Single mode optical fiber with low bending losses

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DESCRIPTION

Singlemode fiber optic with low bending losses.

The present invention relates to the field of optical fiber transmissions, and more specifically, to a line fiber with low bending and microbending losses.

In the case of optical fibers, the index profile is usually scored in relation to a graph plot showing the function that associates the refractive index of the fiber with the radius of the fiber. Conventionally, the distance r to the center of the fiber is shown along the abscissa axis, and the difference between the refractive index and the refractive index of the fiber cladding is shown along the ordinate axis. Therefore, the index profile is described as "stepped", "trapezoidal" or "triangular" by the graphs describing, respectively, stepped, trapezoidal, or triangular shapes. These curves are generally representative of the theoretical or preset profile of the fiber, and it is possible that the stresses induced during fiber manufacture will produce a substantially different profile.

Conventionally, an optical fiber consists of an optical core whose function is to transmit, and optionally amplify, an optical signal, and an optical cladding whose function is to confine the optical signal inside the core. For these purposes, the refractive indices of the nucleus n_c y of cladding n_g are such that n_c > n_g. As is well known, the propagation of an optical signal in a single-mode optical fiber is broken down into a fundamental mode guided in the core and secondary modes guided at some distance in the whole of the cladding of the core, and called cladding modes.

As line fibers for fiber optic transmission systems, single mode fibers (SMF) are commonly used. These fibers have chromatic dispersion and a chromatic dispersion slope that meets specific telecommunications standards.

For compatibility needs between the optical systems of different manufacturers, the International Telecommunications Union (ITU) has established a standard called ITU-T G.652 that must be met by a standard single-mode fiber (SSMF).

This G.652 standard for transmission fibers recommends, among other things, a range of [8,6; 9.5 pm | for the mode field diameter (MFD) for a wavelength of 1310 nm; a maximum of 1260 nm for the cut-off wavelength of the cable; a range of [1300; 1324 nm] for the scattering cancellation wavelength, called λ₀; a maximum of 0.093 ps / nm²-km for the chromatic dispersion slope. The cut-off wavelength of the cable is conventionally measured as the wavelength at which the optical signal ceases to be single-mode after propagation along twenty-two meters of fiber, as defined by Subcommittee 86A of the Commission. International Electrotechnical in accordance with the IEC 60793-1-44 standard.

Likewise, for a specific fiber, the so-called MAC value is defined as the ratio between the mode field diameter of the fiber at 1550 nm and the effective cut-off wavelength dc_eff, also called the cutoff wavelength. The cut-off wavelength is conventionally measured as the wavelength for which the optical signal ceases to be single-mode after propagation through two meters of fiber, as defined by subcommittee 86A of the International Electrotechnical Commission of in accordance with standard IEC 60793-1-44. The MAC value is used to evaluate the performance of the fiber, specifically, to reach a compromise between the diameter of the field so the effective cut-off wavelength and the losses due to bending.

Figure 1 shows the applicant's experimental results, with bending losses at a wavelength of 1625 nm with a radius of curvature of 15 mm in a standard SSMF fiber in relation to the MAC value at a wavelength of 1550 nm. . It can be seen that the MAC value influences the bending losses of the fiber, and these bending losses can be reduced by lowering the MAC value.

However, a reduction in the MAC value by reducing the mode field diameter and / or by increasing the effective cut-off wavelength can cause the G.652 standard to be exceeded, rendering the fiber commercially incompatible with some transmission systems.

Compliance with the G.652 standard and the reduction of bending losses represents a real challenge for fiber applications designed for home fiber optic systems, called fiber optic systems to the home (FTTH). The Home) or fiber optic systems to the sidewalk or to the building, also called Fiber to the Curb Systems (FTTC).

