BRPI0418210B1 - Optical Fiber Link, Optical Cable Line, and Methods for Obtaining an Optical Fiber Link, to Produce an Optical Cable, and to Obtain an Optical Cable Line - Google Patents

Description

"OPTICAL FIBER LINK, OPTICAL CABLE LINE, AND METHODS FOR OBTAINING AN OPTICAL FIBER LINK, FOR PRODUCING AN OPTICAL CABLE" DESCRIPTION The present invention relates generally to the field of optical fiber and to their manufacturing methods. More particularly the invention relates to a fiber optic link that has a low polarization mode dispersion (in short, PMD) and a method of doing so.

Optical signals transmitted via single mode optical fibers comprise two orthogonal polarization modes, typically indicated Transverse Electric, or TE, or Transverse Magnetic, or TM. In case the fiber has a perfectly cylindrical core of uniform diameter, the two Te modes and TM propagate at a common rate. However, in real optical fibers, the cylindrical symmetry of the core may be disrupted due to non-uniform shape or stress defects. As a result, a phase difference can accumulate between the two modes when they propagate and the fiber is said to have "birefringence." In particular the birefringence introduced by the asymmetry of form and tension is known as “intrinsic linear birefringence”.

The structural and geometric irregularities of the optical fiber that give rise to birefringence typically originate from the fiber preform itself, and are modified during the fiber stretching process. This process is usually carried out by means of an apparatus known as a "stretching tower" starting from a glass preform. In practice, after the preform has been placed in an upright position and heated to a temperature above the softening point within a suitable oven, the melted material is stretched down at a controlled rate to produce an element. as a filament that forms the optical fiber itself. In this process asymmetric stresses are typically applied to fiber.

In a birefringent fiber the two fundamental optical mode TE and TM components initially in phase with each other are in phase again only after a certain preparation length, commonly known as the "beat length" (L B). In other words, the stroke length is the period of repetition of a certain state of polarization, upon admission that the fiber maintains a constant birefringence over this length. The other characteristic parameter of a birefringent fiber is the “correlation length” (LF) which is defined as the distance over which the birefringence self-correlation function is 1 / e times its maximum value.

In so-called “polarization preservation” of optical fibers, asymmetry is deliberately introduced into the fiber to generate birefringence. However, in ordinary (i.e. non-polarizing preservative) fibers, birefringence is detrimental to fiber performance.

In fact, when pulsed signals are transmitted on an optical fiber, birefringence is a cause of pulse scattering, since the two polarization components TE and TM travel at different rate groups, that is, they become scattered. This phenomenon, known as polarization mode scattering (PMD), has been widely studied in recent years due to its importance in periodically amplified light guide systems.

Typically, the PMD phenomenon leads to a limitation of the signal transmission bandwidth and, consequently, a degradation of optical fiber performance over which the aforementioned signals are transmitted. This phenomenon is therefore undesirable in signal transmission systems. signal over optical fibers, especially those operating over long distances, where any form of signal attenuation or dispersion must be minimized to ensure high transmission and reception performance. UK Patent Application GB-A-2101762 considers the effects on post-stretched fiber twist PMD and notes that while such a twist reduces the PMD that results from intrinsic linear birefringence, it introduces torsional stresses that generate substantial circular birefringence due to photo-elastic effect. Twisting a stretched fiber thus reduces bandwidth limitation due to one effect while replacing it with another. The same patent application thus proposes to rotate the preform during stretching so that twisting can be effected while keeping the fiber material substantially tension free. Spin is performed at a relatively high rate, so that its spatial repetition frequency, or spin step, is small compared to the stroke length due to intrinsic birefringence; As a result, an optical fiber can be produced in which the contribution of birefringence due to shape and stress asymmetries is greatly reduced. Such a fiber is called a "spun" fiber to distinguish it from a twisted (post-stretched) fiber. Conveniently, the preform is rotated at a substantially constant rate, but could even reverse in direction by swinging from a right twist to a left twist.

In the present description the same distinction as above will be made between "turning" and "twisting". More precisely, the terms "spin" and "twist" are used herein to identify two different types of fiber twist: spin identifies a twist that is frozen during stretching, which is applied to a viscous portion of the fiber and maintained as a structural modification. fiber when cooling; In contrast, twisting identifies an elastic twist of the fiber, which is present when a torque is applied to a portion of the fiber whose ends are restrained against rotation. In other words, while both twist and twist change the shape fiber, so that parts previously on the same straight line are located in a spiral curve, a twisted fiber will spin back to its original shape when its ends are released from the fiber. spin restriction, while a spun fiber will maintain its change as an intrinsic and permanent deformation. Due to the spin the fiber undergoes a rotation of its polarization axes. As a result, when optical pulses are transmitted in the optical fiber they propagate greatly over the slow and fast birefringence axes, thereby compensating for relative delay and reducing pulse dispersion. This is equivalent to having an effective local refractive index for the optical pulses equal to the average refractive index on both axes, the average being taken over the pulse length along the fiber.

Theoretical studies have shown that the dominant process for PMD reduction in a spun fiber is the average of the local anisotropy of the fiber by rapidly cutting the asymmetry axes along the fiber. United States Patent 4,504,300 on a technique for making an optical fiber having a chiral structure faces disadvantages related to preform rotation and proposes a new spinning technique which consists of rotating the fiber instead of preform. In particular, a device is disclosed which comprises device placed just below the preform for twisting the fiber during fiber stretching. The twisting device will comprise a rotary arc that supports three pulleys. The twisted fiber is coated by a coating device, followed by rapid cooling device cooling, which facilitates twist freezing. United States Patent US 5,418,881 proposes arranging the device adapted to torque the fiber downstream of the coating station to prevent damage to the fiber surface. In particular, torque is applied by tilting the clockwise and counterclockwise direction a fiber guide roll having a pivot axis that extends perpendicular to the fiber stretch axis. Thus, in at least a portion of the fiber the spin printed on the fiber is highly clockwise and counterclockwise. The same patent describes that applying clockwise and counterclockwise torque to the fiber substantially prevents the introduction of an elastic twist to the fiber. United States Patent Application US 2001/002374 proposes a new device that overcomes some disadvantages of the inclined roll technique and allows both unidirectional and alternate spinning, but also describes that alternate spinning should be considered as preferable since It prevents residual twisting, that is, residual twisting of the fibers coiled over the take-up reel, thus making both unwinding and installation operations easier.

In United States Patent 5,943,466 it is proposed to rotate the fiber during stretching according to spin functions that are not substantially constant (in the sense that they change substantially as a function of distance along the length of a fiber or as a function not substantially sinusoidal, and have sufficient variability (e.g., sufficient harmonic content) to provide a substantial reduction in PMD for a plurality of beat lengths.

There were some other disadvantages of the alternating spin technique not previously highlighted. Alternate turning can, for example, cause relatively low mechanical performance of the turning device due to continuous accelerations and decelerations. In addition, with respect to one-way gyration, an alternate gyrus requires a relatively high peak profile amplitude to compensate for those profile positions where rotation reduces to change direction and thus to ensure a sufficient average gyration rate. In addition, places where the gyration rate is zero are detrimental to PMD, as there is an increase in effective pulse birefringence, and thus a higher contribution to PMD. The document by A. Galtarossa and others “PMD statistical properties of constantly spun fibers” ECOC-IOOC 2003 Proceedings, Vol. 4, Th. 1.7.4, and the document by A. Galtarossa and others, “Polarization mode dispersion properties of constantly spun randomly birefringent fibers ”, Optics Letters, vol 28 No. 18, September 2003, pp. 1639-1641 report the PMD-induced delay (i.e. the PMD-induced mode-delay in ps or, equivalently, the Medium Fiber Differential Group Delay or "DGD") of unidirectionally spun fibers. It can be shown that while in a non-spun fiber or a highly spun fiber the induced PMD delay increases proportionally to the square root of the fiber length, in a unidirectionally spun fiber the induced PMD delay has a higher rate of increase, and it only increases asymptotically proportionally to the square root of the length. In particular, the PMD-induced delay in a unidirectionally spun fiber increases asymptotically at the same rate as the PMD-induced delay of a spun fiber that has the same beat length LB and the same correlation length LF. Advantageously, a coefficient of PMD, hereinafter indicated with PMDC, defined as the average fiber DGD divided by the square root of the length, is introduced. For non-spun or heavily spun fibers, this parameter is independent of fiber length.

