Single mode optical fiber with low bending losses

[0001] The present invention relates to the field of fiber optic transmission, and specifically to linear fiber having reduced bending and micro-bending losses.

[0002] For optical fibers, the [refractive index] profile is generally classified with reference to the graph showing the function combining the refractive index of the optical fiber with the optical fiber radius. Typically, the distance r to the center of the optical fiber is shown on the abscissa axis, and the difference between the refractive index and the refractive index of the optical fiber jacket is shown on the ordinate axis. The [refractive index] profile is therefore referred to as "stepped", "trapezoidal" or "triangular" for graphs showing the degree, trapezoid or triangular shape, respectively. These curves are generally representative of the theoretical or given fiber profile [and] the stresses resulting from fiber production can lead to essentially different profiles.

[0003] An optical fiber typically consists of an optical core whose task is to transmit and optionally amplify the optical signal and the optical cladding whose task is to keep the optical signal inside the core. For this purpose, the refractive indexes of the core nc and the jacket ng are such that nc> ng. It is known that the propagation of an optical signal in single-mode fiber is broken down into a basic mode, guided in the core, and higher-order modes, carried out a certain distance in the core-mantle assembly and called mantle modes.

[0004] Single Mode Fibers (SMFs) are usually used as fiber optics in fiber optic transmission systems. These optical fibers exhibit chromatic dispersion and chromatic dispersion slope that meet specific telecommunications standards.

[0005] For the purpose of compatibility between optical systems of different manufacturers, the International Telecommunication Union (ITU) has established a standard known as ITU-T G.652, which must be met by Standard Single Mode Fiber (SSMF).

[0006] This G.652 standard for transmission optical fibers recommends, inter alia, the interval [8,6; 9.5um] for the diameter of the mode field at 1310nm wavelength; maximum

1260nm for cable cut-off wavelength; range [1300; 1324nm] for the zero dispersion wavelength denoted by λ₀; maximum 0.093ps / nm²-km for the dispersion slope

Chromatic EP 1 785 754 B1. Cable cut-off wavelength is typically measured as the wavelength at which the optical signal ceases to be single-mode after traveling twenty-two meters of fiber, as defined by subcommittee 86A of the International Electrotechnical Commission in the IEC 60793-1-44 standard.

[0007] Furthermore, for a given optical fiber, the so-called MAC value is defined as the ratio of the optical fiber mode field diameter at 1550nm to the effective cut-off wavelength Xc_eff also known as cutoff wavelength. The cut-off wavelength is typically measured as the wavelength at which the optical signal ceases to be single-mode after passing two meters of optical fiber, as defined by subcommittee 86A of the International Electrotechnical Commission in the IEC 60793-1-44 standard. MAC values ​​are used to evaluate fiber optic performance, in particular to find a compromise between mode field diameter, effective cut-off wavelength, and bending losses.

[0008] Fig. 1 illustrates the applicant's experimental results, giving bending losses at 1625nm wavelength, with a bending radius of 15mm, in a standard SSMF optical fiber relative to the MAC value at 1550nm. It can be seen that the MAC value affects the fiber bending losses and that these bending losses can be limited by reducing the MAC value.

[0009] However, reducing the MAC value by reducing the mode field diameter and / or by increasing the effective cut-off wavelength may lead to exceeding the G.652 standard, making the fiber commercially incompatible with some transmission systems.

[0010] Compliance with the G.652 standard and reduction of bending losses is a real challenge for the use of optical fibers for home fiber optic systems, called Fiber To The Home (FFTH) systems, or fiber optic systems for curb or building Fibers To The Curb (FTTC).