In fact, a fiber optic transmission system includes storage boxes in which additional lengths of fiber are available for future interventions; these extra lengths are rolled up in the boxes. Due to the intention of miniaturizing these boxes for FTTH or FTTC applications, single-mode fibers, in this context, are designed to be wound with increasingly smaller diameters (reaching bend radii of only 15mm or 11mm). In addition, within the scope of FTTH or FTTC applications, fiber runs the risk of being subjected to more arduous installation problems than in the case of applications at greater distances,

ES 2 344 992 T3, that is, the presence of accidental flexing related to the low cost of the installation and the environment. The presence of an accidental bend radius equivalent to 7.5 mm or even 5 mm must be taken into account. Therefore, it is absolutely necessary, in order to meet storage box constraints and installation constraints, that single-mode fibers used for FTTH or FTTC applications exhibit limited bending losses. However, it is understood that this reduction in bending losses should not be achieved to the detriment of a loss of the single-mode character of the signal, which would seriously deteriorate the signal, or to the detriment of the introduction of significant optical losses caused by splices.

The publication of S. Matsuo et al. "Bend-Insensitive and Low Splice-Loss Optical Fiber for Indoor Wiring in FTTH", OFC'04 Proceedings, paper Th13 (2004) describes an index profile for single-mode fiber (SMF) that allows a reduction in bending losses. However, this fiber has a chromatic dispersion between 10.2 ps / nm-km and 14.1 ps / nm-km, which is outside the scope of the G.652 standard.

The publication of I. Sakabe et al. "Enhanced Bending Loss Insensitive Fiber and New Cables for CWDM Access Networks", 53rd IWCS Proceedings, pages 112-118 (2004) proposes reducing the field diameter so to reduce bending losses. However, this reduction in mode field diameter exceeds the G.652 standard.

The publication of K. Bandou et al. "Development of Premise Optical Wiring Components Using Hole-Assisted Fiber" 53rd IWCS Proceedings, pages 119-122 (2004) proposes a hollow fiber with the optical characteristics of an SSMF fiber with lower bending losses. The cost of manufacturing such fiber and the current high levels of attenuation (> 0.25 dB / km) make it difficult to use commercially in FTTH systems.

The publication of T. Yokokawa et al. "Ultra-Low Loss and Bend Insensitive Pure-Silica-Core Fiber Complying with G.652 C / D and its Applications to a Loose Tube Cable", 53rd IWCS Proceedings, pages 150-155 (2004) proposes a silica core fiber pure PSCF, with low transmission and bending losses, but with a smaller mode field diameter, thus exceeding the G.652 standard.

US 6771865 describes a transmission fiber profile with low bending loss. The fiber has a central core, an annular inner cladding, and an outer optical cladding. The ring liner is coated with germanium and fluorine. The information provided in this document does not allow to determine whether or not the fiber satisfies the criteria established by the G.652 standard.

US 4852968 describes the profile of a transmission fiber with low bending losses. However, this fiber has a chromatic dispersion that does not meet the criteria of the G.652 standard. The G.652 standard requires the cancellation of the chromatic dispersion at wavelengths between 1300 nm and 1324 nm, but the fiber described in document US 4852962 exhibits a cancellation of the chromatic dispersion at wavelengths between 1400 nm and 1800 nm.

Additionally, document US 4852968 refers to a single-mode optical fiber with a sunken index ring or peripheral region located outside the core of the fiber, and inside the cladding of the fiber, said cladding including a first cladding region that extends from a radio ai to a radio a₂ and that has a refractive index n₂(a), an index bed extending from a radius to₂ to a radius to₃ and that has a refractive index n₃(a) and a second cladding region extending outward from a radius a₃and that has a refractive index n₄ (a), being a₁<a₂<a₃, and with a maximum value of n₃ (a) less than the minimum value of n₂(a) and which is also less than n₄(a = a₃), the refractive index of the second cladding in a₃.

Document US 2003/0223717 refers to an optical fiber for propagation of an optical signal with a wavelength, said optical fiber having a center line, said optical fiber including:

- a core; Y

- a cladding layer surrounding the core, said cladding layer having an outer radius r_c and a mean refractive index n_c. The core includes a central region disposed around the center line of the fiber, and an annular region surrounding the central region, the annular region being surrounded by the cladding layer, and the annular region having a minimal difference Δ₂ between -0.1 and 0.05% approximately, a refractive index profile, a maximum concentration of germanium between 2% and 22% by weight, a concentration profile of germanium, a maximum concentration of fluorine between between 0.5% and 3.5% by weight, and a fluorine concentration profile.

