Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2000-12-22
2003-04-22
Derrington, James (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S430000, C065S431000, C065S432000, C065S443000, C065S504000
Reexamination Certificate
active
06550282
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in a first aspect thereof, to a method for manufacturing an optical fiber from a preform, comprising the steps of:
a) heating at least one end portion of said preform;
b) drawing an optical fiber from a free end of said heated end portion along a fiber drawing axis;
c) coating said optical fiber with a suitable coating material;
d) applying to said coated optical fiber a torque about said fiber drawing axis,
e) winding said coated optical fiber onto a collecting spool.
More specifically, this invention relates to a method for manufacturing an optical fiber having low PMD for use in telecommunication field.
The present invention also relates to a device for applying a torque to an optical fiber about its drawing axis, as well as to an apparatus for carrying out the above-mentioned method.
PRIOR ART
As is known, in signal transmission systems using optical fibers, above all in those systems operating over long distances, the need arises of minimizing any kind of attenuation or dispersion of the signals, in order to assure high transmission and reception standards. In particular, the need arises of minimizing a specific dispersion phenomenon known as PMD (Polarization Mode Dispersion), that implies a restriction in the width of the signal transmission band and, accordingly, a worsening in the performances of the optical fibers through which the above-mentioned signals are transmitted.
As is known, the fundamental propagation mode of a signal in an optical fiber may be seen as the resultant of two linearly polarized waves on orthogonal planes. Theoretically, in a perfectly symmetric circular section fiber (i.e., in monomode fibers) the propagation constants of the two components are identical and therefore the mode is propagates unaltered and according to a cylindrical symmetry along the fiber itself, with the two components traveling at the same speed. In contrast thereto, however, optical fibers possess in practice structural and geometrical irregularities that, by altering the above-mentioned cylindrical symmetry, bring about asymmetric stress conditions in the fibers themselves and anisotropy in their optical properties; accordingly, the two mode components meet zones with different refraction index and travel with different propagation speeds, thus causing the PMD phenomenon.
A parameter of particular importance in this respect, is the so-called coherence length or fiber beat length, that is, the necessary length of fiber needed for the two components of the fundamental mode to get in phase again with one another.
Other than by structural and/or geometrical defects which are intrinsic in the fiber, such as, for example, the presence of not perfectly circular cores, the PMD phenomenon may also be originated by asymmetric stress conditions caused by outer stresses such as, for example, those generated in operation during the spool winding or wiring steps.
It has been experimentally shown that it is possible to reduce the PMD of an optical fiber by submitting the latter to suitable external stresses during the drawing process, in particular by applying a torque to such an optical fiber.
To this end, various devices and methods for manufacturing an optical fiber having low PMD have been proposed in the past, the earlier ones based upon the technique of rotating the preform (a technique which was soon abandoned due to the evident technological difficulties and to the impossibility of obtaining high rotational speed), the most recent ones based upon the technique of applying during the drawing process a torque to the fiber about its axis (fiber spinning/twisting).
Therefore, for example, U.S. Pat. No. 4,504,000 (Thomson—CSF) discloses a method for manufacturing an optical fiber having a chiralic structure with high circular birefringence, wherein an equipment provided with three pulleys applies to a fiber produced from a heated portion of a preform a torque about the axis thereof. The applied torque is subsequently “frozen” in the fiber structure by means of a suitable coating film, in glass or glass-ceramic, applied on the fiber itself in a dedicated coating station.
A drawback associated to the above disclosed method is connected to the high risk of damaging the fiber surface due to the fact that the latter gets in touch with the above mentioned pulleys before being properly protected by a suitable coating film.
In order to overcome the above mentioned drawback it has been proposed, as disclosed in U.S. Pat. Nos. 5,298,047 and 5,418,881 (AT&T), to arrange the device adapted to apply the torque to the fiber downstream of the coating station. In particular, in the above mentioned patents, torque is applied by means of a fiber guiding roll having a rotation axis which extends perpendicularly to the drawing axis of the fiber and which is alternatively canting in clockwise and counterclockwise direction.
Although substantially achieving the object of reducing the PMD of the fiber, the Applicant has however noticed that the above disclosed method shows a series of drawbacks to which no adequate solution has been given up to now.
A first drawback observed by the Applicant is connected to the need of limiting the canting frequency of the guiding roll in order to avoid that, during the drawing process, a relative sliding between the optical fiber and the roll could take place. Actually, there are two reasons for such a sliding to be particularly disadvantageous: firstly because it could cause a mechanical abrasion of the optical fiber surface and, accordingly, a worsening of the mechanical resistance properties and of the performances of the fiber itself; secondly, because it does not allow the torque to be imparted to the optical fiber according to a desired law of variation, thus limiting the advantageous effects given by the applied twisting regarding the reduction of the PMD phenomenon.
The limitation of the canting frequency of the guiding roll implies, furthermore, a corresponding limitation in the drawing speed of the fiber (the two speeds being unavoidably related to one another in order to apply the desired torque to the fiber) and, therefore, a limitation in the amount of fiber produced per unit of time.
A further drawback associated with the above disclosed method is connected to the fact that the oscillations of the fiber during the drawing may induce displacements of the drawing axis which cause undesired fluctuations in the diameter of the fiber produced or unevenness and/or imperfect coaxiality between the fiber and the coating layer applied thereon.
Furthermore, such method does not allow to apply a continuous unidirectional rotation to the optical fiber, because the device which is used necessarily imparts an alternate twisting to the optical fiber.
Other methods are known for the manufacture of optical fibers, comprising the step of applying a torque to the fiber about its drawing axis and “freezing” the applied torque by means of a suitable coating film before the fiber reaches the twisting means are also disclosed by Japanese Patent Application No. JP 58-020746 and by German patent DE 3010005.
JP 58-020746 discloses a method for the manufacture of optical fibers adapted to maintain a single mode circular polarization of the fiber, that is a method wherein the plane of polarization is uniformly rotated across the length of the fiber itself. In order to produce the birefringence required for the circularly polarized mode, the fiber is drawn from the free end of the preform by drawing means located upstream of the coating station and the twist is imparted to the fiber by means of a collecting spool located downstream of a coating station.
Similarly, German Patent DE 3010005 discloses a method for the manufacture of twisted optical fibers wherein the fiber is drawn and twisted by means of a collecting spool located downstream of a number of coating stations.
According to the above disclosed methods, the twist is however imparted to the solidified fiber in the region between said drawing means and said collecting spool,
Pata Roberto
Roba Giacomo Stefano
Derrington James
Pirelli Cavi e Sistemi S.P.A.
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