In more detail reference is made to Figure 1, in which a theoretical diagram of the mean square DGD <Δτ> (in ordinate, unit ps2) as a function of propagation distance (in abscissa, unit km) is shown for a fiber untwisted (curve (a)) with a typical 1/9 PMDC (for example 0.1 ps / km), a highly spun fiber (curve (b)) with a typical (constant) PMDe (for example , 0.04 ps / km), and a unidirectionally spun fiber (curve (c)) with the same beat length Lb and the same correlation length LF as the spun fiber. From the diagram it can be appreciated that the slope of the curve (c) (ie the rate of increase of <Δτ> is not constant but increases with the propagation distance to a constant value corresponding to the slope of the curve ( The length over which the slope changes may be indicated as a transient length, since the PMDC is proportional to the square root of <Δτ> divided by the square root of the fiber length, it is expected that this coefficient will increase with the propagation distance, that is, with the fiber length, unlike the PMDC of unchanged and highly generated fibers, which is constant, in particular for unidirectionally spun fiber, the increase in PMDC will be faster in the initial transient than the rate increase of the PMDC becomes similar to that of the non-spun fiber, after the transient PMDC increases very slowly reaching asymptotically the non-spun fiber PMDC. r Galtarossa et al., “Optimized Spinning Design for Low PMD Fibers: On Analytical Approach” Journal of Lightwave Technology Vol. 19, no. 17 Oct. 2001 pp. 1502-1512, the initial PMDC increase is that predicted in the deterministic regime.

In the above cited articles from Galtarossa, it is also described that the magnitude of the spin period changes the length of the aforementioned transient regime, and that a characteristic transient LT length can be defined for unidirectionally spun fibers (curve (c) in Figure 1): where p is the turning period, LF is the correlation length, Lb is the tapping length. The characteristic transient length LT is equal to the intersection of the linear asymptotic behavior of the curve (c) with the abscissa axis. The propagation distance of the fiber length required to approximate the PMD behavior regime of the non-spun fiber is estimated to be of some characteristic transient lengths.

Assuming that the parameters appearing in the above formula fall within typical ranges: LF = 1 - 20 m, LB = 5 + 15 m; and p = 0.1 + 1 m, the characteristic transient length LT may vary between 0.1 and 1800 km, covering four orders of magnitude. If the transient characteristic length LT is much larger than the link length, the increase in PMDC remains moderate. In contrast, when the characteristic LT transient length is comparable to or less than the link length, the increase in PMDC over the link becomes significant and may be detrimental to signal transmission.

Thus, unidirectionally spun fibers with characteristic short transient lengths suffer from a growth in fiber length PMDC which negates the advantage of using a spun fiber.

Another prediction made on paper quoted by A. Galtarossa published in Optics Letter is that the statistical distribution of DGD for unidirectionally short enough spun fibers may deviate from the typical Maxwell distribution presented by both spun and spun fibers. In view of the state of the art outlined above, it seems that an optimal solution to the fiber PMD problem does not exist: un-spun fibers actually have a PMD which for many applications is very high; On the other hand, highly spun fibers present the series of problems mentioned above. From the above theoretical considerations it also emerges that unidirectionally spun fiber may be preferable to unpunched fibers only for relatively short fiber lengths, since they experience an increase in their PMDC when the length increases, which thus becomes roughly equal. that of non-spun fibers.

Thus, it was an object of the present invention to provide a solution to these problems.

In particular, it was an object of the present invention to provide a fiber optic link and a method for carrying it out which have a significant limitation of the increase in PMDC with fiber length.

With these objectives in mind, it has been found that the increase in PMDC presented by unidirectionally spun fibers can be completely eliminated or substantially reduced if an optical fiber link is made of unidirectionally spun fiber sections of appropriate "opposite" and spliced lengths. each other to form the optical fiber. By "helicoidality" is meant here the direction of rotation of the fiber which may be either right or left, that is, clockwise or counterclockwise.

Therefore, a fiber optic link according to the present invention includes at least one first and a second portion of optical fiber rotated unidirectionally in opposite directions and joined together. Preferably the fiber optic link comprises a first type of unidirectionally spun fiber in a first direction and a second type of unidirectionally spun fiber in the opposite direction, the fibers of the first type being alternated with fibers of the second type, i.e. fiber sections opposite “helicoidality” are alternated with each other.

According to one aspect of the present invention, a fiber optic link is provided as described in the fiber optic link in appended independent claim 1.

In summary, the fiber optic link comprises a plurality of fiber optic portions joined together, said plurality of fiber optic portions including at least one unidirectionally spun optical fiber portion and at least a second optical fiber portion. unidirectionally rotated, which have mutually opposite directions of rotation.

For purposes of the present invention, the terms "spin", "spin", and "spin" all pertain to a twist that is frozen during stretching that is applied to a viscous portion of the fiber and maintained as a structural modification of the fiber. when it cools down. In other words, a spun fiber will maintain this change as an intrinsic and permanent deformation.

Also for purposes of the present invention, "unidirectional spinning" means a spinning that occurs in the same direction and away from possible local inversions, for example due to fiber slip in the spinning device or traction device. Preferably, the unidirectional gyrus considered herein is constant, but it can also derive from the superposition of a constant gyrus function and a variable gyrus function, the variable gyrus function having preferably small amplitude and long period.

Preferably, the first unidirectionally rotated optical fiber segment and the second unidirectionally rotated optical fiber segment are joined together.

In a preferred embodiment of the present invention, the plurality of fiber optic runs includes a plurality of first fiber optic runs and a plurality of second fiber optic runs, the first fiber optic runs and the second fiber optic runs being unidirectionally spun optical fibers that have mutually opposite directions of rotation. The first fiber optic runs and the second fiber optic runs alternate with each other on the fiber optic link. The first unidirectionally spun fiber optic run and the second unidirectionally spun fiber run may have substantially the same length of run.

Defined a spin period p, a correlation length Lp and a beat length LB for the fiber, the length of the first unidirectionally spun fiber optic run and / or the second unidirectionally spun fiber run is preferably less than 10. times the transient characteristic length LT defined as f 4L2A W, 1 + -J * VJ

More preferably, said stretch length is less than 5 times the characteristic transient length LT.

In one embodiment of the present invention said stretch length is equal to or less than approximately 3 km, preferably equal to or less than approximately 1 km.

In particular, the first unidirectionally rotated optical fiber segment and the second unidirectionally rotated optical fiber segment may have substantially the same spin rate.

Preferably the number of first fiber optic runs and second fiber optic runs is odd.

According to another aspect of the present invention, an optical cable line as described in appended claim 10 is provided.

In summary, the optical cable line includes a plurality of optical cable trunks joined together. Said plurality of optical cable trunks comprises at least one first optical cable trunk and a second optical cable trunk, the first optical cable trunk including a first unidirectionally rotated fiber optic run in a first direction and the second optical trunk trunk. optical cable including a second portion of optical fiber rotated unidirectionally in a second direction opposite the first direction, the first and second portions of optical fiber being optically entwined together.

It was particular, the first and second stretches of fiber optic are joined together. The first and second fiber optic runs may have substantially the same stretch length.

Preferably, the stretch length of the first and / or second fiber optic stretch is less than 10 times the LT transient characteristic length defined above, more preferably less than 5 times the LT transient characteristic length. In particular, the fiber stretch is preferably equal to or less than approximately 3 km, more preferably equal to or less than approximately 1 km.

In particular, the first and second fiber optic portions may have substantially the same spin rate.

According to one embodiment of the present invention, the plurality of optical cable trunks include a plurality of first fiber optic runs and a plurality of second fiber optic runs joined together to form a fiber optic link, the first fiber runs. optical fiber and the second fiber optic portions being unidirectionally spun optical fibers having reciprocally opposite directions of rotation, and the first optical fiber portions and the second optical fiber portions being alternating with each other on the fiber optic link.

In particular, in one embodiment of the present invention, at least one fiber optic trunk of said plurality of optical cable trunks has an optical core that includes a plurality of unidirectionally rotated fiber optic portions having the same direction of rotation.

