Multimode optical fibre having improved bending losses

The present invention relates to the field of optical fiber transmission, and more particularly to multimode optical fiber with reduced bending losses without significantly increasing its numerical aperture.

[0002] Normally, the optical fiber is composed of a fiber optic core whose function is to transmit and optionally amplify the optical signal, and from an external optical sheath, the function of which is to close the optical signal within the core. For this purpose, refractive indices for the core nc and the jacket ng are such that nc> ng.

[0003] The refractive index profile is a graph of a function that associates a refractive index with a fiber optic radius. As a standard, the distance to the center of the optical fiber is shown on the x-axis, and on the y-axis the difference between the refractive index in this radial position and the refractive index of the external optical sheath. In general, the refractive index profile qualifies according to its appearance. Thus, the refractive index profile is described in terms of "degree", "trapezoid", "triangle" or "alpha" in the case of graphs that have the shape of a degree, a trapezoid, a triangle or a gradient, respectively. These curves represent the theoretical or fixed profile of the optical fiber, while limitations in the production of an optical fiber can cause a slightly different profile.

[0004] There are two main types of optical fiber, namely multimode fiber and single-mode fiber. In a multimode optical fiber, for a given wavelength, several optical modes are propagated along the optical fiber, while in the single-mode optical fiber the higher order modes are strongly suppressed.

[0005] Multimode optical fibers with a gradient coefficient of "central alpha" profile have been used for many years. Their characteristics are described in particular in "Multimode theory of graded-core fibers" by D. Gloge et al., Bell system Technical Journal 1973, pp. 1563-1578, and summarized in "Comprehensive theory of dispersion in graded-index optical fibers "by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, no. 2, pp. 166-177.

[0006] The gradient coefficient profile or alpha coefficient profile (α) - these two terms are equivalent - can be determined by the relationship between the refractive index value n at a certain point as a function of the distance r from this point to the center of the fiber:

η = ιη 1-2Δ - ν L^r<J where α> 1;

n1, the maximum multimode core factor;

r1, multimode core radius; and_AND f ~ ^ o) «,² where n0 is the minimum coefficient of the central multimode core, generally corresponding to the ratio of the external optical sheath (most often made of silica).

[0007] However, each mod is propagated with its own propagation constant with which the effective refractive index neff can be associated, which is a function of the refractive index profile of the optical fiber and the wavelength.

[0008] Figure 1 shows the refractive index profile of an α-shaped optical fiber according to the prior art. The lower x-axis shows the optical fiber radius, and the y-axis on the left shows the refractive index of the α-profile of the optical fiber. Therefore, the multimode optical fiber with the α profile has the profile of the middle core with rotational symmetry, so that in any radial direction the value of the refractive index decreases continuously from the center of the optical fiber to its periphery. The chart also shows the fashions that are propagated in optical fiber. The y-axis on the right represents the relative effective refractive indices for the propagation modes, i.e. the difference between the effective refractive index for the mode and the refractive index for the outer optical sheath. The reference is determined by the azimuthal coefficient, shown on the top x axis, matches each mode. Usually, the fashions gather together in groups of visible mods in the horizontal

In the direction of the graph. For example, the illustrated optical fiber includes 18 modes groups.

[0009] The numerical aperture (NA) of a fiber is defined by the expression:

<img file="PL2299302T3_D0001.tif" />

where neff, min and neff, max are respectively the minimum and maximum effective refractive index for the modes contained in the signal measured at the output of the optical fiber under OFL conditions (saturated excitation), i.e. when the signal stimulation at the fiber input is even across all propagation modes.

[0010] However, a good approximation of the numerical aperture is obtained by means of the expression:

<img file="PL2299302T3_D0002.tif" />

where nmax and nmin are respectively the minimum and maximum refractive index for the refractive index profile in the optical fiber.

[0011] It is known that the flexural losses of multimode fiber with a gradient coefficient can be reduced by adding a recessed groove between the central core and the outer optical sheath. However, the addition of such a recessed groove results in the development of additional propagation modes known as leaking fashions.