WO-A-2004/092794 describes a transmission fiber profile with low bending losses. The fiber has a central core, a first inner cladding, a second buried inner cladding, and an outer optical cladding. Some of the fiber examples described in this document also meet the criteria of the G.652 standard. The fiber described in this document has been manufactured by techniques of the type of axial vapor deposition (VAD) or Chemical vapor deposition (CVD). However, the fiber described in this document does not identify microbending loss problems.

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Therefore, a fiber for transmission is needed with which it is possible to meet the criteria of the G.652 standard, that is, that can be used commercially in transmission systems of the FTTH or FTTC type, and that presents both low bending losses and low microbending losses. In FTTH or FTTC applications, fibers are subjected to higher bending and micro-bending stresses than in long-distance transmission applications. In fact, in FTTH or FTTC applications, excess fiber lengths are often wound into increasingly miniaturized storage boxes; In addition, the fiber will be subject to significant bending stresses related to the environment of its installation.

To this end, the invention proposes a fiber profile comprising a central core, a first inner cladding, a second deep buried inner cladding and an outer cladding. The second inner lining contains germanium.

The presence of germanium in the deep-buried coating, despite the fact that germanium is a coating whose effect is to increase the silica index, makes it possible to increase the elastic-optical coefficient of the buried coating. Therefore, when stresses are applied to the fiber, especially when the fiber is subjected to flexing or micro-flexing, the presence of the deep-buried, germanium-containing coating allows to limit the effects of stresses on changes in the index. refraction of the fiber. Optical losses, therefore, are reduced when these stresses are applied to a fiber that has a second deep-buried liner containing germanium.

More specifically, the invention proposes a transmission optical fiber comprising:

- a central core with an Ani index difference with an external optical coating;

- a first interior lining with an index difference An₂ with the external cladding;

- a second internal lining buried with an index difference An₃ with external coating less than -3 · 10 ³, and containing germanium with a concentration by weight comprised between 0.5% and 7%.

According to a characteristic, the index difference An₃ of the second inner liner with the outer liner is greater than -15 · 10 ³.

According to another characteristic, the index difference between the central core and the first inner lining (An₁-An₂) is between 3.9 · 10³ and 5.9 10 ³.

According to another characteristic, the second buried liner has a radius between 12 pm and 25 pm.

According to another characteristic, the central core has a radius between 3.5 pm and 4.5 pm, and presents an index difference with the outer cladding between 4.2 · 10³ and 6.1 10 ³.

According to another characteristic, the first internal cladding has a radius between 7.5 pm and 14.5 pm, and has an index difference with the outer cladding between -1.2 · 10³ and 1,210 ³.

According to another characteristic, the integral of the central nucleus, defined as:

It is between 17 · 10³ pm and 24 · 10 ³ p.m.

According to another characteristic, for a wavelength of 1310 nm, the current fiber has a chromatic dispersion differential of 0.093 ps / nm²-km or less.

According to another characteristic, the current fiber shows a cancellation of the chromatic dispersion for a wavelength between 1300 nm and 1324 nm.

According to another characteristic, the present fiber exhibits a cable cut-off wavelength of 1260 nm or less.

According to another characteristic, at a wavelength of 1625 nm, the present fiber shows bending losses, for a winding of 100 turns around a radius of curvature of 15 mm, of 1 dB or less.

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According to another characteristic, for a wavelength of 1625 nm, the present fiber shows bending losses, for a 1 turn winding around a radius of curvature of 11 mm, of 0.5 dB or less.

According to another characteristic, for a wavelength of 1625 nm, the present fiber shows bending losses, for a 1 turn winding around a radius of curvature of 5 mm, of 2 dB or less.

According to another characteristic, up to a wavelength of 1625 nm, the present fiber exhibits bending losses, measured with the so-called fixed diameter drum method, of 0.8 dB / km or less.