In another embodiment of the invention, at least one optical cable trunk of said plurality of optical cable trunks has an optical core that includes at least two unidirectionally rotated fiber optic sections having opposite rotational directions.

Preferably the total number of optical cable trunks is odd.

In accordance with yet another aspect of the present invention, a method of performing a fiber optic link as described in the attached independent claim 21 of the method is provided. The method comprises: providing at least a first stretch of unidirectionally rotated optical fiber in a first direction; providing at least one second portion of unidirectionally rotated optical fiber in a second direction opposite the first direction; and join the first and second portions together at their respective ends.

According to another aspect of the present invention, a method of producing an optical cable as described in appended claim 22 is provided. The method comprises providing a plurality of optical fibers to a cable manufacturing line, wherein said plurality of optical fibers comprises at least one first optical fiber which is unidirectionally rotated in a first direction and at least a second optical fiber. which is unidirectionally rotated in a second direction opposite the first direction.

In accordance with yet another aspect of the present invention, a method of realizing an optical cable line as described in appended claim 23 is provided. The method comprises forming a plurality of optical cable trunks, each including at least one fiber optic section and joining the optical cable trunks together. The step of forming a plurality of optical cable trunks comprises forming at least one first trunk that includes a first unidirectionally rotated fiber optic run in a first direction and forming at least one second trunk that includes a second fiber run optics unidirectionally rotated in a second direction opposite the first direction; said joining the optical cable trunks together including optically interlacing the first optical fiber portion to said second optical fiber portion.

These and other aspects and advantages of the present invention will be apparent from the following detailed description of a configuration provided merely by way of non-limiting example, which description will be conducted with reference to the accompanying drawings, in which: Figure 1 is a diagram showing shows the predicted variation of the Mean Square Differential Group Delay (DGD) in ordinate with the propagation distance (in abscissa) for: a non-spun fiber (curve (a)), a highly spun fiber (curve (b)) and a unidirectionally spun fiber (curve (c)) with the same tap length LB and the same correlation length LF as the unpinned fiber; Figure 2 shows schematically a portion of a fiber optic link in accordance with an embodiment of the present invention comprising unidirectionally alternating spun fiber sections having reciprocally "helical".

Figures 3A and 3B show diagrams of the predicted PMDC variation in ordinate (unit ps / km), with the abscissa propagation distance (unit, km) for the fiber of Figure 2 for various alternate fiber lengths, and for two different values of the characteristic fiber transient length; Figure 4A shows, in cross section, an optical cable containing optical fibers according to an embodiment of the present invention; Figure 4B schematically shows in side view a portion of an optical cable line in accordance with the present invention; Figure 5 is a diagram showing the predicted variation with the abscissa propagation length (unit, km) of the relationship between the mean of the DGD values squared to the square of the average DGD values in ordinate for a fiber with the same parameters as the one. Figure 3A, with alternating stretches of 5 km length;

Figures 6A through 6F are diagrams showing the statistical distribution of DGD values for the same fiber as Figure 5 at propagation distances indicated in Figure 5 with the letters a) through f), respectively; Figure 7 shows a stretching tower adapted to stretch unidirectionally spun fibers; Figure 8 illustrates a turning device suitable for use in the stretching tower of Figure 7; Figure 9 shows a torsion apparatus suitable for use in the stretching tower of Figure 7; Figure 10 illustrates a reel relocation apparatus; Figure 11 shows a twisting apparatus to be used in the stretching tower of Figure 7 as an alternative to the apparatus of Figure 9; and Figures 12 and 13A through 13D are diagrams showing the results of conducted experiments.

Referring to the drawings, in Figure 2 a portion of a fiber optic link according to one embodiment of the present invention is shown very schematically.

By "fiber optic linkage" is meant a fiber made up of two or more pieces of optical fiber joined together. The fiber optic link indicated globally as 300 is, for example, of the type used in fiber optic cables for optical communication system. Fiber optic link 300 (whose portion shown in Figure 2 is for example an intermediate portion along the overall length of the fiber optic link) comprises a plurality of fiber optic segments or portions ..., 305 (k.,) , 305k, 305 (k + 1), 305 (k + 2), 305 (k + 3), 305 (k + 4), 305 (k + 5), 305 (k + e)), ... of shorter length, joined together at respective free ends, to form a fiber optic link 300; in jargon, the operation of joining the two fiber optic segments together is referred to as "splice"; in the drawing, the points where the two generic fiber optic sections ..., 305 (ki), 305k, 305 (k + i), 305 (k + 2), 305 (k + 3), 305 (k + 4) ), 305 (k + 5), 305 (k + 6), ..., are spliced together and are schematically indicated by 310. According to one embodiment of the present invention, the fiber optic sections ..., 305 ( ki), 305k, 305 (k + i), 305 (k + 2), 305 (k + 3), 305 (k + 4), 305 (k + 5), 305 (k + 6)), .. .. are unidirectionally spun segments or segments of optical fibers. In particular, stretches of unidirectionally spun optical fibers with reciprocally opposite "helicoidality" (right, or "helical" σ +, and left or "helical" σ-) are exploited to form the fiber optic link 300, and the portions unidirectionally spun fiber strands with right-spinning “helicoidality” or σ +, are alternated with the unidirectionally spun fiber sections with left-spinning “helicality” σ- as outlined schematically in the drawing. Preferably, the unidirectional spin of the different fiber runs is constant in modulus.

Due to the fact that splicing together fibers with opposite “helicality” interrupts the PMDc transients of a unidirectionally spun fiber towards the value of the spun fiber, the PMDC growth of fiber optic link 300 with the fiber link length discussed in introductory part of the present disclosure may be substantially reduced by the provision described above in the fiber link 300 of both types of fiber sections.

In principle, the individual fiber lengths ..., 305 ^,), 305k, 305 (k + i), 305 (k + 2), 305 (k + 3), 305 (k + 4), 305 (k + 5), 305 (k + 6) ,,,. can be any, however, as will be shown below, a careful choice of such lengths allows to substantially reduce or even eliminate the growth effect of PMDC with fiber length (whereby, after a certain length, a practically constant PMDC is achieved, lower than that of the unidirectionally spun fiber with a unique "helicoidity".

In particular, if the spin rates of unidirectionally spun optical fibers with “helicoidality” σ + have substantially the same magnitude (modulus) as the spin rates of unidirectionally spun “optical fibers”, the best results in terms of Growth suppressors of PMDc with fiber link length are achieved by alternating along fiber link 300, substantially equal lengths of σ + and a- fiber lengths. However, if the spin rates of unidirectionally rotated optical fibers with “helicoidality” σ + have a different magnitude (modulus) than the spin rates of unidirectionally rotated optical fibers with “helicality” a-, the lengths of the cr + optical fiber sections and σ-, should depend on the absolute values of the respective turnover rates. Reference is now made to Figures 3A and 3B, which are diagrams of the predicted variation of PMDC (ordinate, unit ps / km) with propagation distance (in abscissa, unit km) for fiber link 300 for various lengths of runs. alternate fiber ..., 305 (ki), 305k, 305 (k + I), 305 (k + 2), 305 (k + 3), 305 (k + 4), 305 (k + 5), 305fk +6), ..., which constitute fiber link 300, and for two different values of the characteristic LT fiber transient length. The curves were derived according to the teaching of Galtarossa and others, "Polarization mode dispersion properties of constantly spun randomly bireffingent fibers", Optics Letters, vol 28 No. 18, September 2003, pp. 1639-1641, relating to fibers with a single direction of rotation.

In particular, the diagram of Figure 3A is relative to the fiber optic link 300 consisting of unidirectionally rotated spun fiber sections of opposite "helicality" having a spin period p = 0.25 m, a stroke length LB equal to at 7 m, a correlation length LF = 10 m and, consequently, a transient characteristic length of LT equal to 32 km. On the contrary, the diagram of Figure 3B relates to a similar fiber optic link 300, but which has a pivoting period p = 0.5 m thus presenting a characteristic transient length LT = 8 km. PMDC with the propagation distance for alternating fiber runs ..., 305 (k_i), 305k, 305 (k + i), 305 (k-2), 305 (k + 3), 305Ck + 4), 305 ( k + 5), 305 (k + 6), ..., of length Lc equal to 5 km, 10 km, 20 km, 40 km, and to an infinite stretch length (ie for a “helicoidity” fiber ”Only) is shown.

