened to normal condition.

Summary of the invention. In the present invention, the Applicant realizes that advantages may arise from using the plastic deformation portion of the metal tube loading / elongation diagram better than the elastic deformation portion of the metal tube loading / elongation diagram.

However, the Applicant has experienced that by providing an additional fiber length by corrugating a pipe, the resulting cable has an EFL ranging near acceptable limits along the longitudinal extension of the cable.

In particular, within the present invention the Applicant has observed that while shortening a tube, in particular shortening a metal tube through plastic deformation, is a difficult operation to produce to produce a constant result, Plastic pipe stretching is a reproducible process that can be precisely controlled in industrial production.

The Applicant has found that an optical cable with an EFL substantially constant along its longitudinal extent can be produced by granting a higher than desired EFL during at least one step of the manufacturing process and subsequently plastic stretching the tube. conditioning the fibers in a controlled manner to the final EFL value.

In a first aspect, the present invention relates to a method for manufacturing an optical cable comprising at least one metal tube carrying at least one optical fiber, said cable having a predetermined additional fiber length (EFL) method. mentioned comprising the steps of: - advancing said optical fiber; - measure a length of the advanced optical fiber in a time interval; forming a metal tube around the mentioned fiber; - obtain the congency between the metal tube and the mentioned fiber; plastically deforming the metal tube by shortening the metal tube by a predetermined amount (St) greater than the predetermined EFL, during advancement of the metal tube; plastically deforming the metal tube after shortening to provide an elongation thereof during advancement of the metal tube; measuring an advanced metal tube length at said time interval after said stretching; assessing a resulting additional fiber length as a function of the length of said optical fiber and the length of said measured metal tube; adjusting the resulting additional fiber length by controlling the elongation of the metal tube to achieve the predetermined additional fiber length.

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

In the present description and claims, as additional fiber length (EFL) is meant the value given by the following formula where Lf is the length of at least one optical fiber and Lt is the length of the metal tube packing the fibers.

The EFL obtained by the method of the invention may preferably reach from -1.5% to 2.5% depending on the requirements of the specific optical cable to be manufactured. Typically, a positive EFL is advantageous in all cases where the cable may be subjected to stresses and stretching, for example in overhead optical cables; however, in some cases a negative EFL may be advantageous, for example in underground cable, especially when buried in freezing ground.

In view of the above definitions, the additional fiber length (EFL) can easily be measured on the terminated cable.

In order to measure EFL for control purposes during the cable manufacturing process, EFL is determined as a function of the measured advanced length of the optical fibers over a given time period and the measured advanced length of the metal tube. in the same time period. If this value is calculated on the basis of the measured forward length of the metal pipe at the end of the manufacturing line (ie when the pipe is not expected to undergo any additional size changes), the value obtained also corresponds to the final EFL of the pipe. the fiber being substantially non-extensive throughout the cable manufacturing line.

[00022] In the present description and claims, as "time interval", I mean an arbitrary period of time. Preferably, a useful time span in the manufacture of the cable according to the present invention is out of order of magnitude of the time taken by an optical fiber to travel along the manufacturing line at the usual manufacturing speed to go along of the entire full length of the manufacturing line (typically 10 to 50 m).

Any different times may be used in view of specific manufacturing needs, for example, shorter time intervals may be useful for achieving a particularly high constancy of the EFL value along the cable where the cable application This specific requirement requires either at high manufacturing speeds, while a longer time frame may allow the use of simpler measuring devices and control devices, or be used at low manufacturing speeds.

[00024] The step of forming the metal tube around the fiber optic can be done by a technique known in the art, for example by providing a metal tube with a longitudinal slit that is sealed upon insertion of at least one. fiber, or forming the metal tube in situ by extruding a plasticized metal, the latter being preferred.

Advantageously, the step of forming the metal tube around at least one optical fiber includes monitoring the speed of the formed metal tube.

Advantageously, the method of the invention also comprises a step of providing a filler material in said metal tube.