[0011] Indeed, the optical fiber transmission system includes storage boxes in which the excess length of optical fibers are placed for future intervention; this excess is rolled up in boxes. Due to the intention of miniaturization of these boxes, for FTTH or FTTC applications, in this context single-mode optical fibers are going to be wound on ever smaller diameters (until

EP 1 785 754 B1 to achieve bending radii as small as 15mm or 11mm). In addition, in the scope of FTTH or FTTC applications, there is a risk of subjecting the fiber to stricter installation constraints than in the case of applications over longer distances, i.e. the presence of accidental bends associated with low installation costs and the environment. The presence of accidental bending radii equal to 7.5mm or even 5mm should be assumed. It is therefore absolutely necessary to meet the requirements of storage boxes and installation requirements, so that single-mode optical fibers used for FTTH or FTTC applications have limited bending losses. Regardless, it is understood that this reduction of bending losses should not be achieved at the expense of deterioration or loss of single-mode signal character, which could significantly worsen the signal, or at the expense of introducing significant losses on optical connectors.

[0012] Publication of S. Matsuo et al. "Bend-Insensitive and Low Splice-Loss Optical Fiber for Indoor Wiring in FTTH", OFC'04 Proceedings, art. Th13 (2004) describes the [refractive index] profile for single-mode fiber (SMF), which reduces bending losses. However, this fiber has a chromatic dispersion between 10.2ps / nm-km and 14.1ps / nm-km, which is beyond the G.652 standard.

[0013] Publication by I. Sakabe et al. "Enhanced Bending Loss Insensitive Fiber and New Cables for CWDM Access Networks", 53rd IWCS Proceedings, pp. 112-118 (2004) proposes to reduce the diameter of the mode field to reduce bending losses. Such a reduction of the mode field diameter, however, leads to exceeding the G.652 standard.

[0014] Publication by K. Bandou et al. "Development of Premise Optical Wiring

Components Using Hole - Assisted Fiber ”53rd IWCS Proceedings, pp. 119-122 (2004) proposes a leaky fiber with optical characteristics of SSMF fiber with reduced bending losses. The cost of producing said fiber and the current high attenuation (> 0.25dB / km) make it difficult to use in commercial FTTH systems.

[0015] Publication by T. Yokokawa et al. "Ultra-Low Loss and Bend Insensitive PureSilica-Core-Fiber Complying with G.652 C / D and its Applications to a Loose Tube Cable", 53rd IWCS Proceedings, pp. 150-155 (2004) proposes fiber with pure silica PSCF , having reduced transmission losses and bending losses, but with a reduced mode field diameter, outside the G.652 standard.

EP 1 785 754 B1 [0016] US 6,771,865 describes a transmission fiber profile with reduced bending losses. The optical fiber has a core, cylindrical inner jacket and optical outer jacket. The cylindrical coat is doped with germanium and fluorine. The information provided in this document does not make it possible to determine whether this fiber meets the criteria established by the G.652 standard or not.

[0017] US 4,852,968 describes a transmission optical fiber profile having reduced 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 a zero chromatic dispersion wavelength between 1300nm and 1324nm, and the optical fiber described in US 4,852,962 has a zero chromatic dispersion wavelength between 1400nm and 1800nm.

[0018] US 4,852,968 further relates to single-mode fiber with a reduced refractive index ring or "groove" outside the fiber core and inside the outer sheath, [wherein] the sheath comprises a first sheath area that extends from radius a1 to a2 and has a refractive index n2 (a), a "refractive index groove" that extends from the radius a2 to a3 and has a refractive index n3 (a) and a second mantle region, which extends outwardly from a3 and has a refractive index n4 (a), where a1 <a2 <a3, and with a maximum value of n3 (a) less than the minimum value of n2 (a) and also less than n4 (a = a3), [ that is] the refractive index of the second coat in a3.

[0019] US 2003/0223717 relates to an optical fiber for the propagation of an optical signal of a specific wavelength having an axis, wherein the optical fiber comprises: a core; and the mantle surrounding the core, said mantle having an outer radius rc and an average refractive index nc. The core includes a central area distributed around the fiber axis and a cylindrical area surrounding this central area, where the cylindrical area is surrounded by a mantle, the cylindrical area has a minimum [factor] delta Δ₂between about -0.1% and about 0.05%, refractive index profile, maximum germanium concentration between about 2% and 22% by weight, germanium concentration profile, maximum fluorine concentration between 0.5% and about 3 , 5% by weight and fluorine concentration profile.