It can be appreciated that in both cases, when alternating fiber optic sections are unidirectionally rotated with opposite "helical", the PMDC after one. The transient reaches a substantially constant value which is lower than that of the unidirectionally spun single-helical fiber, and hence the spun fiber with the same beat length LB of correlation length LF. Thus, the typical behavior of the unidirectionally spun single-helical spun fiber is substantially transformed into a behavior similar to that of a spun fiber.

Comparing the two diagrams, it can also be appreciated that the smaller the value of the characteristic transient length LT, the smaller the length of Lc required to achieve the same value of the PMDC. It may be appreciated by those skilled in the art that an optimal Lc value can always be evaluated for the link length, the maximum allowed number of stretches being the characteristic transient length. From the two diagrams of Figures 3A and 3B it can also be seen that for a beat length value LB = 7 m and a fiber correlation length value LF = 10 m, a stretch length Lc substantially equal to the characteristic transient length LT provides a PMDC of about 0.4 ps / km, which is comparable to that of the commercially available highly available spun optical fibers.

The fiber optic sections ..., 305 (ki), 305k, 305 ^), 305 (k + 2), 305 (k + 3), 305 (k + 4)> 305 (k + 5), 305 ( k + (5), ..., are typically wired and the fiber optic link 300 described above is therefore typically part of an optical cable line.As shown schematically in Figure 4B (the drawing is not to scale), a optical cable line 80 typically comprises a plurality of optical cable trunks ..., 805 (ki), 805k, 805 (k + i), 805 (k + 2), 805 (k + 3), 805 ( k + 4), 805 (k + 5), 805 (k + 6), ..., united in series, that is, concatenated with each other. Each cable trunk ..., 805 ^, 805k, 805 (k + i), 805 (k + 2), 805 (k + 3), 805.1 (-: - 4), 805 (k + 5) , 805 (k + 6), ..., includes a respective fiber optic portion ..., 305 (ki), 305k, 305 (k + i), 305 (k + 2), 305 (k + 3) , 305 (k + 4), 305 (k + 5), 305 (k + 6), ..., Each trunk of optical cable ..., 805 (ki), 805k, 805 (k + 1), 805 + 2), 805 (k + 3), 805 (k 14), 805 (k + 5), 805 (k + 6), ..., have a typical length in the range of about 2 km to about 10 km.

Referring to Figure 4A, a cross-sectional view of an optical cable along the optical cable line 80 is shown; the optical cable typically comprises an optical core 81 containing a plurality of optical fibers 800. The optical core 81 may be of the "tight" type as shown in the drawing, wherein the optical fibers 800 are embedded in a polymer matrix disposed around it. of a resistor element 83, or of the "loose" type, in which the fibers 800 are loosely housed within a single accumulator tube placed centrally within said cable, within a plurality of accumulator tubes wrapped around a central element of resistance. Around optical core 81 optical cable 80 is provided with reinforcing elements 84 and protective sheaths 85,86.

In the "tight" cabling type the contact between the fiber and polymer matrix prevents the printed twist to the fibers from being released. In 'loose' cabling the fiber-printed twist is not released at typical cable lengths due to friction between the fiber and the accumulator tube, possibly enhanced by the presence of a gelatinous filler.

From a manufacturing point of view, the fiber optic link 300 can be obtained by starting to produce two sets of unidirectionally spun optical fibers that have opposite "helical" spinning. The two sets of fibers are suitably labeled, for example σ + and σ-, so as to be able to distinguish fibers from one set from those from the other. Consequently, the first set will be said to have a "helicoidality" c + and the second set to have a "helicoidality" o-.

Preferably, to facilitate the task of alternating oppositely rotating "helical" fiber runs, the unidirectionally spun optical fiber σ + has substantially the same spin rate as the unidirectionally "helical" spun optical fiber σ-.

Later in the present disclosure, an apparatus suitable for producing unidirectionally spun optical fibers will be described in detail, and it is designed that the manner and apparatus by which unidirectionally spun optical fibers are obtained are not limiting to the present invention.

Since two sets of fiber (σ + and σ-) with opposite “helicality” have been produced, predetermined lengths of these fibers are used in a threading process of a known type to produce an optical cable, such as that illustrated in Figure 4A.

A plurality of optical cable runs are thus formed. These optical cable runs are then connected to each other by known techniques to form an optical cable transmission line such as that illustrated in Figure 4B.

According to a first embodiment, each optical cable trunk can include in its optical core a certain number, for example half the total number of fibers, with an hourly "helical" and a certain number, for example half the total number of fibers. , with an anti-clockwise “helicoidality”. In this case, the optical cable trunks may be identical to each other.

According to a second embodiment, each optical cable trunk may include fibers of a single type, that is, either of hourly "helical" or "counterclockwise" helical "fibers. In this case, cable trunks that include only σ + fibers and cable trunks that include only α- fibers are produced.

Then, the optical cable trunks are concatenated together to form optical cable line 80. To join two optical cable trunks together, a connector of a known type can be used, such as the fiber connector assembly. optics described in U.S. Patent 5,778,131 or Oasys®, a compact joint by Pirelli. In practice the fibers exiting the ends of the two cable trunks are housed and routed in the connector, and then may be spliced end by fusion splicer of a known type, such as model FSM-40S / 40S-B by Fujikura.

The fiber optic sections ..., 305 (ki), 305 (k + 1), 305tk + 3), 305 (^ 5), ... are thus amended to form the fiber optic link 300. In particular , the fiber optic link 300 is formed by severely splicing a fiber portion ..., 305 (ki), 305 (k + i), 305 ^ + 3), 305 (k + 5), ... of the right spun fiber (left) o + (σ-), with one stretch of fiber ..., 305k, 305 (k + 2), 305 (k + 4), 305 (k + 6), .., of the set spun fiber left (right) σ- (σ +).

By appropriately choosing the spin rate of the + and o- fiber sections, in particular by making the LT transient characteristic length suitably longer than the typical cable trunk length, the optical cable line obtained by joining optical cable trunks which include sections of opposite “helical” fiber optics (σ + and cr-) have a low and substantially constant PMDC.

If the optical cable trunks include fiber optic sections of the same "helicality" (either right, ie σ +, or left, ie σ-), optical cable 80 is preferably made by alternating cable trunks that include fiber optic sections. u + fiber with cable trunks that include cr- fiber optic sections.

Alternatively, if the optical cable trunks include both σ + and o- fiber optic sections, the optical cable is preferably made by joining the different cable trunks such that σ + fiber sections are spliced with σ- fiber sections. .

The statistical properties of PMD of a fiber optic link such as link 300 have been investigated. It is known in the art that both un-spun and highly spun optical fibers present a Maxwelian statistical distribution of DGD values. The Maxwelian distribution is characterized by a relationship between the mean DGD squared <Δτ>, and the mean DGD squared <Δτ>, equal to In Figure 5 the numerically computed (predicted) relationship r is plotted as a function of propagation length . The fiber optic link parameters are the same as for the fiber of the diagram of Figure 3A, with an Lc section length of 5 km. The ratio r presents strong oscillations superimposed on a monotonic rise towards the asymptotic value equal to 1.18. A value of r greater than 1.18 indicates a statistical dispersion of the distribution of DGD values greater than that typical of the Maxwelian distribution. On the other hand, a value of r less than 1.18 indicates that the DGD values are less dispersed than in Maxwell's case. Figures 6A through 6F are diagrams showing the statistical distribution of DGD values at points a) through f ) of Figure 5, respectively.

According to these results, an odd number of pieces of fiber joined together guarantees a narrower Gaussian DGD statistical distribution than that of Maxwelian as shown in Figures 6A-6D, and correspondingly at the points marked (a) - (d) In Figure 5. Here and in the following Figures, the dashed lines indicate the Maxwelian fit and the full lines the Gaussian fit. However, the narrowing of the distribution below the Maxwelian limit decreases as the number of stretches increases. On the other hand, an even number of runs gives a DGD dispersion greater than and asymptopically equal to the Maxwelian distribution, as shown in Figures 6E and 6F, and correspondingly at the points marked as (e) and (f) in Figure 5. The prediction The DGD statistical distribution deviation, mentioned in the introductory part of this description, is based on the following considerations. The DGD statistical properties are determined by the three Gaussian distributed stochastic components of the polarization dispersion vector Oi, with i = 1, 2, 3 according to the formula reported in a document by A. Galtarossa et al., Optics Letters, Vol. 28 , No. 18, September, 2003: 0 PMDC is the expected value of the DGD statistical distribution divided by the square root of the L fiber length.