Preferably, the filler material is advanced separately from the optical fiber until after the metal tube is formed around the optical fiber.

In a preferred embodiment, the method of the invention also comprises the steps of feeding a plasticized metal through a nozzle to form a metal tube; conducting at least one optical fiber into the metal tube through a first feed channel having an end disposed downstream of the nozzle; and conducting a filler material into the metal tube through a second feed channel having an end disposed downstream of the nozzle.

In the present description and claims, "achieving congruence" means causing the optical fiber and metal tube to be longitudinally bonded together.

If congruence is obtained by winding the metal tube containing the optical fiber into a pulley or the like, the length of the optical fiber may turn out to be shorter than that of the metal tube, because the fiber is wound to a diameter that is smaller than the rotating diameter of the pipe shaft. In such a case, in order to reach the final EFL, the shortening of the metal pipe must also take into account this reduction in fiber length.

Preferably, because in such a case the fiber is subjected to stress pressures after its rotation within the tube in the congruence pulley, the subsequent shortening of the tube should be done immediately after the fiber and tube have come out of the congruence pulley. in order to reduce the time in which the fiber is exposed to such stress pressures.

In the present description and claims, as "plastically deforming", that is, causing a change in the length of the metal tube by applying a force, the mentioned change lasting after the applied force is removed. For example, the mentioned change may result from the application of sustained pressure beyond the elastic limit of the pipe material.

Preferably, the shortening step is performed by corrugating, wrinkling, indenting the at least one of said metal tube. In the present description and claims, as "indenting", it should also encompass the term "wrinkling" and "corrugating" unless otherwise specified.

For example, shortening is obtained by indenting the metal tube with a helical deformation having predetermined pitch and depth values. Preferably, a pitch of 5 to 15 mm is established.

Preferably, the amount of St shortening is 0.2% to 0.4% greater than one corresponding to the predetermined EFL.

The resulting EFL evaluation step is performed by comparing the length of the metal tube after stretching with the length of the optical fiber as measured upstream of the congruence.

Preferably, the step of adjusting the resulting EFL by controlling the elongation of the metal tube is effected by adjusting the speed in the apparatus by stretching the tube. More particularly, tube elongation is effected by increasing the tube extraction rate when the resulting EFL is higher than the predetermined one, or reducing the tube extraction rate when the resulting EFL is lower than the predetermined one.

In another aspect, the present invention relates to an optical cable comprising at least one metal tube and at least one optical fiber packaged therein, said cable having a local additional fiber length (EFL) ranging from, less than 0.2% along the longitudinal extension of the cable with respect to an average cable EFL.

In the present description and claims, as "average EFL" is an EFL value resulting from measurements of fiber optic and metal tube lengths taken from an optical cable of at least 1 km in length.

In the present description and claims, as "local EFL", I mean an EFL value resulting from measurements of fiber optic and metal tube lengths taken from a portion of the cable having a length of 10 m to 50 m.

Preferably, a cable according to the present invention, the local EFL measured at distal cable portions of at least 100 m from one another in a cable of at least 1 km, ranges from, or less than 0.2 % along the longitudinal extension of the cable with respect to the average EFL of the length of said cable.

Examples of optical cables according to the invention are cables for aerial use such as optical ground wire (OPGW) and optical phase conductors (OPPC). and ground cables for use in harsh environments, or in general when a desired metal shield.

In one preferred embodiment according to the invention, the cable of the invention has an average EFL of 0.2% to 1.5%, more preferably 0.2% to 0.9%. In another embodiment, the average EFL may have negative values, for example of -0.5%.

Preferably, the local EFL differs from the average EFL by or less than 0.1%.

Preferably, the optical cable of the invention comprises at least one metal tube housing from 12 to 96, more preferably from 24 to 48 optical fibers.

[00046] The metal tube is made of a plastic deformable metal material. Examples of such metal materials are steel, aluminum, aluminum alloy and copper.