The invention also relates to a method of manufacturing an optical transmission fiber according to the invention, said method including the following steps:

- providing a tube of silica and placing said tube on a lathe;

- inject a gaseous oxygen mixture, OR₂, silicon SiCl₄, fluorine C₂F₆ and germanium GeO₂ in the tube;

- ionization of the gaseous mixture to obtain a plasma by microwave heating, to deposit a layer of silicon coating that forms the second buried internal coating;

- subsequently, injection of gaseous mixtures and ionization of said mixtures to deposit silica cladding layers to form the first inner cladding and the central core.

The invention also relates to a Fiber to the home (FTTH) or Fiber to the curb (FTTC) optical system, comprising at least one optical module or a storage box according to the invention.

Other characteristics and advantages of the invention will be observed by reading the following description of the embodiments of the invention, provided by way of example, with reference to the attached figures, which show:

Figure 1, previously described, consists of a graph showing the bending losses at a wavelength of 1625 nm with a radius of curvature of 15 mm in a standard single-mode fiber (SSMF) in relation to the MAC value. at a wavelength of 1550 nm.

Figure 2 is a graph showing the preset profile of a single mode fiber (SMF) according to an embodiment of the invention.

Figures 3a to 3c are graphs showing, for different radii of curvature, the bending losses at a wavelength of 1625 nm in relation to the MAC value at a wavelength of 1550 nm for different single-mode fibers standard (SSMF) and for different fibers according to the invention.

Figures 4a and 4b consist of a graph showing the microbending losses.

The present fiber has a central core, a first inner cladding, and a second buried inner cladding. Buried cladding means a radial portion of the fiber whose refractive index is less than the index of the outer cladding. The second buried inner liner has an index difference with the outer liner that is less than -3 · 10^-3 and that can go up to -15 · 10^-3. Also, the buried coating contains germanium in a concentration by weight of between 0.5% and 7%.

As is known per se, an optical fiber is obtained by stretching a preform. For example, the preform can be a very high quality glass tube (pure silica) that forms part of the outer cladding and surrounds the central core and inner cladding of the fiber; this tube can be covered or coated to increase its diameter before proceeding to the drawing operation in a drawing tower. To manufacture the preform, the tube is usually mounted horizontally and held at both ends by glass bars on a lathe; the tube is then rotated and locally heated to deposit the components that determine the composition of the preform. This composition determines the optical characteristics of the future fiber.

The deposition of the components in the tube is often called "doping", that is, impurities are added to the silica to modify its refractive index. Thus, germanium (Ge) or phosphorus (P) increase the refractive index of silica; they are frequently used to adulterate the central core of the fiber. Likewise, Fluorine (F) or Boron (B) decrease the refractive index of silica; they are often used to form buried coatings or as "co-dopants" with germanium when it is desired to compensate for the increase in refractive index in a photosensitive coating.

It is difficult to make a preform with a buried liner. Fluorine is not easily incorporated into silica when heated above a certain temperature, at the same time that a high temperature is required for the manufacture of glass, and a lower temperature that promotes the incorporation of fluorine does not allow to obtain indices much smaller than that of silica.

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The manufacture of the preform of the present invention is proposed using a PCVD (Plasma Chemical Vapor Deposition) technique, since it allows reactions at lower temperatures than with conventional techniques (CVD, VAD, OVD) by ionizing the reaction components. Said manufacturing technique is described in documents US RE 30635 and US 4314833; allows the incorporation of large amounts of fluorine into silica to form deep buried coatings.

The use of the PCVD technique for the manufacture of the fiber of the invention also makes it possible to add germanium to the buried coating. As indicated above, germanium increases the refractive index of silica; therefore, its incorporation in a fiber section for which it is desired to obtain a refractive index lower than that of silica is generally not recommended. However, PCVD makes it possible to manufacture a large number of highly reactive fluorine ions; Germanium can then be added to the reaction, and yet a buried lining is obtained.

Therefore, the present fiber includes germanium in the assembly of the inner liners, including the cladding whose index is less than -3 · 10 ³. The presence of germanium in the buried coating modifies the viscosity of the silica and the elastic-optical coefficient of said coating.