In unidirectionally spun fibers Ω32 behaves Λ A markedly differently than Ω, and Ω2 ■ For small z 0 DGD is mainly determined by the component Ω32 so that A A tends to obey a distribution like Gauss. When z increases Ω1 and Ω2 together, the three components tend to acquire the same statistical weight and DGD becomes distributed as Maxwell.

In unidirectional fibers the increase in PMD with z follows the increase (asymptotically linear) with z of means <Ω2 (z)>, i = 1,2, 3. It was found that by alternating sections of opposite, these means can be substantially reduced with respect to the case of single “helicoidality”, and that the shorter the stretch length the stronger the reduction.

In the following, an apparatus and method for producing unidirectionally spun optical fibers will be described in detail. It is understood that this apparatus and method is not limitative of the present invention, any other method and apparatus adapted to produce unidirectionally spun fibers being suitable.

Referring to Figure 7, a stretching tower 1 comprises a plurality of devices that are substantially aligned along a vertical stretching axis 2 (hence the term "tower"). The choice of a vertical direction to carry out the main steps of the stretching process arises from the need to exploit the force of gravity in order to obtain from a glass preform melted material from which an optical fiber 4 can be stretched.

In detail, tower 1 comprises a mold 6 for controlled melting of a lower portion of the preform 3 (also known as the lower neck of the preform), a feeding device 7 for supporting the preform 3 and feeding into the furnace 6 from above, a traction device 8 at a lower end of the tower for pulling fiber 4 from preform 3, and a winding device 9 for storing fiber 4 on top of it. a reel 10. The furnace 6 may be of any type designed to produce controlled melting of a preform. Examples of cores that can be used in tower 1 are described in US 4,969,941 and US 5,114,338.

Preferably, a cooling device 12, for example of a type having a cooling cavity designed to be traversed by a flow of a cooling gas, is situated below the oven 6 to cool the fiber 4 upon leaving it. The cooling device 12 is arranged coaxially to the axis 2, so that the fiber 4 leaving the oven 6 can pass through it. Tower 1 may also be provided with voltage monitoring device 13 (e.g., of the type described in United States Patent 5,316,562), and a diameter sensor 14 of a known type, preferably positioned between shape 6 and 1. cooling device 12 for measuring the tension and diameter of fiber 4 respectively.

Preferably the tower 1 further comprises a first and a second lining device 15, 16 of a known type, positioned below the cooling device 12 in the vertical stretching direction, and designed to deposit a first lining onto fiber 4 as it passes through it. and respectively a second backing. Each coating device 15, 16 comprises in particular a respective application unit 15a, 16a which is designed to apply a predefined amount of resin to the fiber 4, and a respective curing unit, 15b, 16b, for example a dome. ultraviolet lamp to cure the resin, thus providing a stable coating. Traction device 8 may be of single pulley or double pulley type. In the illustrated embodiment the traction device 8 comprises a single motor driven (or capstan) pulley 18 which is designed to stretch the already coated fiber 4 in the vertical stretching direction. Traction device 8 may be provided with an angular rate sensor 19 which is designed to generate a signal indicating the angular rate of pulley 18 during its operation. The rotation rate of the pulley 18, and therefore the stretching rate of fiber 4, may be varied during the process, for example as a response to a diameter variation detected by detector 14. Tower 1 further comprises a device for pivot 20 positioned between the lining devices 15, 16 and the traction device 8 to impress the pivot 4 about its axis during stretching. For purposes of the present disclosure, the term "gyrus" indicates the ratio (neglecting a constant multiplication factor) between the optical fiber rotation rate dq / dt (where q is a measured optical fiber rotation angle relative to a fixed reference point) and the drawing rate. The turn defined in this manner is typically measured in turns per minute.

In a possible configuration illustrated in Figure 8, the swinging device 20 comprises a fixed support structure 21, with a direct current motor 22 maintained by the structure 21, and a rotating element 23 maintained by the structure 21 and coupled to the motor 22 via a belt drive 24. Belt drive comprises a first drive pulley 24a rigidly coupled to motor 22, a second drive pulley 24b rigidly coupled to rotary member 23, and a belt 24c that connects first drive pulley 24a to the second pulley drive 24b. The rotating element 23 has a pivot axis corresponding to the axis 2, i.e. the axis of movement of the fiber 4 when entering and leaving the device 20. The rotating element 23 comprises first and second end portions like sleeve 23a, 23b respectively upper and lower, which are rotatably coupled to the support structure 21 by means of respective bearings 26 and allowing the fiber to pass through. The second extreme notion 23b is coupled with the second drive pulley 24b. The rotating element 23 comprises two arms 27a, 27b extending from the first end portion 23a to the second end portion 23b. Arms 27a, 27b are substantially C-shaped with a central straight main region parallel to axis 2, and are symmetrically arranged with respect to axis 2. One of the two arms (the one indicated with 27b in the drawing) carries a first , a second and third madly mounted rotating pulley 28a, 28b, 28c from top to bottom in the drawing, substantially aligned in a direction parallel to axis 2. The three pulleys 28a, 28b, 28c have corresponding axes perpendicular to axis 2. , and are sized such that the corresponding guide grooves are substantially tangent to axis 2.

Referring again to Figure 7, tower 1 may also comprise a tension control device 30, commonly known as a "dancer" to adjust the tension of fiber 4 downstream of the traction device 8. The tension control device 30 is designed to balance any variations in fiber tension 4 between the pulley 18 and the winding device 9. The tension control device 30 may comprise, for example, a first and a second pulley 30a, 30b, which are mounted madly and in a fixed position and a third pulley 30c which is free to move vertically under the action of its own weight and the tension of the fiber 4. In practice the pulley 30c is lifted if there is an undesired increase in the tension of the fiber 4, and lowered whether there is an undesirable decrease in fiber tension 4 so as to maintain said tension substantially constant. The pulley 30c may be provided with a vertical position sensor (not shown) which is designed to generate a signal that indicates the vertical position of the pulley 30c and thus indicates the fiber tension 4.

One or more pulleys 31, or guide elements of other types, are advantageously provided to guide the fiber 4 from the tension control device 30 to the winding device 9. The winding device 9 comprises, in the illustrated configuration, a first a second, third and fourth guide pulleys 36a, 36b, 36c, 36d held by a support element 37 to guide the fiber 4 over the reel 10. The winding device 9 further comprises a motorized device 33 for adjusting the reel 10 rotating about its axis 34. The motorized device 33 may also be suitable for alternating movement of reel 10 along axis 34 so as to allow helix winding of fiber 4 thereon during stretching. Alternatively, the carriage 10 may be axially fixed and the support member 37, together with the pulleys 36a, 36b, 36c, 36d, may be mounted on a motorized slider (not shown in the drawing) designed to have alternate movement along an axis. parallel to the reel axle 34.

A twisting apparatus 40 is advantageously used to untwist the fiber, that is, to remove an unwanted elastic twist stored in the fiber 4 when rotated. This unwanted twist that tends to generate circular birefringence in the fiber is produced during fiber spin due to the presence of a restriction on fiber spin downstream of the spin point. The twisting apparatus 40 may be used at the stretch stage, in particular to untwist fiber 4 during its winding, or may be used at a subsequent stage, for example, during unwinding of fiber 4, for its repositioning on a reel in a spool suitable for shipment as described below.