In a preferred embodiment of the invention, the metal tube is longitudinally shortened with respect to at least one optical fiber contained therein. More particularly, the metal tube is longitudinally shortened by wrinkling, indentation or corrugation. Most preferably the metal tube is retracted.

In a preferred embodiment, the wrinkling, indentation or corrugation of the metal tube has a depth of 0.05 mm to 2 mm, more preferably 0.1 mm to 1 mm of the outer envelope surface of the tube.

In a preferred embodiment, wrinkling, indentation or corrugation is a propeller with a pitch of 5 mm to 15 mm.

Preferably, the metal tube has an internal diameter of 2 to 11 mm, more preferably 2 mm to 4 mm.

Preferably, the metal tube has an outer diameter of 3 to 15 mm, more preferably 3 mm to 6 mm.

For the purpose of the present description and claims, the inner and outer diameters of the tube are referred to as the inner and outer cylindrical surfaces of the tube and do not take into account their wrinkling, indentation or corrugation.

[00053] In a preferred embodiment, the optical fiber contained in the metal tube is embedded in a filler material. An example of a filler is a gel, gelatin or grease. Preferably, said filler material has at least one selected feature of hydrophobicity, hydrogen absorption, thixotropy. More preferably, said filler material is hydrophobic and thixotropic. More preferably, said filler material is hydrophobic and thixotropic. More preferably, it is a mentioned filler material that is hydrophobic, thixotropic and capable of absorbing hydrogen.

Preferably, a filler material suitable for the invention has a hydrogen absorption equal to or greater than 0.1 cm3 STP / g plus a filler preferably equal to or greater than 0.5 cm3. STP / g.

Preferably, a filler material suitable for the invention has a local temperature viscosity of 40 to 300 Pa.s (shear rate of 1.56 / s), more preferably 50 to 120 Pa.s. Viscosity values are derived from a rheological measurement using a Bohlin CVO 120 pressure control rheometer with a 2740 mm cone and plate measuring system at constant temperature.

In another aspect, the present invention relates to an apparatus for manufacturing an optical cable comprising at least one metal tube containing at least one optical fiber, said cable having a predetermined additional fiber length (EFL), the apparatus comprising - a device for continuously forming a metal tube; a first feed channel for conducting at least one optical fiber within said metal tube; a first puller for making the metal tube and at least one congruent optical fiber - a corrugation device for shortening said metal tube; - a second pulling device for extending said metal tube. For example of device for forming the metal tube according to the invention is a metal forming machine continuously.

Advantageously, said apparatus comprises a second feed channel for conveying a filler material into said metal tube.

Advantageously, the second feed channel is coaxial and externally radially with respect to the first feed channel.

The apparatus of the invention advantageously comprises a first cooling device involving the first feed channel.

Preferably, said first cooling device comprises a cooling pipe, coaxially and externally radially with respect to the first feed channel, said cooling pipe conditioning a flow of a cooling fluid.

More preferably, said cooling tube is coaxial and in an external position radially to the second feed channel.

A second cooling device, for example a cooling ditch operating by spraying water, is preferably provided downstream of the device to form the metal tube.

Optionally, upstream of the first puller, a die may be provided to equalize the size or surface of the metal tube. Said matrix may also provide a reduction in the diameter of the metal tube.

The first pulling device is, for example, a rotary shaft, or a track, the former being preferred.

Within the present invention and claims, by "rotary shaft" is meant a motor driven pulley around which the cable or pipe is wound at least one turn. If necessary, such a pivoting shaft includes a track acting on the cable or pipe immediately downstream of the cylinder or pulley to pull the cable or pipe high enough to generate friction preventing the cable or pipe from slipping on the surface of the roller or pulley. .

Within the present invention and claims, by caterpillar means two motor-driven endless metal belts carried by relevant pulleys, between which the cable or pipe is tightened. When a caterpillar is used with the puller, the clamping force of the belts is high enough to allow the cable to be transmitted through the pull without slipping.