[0020] WO-A-2004/092794 describes a transmission optical fiber profile with reduced bending losses. The optical fiber has a core, the first inner mantle, the second

EP 1 785 754 B1 hidden inner coat and outer coat. Some of the fiber examples described in this document also meet the criteria of the G.652 standard. The optical fiber described in this document is produced by the methods of volumetric glass axial deposition (VAD) or chemical vapor deposition (CVD). The optical fiber described in this document, however, does not identify the problem of micro-twisted losses.

[0021] There is therefore a need for a transmission optical fiber that can meet the criteria of the G.652 standard, i.e. which can be given commercial application in FTTH or FTTC transmission systems, and which exhibits both reduced bending losses and reduced micro-bending losses. In FTTH or FTTC applications, optical fibers are subjected to higher bending and micro-bending stresses than in long-range transmission applications. Indeed, in FTTH or FTTC applications, excess lengths of fiber are generally wound up in increasingly miniaturized storage boxes; in addition, the optical fiber will be subjected to significant bending stress related to the surrounding of its installation.

[0022] To this end, the invention proposes an optical fiber profile comprising a core, a first inner jacket, a deeply hidden second inner jacket and an outer jacket. The second inner coat contains germanium.

[0023] The presence of germanium in a deeply hidden coat, although germanium is an admixture that results in an increase in silica's refractive index, makes it possible to increase the elastic-optical factor of the hidden coat. Therefore, when the optical fiber is subjected to stress, in particular when the optical fiber is bent or micro-bent, the presence of a deeply hidden sheath containing germanium reduces the effects of stress on changes in the refractive index in the optical fiber. Optical losses are thereby reduced when stresses are applied to an optical fiber having a second deeply hidden inner jacket containing germanium.

[0024] More specifically, the invention proposes a transmission optical fiber comprising:

_- a core having a difference in [refraction] Δπι relative to the outer shell;

EP 1 785 754 B1

- the first inner cladding, having the difference [refraction] Δπ₂ relative to the outer jacket;

- a second hidden inner coat, having a difference of [refraction]

Δπ₃ relative to the outer jacket less than -3.10 and containing germanium in a concentration by weight between 0.5 and 7%.

[0025] In one case, the difference in [refraction] Δπ₃ between the second inner jacket and the outer jacket is greater than -15.10^-3.

[0026] In another case, the difference of the [refraction] coefficient between the core and -3 -3 the first inner jacket (Δπι - Δπ₂) lies between 3.9.10^-3 and 5.9.10^-3.

[0027] According to another case, the second hidden coat has a radius between 12μm and 25μ ^ ι.

[0028] According to another example, the core has a radius between 3μm and 4.5um and exhibits a difference in [refraction] relative to the outer jacket between

4,2.10^-3 and 6.1.10^-3 [0029] According to another example, the first inner jacket has a radius between 7.5um and 14μm and shows a difference in [refraction] in relation to the outer jacket between -1.2.10^-3 and 1.2.10^-3.

[0030] According to another example, the core integral defined as:

/, = jz) / 2 (r) z / r ~ η xDr \ u is between 17.10^-:Vm a 24.10^-:Vm.

[0031] According to another example, at a wavelength of 1310nm, the current fiber exhibits a chromatic dispersion slope of 0.093ps / nm²-km or less.

[0032] According to another example, the current optical fiber exhibits zero chromatic dispersion at a wavelength between 1300nm and 1324nm.

[0033] According to another example, the current fiber has a cable cut-off wavelength of 1260nm or less.

[0034] According to another example, at a length of 1625nm, the current optical fiber exhibits bending losses for winding 100 turns around a 15mm bend radius that is 1dB or less.

[0035] According to another example, at a wavelength of 1625nm, the current fiber exhibits bending losses for one winding around a bending radius of 11mm [of] 0.5dB or less.

[0036] According to another example, at a wavelength of 1625nm, the current fiber exhibits bending losses for one winding around a bending radius of 5mm [of] 2dB or less.

[0037] According to another example, up to a wavelength of 1625nm, the current optical fiber exhibits micro-bend losses as measured by the so-called fixed drum method [of] 0.8dB / km or less.