In practice, the twisting apparatus 40 expressly applies a twist to the fiber (which will be called "twisting") in a direction opposite to that of the unwanted elastic twist resulting from spinning. In the following, by "direction opposite to direction of rotation", with reference to the direction of distortion, is meant the direction opposite to the direction of twist resulting from rotation. Advantageously, the torsion apparatus 40 may be integrated into the reel 9 winding device 9 in particular. The support member 37 and the pulleys 36a, 36b, 36c, 36d may be part of the torsion apparatus 40. Figure 9 illustrating a possible configuration of the torsion apparatus 40, the support member 37 is a rotating member that is in the shape of a two-pronged fork and comprises a hollow spindle 41 and a first and second extending arms 45, 46 from one end 41a of hollow spindle 41. Spindle 41 is kept coaxial to shaft 34 by a fixed frame 43, and is rotatably mounted thereon through bearings 44. Spindle 41 is driven by a DC motor (not shown in the drawing) via a belt drive (also not shown in the drawing). In use, spindle 41 is designed to be traversed by fiber 4 along axis 34.

First and second arms 45, 46 are symmetrical with respect to axis 34, and have respective first portions 45a, 46a rigidly connected to spindle 41 and extending away from opposite axis 34 and respective portions 45b 46b parallel to the axis 34. The first portions 45a, 46a have a radial extension greater than the radius of the reel 10 and the second portions 45b, 46b have a length that substantially corresponds to the length of the reel 10. The reel 10 is located between the second portions 45b, 46b of the arms 45,46. The first pulley 36 is positioned at the end of the spindle 41 facing the reel 10 and designed to divert the fiber 4 to the first arm 45. The second, third and fourth pulleys 36b, 36c and 36d are positioned along the second portion. 45b of the first arm 45, and define a corrugated path for fiber 4 before it is fed to the reel 10. The function of the third pulley 36c which is intermediate between the second pulley 36b and the fourth pulley 36d is to prevent the fiber 4 from slipping pulleys 36b and 36d and it can be dispensed with. Second arm 46 has only a balancing function, and can load three pulleys identical to pulleys 36b, 36c, 36d to have the same weight distribution as first arm 45.

While the first, second and third pulleys 36a, 36b and 36c preferably have their respective axes parallel to each other and perpendicular to the axis 34, the fourth pulley 3od is preferably inclined around an axis parallel to the axis 34 of such an angle It lies on a plane that is tangent to the fiber spool when the reel 10 is half full. The torsion apparatus 40 preferably comprises a fiber position sensor 48 (e.g., a Keyence FS-V1 IP FU-35FA model device) positioned between the fourth pulley 36d and the reel 10 to provide a control signal for the alternate axial movement of the reel 10 (Figure 9 shows, for example, two different positions of the reel 10) or of the support element 37. In fact, as described above, relative alternate movement must be provided between the reel 10 and the reel bracket 37 to allow fiber 4 helix winding. Stretch tower 1 may further comprise a control unit (not shown in the drawing) electrically connected to all tower 1 devices to be controlled from outside and to all sensors and sensors. detectors present along tower 1. Stretch tower 1 operates as follows. The support device 7 feeds the preform 3 to the oven, where a lower portion thereof (the lower neck) is melted. The fiber 4 stretched from the lower neck is pulled down from the traction device 8 and wound onto the reel 10 by means of the winding device 9. Between the capstan 18 and the reel 10, the tension control device 30 regulates fiber tension 4.

When fiber 4 is stretched, sensors 13 and 14 monitor its tension and diameter. Such monitoring can be used to control the stretching process, for example by acting on the traction rate. Upon leaving oven 6 the fiber 4 is cooled by the cooling device 12, and is coated with two protective layers by the coating devices 15, 16. The coated fiber 4 is then subjected to a one-way and substantially constant rotation by means of the pivoting device 20. This is achieved by rotating the rotating element 23 around axis 2 at a constant rate. Each turn of the rotary element corresponds to one turn of fiber 4 about its axis. The spin rate is selected such that the effects of fiber 4 imperfections and irregularities are taken substantially uniform at a fiber 4 length equal to at least the shortest typical stroke length LB. As a result, when signals are transmitted on the fiber, there is a power exchange between the fundamental propagation modes and thus a reduction in PMD. Thus it is possible to significantly reduce the negative effects caused by asymmetric stress conditions and imperfections intrinsically present in the fiber 4.

It was observed that the higher the spin rate the better the fiber performance in terms of PMD. However, the higher the gyration rate, the greater the elastic twist to be removed. It has been found that a gyration rate of between 1 and 8 turns / m enables the PMD to be reduced to acceptable values and at the same time introduces an amount of elastic torsion that can be efficiently removed by the technique described herein.

When spun, fiber 4 transmits an upstream and downstream torque. Upstream torque is transmitted to the lower neck of the preform where the plastic deformation of the molten glass absorbs the torque and transforms it into an intrinsic orientation of the fiber birefringence axes 4. This intrinsic twist is frozen in fiber 4 when the fiber It cools. Downstream and in the absence of any countermeasure, the torque could be transmitted as far as the reel 10, where the fiber 4, once coiled, could maintain a residual elastic twist. This elastic twisting could introduce, if not controlled, an unwanted circular birefringence in the fiber 4.

In order to control the residual twisting in the coiled fiber 4, the fiber 4 is distorted by the twisting apparatus 40. In practice, the rotating support member 37 is rotated about the axis 34 in a direction opposite to the direction of rotation or, more precisely as described above, in a direction opposite to that of the elastic twist generated by the gyrus. Each turn of the support member 37 about axis 34 corresponds to one revolution of fiber 4 about its axis. The torque transmitted along the fiber 4 downstream of the pivoting device 20 is then at least reduced by the twisting apparatus 40 before the fiber is wound onto the reel 10.

In detail, the fiber 4 after traversing the spindle 41 is deflected by the first pulley 36a towards the first arm 45, and is conveyed therein along the second portion 45b with the tension required by the second and third pulleys 36b, 36c and is finally fed to the reel 10 by means of the fourth pulley 36d in a direction substantially perpendicular to the axis 34. While being rotated around the axle 34, the reel 10 also has alternate movement along the axis 34 so as to allow helical winding The signal from sensor 48 is used to control the speed of alternating movement of reel 10 so that fiber 4 is always passed at a predetermined position of sensor 48.

It has been found that the PMD of fiber 4 can be reduced to a minimum by printing the fiber after it has been spun, a twist that not only removes the elastic twist generated by the spin action but also introduces a positive residual twist, that is. it is a twist in the opposite direction. A positive residual torsion between 0 and 1.5 turns / m, preferably between 0.3 and 1 turns / m, has been found to reduce the PMD of spun fibers over a wide range of spin rates, up to at least 8 turns. / m

As previously described fiber distortion may be performed, rather than during the stretching process, at a stage subsequent to stretching, and may be associated with the reel operation of fiber 4 of the reel 10. For example, distortion may be performed during repositioning the fiber 4 reel onto a boarding reel to be shipped to a customer, or during screening operations. Screening is a test operation performed on an optical fiber to verify its strength, which comprises applying a predetermined longitudinal tension to the fiber 4 as it runs on a predetermined path usually defined by pulleys.

As shown in Figure 10, the twisting apparatus 40 may, for example, be used with the fiber 4 moving in the opposite direction so as to perform fiber distortion while the fiber 4 is being unwound. In particular, Figure 10 illustrates a reel assembly 70 which comprises an unwinding device 9 'for unwinding fiber 4 from reel 10 and another winding device 71 including guide pulleys 73 for rewinding the fiber 4 over a different reel 74. The unwinding device 9 'substantially corresponds to the winding device 9, but operates in the opposite direction to unwind the fiber 4. In this case the twisting apparatus 40 is integrated into the unwinding device 9r to untwist the fiber 4 when it is unrolled from reel 10. Replacement assembly 70 may also comprise a screening device 72, for example of the type described in US 5,076,104. Figure 11 shows a different configuration of the twisting apparatus indicated with reference numeral 50. The twisting apparatus 50 comprises a fixed frame 51 which supports the reel 10 along the axis 34, and a rotating member 52 for twisting the fiber 4 when it is rolled over or reeled from reel 10. The rotating element 52 comprises first and second spindles 53, 54 supported by frame 51 coaxially with the axis 34, and a flexible bow element 55 which connects the two spindles 53, 54 onto the reel 10 for the passage of fiber 4 The fixed structure 51 comprises two outer support members 56, 57 and two inner support elements 58, 59 substantially aligned with each other along the axis 34. The outer support elements 56, 57 are cylindrical and the element 57 has an internal passage for the fiber 4 along the axis 34. The reel 10 is positioned between the inner support members 58, 59 and is supported therewith. The reel 10 is connected to a motor (not shown in the drawing) by a belt drive 60.