In the present description and claims, "corrugation device" means a device capable of shortening the metal tube by plastic deformation, for example by indenting or corrugating or corrugating the metal tube walls.

The second pulling device may be selected from a track or a rotary axis, the latter being preferred.

Advantageously, the apparatus of the invention also comprises at least one length measuring device for checking the progress of the manufacturing process. Preferably, length measuring devices are provided, for example, upstream of the first feed channel, upstream and downstream of the first puller, downstream of the second puller. Advantageously, the length measuring devices mentioned measure the length of the optical fibers, the metal tube or both while advancing along the apparatus. Each of the length measuring devices mentioned is advantageously connected to a counter.

[00071] Examples of length measuring devices used in the present manufacturing apparatus are laser-based noncontact encoders and measuring instrument using Doppler effect.

Preferably at least one measuring device of said length is an encoder.

Brief Description of the Drawings The invention will be further illustrated with reference to the following examples, where: Figure 1 shows a cross-sectional view of an optical cable according to the invention; Figure 2 is a schematic side view of an apparatus according to the invention; Figure 3 shows a cross-sectional view of a tube forming device for the present invention; Figure 4 shows a metal tube in a partial longitudinal section housing an optical fiber; Figure 5 refers to the variation of EFL in a cable manufactured according to the prior art; Figures 6 and 7 refer to the variation of EFL in a cable manufactured in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In Figure 1 an example of an OPGW 100 cable according to the invention is shown.

Cable 100 comprises a bundle of optical fibers 101 embedded in a filler 102, the assembly being housed in an aluminum tube 103. Shielding elements 104, 105 are provided in a radially external position with respect to the dc tube. aluminum 103.

Preferably, the shielding elements are helically wound around tube 103 in a single layer, coaxial to tube 103.

In a preferred embodiment, in particular for use as OPGW (Optical Ground Wire), the shielding elements include aluminum-coated steel wire 104, and aluminum-wire 105. The number and diameter of aluminum-coated steel wire 104 and aluminum wires 105 are determined on a case-by-case basis in view of the required tensile strength of the cable and the required electrical resistance of the cable (in particular for use as a ground wire in high voltage power line areas).

For cables for different uses, shielding elements may not be present or made of different materials, or metallic or dielectric.

Depending on use, one or more coating or polymer material may be present outside of aluminum tube 103 (not shown).

[00080] In Figure 2, an apparatus for manufacturing a cable according to the invention is illustrated.

In apparatus 200 of FIG. 2, a storage compartment 201a and a feed system 201b are provided for respectively supplying the optical fibers and the filler material.

Typically, the fiber storage compartment 201a comprises one or more fibers, carries cylinders from which the fibers are unwound, with controlled friction, and conducted to form a fiber bundle to be fed to subsequent elements of the apparatus.

At the exit of the fiber storage compartment 201a a first encoder 202 is provided, suitable for measuring the length of the advanced optical fibers over a given period of time.

The encoder is a commercial apparatus and generally includes a roller, friction driven in rotation through the moving fibers without slipping. The roll speed is monitored and an electronic circuit is provided to calculate the advanced length as a function of the detected roll turns.

Following the first encoder 202 the optical fibers are driven into an injection system 203.

[00086] The injection system 203 is operatively connected to a metal extrusion machine 204, where the metal tube is extruded. An example of a metal extrusion machine 204 is a Conform ™ machine.

[00087] At the outlet of the metal extruder 204 a cooling trough 205 is provided, which cools the extruded metal tube by means of a cooling fluid, preferably by water jet.

[00088] A second encoder 206 is provided to monitor the advancement of the metal pipe by wrapping the optical fibers at the outlet of the cooling ditch 205.

Following the second encoder 206, a rotary shaft 207 is provided, around which cylinder, the tube housing the fiber or fibers is wound with one turn.

The winding of the tube around the swivel shaft drum 207 achieves congence between optical fibers and metal tube.