[0012] Figure 2 shows the refractive index profile of an optical fiber according to the prior art as shown in Figure 1, to which a recessed groove was inserted between the central core and the outer optical sheath. Additional propagation modes have been observed with reference to Figure 1, below the zero value of the relative effective refractive index. These additional bubbling fashion or fashion are placed in 5 groups of modes. Fashion leaks have effective refractive indexes, which are lower than those for conducted mods. This leads to an increase in the NA numerical aperture in gradient-based optical fibers containing a recessed groove, as compared to the gradient-based optical fibers without a recessed groove. Such a difference in the numerical aperture can lead to

Loss during connections in a system comprising fiber optics with a gradient coefficient with a recessed groove and fiber optics with a gradient coefficient without a recessed groove. Hence, the addition of a recessed groove to the profile with a gradient coefficient causes an increase in NA, which is undesirable. Therefore, it is preferable to limit the increase in NA by adding a recessed groove.

[0013] Documents US-A-2008/0166094 and WO-A-2008/085851 disclose the use of a recessed groove for reducing bending losses in a gradient coefficient optical fiber. However, these documents do not state how to ensure that the numerical aperture increases only slightly in relation to the numerical aperture of the gradient-type optical fiber without the recessed groove. In other words, how to avoid a large increase in the numerical aperture due to the addition of a recessed groove.

[0014] WO-A-2006/010798 describes an optical fiber comprising a center core with a gradient coefficient and a recessed groove. The profile of the gradient core coefficient is extended by the refractive index for the outer optical sheath, down to the bottom of the recessed groove. In other words, there is no sharp drop in the refractive index at the beginning of the recessed groove, but instead there is a gradual decrease until the bottom of the recessed groove is reached. Extension of the middle alpha-shaped core under the refractive index of the external optical sheath up to the bottom of the recessed groove reduces the reduction of bending losses, additionally increasing the numerical aperture. Therefore, this is undesirable. Furthermore, document WO-A-2006/010798 does not indicate how to ensure that groove.

[0015] WO 2009078962 relates to a bend resistant multimode fiber comprising: a glass core with a gradient coefficient; and an inner sheath surrounding and contacting the core and a second jacket comprising a annular fragment with a depth factor surrounding the inner jacket, said annular fragment having a refractive index having a refractive index delta below about -0.2% and a width of at least 1 micron, wherein the width of said inner shell is at least 0.5 micron and less than 4 microns.

[0016] WO 2010/138653 relates to a fiber, which is multimode at 850 nm, comprising a multimode glass core with a gradient coefficient having an optical core radius between 21 and 27 microns and a maximum delta of refractive index at 850 nm, delta1MAX, an annular region with a sinking coefficient surrounding said multimode core exhibiting a minimum delta of refractive index at 850 nm, delta2MIN, wherein the internal boundary of said region in the recessed coefficient is an extension of the gradient coefficient core, a region with a recessed coefficient having a limiting volume above 105% -urn2 and an area of ​​the outer mantle surrounding said region with a recessed coefficient and exhibiting a refractive index delta at 850 nm, A3,wherein delta1MAX&gt; A3&gt; delta2MIN, and wherein the optical fiber exhibits (a) bandwidth at saturation above 1.5 GHz-km at 850 nm and (b) increase damping at 1 turnover per mandrel with a diameter of 10 mm for length they gave 1550 nm below 0.28 dB.

[0017] Therefore, there is a need for an optical fiber with a gradient coefficient having reduced flexural losses, which, however, does not show a significant increase in its numerical aperture.

For this purpose, the invention proposes a multimode optical fiber according to claim 1.

[0019] According to an embodiment, the optical fiber has an extension below 0.015 of a numerical aperture relative to an optical fiber having the same ratio profile but no recessed groove.

[0020] According to an embodiment, the optical fiber has an extension below 0.010 of the numerical aperture relative to the optical fiber having the same ratio profile, but without the recessed groove.

[0021] According to an embodiment, the optical fiber has a central core with a diameter of 62.5 ± 3 Pm and a numerical aperture of 0.275 ± 0.015.

[0022] According to an embodiment, the optical fiber has a central core with a diameter of 50 ± 3 μm and a numerical aperture of 0.2 ± 0.015.