Spindles 53,54 are opposed to one another with respect to reel 10, and are connected to the same motor other than that of reel 10 (not shown in the drawing) through respective belt drives 62, only one of which is illustrated in such a manner. that they can be rotated at the same speed. Each of the spindles 53, 54 is positioned between a corresponding outer support member 56, 57 and a corresponding inner support member 58, 59. The first spindle 53 internally carries a pulley 67 tangent to the axis 34 which allows fiber 4 to pass between the arc member 55 and another pulley 69 tangential to axis 34 carried by inner support member 58. The second spindle 54 internally carries another pulley 68 tangent to axis 34 allowing passage of fiber 4 between outer support element 57 and arc element 55. One or more other pulleys are provided to guide the fiber to or from the reel 10. The flexible arc element 55 is preferably made of triphenyl methane and bridges the reel 10 for the passage of the fiber 4 between the spindles. 53, 54. The bow member 55 may be provided with equidistant guide screws 61, preferably made of ceramic and suitable for guiding the fiber 4 along the bow member 55. Alternatively, the bow member 55 p It may be provided with a guide tube (not shown in the drawing) and which offers the advantage of easier adjustment prior to the start of the process allowing the fiber 4 to be blown from one end of the bow member 55 to the other. The apparatus 50 is described here below when operating to roll the fiber over reel 10. Similar to apparatus 40, apparatus 50 may operate in the opposite direction to unwind fiber 4 from reel 10, for example in the reel assembly. 70 of Figure 10. Fiber 4 is received through element 57 and a first portion of second spindle 54 where it is deflected by pulley 68 to arc element 55; the fiber 4 then runs over the entire arc element 55 and penetrates the first spindle 53 where it is still deflected by the pulley 67 towards the inner support element 58 along the axis 34; then the fiber is further deflected by the pulley 69 and is finally fed to the reel 10. The amount of twist to be applied to the optical fiber 4 to obtain the desired amount of residual twist can be determined according to the following technique. In a first step a spin-only test fiber section is stretched. This test fiber section can be obtained, for example, by operating the stretching tower 1 of Figure 7 with the twisting apparatus 40 off, that is, with the rotating element 37 in a stopped condition for a predetermined period of time. Then, the residual torsion accumulated in the test fiber section wound over the reel 10 is measured as follows. The reel 10 is suspended on a support located at a predetermined height, for example 2 m above the ground. A corresponding length of fiber is unwound from the reel 10 keeping it under a moderate tension. The upper end of the unwound fiber section is attached to the surface of the reel while the free end is marked, for example, with a small piece of tape that has negligible weight and is left free to rotate. The measurement resolution depends on the length of the unwound fiber section. For a fiber length of 2 m, the number of turns can be measured with a resolution of about one quarter turn over 2 m, so that a resolution of about 0.125 turns / m can be obtained. If a higher resolution is required, a longer fiber may be used.

It has been noted that the presence of the fiber sheath can be taken into account for accurate measurement of the residual twist due to spin, as a residual twist is also accumulated in the fiber under the sheath. Accordingly, after the residual twist of the coated fiber has been measured in the manner described above, the free end of the coated fiber is blocked and the coating is completely removed using a conventional Miller scraper. The fiber is then left free to rotate again, and the additional fiber rotation is measured at the same resolution as above. The operation is repeated on consecutive fiber sections of predetermined length, for example every 2 m, to achieve a predetermined total measured length, for example between 20 and 60 m. The average value is used to label the fiber twist value.

After the residual torsion due to the spin has been measured, the stretching of the fiber can be continued with the torsion apparatus 40 attached, adjusted accordingly to obtain the desired residual torsion. It is thus possible to obtain an optical fiber which has a unidirectional intrinsic gyrus and an elastic modulus torsion of zero, or opposite to that gyrus and greater than zero in modulus. The unidirectional intrinsic gyrus may be substantially constant or variable. In this second case the gyrus function is preferably obtained by superimposing a substantially constant function and a periodic function, and the torsion is applied to vary the mean value of the residual torsion to the desired value. The elastic torsion applied to the fiber is preferably between 0 and about 1.5 turns / m, more preferably between about 0.3 and 1 turns / m.

Fibers with both clockwise and counterclockwise helicalities are produced by the process described above, changing the direction of rotation of the turning device and the twisting device. Once two sets of fibers (σ + and σ-) with opposite “helicality” have been produced, predetermined lengths of these fibers are used in a threading process of a known type to produce an optical cable as described above.

While the present invention has been disclosed and described by some embodiments, it is apparent to those skilled in the art that various modifications to the described embodiments as well as other embodiments of the present invention are possible without departing from its scope as defined in the appended claims. .

For example, while in the embodiment of the invention shown in Figure 2 a strict alternation of unidirectionally spun fiber sections having mutually opposite "helicality" is provided, this should not be construed as a limitation of the present invention, since a link Fiber optic cables can also be produced by splicing unidirectionally rotated fiber optic sections of opposite spinning "helical" without necessarily respecting such strict alternation.

In addition, the fiber optic link may comprise one or more stretches of untwisted optical fibers or highly spun optical fibers, and spliced to unidirectionally spun fibers or arranged between two stretches of unidirectionally stretched optical fiber. Experimental Results The predicted increase of PMDC in unidirectionally spun fibers was confirmed experimentally.

To do this two G.652 fibers were stretched at a unidirectional spin rate of + 3 turns / m and -3 turns / m and completely distorted after the stretching process to eliminate any residual elastic twisting. The fibers were then loosely wrapped around a large diameter coil and to ensure that the fullest possible range of DGD values is explored, repeated DGD measurements were taken at each moment slightly disturbing the fiber development. In particular, the measurement was performed according to the Eingenanalysis Matrix Jones technique using a PAT9200 polarimeter and a Tunics-Plus tunable laser. The wavelength range from 1530 nm to 1620 nm was scanned using a 10 nm step. Up to 1200 DGD values in a 1 hour time were thus obtained. Circles and squares in Figure 12 show the measured PMDC as a function of z for + 3 turns / m and -3 turns / m, respectively: it can be appreciated that the PMDC increases with the propagation distance and converges to an asymptotic value. in line with the predictions reported in the document previously mentioned by A. Galtarossa et al, "Polarization mode dispersion properties of constantly spun randomly birefmgent fibers", Optics Letters, vol 28 No. 18 September 2003.

It was also experimentally confirmed the deviation of the DGD statistical distribution from the typical Maxwell distribution which was suggested in the same document as affecting short stretches of unidirectionally spun fibers. Referring to Figures 13A through 13D showing the measured DGD distributions of the + 3 turns / m fiber of Figure 12 for z = 1, 2, 3 and 4 km respectively (in the diagrams the solid lines and dashed lines represent respectively , Maxwell and Gauss adjustments, X and Y axes represent DGD (in ps) and counts), it can be appreciated that unidirectional turning can indeed severely affect DGD statistics. In particular for short values the DGD distribution is well adjusted by a very narrow Gauss curve with an R ratio (ratio between expected value and standard deviation much greater than 2.4 which is the value of the Maxwell distribution. As z increases, the dispersion of the data around the expected value increases (the ratio R decreases), and at the same time the distribution becomes more and more like Maxwell's.

The achievable PMd growth reduction was experimentally confirmed by concatenating opposite "helical" fiber sections. Triangles in Figure 12 report the experimentally measured PMDC on alternately spliced fibers, 1-kilometer long stretches of unidirectionally spun fiber of opposite "helicality". In particular, 1 kilometer samples of the previously mentioned G.652 were spliced together taking care to toggle the "helicoidality". It may be appreciated that the PMDc stabilizes to a value of about 0.03 ps / km. middle. The PMDc measured at the splicing points has an oscillating behavior with z, with the minimum and maximum corresponding to an even and odd number of sections, respectively. As indicated by the values of the ratio R (reported on the side of the triangles) concatenation of an odd number of stretches always provides a larger DGD dispersion than concatenation of an even number. When z increases and the PMDC tends to stabilize, the DGD distribution becomes larger and Maxwell-shaped, and the R value decreases to 2.4.