The winding of the tube and fibers around the swivel axis cylinder 207 causes the length of the optical fibers in the tube to be shorter than that of the metal tube, as explained above.

The amount of subsequent metal tube shortening is selected to take this fact into account as well.

From the revolving shaft 207, the metal tube is pulled through a first track 209 and taken to the entrance of a corrugating device 210.

Between the rotary shaft 207 and the first track 209, a third encoder 208 is provided to measure the length of the tube. Alternatively, said encoder 208 may be positioned on the rotary axis 207 itself.

The first caterpillar 209 is preferably positioned near the corrugating device 210.

Corrugation device 210 is a machine capable of causing plastic deformation of the tube in the form of a corrugation, indentation or wrinkling. Preferably, such corrugation or indentation is obtained through an indentation head, including a deflection ring through which the tube is extracted, forced to rotate about the tube and interfering with its outer diameter for a part of its rotation.

The action of the ring on the tube causes the tube itself to be plastically deformed according to an indentation helix, whose pitch, width and depth are determined by the geometric parameters of the corrugation device, which, in combination, determines the amount shortening of the corresponding tube.

Such shortening parameters are set in corrugating device 210 and may be varied during operation. In particular, the indentation head varies its speed of rotation following variations in the measured tube speed through a third 208 to maintain a constant pitch along the length of the tube.

Following corrugation device 210, a second caterpillar 211 pulls the corrugated tube and supplies it to a collecting cylinder 213.

The metal tube is pulled by the second track 211 at a predetermined speed as well as to lengthen the metal tube and achieve the desired EFL.

The shortening of the metal pipe obtained is measured by comparing the pipe length measured by the third encoder 208, provided between the first track 209 and the corrugation device 210, and the pipe length measurement through a fourth encoder 212, downstream of the corrugation device 210 and the second track 211.

The fourth encoder 212 measures the length value after shortening and stretching, ie the final length of the tube in the cable.

In operation, the fourth encoder 212 measures the length passed in the time period when the third encoder 208 measured the passage through a predetermined length. Preferably, such predetermined length measured through the third encoder 208 is out of order of magnitude from the manufacturing line (e.g. 10 to 50 m).

The resulting additional fiber length is measured by comparing the measurements of the first encoder 202 - which measures the optical fiber length from the optical fiber provider 201 - and the fourth encoder 212 which measures the tube length after the second caterpillar 211. .

Finally, the tube, with the optical fibers packed in it, is wound into a cylinder 213; The pipe is subsequently supplied to other stages, where shielding and casing are applied to it as required. At such stages the EFL value is generally not changed, such that the measured EFL on the line is substantially the same as on the terminated cable, however, if a special treatment made at these stages is expected to modify such value, the line parameters described above may be selected to take account of such modifications.

In operation, the shortening caused by corrugating device 210 is selected to a value exceeding that corresponding to the final EFL in the cable.

Caterpillar 211 stretches the corrugated or retracted tube by a specified amount to achieve the desired final EFL at encoder 212.

In such manner, any uncontrollable variation and tolerance in shortening obtained by the corrugation device is taken into account thereby achieving the desired accuracy of the final EFL value.

As an example, if an optical cable with a 0.5% EFL is desired, a shortening of the metal tube (St) higher than such value, for example 0.7% is established. In this way, a metal pipe having an original length of 10 m, measured at 9.993 after shortening.

If the speed of the second track 211 is 0.7% lower than the speed of the revolving axle 207, the metal tube maintains the set percentage of shortening St. In order to achieve the desired EFL of 0.5%. , the relative speed of the second track 211 is set to be 5% lower than that of the revolving shaft 207, i.e., a 0.2% elongation (St - EFL) is performed on the pipe, just as the final product has a shortening of 0.5%, ie a length of 9,995 m.

As the speed of the second track 211 can be adjusted on the basis of the final measured EFL with relatively high accuracy (eg within ± 0.00025 m / s or ± 0.15 m / min), any error in the value Stopping shortening can be compensated.