Figure 2 shows an index profile for a transmission fiber according to the invention. The profile shown is a preset profile, that is, it represents the theoretical profile of the fiber, and it is possible that the fiber actually obtained by stretching a preform gives a substantially different profile.

The single-mode transmission fiber of the present invention comprises a central core with a difference of indices An_B with an outer coating that acts as an optical coating; a first internal lining with an index difference An₂ with the exterior cladding; a second internal lining, deeply buried and with an index difference An₃ with the outer cladding. The refractive indices of the central core, the first cladding, and the second inner cladding are substantially constant over the entire width of the preset profile, and therefore, it is a true single-mode fiber. The width of the nucleus is defined by its radius r₁, and the width of the coatings by their respective outer radii r₂ yr₃.

To define a preset index profile for an optical fiber, the index of the outer sheath is often taken as a reference. The index values ​​of the central core, the buried linings and the annulus are then given as differences of indices An₁₂,₃. Generally, the outer coating is made of silica, but this outer coating can be doped to increase or decrease its refractive index, for example to modify the characteristics of the signal propagation.

Therefore, each section of the fiber profile can be defined using integrals that associate the variations of the indices with the radius of each section of the fiber.

Therefore, three integrals can be defined for the current fiber, representing the surface of the core I₁, the surface of the first inner liner I2 and the surface of the second buried inner liner I3. The expression “surface” should not be interpreted geometrically, but corresponds to a value that takes two dimensions into account. These three integrals can be expressed as follows:

<img file="ES2344992T3_D0001.tif" />

Table I below shows the limit values ​​of the radii and the index differences, and the limit values ​​of the integral I1 that are required for the fiber to show lower bending losses and micro-bending losses and still meet the criteria optical propagation of the G. 652 standard for transmission fibers. The values ​​provided in the table correspond to the predetermined profiles of the fibers.

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TABLE I

<td>ri (pm)</td><td>r₂ (p.m)</td><td>r₃(p.m)</td><td>ri / r₂</td><td>Δπι (.10³)</td><td>Δπ₂ ( .10³)</td><td>Δπ₃ ( .10³)</td><td>Δπι-Δπ₂</td><td>Ι₂ (pm. 10³)</td>

<td> 3,5</td><td> 7,5</td><td> 12, 0</td><td> 0,27</td><td> 4,2</td><td> -1,2</td><td> -15</td><td> 3, 9</td><td> 17</td>

<td> 4,5</td><td> 14,5</td><td> 25, 0</td><td> 0,5</td><td> 6,2</td><td> 1,2</td><td> -3</td><td> 5, 9</td><td> 24</td>

The value of the integral Ij of the central core influences the shape and size of the fundamental propagation mode of the signal in the fiber. An integral value for the central nucleus between 1710³ pm and from 24 · 10 ³ pm specifically allows you to maintain a field diameter so that it is compatible with the G. 652 standard.

Table II provides examples of possible index profiles for a transmission fiber according to the invention. In the first column a reference is assigned to each profile. The following columns provide the values ​​of the radii of each section (η ar₃); and the following columns offer the values ​​of the index differences of each section with the exterior cladding (An₁ to An₃). Index values ​​are measured at a wavelength of 633 nm.

TABLE II

<td>Example</td><td>Γι (pm)</td><td>r₂ (p.m)</td><td>r₃(p.m)</td><td>Δπι (.10³)</td><td>Δπ₂ ( .10³)</td><td>Δπ₃ ( .10³)</td>

<td> 1</td><td> 2,86</td><td> 6, 90</td><td> 13,24</td><td> 5, 41</td><td> 2, 00</td><td> -3, 70</td>

<td> 2</td><td> 3,86</td><td> 9,50</td><td> 15</td><td> 5,16</td><td> 0, 69</td><td> -5, 0</td>

<td> 3</td><td> 4, 02</td><td> 9,55</td><td> 15</td><td> 5, 31</td><td> 0, 45</td><td> -5, 0</td>

<td> 4</td><td> 3,86</td><td> 8, 66</td><td> 15</td><td> 5, 41</td><td> 0, 85</td><td> -5, 0</td>

The present transmission fiber, which has an index profile as described above, exhibits lower bending losses and microbending losses at useful wavelengths.