[0038] The invention also relates to a method for producing a transmission optical fiber according to the invention, which method comprises the steps of:

- providing a silica tube and placing said tube on a slat;

- injection of a gaseous mixture of oxygen 02, silica SiCl4, Fluorine C2F6 and germanium GeO2 into the pipe;

- ionizing the gas mixture to obtain plasma by microwave heating to deposit a layer of doped silica forming a second, hidden, inner mantle;

- subsequent injection of the gas mixtures and ionization of said mixtures to deposit layers of doped silica forming the first inner jacket and the core.

[0039] The invention also includes a Fiber To The Home (FTTH) or Fiber To The Curb (FTTC) optical system, including at least an optical module or storage box according to the invention.

[0040] Other features and advantages of the invention will become apparent after reading the following description of embodiments of the invention, with reference to the accompanying drawings showing:

Fig. 1, previously described, a graph illustrating bending losses at a wavelength of 1625nm with a bending radius of 15mm in standard single mode fiber (SSMF) as a function of MAC value at 1550nm;

Fig. 2, graph showing a set single-mode fiber (SSMF) profile according to one embodiment of the invention, Fig. 3a to 3c, graphs illustrating, for different bending radii, bending losses at 1625nm wavelength as a function of MAC values ​​at 1550nm wavelength different standard single-mode optical fibers (SSMF) and for different optical fibers according to the invention, Figs. 4a and 4b, graphs illustrating micro-bend losses.

[0041] The current optical fiber has a core, a first inner mantle and a second hidden inner mantle. Hidden cladding is understood to be the radial part of the optical fiber whose refractive index is lower than the refractive index of the outer jacket. The second, hidden, inner coat has a difference of [refraction] relative to the outer coat, which is less than -3.10^-3 and which can reach -15.10^-3. In addition, the hidden coat contains germanium in a concentration between 0.5% and 7% by weight.

[0042] As is known per se, an optical fiber is obtained by pulling a preform. For example, the preform can be a glass tube (pure silica) of very high quality that forms part of the outer jacket and surrounds the core and inner jackets of the optical fiber; this pipe can then be bushed or filled to increase its diameter before proceeding with the drawing operation on the drawing tower. To make the preform, the tube is generally mounted vertically and held in place by its two ends by glass rods in the strip. Then the tube is rotated and locally heated to embed the components determining the composition of the preform. This composition determines the optical characteristics of the future optical fiber.

[0043] The embedding of ingredients in a pipe is usually called "doping", i.e.

"Admixtures" are added to the silica to modify its refractive index.

Thus, germanium (Ge) or phosphorus (P) increases the refractive index of silica;

they are often used to dope the fiber core. Also, fluorine (F) or boron

(B) reduce the refractive index of silica; they are often used for the production of hidden coats or as a co-admixture with germanium when it is desired to compensate for the increase in refractive index in the photosensitive coat.

[0044] A hidden jacket preform is difficult to fabricate. Fluorine is not readily incorporated into silica when heated above a certain temperature, while high temperature is required for making glass. The compromise between the high temperature needed for glass production and the low temperature that promotes proper incorporation of fluorine does not allow obtaining refractive indexes much lower than those for silica.

[0045] It is proposed to make a preform of the current optical fiber using the PCVD (Plasma Chemical Vapor Deposition) technique because it allows reactions at lower temperatures than conventional techniques (CVD, VAD, OVD) by ionization of reagents. Said manufacturing technique is described in documents US RE 30,635 and US 4,314,833; it enables the significant incorporation of fluorine in silica to form deeply hidden coats.

[0046] The use of the PCVD technique for producing the optical fiber of the invention also allows the addition of germanium to the hidden sheath. As noted earlier, germanium increases the refractive index of silica; it is therefore generally inadvisable to incorporate it into the part of the optical fiber for which a lower refractive index is desired than for silica. PCVD, however, allows the production of a significant number of highly reactive fluorine ions; then it becomes possible to add germanium to the reaction and yet obtain a hidden inner mantle.