The method of the invention makes it possible to produce a predetermined EFL with a much lower variability along the longitudinal direction of the optical cable than that of the prior art process.

A real-time measurement and, in this case, adjustment of the resulting EFL is performed by controlling the relative speed of the second track 211 with respect to the speed of the revolving axis 207.

The resulting EFL is calculated according to formula (1), wherein Lf is the length of the optical fiber measured through the first encoder 2-2, and Lt is the tube length measured through the fourth encoder 21, both the lengths being measured at the same time interval.

In case the resulting EFL is higher than desired, ie the optical fibers in the tube are "longer" than expected, the relative speed of the second track 211 with respect to the pivot axis 207 is decreased by a corresponding amount to provide a higher pipe elongation and consequently increase the resulting EFL.

In case the resulting EFL is lower than desired, i.e. the optical fibers are "shorter" than expected, the relative speed of the second track 211 with respect to the revolving axis 207 is decreased by a corresponding amount. to provide lower tube elongation and therefore increase the resulting EFL.

Figure 3 illustrates a part 300 of an apparatus of the invention, in particular a Conform ™ machine 301 and partly an injection system 302 for the filler material. A metal rod 303 suitable for producing a tube for the optical cable of the invention is driven from a rotating friction wheel 304 through an opening 305 in a housing 306. The increase in surface temperature of the friction wheel 305 and the The pressure produced through the continuously driven material causes the metal rod 305 to become plastic. The plasticized metal 309 'is then forced through an outlet opening of a nozzle 307 to form the metal tube 308.

An optical fiber 309 (or an optical fiber bundle) is conducted through the first feed channel 310 terminating downstream in the nozzle 307. A filler material (arrow F) is driven through a second feed channel 311 provided. at a radially external position with respect to the first feed channel 310, and terminating downstream at the nozzle 307. A cooling circuit 312 for a cooling fluid is provided coaxially and at the radially external position with respect to the first and second supply channel 310, 311. Coolant inlet and outlet are shown by arrows CFi and CF2.

[000119] In Figure 4, a metal tube 401 housing an optical fiber 402 is shown partially a cross section in an axial plane, to show an example indentation profile.

The metal tube 401 is helically retracted with one pitch A and one depth B (from the outer untreated surface of the pipe).

The width C of the indentation (determined by the indentation tool) is preferably relatively small relative to step A. For example, the width C may vary between 10 and 50% of step A, and preferably between 20 and 30. % of step A.

Preferably, the indentation depth B is between 2% and 15% of width C.

Preferably, the indentation depth B is such that it does not reduce the inner diameter of the pipe to more than 10% of the original unreturned pipe diameter.

While a preferred embodiment is a helical indentation, in particular for manufacturing reasons, other types of indentation may be used for the purposes of the present invention, for example annular indentation at regular or non-regular distances along the tube. EXAMPLE 1 (Comparative) An aluminum tube having a outermost diameter of 4.6 mm and a wall thickness of 1.7 mm was produced, forming 6 optical fibers.

[000126] The tube was produced with a Conforni ™ machine and was shortened by corrugation.

A constant pulling force of 40 kgf was applied downstream of the corrugation device through a caterpillar.

[000128] A 10 km length of fiber optic conditioning tube was produced.

The desired EFL was 0.7%.

The resulting metal tube provided an average EFL of 0%, which means that shortening due to corrugation was completely canceled by subsequent extraction.

Frequent shutdowns were also observed during production. EXAMPLE 2 (Comparative) An aluminum tube having an outermost diameter of 4.95 mm and a wall thickness of 1.25 mm was produced, forming 6 optical fibers.

[000133] The tube was produced with a Conform ™ machine. [000134] The 6 optical fibers were supplied into the tube as it was formed, along with a Seppigel® H-Lav gel (Seppigel® is a registered trademark of Seppic).

[000135] The tube was cooled by water jets.