[0023] According to an embodiment, the optical fiber exhibits a difference in refractive index for the recessed groove Δ ^ relative to the external groove.

The optical jacket, which is comprised between -6 x 10^-3 a -15 x 10^-3.

[0024] According to an embodiment, the optical fiber has a difference in the refractive index for the recessed groove Δ ^ with respect to the external optical jacket, which is comprised between -7x10.^-3 a -10x10^-3.

[0025] According to an embodiment, the optical fiber has a width Wt of the recessed groove which is smaller than 4.5 μm, preferably equal to or larger than 1.5 μm or equal to or larger than 2 μm.

[0026] According to an embodiment, the difference between the radius r2 of the inner shell and the radius r1 of the central core (i.e., the width of the inner shell) is between 0 and 5 Pm.

[0027] According to an embodiment, the optical fiber has a difference between the radii r2 of the inner shell and the radius r₁ the central core, which is 0 μm (i.e., r1 = r2), and thus the recessed groove is in direct contact with the central core.

[0028] According to an embodiment, the optical fiber exhibits bending losses at a wavelength of 850 nm for two revolutions around a radius of curvature of 15 mm reduced by at least 40% relative to an optical fiber having the same ratio profile but without a recessed groove.

[0029] According to an embodiment, the optical fiber exhibits bending losses at a wavelength of 850 nm for two revolutions around a radius of curvature of 10 mm reduced by at least 30% relative to an optical fiber having the same coefficient profile but without a recessed groove.

[0030] According to an embodiment, the optical fiber exhibits bending losses at a wavelength of 850 nm for two revolutions around a radius of curvature of 7.5 mm reduced by at least 20% relative to an optical fiber having the same coefficient profile but without a recessed groove.

[0031] According to an embodiment, the optical fiber exhibits bending losses at a wavelength of 850 nm for two revolutions around a radius of curvature of 5 mm reduced by at least 20% relative to an optical fiber having the same ratio profile but without a recessed groove.

[0032] Other features and advantages of the invention will become apparent after reading the following

A detailed description of the embodiments of the invention given merely by way of example and with reference to the drawings which show:

Figure 1, the refractive index profile of an optical fiber having an alpha refractive index (α) profile and fashion propagated in an optical fiber;

Figure 2, the refractive index profile of the optical waveguide of figure 1 including the added recessed groove and the fashion propagated in the optical fiber;

Figure 3, an example of a profile of the refractive index of an optical fiber according to the prior art;

Figure 4, a graph showing leakage losses of leaked modes as a function of relative effective refractive indices for modes, in the optical fiber described in figure 2;

Figure 5, power distribution of modes in modes propagated in optical fibers having different volumes of recessed groove, as a function of the number of major modes for modes;

Figure 6, the resulting increase in numerical aperture induced in an optical fiber to which a recessed groove has been added, as a function of the volume of the recessed groove;

Figure 7, bending losses for 2 revolutions around a radius of curvature Rc ranging between 5 mm and 15 mm, as a function of the volume of the recessed groove;

Figure 8, another example of a profile of the refractive index of an optical fiber according to the present invention;

Figure 9, showing the power distribution of modes at 2 m under OFL conditions for multimode fiber-aided fibers with groove width μm and different depths.

[0033] The profile of the refractive index for a fiber of the prior art will be better described with reference to Figure 3. The refractive index profile of an optical fiber according to the present invention will be better described with reference to Figure 8. The prior art fiber is a multimode optical fiber having a central core having radius r1 and alpha coefficient profile relative to the external optical sheath, and a recessed groove having a width W_t ^ m) and the difference of Δ ^ between its