[0047] Therefore, the current fiber contains germanium in the inner cladding assembly -3 including the cladding whose refractive index is less than -3.10 The presence of germanium in the hidden cladding changes the silica viscosity and the elastic-optical coefficient in said cladding.

[0048] Fig. 2 illustrates the [refractive index] profile for a transmission optical fiber according to the invention. The illustrated profile is a predefined profile, ie it represents the theoretical profile of the fiber, and [...] the fiber actually obtained after drawing from the preform may have a significantly different profile.

[0049] The single-mode transmission optical fiber according to the invention comprises a core having a refractive index difference Δπι relative to the outer jacket,

EP 1 785 754 B1 serving as an optical jacket; the first inner cladding, having a difference [refraction] Δη₂ relative to the outer jacket; the second inner jacket deeply hidden and having the difference [refraction] Δπ₃ relative to the outer jacket. Refractive indexes in the core, in the first coat and in the second inner coat, are substantially constant over their entire width; the given profile is therefore a truly single-mode fiber. The width of the core is defined by the radius r1, and the width of the coats by their respective external radii r2 and r3.

[0050] To define the [refractive index] profile of an optical fiber, the outer [refractive index] coefficient is generally taken as reference. Values ​​of the [refraction] core, hidden coats and ring are then given as differences Δπ₁,₂,₃. In general, the outer jacket is made of silica, but this outer jacket can be doped to increase or decrease its refractive index, e.g. to modify the signal propagation characteristics.

[0051] Each part of an optical fiber profile can therefore be defined by the use of integrals that combine changes in refractive indices with the radius of each part of an optical fiber.

[0052] Thus, for the current fiber, three integrals can be defined, which represent the surface of the core I1, the surface of the first inner jacket I2 and the surface of the second, hidden inner jacket I3. The expression "surface" is not understood geometrically, but it corresponds to a value that includes two dimensions. These three integrals can be expressed as follows:

<img file="PL1785754T3_D0001.tif" />

[0053] Table I below lists the limits of radii and differences of the [refraction] coefficients and the limits of the I1 integral, which are required for the optical fiber to have reduced bending losses and micro-twisted losses, while meeting the optical propagation criteria of the G.652 standard. for transmission optical fibers. The values ​​given in the table correspond to the given optical fiber profiles.

TABLE I

<td>r1 (Lm)</td><td>r2 (Lm)</td><td>r3 (Lm)</td><td>r1 / r2</td><td>ΔΠ1 (.10³)</td><td>ΔΠ2 (.10³)</td><td>Δπ3 (.10³)</td><td>Δπ1-Δπ₂</td><td>I1 (Lm.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> 4,4</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>

[0054] The value of the I1 integral of the core affects the shape and size of the basic mode of signal propagation in the optical fiber. The integral value for the core between 17.10 Lim and

24.10^-3In particular, L m enables the field diameter to be maintained which is compatible with the G.652 standard.

[0055] Table II below gives examples of possible coefficient [refraction] profiles for a transmission optical fiber according to the invention. The first column indicates the reference to each profile. The next columns give the radius values ​​of each part (r1 to r3); and the next columns give the values ​​of the difference [refraction] for each part in relation to the outer jacket (Δπ₁ to Δπ₃). Coefficient values ​​are measured at 633nm wavelength.

TABLE II

<td>Example</td><td>r1 (Lm)</td><td>r₂(Lm)</td><td>r3 (Lm)</td><td>ΔΠ1 (.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>

[0056] The current transmission optical fiber having a [refractive index] profile as described previously shows reduced bending losses and micro-bending losses at useful wavelengths.

[0057] In addition, the current optical fiber meets the criteria of the G.652 standard.

[0058] Tables III and IV below illustrate the simulated optical characteristics of transmission optical fibers corresponding to the [refractive index] profiles in Table II.

[0059] In Table III, column one reproduces the reference numbers in Table II. Subsequent columns for each fiber profile provide the effective TC cut-off wavelength values_eff, TCC cable cut-off wavelength, 2W02 mode field diameter for 1310nm and 1550nm wavelength, zero chromatic dispersion wavelength λ₀, slope of dispersion P0 at T0, chromatic dispersion C for wavelengths 1550nm and 1625nm.