After cooling, the tube was shortened by 0.9% by corrugation, [000137] Corrugation was applied with the following parameters: - pitch: 10 mm; - width: 3 mm; and - depth: 0.25 mm [000138] The shortened tube was then elongated by 0.2% by applying a pulling force of 20 kgf downstream of the corrugation device.

The pulling force mentioned was applied through a motor-driven track whose operation was controlled to keep the pulling force substantially constant (± 10%).

[000140] A 6 km length of fiber optic tube was produced.

The desired EFL was 0.7%.

The resulting EFL was found ranging from -0.22% to + 0.9% * as shown in Figure 5. EXAMPLE 3 (Invention) [000143] An aluminum tube having outer diameter of 5 * 6 mm and thickness of 1.8 mm was produced, packing 24 Optical Works.

The fibers were supplied in together with a SepP®® H-Lav gel.

[000145] The tube was cooled by spraying water.

After cooling, the tube was shortened by 0.9% through corrugation.

Corrugation was applied with the following parameters: - pitch: 11 mm; - width: 3.55 mm; and - depth: 0.3 mm The shortened tube was then elongated by 0.2% by applying a pulling force of 20 kgf downstream of the corrugation device.

The pulling force mentioned was applied through a motor driven caterpillar. The operation of the caterpillar mentioned was controlled to vary the pulling speed, as a function of the EFL resulting from measurements of tube length measured through the fourth downstream elongation encoder and fiber length measured through the first encoder, mentioned lengths being same time period (eg 40 m / min).

[000150] A 2.6 km length of fiber optic tube was produced.

The desired EFL was 0.7%.

The resulting EFL was found ranging from 0.8 5 to 0.6%, as shown in Figures 6 and 7 reporting the% EFL on the ordinate axis and the produced tube length (m) on the horizontal axis. In particular, Figure 6 shows the EFL variation through measurements taken every 15 m, and Figure 7 shows the EFL variation through measurements taken every 45 m.

As evidenced by Example 1 above, the lack of pull force control after metal tube corrugation has been considered to cause tube shortening resulting from tube corrugation to be virtually removed in subsequent operations, thereby preventing a significant EFL from being obtained.

In Example 2 a substantially constant pulling force after corrugation of the metal tube was applied.

Pulling force control has proven effective in maintaining a certain amount of tube shortening applied, such that the obtained tube has an average EFL value close to the desired value. However, it was observed that it was not possible to maintain a constant value of local EFL through production, given the fluctuations in the difficulties of corrugation conditions to be avoided, and the uneven behavior of the tube in the corrugation machine such that the local EFL varies beyond acceptable limits with respect to average EFL along the longitudinal extension of the cable.

In Example 3, the elongation was controlled by controlling the speed of the track downstream of the corrugation device. Speed control was considered effective to maintain the desired geometric conditions (ie EFL) within narrow tolerance limits, despite any possible inequality of tube behavior in the corrugation machine and the stretching caterpillar.

Claims (28)