The refractive index and the refractive index of the outer optical sheath. The volume V ^ m) of the recessed groove is determined by the expression V = 1000 χ W_t χ Δ ^. The optical fiber of the present invention is a multimode optical fiber having a central core having a radius r1 and an alpha coefficient profile relative to the external optical sheath, an inner shell having a radius r2 and a refractive index difference Δ ^ relative to the outer optical sheath, and a recessed groove having a width W_t ^ m) and the difference of the Δ ^ factor between its refractive index and the refractive index of the outer optical sheath. The volume V ^ m) of the recessed groove is determined by the expression V = 1000 χ W_t Δ ^. The optical fiber according to the invention is such that the difference in refractive index between the end of the gradient central core and the outer optical jacket is zero, and the volume V of the recessed groove is between -40 μm and -30 μm. The range of values ​​for the volume V of the recessed groove and the difference in refractive index between the end of the gradient central core and the outer optical jacket ensure that the leaking modes that are propagated in the recessed groove have limited power. Thus, leaking fashions contribute to a limited extent to the signal transmitted by the optical fiber according to the invention, and the numerical aperture increases to a limited extent compared to a gradient-type optical fiber with no recessed groove. Thus, the gradient coefficient optical fiber according to the present invention has reduced bending losses without showing a significant increase in its numerical aperture. The advantages of the optical fiber according to the invention are explained in more detail below.

[0034] In a multimode optical fiber with a gradient coefficient comprising a recessed groove, the leakage modes experience leaking losses during their propagation in the optical fiber. Figure 4 depicts leakage losses in dB / m experienced by mods leaking as a function of their relative effective refractive indexes in an optical fiber whose refractive index profile is described in figure 2. Five successive graphs show each of the different groups of modes.

[0035] By comparing the plots of modes groups, it is noted that the mode groups having

EP 2 299 302 B1 ο

effective refractive indices below -2.5x10^-3 they have fashions where bending losses are greater than 1 dB / m. However, groups of modes having effective ο

refractive indices above -2.5x10^-3 they have fashions whose leakage losses may be less than 1 dB / m. Thus, the smaller the effective refractive index for the mod group, the greater the leakage loss. As a result, the groups of leakage modes contribute in different ways to the signal propagated along the optical fiber, depending on the value of their effective refractive index.

The effect of leakage modes on the computation of the numerical aperture will be more clearly understood with reference to Figure 5. Figure 5 shows the power distribution in mode groups after propagation at two meters in an optical fiber which has experienced arousal under OFL conditions (saturated excitation), i.e. with an even distribution stimulation in all modes groups. The x-axis shows the number of main modes or the row of each mode group.

[0037] Curves 1 to 10 are taken from gradient-optic fiber optics having a groove width of 9 μm and a difference o. Refractive index Δ ^ between -1x10 and -10x10. In other words, curves 1 to 10 correspond to optical fibers, which the volume of the recessed groove varies between -9 and -90 μm.

[0038] The curve in Figure 5 denoted "REF" is a reference curve characteristic of a gradient-weighted optical fiber without a recessed groove. It should be noted that 18 modes groups have a growing share of power. 18. group of modes is a group of modes, for which the effective refractive index is used as the minimum effective refractive index neff, min when calculating the numerical aperture of an optical fiber. A numerical aperture of 0.190 is then obtained.

[0039] By adding the recessed groove to the reference optical fiber, further groups of modes are added. For example, curve 6 corresponds to a reference fiber, o

to which was added a groove with a width of 9 μm and a difference of -6x10. It should be noted that curve 6 has 8 additional groups of leaking modes compared to the reference curve, i.e. from the order of 19 to 26 mode groups. When calculating the numerical aperture of an optical fiber corresponding to curve 6, the 26th group of modes must be one whose effective refractive index is used as

The minimum effective refractive index neff, min.

[0040] However, when calculating the numerical aperture, it is considered that the contribution of the mode group is significant if the following relationship is satisfied:

<img file="PL2299302T3_D0003.tif" />

where n is the number of modes in the row group of modes m, a_and is a leakage loss in dB / m for the i-th mode from the mod group in the order m, and m -1 is the integer part of the fraction 2 This relationship can also be represented graphically by the limiting curve shown in the form of steps in figure 5, representing

In the case of an optical fiber, the last group of modes having a point representing its power distribution above the limiting curve is a mode group whose effective refractive index can be used as neff, min when computing the numerical aperture of an optical fiber.