In addition, the present fiber meets the criteria of standard G. 652.

Tables III and IV below show the simulated optical characteristics for transmission fibers corresponding to the index profiles in Table II.

In table III, column one reproduces the references in table II. The following columns, for each fiber profile, show the values ​​of the effective cut-off wavelength d_Ceff, from the wired cut-off wavelength dcc, the mode field diameters 2W02 for the wavelengths 1310 nm and 1550 nm, the chromatic dispersion cancellation wavelength λ₀, the dispersion differential P₀ a λ₀, the chromatic dispersions C for the wavelengths of 1550 nm and 1625 nm.

In table IV, column one reproduces the references in table III. The following columns provide MAC values ​​at a wavelength of 1550 nm. The next three columns provide the values ​​for the bending losses BL for the corresponding radii of curvature of 5, 11 and 15 mm at a wavelength of 1625 nm. The next column, for a radius of 15 mm, gives the relative bending losses normalized to the standard bending losses of an SSMF fiber with the same MAC value at a wavelength of 1550 nm. The penultimate column facilitates the microbending losses obtained by the pin matrix test (10 pins of 1.5 mm) at a wavelength of 1550 nm.

This test uses a matrix of ten polished needles, 1.5 nm in diameter and spaced 1 cm apart. The fiber is woven through the matrix, orthogonal to the axis of the needles. The fiber and matrix are pressed between two rigid plates lined with a layer of approximately 3 mm of high-density polyethylene foam. The layers of the assembly (plates, matrix, fiber) are arranged horizontally, and the assembly is covered with a weight of 250 g. The last column indicates the microbending losses measured using the fixed diameter drum method at a wavelength of 1625 nm. This method is described in the technical recommendations of subcommittee 86A of the International Electrotechnical Commission under the reference IEC TR-62221. The diameter of the drum used is 60 cm; the drum is covered with a superfine sandpaper. The values ​​of the BL bending losses are indicated at a wavelength of 1625 nm.

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TABLE III

<td>No.</td><td>Acefft (p.m)</td><td>Acc, (p.m)</td><td>2W02 @ 1310nm (p.m²)</td><td>2W02 @ 1550nm (p.m²)</td><td>Λ_ο (nm)</td><td>P_or(ps / nm²-km)</td><td>c @ 1550nm (ps / nm -km)</td><td>C @ 1625nm (ps / nm -km)</td>

<td> 1</td><td> 1, 13</td><td> &lt;1,26</td><td> 9, 10</td><td> 10, 18</td><td> 1308</td><td> 0,097</td><td> 19,2</td><td> 23, 9</td>

<td> 2</td><td> 1,23</td><td> &lt;1,26</td><td> 9,16</td><td> 10, 36</td><td> 1312</td><td> 0,091</td><td> 18, 1</td><td> 22, 9</td>

<td> 3</td><td> 1,25</td><td> &lt;1,26</td><td> 9, 01</td><td> 10, 13</td><td> 1318</td><td> 0,089</td><td> 17, 3</td><td> 22, 0</td>

<td> 4</td><td> 1,25</td><td> &lt;1,26</td><td> 9, 00</td><td> 10, 08</td><td> 1318</td><td> 0,091</td><td> 17, 8</td><td> 22,5</td>

TABLE IV

<td>No.</td><td>MAC @</td><td>BL</td><td>BL</td><td>BL</td><td>BLrel</td><td>BLp</td><td>BLp</td>

<td></td><td>1550 nm</td><td>R = 5 mm</td><td>R = ll mm</td><td>R = 15 mm</td><td>R = 15 mm</td><td>Test</td><td>Method</td>

<td></td><td></td><td>@ 1625 nm</td><td>@ 1625 nm</td><td>@ 1625 nm</td><td> @1625</td><td>of</td><td>of the</td>

<td></td><td></td><td></td><td></td><td></td><td>nm</td><td>matrix</td><td>drum</td>