[0060] In Table IV, column one repeats the reference numbers in Table III. The next columns give MAC values ​​at 1550nm wavelength. The next three columns give the values ​​of BL bending losses for the respective bending radii [of] 5, 11 and 15 mm at a wavelength of 1625 nm. The next column, for a 15mm radius, gives relative bend losses normalized to the standard bend losses of the SMMF fiber having the same MAC value at 1550 nm. The penultimate column reports the micro-bend losses obtained in the pin-array test (1ο pins 1.5mm each) at a wavelength of 1550nm.

[0061] This test uses an array of ten polished pins 1.5mm in diameter and 1cm apart. The optical fiber is interwoven through the board perpendicular to the axis of the pins. The optical fiber and array are compressed between two rigid plates covered with a layer of about 3mm high density polyethylene foam. The layers of this assembly (plates, board, optical fiber) are placed horizontally, and the assembly is covered with a 250g weight. The last column gives microbending losses measured by a fixed diameter drum method at 1625 nm. The method is described in the technical recommendation of the International Electrotechnical Commission, sub-committee 86A under reference number IEC TR-62221. The diameter of the drum used is 60cm; the drum is covered with very fine sandpaper. The values ​​of BL bending losses are determined at 1625 nm.

EP 1 785 754 B1

TABLE III

<td>lp</td><td>Źceff (Pm)</td><td>Z-CC (Pm)</td><td>2W02 @ 1310nm (pm²)</td><td>2W02 @ 1550nm (pm²)</td><td>^FROM0 (Nm)</td><td>P0₂(Ps / nm²-km)</td><td>C @ 1550nm (Ps / nm-km)</td><td>C @ 1625 nm (Ps / nmkm)</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>lp</td><td>MOTHER @ 1550nm</td><td>BL R = 5mm @ 1625 nm (DB / turn)</td><td>BL R = 11mm @ 1625 nm (DB / turn)</td><td>BL R = 15 mm @ 1625 nm (DB / 100 coils)</td><td>BLrel R = 15 mm @ 1625 nm</td><td>BLP Pin board test @ 1550nm (DB)</td><td>BLP Drum method @ 1625 nm (DB / km)</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>

[0062] In Table III it can be seen that examples 2 to 4 actually meet the G.652 standard, example 1 shows the dispersion slope P0 lying slightly outside the G.652 standard.

[0063] In particular, the optical fiber in Examples 2 to 4 shows zero chromatic dispersion at a wavelength between 1300nm and 1324nm; this is in line with the G.652 standard. The fiber in examples 2 to 4 also shows for wavelengths

1310nm, the slope of the chromatic dispersion is 0.093ps / nm²-km or less;

which agrees with the G.652 standard. Also, the optical fiber in examples 2 to 4

EP 1 785 754 B1 has a cable cut-off wavelength that is 1260nm or less, meeting the criteria of the G.652 standard that requires a cable cut-off wavelength of 1260nm or less.

[0064] In addition, it can be seen in Table IV that Examples 2 to 4 show significantly improved bending losses relative to SSMF transmission optical fiber losses. Micro-flexion losses are also improved.

[0065] The graphs in Figs. 3a, 3b and 3c show measurements of bending loss obtained for optical fibers produced according to the invention and for standard optical fibers, with bending radii R = 5mm, R = 11mm and R = 15mm, with a wavelength equal to 1625nm. Here the bending losses are given at the end of one turn (for R = 5mm and R = 11mm) or at the end of 100 turns (R = 15mm).

[0066] Fig. 4a shows micro-bend losses for optical fibers manufactured according to the invention, characterized by a pin array test and measured for a wavelength of 1550nm versus MAC value at 1550nm for different SSMF optical fibers and for different optical fibers according to the invention.

[0067] Fig. 4b shows micro-bend losses [measured] using a fixed diameter drum test as a function of wavelength for SSMF fiber and for an optical fiber according to the invention having MAC values ​​at 1550nm [equal], 8.11 and 8.31, respectively.

