Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Optical fiber – waveguide – or preform
Reexamination Certificate
2001-06-28
2004-12-07
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Optical fiber, waveguide, or preform
C264S001290, C264S002700
Reexamination Certificate
active
06827884
ABSTRACT:
The present invention relates to a method for manufacturing optical cables, particularly for manufacturing optical cables containing optical fibres loosely placed and a cable so manufactured.
More specifically, one aspect of this invention concerns a method for controlling the amount of optical fibre in an elongated jacket suitable for containing the optical fibre, specifically a tubular element, typically made of plastic.
Additional aspects of this invention concern an optical element, consisting of a tubular element containing one or more optical fibres of controlled length, a method for manufacturing this optical element and a cable comprising this optical element.
Currently, the optical fibre manufacturing method consists in loosely inserting one or more optical fibres inside a plastic tube to form the so-called “optical core” of the cable. This element, also known as “loose tube” or “buffer tube”, can then be used, in different configurations, to manufacture optical cables, singly or in groups of several tubes. These tubes can contain either single optical fibres, or groups of optical fibres grouped in one or more bundles, or one or more ribbons. Typically, the tubes also contain a filler, e.g. grease, to prevent water from accidentally seeping into the tube and propagating longitudinally inside.
The length of the fibres in the tubes (single, bundles or ribbons) can be equal to, longer or shorts than the (axial) length of the tube. For the purpose of this description, the difference in length between fibre and tube will conventionally be called “excess fibre”. In particular, when the fibre is longer than the tube containing it, the term “positive excess fibre” will be used. On the contrary, when the fibre is shorter than the tube containing it, the term “negative excess fibre” will be used. Finally, the term zero excess fibre will be used to indicate that the length of the fibre is substantially the same as that of the tube containing it.
Typically, the difference in length of the fibre in the tube allows cable structure stretching and shrinking caused by, for example, thermal variations or mechanical handling, to avoid cable length variations from affecting the fibre. In fact, unlike polymers, the vitreous material forming the optical fibre is not very sensitive to the temperature variations that the cable is subjected to during use, but it can present problems if mechanically stretched. Consequently, the length of the fibre in the tube should generally allow the tube to follow the length variations associated with the stresses (mechanical and thermal) it is subjected to, without imposing undesired mechanical traction or other attenuation-causing phenomena on the fibre. For example, positive excess fibre is suitable for high temperature environment or overhead cable optical fibre applications (subject to stretching due to own weight) to compensate for the structural stretching of the cable in order to allow the fibre to follow such stretches without suffering undesired stretches. This ensures that the fibre can follow the stretching without being undesirably stretched itself. On the other hand, for low temperature environment applications of an optical cable, the structural contraction of such cable tends to increase the excess fibre value. In this case, if a positive excess fibre were used, the additional increase of the value could cause excessive fibre bending in the tube, with the risk of inducing signal attenuation. In these cases, the use of negative excess fibre may be suitable.
Typically in the production of loose optical cores, the plastic material is extruded at high temperature around the fibres to form a tube which, once cooled, is wound on special reels.
One method for making loose cables and controlling excess fibre is described in U.S. Pat. No. 4,414,165 by Oestreich et al. This patent describes a method and equipment for forming an optical transmission element with loose optical fibres in a tubular jacket containing filling material.
Another method for producing loose cables and controlling fibre length, with respect to the length of the tube containing the fibre, is described in U.S. Pat. No. 5,372,757 by Schneider et al. In particular, as described in this patent, a traction force at high temperature is applied to the plastic tube and to the optical fibres. The tube is then cooled, maintaining the traction force. The applicant, however, has observed that in the lapse of time between tube production and subsequent application, e.g. to make an optical cable employing this tube, undesired and unforeseeable longitudinal shrinking can occur, with consequent uncontrollable variations of the ratio between tube length and fibre length.
Consequently, as observed by the applicant, excess fibre variations must be controlled both during the excess fibre controlling stage on the extrusion line and during the period from production of the tube, which is typically wound on a reel at the end of the production process, to its subsequent employment for making the cable. Typically, storage times (i.e. the time in which the tubes are wound on the reel before being used to make the cable) vary from several hours to approximately one week.
In particular, the applicant has observed that once the optical cores, made according to known techniques, are collected on- a reel, the plastic material forming the tube tends to additionally settle and, in particular, shrink. This settling generally cannot be foreseen; however, it usually causes additional tube shrinking leading to uncontrollable variations—usually increases—of the set excess fibre values.
The shrinking observed by the applicant in some cases results in sizes comparable to the excess fibre value set in production, with the result of substantially modifying the final excess fibre value and creating problems in the subsequent use of the tube in making the optical cables.
In particular, the applicant has observed that, at high production speeds, the tube is typically wound on the reel in random crossed turns. This unorderly tube winding generates gaps randomly distributed on the tube skein collected on the reel. The tube may detensionate more easily near these gaps and shrink, while detensioning may be obstructed in other areas. This causes different, uncontrolled shrinking of the tubes wound on different reels and also along different lengths of the same tube wound on the same reel.
Having defined the problem, the applicant has found a solution to eliminate, or at least minimize, these length variations during the storage of plastic tubes containing optical fibres, by stretching the material forming the tube containing the optical fibres by a predefined amount.
One aspect of this invention, therefore, relates to a method for producing polymeric material tubes associated with one or more optical fibres comprising the following steps:
feeding at least one optical fibre along a path to an extruder;
extruding the polymeric material around said optical fibre to form the tube;
cooling the tube to a predefined final temperature; the following steps are performed during cooling:
applying a first traction force to the tube containing said optical fibre in a first section of said extrusion line;
applying a second traction force to said tube in a second section of said extrusion line, in substantial absence of congruence between said fibre and said tube since said second traction force is greater than said first traction force;
applying a third traction force to said tube in a third section of said extrusion line, said third traction force being less than said second traction force;
said second traction force will reduce tube longitudinal shrinking by at least 20% after a storage period of one week or longer immediately after extrusion, compared to a similar tube which is not stretched.
Preferably, such second traction force is applied at a tube temperature when the modulus of elasticity of the polymeric material is approximately 2000 Mpa, preferably between approximately 100 Mpa and approximately 2000 Mpa, or more preferably betwee
Bau' Fabio
Marelli Paolo
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Pirelli Cavi e Sistemi S.p.A.
Vargot Mathieu D.
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