By splicing together 2km alternating fiber runs, the PMDC tends to about 0.4 ps / km1 / 2, while alternating 3km runs does not provide a significant reduction in PMDC over unidirectionally spun fiber. Thus, the fiber length equal to or less than 1 kilometer seems to provide good results, at least in the fiber considered here.

Numerical results All of these experimental observations were numerically verified with a code based on the fiber birefringence random modulus (RMM) model and are explained in PKA Eai and CR Manyuk, “Polarization Mode Dispersion, Decorrelation, and Diffusion on Optical Fibers with Randomly”. Varying Birefringence ”, Journal of Lightwave Technology, Vol. 14, No. 2, February 1996. The simulation confirmed that PMDC compensation holds for the arbitrary number of alternating runs, so that any fiber of any length with PMDC value controlled can be manufactured. The shorter the length of runs, the smaller the asymptotic value of PMDC.

In Figure 12 the dotted and filled dashed lines represent the result of numerical simulation of + 3 turns / m unidirectional fiber, -3 turns / m unidirectional fiber and concatenated alternate “helicoidality” link (± 3 turns / m), respectively. . The fit with experimental data is very good. The fiber parameters used for the simulations are as follows: for the +3 turns / m fiber and for the + 3 turns / m stretches on the alternating “helical” link LB = 4 m LF = 2.34 m; for -3 turns / m and for -3 turns / m on alternating “helicoidity” link LB = 5.6 m LF = 3.45 m.

Claims (23)

1. Fiber optic link (300) comprising a plurality of fiber optic runs (305 (μ>, 305k, 305 ^ + 1>, 305 ^ + 2), 303 (k + 3), 305 (k + 4 >, 305 (i + 5), 305 (k + fi)) joined together, characterized in that said plurality of fiber optic sections includes at least one first unidirectionally spun skeptic fiber portion (305 <kn, 305 < k + i), 305 (i; +3), 305 (k + 5)) and at least one second unidirectionally rotated pound optic stretch (305k, 305 ^ + 2), 305 (^ + 4), 305 ^ + (6)) having reciprocally opposite directions of rotation, wherein at least one first unidirectionally rotated optical fiber segment is obtained from at least one first unidirectionally rotated optical fiber, at least one second unidirectionally rotated optical fiber segment is obtained from at least one unidirectionally spun second optical fiber, the first and second unidirectionally spun optical fiber being distinct optical fibers and having reciprocally helicality opposite, Fiber optic link according to Claim 1, characterized in that the first unidirectionally rotated optical fiber segment and the second unidirectionally rotated optical fiber segment are joined together. Fiber optic linkage according to Claim 1, characterized in that said plurality of fiber optic runs include a plurality of first fiber optic runs and a plurality of second fiber optic runs with the first fiber optic runs. and the second fiber optic portions being unidirectionally rotated optical fiber portions having mutually opposite directions of rotation, and wherein the first optical fiber portions and the second optical fiber portions are alternating with each other on the fiber optic link. Fiber optic link according to Claim 1, characterized in that the first unidirectionally spun optical fiber portion and the second unidirectionally spun optical fiber portion are substantially the same length. Fiber optic link according to Claim 1, characterized in that the fetus of each of said first and second unidirectionally rotated optical fiber sections has a stretch length, a spin period p, a correlation length Lp, and a length. beat length LB and said stretch length is less than 10 times the characteristic transient length L, defined as Fiber optic link according to Claim 4 or 5, characterized in that the fetus of said stretch length is equal to or less than approximately 3 km. Fiber optic link according to Claim 6, characterized in that said stretch length is equal to or less than approximately 1 kilometer. Fiber optic link according to Claim 1, characterized in that the fetus has the first unidirectionally spun fiber optic section and the second unidirectionally spun fiber segment extending substantially the same spin rate. Fiber optic link according to Claim 3, characterized in that the number of the first fiber optic runs and second fiber optic runs is odd. Optical cable line (80) comprising at least one fiber optic link as defined in claim 1, wherein the optical cable line (80) includes a plurality of optical cable trunks (805 (ki j, 805k, 805 (k + n, 805 (1 + 2), 805 (i +; i), 805 ^ + 4¾) joined together, characterized in that said plurality of optical cable trunks comprises at least one first optical cable trunk and a second optical cable trunk, the first optical cable trunk including a first fiber optic portion (305 <ki>, 305 (k + i), 305 (k + n), 305 (1 + 5)) uni directionally in a first direction, and the second optical cable trunk including a second fiber optic run (305ií, 305 * 1 + 2), 305 (14 + 41, 305 (k + ó>)> unidirectionally rotated in a second opposite direction to the first direction, the first and second fiber optic portions being optically intertwined, Optical cable line according to Claim 10, characterized in that the first and second fiber optic sections are joined together. Optical cable line according to claim 10, characterized in that the first and second fiber optic runs are substantially the same length of the run. Optical cable line according to claim 10, characterized in that each of said first and second fiber optic sections has a stretch length, a spin period p, a correlation length Lp and a beat length. Lb, and said stretch length is less than about 10 times the characteristic transient length Lt defined as Fiber optic link according to Claim 12 or 13, characterized in that said stretch length is equal to or less than approximately 3 km. Fiber optic link according to claim 14, characterized in that said stretch length is equal to or less than approximately I km. Optical cable line according to claim 10, characterized in that the first and second fiber optic sections have substantially the same spin rate. Optical cable line according to claim 10, characterized in that the plurality of optical cable trunks include a plurality of first fiber optic runs (305 (kp, 305 (k + i), 305 (k + 3 ), 305 (k + 5)) and a plurality of second fiber optic runs (305k, 305 (k + 2), 305 ^ + 4), 305 (k4 ')) joined together to form a fiber link (800), the first fiber optic portions and the second optical fiber portions being unidirectionally spun optical fibers, which have mutually opposite directions of rotation, and in which the first optical fiber portions and the second optical fiber portions are alternated with each other on the fiber optic link. Optical cable line according to claim 10, characterized in that at least one optical cable trunk of said plurality of optical cable trunks has an optical core that includes a plurality of unidirectionally spun optical fiber sections having a same direction of rotation. Optical cable line according to claim 10, characterized in that at least one optical cable trunk of said plurality of optical cable trunks has an optical core comprising at least two unidirectionally rotated optical fiber sections having directions. opposite rotation. Optical cable line according to claim 10, characterized in that the total number of optical cable trunks is odd. A method for obtaining a fiber optic link (300) as defined in claim 1, comprising: providing at least one first fiber optic portion (305 (ki), 305 (k + i), 305 (k +3>, 305 (k + 5>), obtained from at least one unidirectionally spun first optical fiber, unidirectionally spun in a first direction, provide at least a second fiber optic run (305k, 305 (k + 2) , 305 (k + 4), 305 (k + 6)), obtained from at least one unidirectionally spun second optical fiber, unidirectionally spun in a second direction, wherein the second optical fiber is distinct from the first optical fiber and the The second optical fiber is opposite the first direction, and joining the first and second segments together at their respective ends. A method for producing an optical cable comprising a fiber optic link as defined in claim 1, comprising providing a plurality of optical fibers to a cable manufacturing line, wherein said plurality of optical fibers comprises at least one fiber optic cable. first optical fiber (305 (ki>, 305 (k + i>, 305 (k + 3), 305 (k + 5)) that is unidirectionally rotated in a first direction, and at least a second optical fiber (305k, 305 (k + 2>, 305 (k + 4>, 305 (k + 6)) which is rotated unidirectionally in a second direction opposite the first direction. A method for obtaining an optical cable line as defined in claim 10, comprising: forming a plurality of optical cable trunks (805k, ... 805 (k + 4>), each including at least one fiber portion. (305 (ki), ... 305 (k + 6)), and joining the optical cable trunks together, characterized in that the step of forming a plurality of optical cable trunks comprises forming at least a first trunk that includes a first stretch of fiber optic (305 (ki), 305 (k + i), 305 (k + 3), 305 (k + s)) unidirectionally rotated in a first direction, and form at least one second trunk which includes a second fiber optic section (305k, 305 (k + 2>, 305 (k + 4>, 305 (k + 6)) rotated unidirectionally in a second direction opposite to the first direction, and by the fact of said union Optical cable trunks to each other include optically looping the first optical fiber portion to said second optical fiber portion.