<td></td><td></td><td>(dB / Turn)</td><td>(dB / Turn)</td><td>(dB / Turn)</td><td></td><td>of pins</td><td> @ 1625</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td> @ 1550</td><td>nm</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td>nm</td><td>(dB / km)</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td>(dB)</td><td></td>

<td> 1</td><td> 9, 0</td><td> &lt;5</td><td> &lt;2</td><td></td><td> 1/5</td><td></td><td></td>

<td> 2</td><td> 8, 4</td><td> 2</td><td> &lt;0,5</td><td> &lt;1</td><td> 1/5</td><td> 0, 025</td><td> &lt;0, 8</td>

<td> 3</td><td> 8, 1</td><td> 1</td><td> &lt;0, 1</td><td> &lt;0, 4</td><td> 1/5</td><td> &lt;0,025</td><td> &lt;0, 8</td>

<td> 4</td><td> 8, 1</td><td> 1</td><td> &lt;0, 1</td><td> &lt;0, 4</td><td> 1/5</td><td> &lt;0,025</td><td> &lt;0, 8</td>

It can be seen from Table III that Examples 2 to 4 comply with the G. 652 standard.

Specifically, the fiber of Examples 2 to 4 shows the cancellation of the chromatic dispersion for a wavelength between 1300 nm and 1324 nm; this is in line with the G.652 standard. The fiber of Examples 2 to 4 also shows, for a wavelength of 1310 nm, a chromatic dispersion slope of 0.093 ps / nm²-km or less, which complies with the G.652 standard. Similarly, the fiber of Examples 2 to 4 exhibits a wireline cut-off wavelength that is 1260 nm or less, meeting the criteria of the G.652 standard requiring a wireline wavelength of 1260 nm or less.

Furthermore, it can be seen from Table IV that Examples 2 to 4 show distinctly improved bending losses over the losses of the standard SSMF transmission fiber. Microbending losses have also improved.

The graphs in Figures 3a, 3b and 3c show measurements of bending losses obtained with fibers manufactured according to the invention and for standard fibers, with radii of curvature of R = 5 mm, R = 11 mm and R = 15 mm at a wavelength of 1625 nm. In this case, the bending losses are provided at the end of a loop (for R = 5 mm and R = 11 mm) or at the end of 100 loops (for R = 15 mm).

Figure 4a shows microbending losses for fibers manufactured according to the invention, characterized by the pin matrix test and measured at a wavelength of 1550 nm, relative to the MAC value at a wavelength of 1550 nm for the different SSMF fibers and for a fiber according to the invention.

Figure 4b shows microbending losses using the fixed diameter drum test in relation to wavelength for an SSMF fiber and for a fiber of the invention with MAC values ​​at a 1550 nm wavelength of 8, 11 and 8.31, respectively.

Likewise, the graphs in Figures 4a and 4b clearly show that the sensitivity of the present fiber to microbending is significantly reduced relative to that of an SSMF fiber. In figure 4a it can be seen that the microbending losses (pin matrix test) measured for a fiber of the invention, with a MAC value of 8.44 at a wavelength of 1550 nm, amount to 0.025 dB, while they are ten times greater for a fiber

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SSMF with the same MAC value. It can also be seen in figure 4b that the micro-bending losses (fixed drum method) for a fiber of the invention increase much more slowly with wavelength than in the case of an SSMF fiber having a higher MAC value. , at the wavelength of 1550 nm. In this graph it can be seen that the current fiber guarantees a sensitivity to micro-bending up to high wavelengths, higher than 1650 nm, which is equivalent to the sensitivity that can be guaranteed for an SSMF fiber up to a wavelength of 1550 nm.

The present transmission fiber can be made by stretching a preform having one of the index profiles described above. Said preform profiles can consist, for example, of a silica hose in which layers of added silica are deposited. The deposition can be carried out by the aforementioned Plasma Chemical Vapor Deposition (PCVD) method. This chemical plasma activated vapor deposition (PCVD) is especially suitable for obtaining a buried inner coating layer for the present fiber; this buried coating layer includes germanium, at a concentration ranging from 0.5% to 7% by weight. The concentration by weight of germanium is preferably between 0.5% and 1.5%, since this allows an optimal balance between lower costs and greater ease of manufacture, on the one hand, and appropriate characteristics of the fiber, Besides.