[0068] Also the graphs in Figs. 4a and 4b clearly show that the sensitivity of the current optical fibers to micro-bends is significantly reduced compared to the SSMF optical fiber. In Fig. 4a it can be seen that the micro-bend loss (pin array test) measured for an optical fiber according to the invention having a MAC value of 8.44 at 1550nm is 0.025dB, while they are ten times higher for an SSMF fiber having the same MAC value. In fig. 4b, it can also be seen that the micro-bend losses (fixed drum method) for an optical fiber according to the invention grow much more slowly with a wavelength than for an SMMF optical fiber, which, however, has a higher MAC value at 1550nm. This graph shows that the current fiber guarantees micro-bending sensitivity up to long wavelengths greater than 1650nm, which corresponds to the sensitivity that can be guaranteed by SSMF fiber up to 1550nm.

[0069] The current transmission optical fiber can be produced by pulling a preform having one of the above described [refractive index] profiles. Mentioned

For example, the preform profiles can be made of a silica sleeve in which doped silica layers are embedded. This deposition can be performed by the Plasma Chemical Vapor Deposition (PCVD) deposition method mentioned earlier. This plasma activated vapor (PCVD) chemical deposition is particularly suitable for obtaining the hidden inner mantle of current fiber; of this hidden inner jacket containing germanium in a concentration between 0.5% and 7% by weight. The weight concentration of germanium is preferably between 0.5% and 1.5% because it allows an optimal balance between lower costs and greater ease of manufacture on the one hand, and good optical fiber characteristics on the other hand.

[0070] A pure silica pipe is supplied and mounted on a batten. The tube is then rotated and a gaseous mixture of silica and dopants is injected into the tube. The pipe passes through a microwave cavity in which the gas mixture is locally heated. Microwave heating generates plasma by ionizing gases injected into the pipe, and the ionized dopants react strongly with silica particles, causing the layers of doped silica to deposit on the inside of the pipe.

[0071] The strong reactivity of the dopants generated by microwave heating enables the incorporation of high concentrations of the dopants into the silica layers. Particularly for fluorine, which is difficult to incorporate into silica by local heating with a burner, the PCVD technique allows doping the silica layer with a high concentration of fluorine to obtain deeply hidden layers.

[0072] Within the scope of the present invention, the production of a second hidden coat is accomplished by depositing a fluorine and germanium doped silica layer; a gaseous mixture containing oxygen O2, silica SiCl4, fluorine C2F6 and germanium GeO2 is injected into the tube. This gaseous mixture is ionized in the microwave cavity of the PCVD installation, [a] fluorine and germanium ions are incorporated into silica particles.

[0073] The proportions of injected gases are monitored to obtain a doped silica layer containing germanium at a concentration of 0.5% to 7% by weight and fluorine at the concentration that is required to obtain the intended refractive index.

[0074] The high concentration of fluorine provides the necessary reduction of the [refractive index] for the hidden cladding, and the low concentration of germanium causes changes in viscosity and the elastic-optical coefficient that are required to reduce bending losses and micro-bending losses in the resulting optical fiber.

[0075] The transmission optical fiber of the invention can be used in a transmitting or receiving module in an FTTH or FTTC system or in a high frequency and long range optical transmission cable with reduced optical losses. The fiber according to the invention is compatible with commercially available systems because it meets the G.652 standard. In particular, the excess length of the optical fiber according to the invention can be rolled up in a storage box associated with the optical module of the FTTF or FTTC system, wherein the optical fiber according to the invention can be rolled up with a bending radius of less than 15mm or even less than 5mm without causing strong optical losses. The fiber according to the invention is also very suitable for handling accidental bends associated with its installation in a human home with bending radii up to 5mm.

[0076] Of course, the present invention is not limited to the embodiments described. In particular, a method of production other than PCVD may be considered, provided that the method [this] enables the incorporation of germanium in a hidden layer, in accordance with the claimed proportions and differences [refraction]. In addition, the optical fiber of the invention can also be used in applications other than FTTH or FTTC.

EP 1 785 754 B1