A method for manufacturing an optical cable comprising at least one metal tube containing at least one optical fiber, said cable having a predetermined additional fiber length (EFL), said method comprising the steps of: advancing said optical fiber; measuring a length of said advanced optical fiber over a period of time; forming a metal tube around said optical fiber; typically deforming the metal tube by shortening the metal tube by a predetermined amount (St) during advancement of the metal tube; characterized in that said predetermined quantity (St) is greater than said predetermined EFL and that the method further comprises: - obtaining the congruence between the metal tube and said optical fiber; wherein the step of obtaining congruence is performed prior to the shortening step of the metal pipe of a predetermined amount (St); plastically deforming the metal tube after shortening to provide an elongation thereof during advancement of the metal tube; measuring an advanced metal tube length at said time interval after said elongation; assessing a resulting additional fiber length as a function of the measured optical fiber length and said measured metal tube length; adjusting the resulting additional fiber length by controlling the elongation of the metal tube to achieve the predetermined additional fiber length. Method according to claim 1, characterized in that the step of forming the metal tube around at least one optical fiber is performed by extruding a plasticized metal. Method according to Claim 1, characterized in that the step of forming the metal tube around at least one optical fiber comprises monitoring the velocity of the metal tube formed. Method according to claim 1, characterized in that it comprises a step of supplying filler material within said metal tube. Method according to claim 4, characterized in that the filler material is advanced separately from the optical fiber until after the metal tube is formed around the optical fiber. Method according to claim 1, characterized in that it comprises the steps of feeding a plasticized metal through a nozzle to form a metal tube; conducting at least one optical fiber into the metal tube through a first feed channel having an end disposed downstream of the nozzle; and conducting a filler material into the metal tube through a second feed channel having an end disposed downstream of the nozzle. Method according to claim 1, characterized in that the shortening step is carried out by corrugating, wrinkling, indenting said at least one metal tube. Method according to claim 1, characterized in that the shortening is carried out with a pitch of 5 to 15 mm. Method according to claim 1, characterized in that the amount of St shortening is 0.2% to 0.4% greater than one corresponding to the predetermined EFL. Method according to claim 1, characterized in that the resultant EFL evaluation step is performed by comparing the length of the metal tube after stretching with the length of the optical fiber as measured upstream of the congruence. Method according to claim 1, characterized in that the step of adjusting the resulting EFL by controlling the elongation of the metal tube is effected by adjusting the speed in the apparatus by stretching the tube. Apparatus (200) for manufacturing an optical cable (100) comprising at least one metal tube (103; 308) carrying at least one optical fiber (101; 309), said cable (100) having an additional fiber length. A predetermined (EFL) apparatus (200) comprising: - a device (204; 301) for continuously forming a metal tube (103; 308); a first feed channel (201a; 310) for conducting at least one optical fiber (101; 309) within said metal tube (103; 308); the apparatus further comprising: - a first pulling device (207) for making the metal tube (103; 308) and at least one congruent optical fiber (101; 309) - a corrugating device ( 210) to shorten said metal tube (103; 308); - a second pulling device (211) for stretching said metal tube (103; 308). Apparatus (200) according to claim 12, characterized in that the device (204; 301) for forming the metal tube (103; 308) is a metal forming machine continuously. Apparatus (200) according to claim 12, characterized in that it comprises a second feed channel (201b; 311) for conducting a filler material (102) into said metal tube (103; 308). Apparatus (200) according to claim 14, characterized in that the second feed channel (311) is coaxial and in an external position radially with respect to the first feed channel (310). Apparatus (200) according to claim 12, characterized in that it comprises a first cooling device (312) involving said first supply channel (310). Apparatus (200) according to claim 16, characterized in that said first cooling device (312) comprises a cooling tube, coaxially and in an external position radially with respect to the first supply channel (310). . Apparatus (200) according to claim 17, characterized in that said cooling pipe is coaxial and in an external position radially to the second feed channel (311). Apparatus (200) according to claim 12, characterized in that a second cooling device (205) is provided downstream of the device (204; 301) to form the metal tube (103; 308). Apparatus (200) according to claim 12, characterized in that, upstream of the first puller (207), a die is provided. Apparatus (200) according to claim 12, characterized in that the first pulling device (207) is selected from a rotary axis or a track. Apparatus (200) according to claim 21, characterized in that the first pulling device (207) is a rotary axis. Apparatus (200) according to claim 12, characterized in that the corrugating device (210) provides shortening at an angle of from 0.1 ° to 15 °. Apparatus (200) according to claim 23, characterized in that the corrugating device (210) provides shortening at an angle of 0.5 ° to 8 °. Apparatus (200) according to claim 12, characterized in that the second puller (211) is selected from a track or a rotary axis. Apparatus (200) according to claim 25, characterized in that the second pulling device (211) is a rotary axis. Apparatus (200) according to claim 12, characterized in that it comprises at least one length measuring device (202, 206, 208,212). Apparatus (200) according to claim 12, characterized in that it comprises at least four length measuring devices (202, 206, 208, 212).