A tube of pure silica is available, which is mounted on a lathe. The tube is then rotated and a gaseous mixture of silica and dopants is injected into said tube. The tube passes through a microwave cavity in which the gas mixture is heated locally. Microwave heating generates a plasma by ionizing the gases injected into the tube and the ionized dopants react strongly with the silica particles, causing the deposit of layers of doped silica inside the tube.

The strong reactivity of the dopants generated by microwave heating allows the incorporation of a high concentration of dopants in the silica layers. In the specific case of fluorine, which is difficult to incorporate into silica by heating in a local burner, the PCVD technique allows the incorporation of a layer of silica with a high concentration of fluorine for the formation of deeply buried layers.

Within the scope of the invention, the creation of the second buried coating is obtained by depositing a layer of silica doped with fluorine and germanium; a gaseous mixture containing oxygen is injected O₂, silica SiCl₄, Fluorine C₂F₆ and Germanium GeO₂ in the tube. This gaseous mixture is ionized in the microwave cavity of a PCVD installation, incorporating the fluorine and germanium ions into the silica particles.

The proportions of the injected gases are monitored to obtain a layer of doped silica containing germanium, at a concentration of 0.5% to 7% by weight, and fluorine, at the concentration necessary to obtain the desired refractive index.

The high concentration of fluorine guarantees the required reduction in the index corresponding to the buried coating, and the low concentration of Germanium provides the changes in viscosity and the elastic-optical coefficient that are required to reduce bending losses and microbending losses. in the fiber thus obtained.

The transmission fiber according to the invention can be used in a transmission or reception module in an FTTH or FTTC system or in an optical cable for transmission over long distances with high transmission speed, with low optical losses. The fiber of the invention is compatible with commercialized systems, since it complies with the G.652 standard. Specifically, the excess length of the fiber according to the invention can be rewound in storage boxes associated with optical modules of FTTH or FTTC systems, the fiber of the invention can be rewound with a radius of curvature of less than 15 mm, or even less than 5 mm without inducing strong optical losses. The fiber according to the invention is also very suitable to withstand accidental bending related to its installation in a single hearth, with radii of curvature that can be reduced up to 5 mm.

Obviously, the present invention is not limited to the embodiments described by way of example. Specifically, a manufacturing method other than PCVD can be envisaged, provided that the method allows the incorporation of Germanium in a buried layer, according to the claimed proportions and the index differences. Additionally, the fiber according to the invention can also be used in applications other than FTTH or FTTC.

References cited in description

The list of references cited by the applicant is for the reader's utility only, not forming part of European patent documents. Even though the references have been carefully compiled, errors or omissions cannot be excluded and the EPO declines all responsibility in this regard.

Patent documents cited in the description • US 6771865 B [0016] · WO 2004092794 A [0020] • US 4852968 A [0017] [0018] · US RE30635 E [0045]

ES 2 344 992 T3 • US 4852962 A [0017] • US 20030223717 A [0019] • US 4314833 A [0045]

Patent bibliography cited in description • S. MATSUO et al. Bend-lnsensitive and Low Splice-Loss Optical Fiber for Indoor Wiring in FTTH. OFC'04 Proceedings, paper Th13, 2004 [0012] • I. SAKABE et al. Enhanced Bending Loss Insensitive Fiber and New Cables for CWDM Access Networks. 53rd IWCS Proceedings, 2004, 112-118 [0013] • K. BANDOU et al. Development of Premise Optical Wiring Components Using Hole-Assisted Fiber. 53^rd IWCS Proceedings, 2004, 119-122 [0014] • T. YOKOKAWA et al. Ultra-Low Loss and Bend Insensitive Pure-Silica-Core Fiber Complying with G.652 C / D and its Applications to a Loose Tube Cable. 53rd IWCS Proceedings, 2004, 150-155 [0015]