[0042] In an example of an optical fiber having a recessed 9 μm wide groove and a refractive index difference with an outer optical sheath of

equal to -6x10^-3, the last mode group, whose point representing its power distribution is above the limiting curve, is a mode group of order 22. Considering the effective refractive index for row groups 1 and 22, a numerical aperture of 0.214 is obtained. In this way, the obtained numerical aperture value is smaller than that obtained by considering the effective refractive index of the 26th mode group, which is the last group of modes propagated in the optical fiber. However, this value reflects the actual numerical aperture of a multimode optical fiber containing a recessed groove under operating conditions.

[0043] Thus, the attenuation experienced by a group of higher order modes allows them to be ignored when computing the numerical aperture of an optical fiber.

[0044] The curves in Figure 5 also show that the number of groups of leakage modes

The additions added by the recessed groove depend on the volume of the recessed groove.

The optical fiber according to the present invention has a volume of recessed groove which is comprised between -40 Pm and -30 Pm, which makes it possible to limit the growth of the numerical aperture relative to the fiber without the recessed groove, while allowing significant reduction of bending losses.

[0046] The range of values ​​of the volume of the recessed groove will be better understood with reference to the diagrams of figures 6 and 7.

The graph of FIG. 6 shows on the y-axis the difference between the numerical aperture of α-type multimode fiber having a recessed groove and multimode reference fiber, i.e. having an equivalent alpha profile (α) without a recessed groove. The volume of the recessed groove is shown on the x-axis. The graph shows results for a reference fiber having a numerical aperture of 0.200 and a reference fiber having a numerical aperture of 0.215.

[0048] The graph of Figure 7 shows y-axis bending losses at a wavelength of 850 nm for 2 turns with radii of curvature Rc equal to 5 mm, 7.5 mm, 10 mm and 15 mm, on α multimode fiber with a recessed groove . The volume of the recessed groove is shown on the x-axis.

[0049] Several pairs depict several pairs (V_t, Δ ^) for the same volume value.

[0050] In comparison with a fiber having a similar refractive index profile but without a recessed groove, it should be noted that a multimode optical fiber having an α profile having a depressed groove with a volume below -40 μm has bending losses at 2 revolutions at 850 nm, which are reduced by at least 60% at Rc = 15 mm, at least 50% at Rc = 10 mm, at least 40% at Rc = 7.5 mm and at least 40% at Rc = 5 mm. However, the numerical aperture of the optical fiber can be increased by more than 0.015 relative to the equivalent multimode optical fiber with the α profile without a recessed groove. This can lead to losses when connected to a fiber-optic cable without a recessed groove.

[0051] It should also be noted that a multimode optical fiber with an α profile having a depressed groove with a volume above -30 Pm shows an increase in the numerical aperture

Below 0.010 in relation to the equivalent α-type multimode optical fiber without a recessed groove. However, with a recessed groove with a volume above -30 μm, the flexure loss for 2 revolutions at 850 nm in relation to the optical fiber having a similar coefficient profile but without a recessed groove is reduced by at most 40% for Rc = 15 mm, at most 30% for Rc = 10 mm, at most 20% for Rc = 7.5 mm and at most 20% for Rc = 5 mm. The addition of a recessed groove is then less significant to reduce bending losses.

The optical fiber according to the invention has a volume of recessed groove that allows the numerical aperture to increase by less than 0.015 or even less than 0.010 when compared to a fiber optic having the same refractive index profile but without a recessed groove. In other words, the optical fiber of the present invention limits the increase in the numerical aperture that occurs due to the addition of a recessed groove. The flexural losses in the optical fiber according to the invention for 2 revolutions at 850 nm, in relation to the optical fiber having a similar coefficient profile, without recessed groove, are reduced by at least 40% for Rc = 15 mm, at least 30% for Rc = 10 mm, at least 20% for Rc = 7.5 mm and at least 20% for Rc = 5 mm.

The optical fiber according to the present invention exhibits a difference in the refractive index between the end of the gradient central core and the outer optical jacket which is zero. The optical fiber according to the present invention therefore has flexural losses below those of an optical fiber having a similar profile, but whose center core is extended under the outer optical sheath (for example according to WO-A-2006/010798). For example, in the case of small radii of curvature, and in the case of higher order modes, the optical fiber of the present invention exhibits ten times less bending losses than a fiber with a similar refractive index profile but whose center core is extended under the outer optical sheath.

[0054] The optical fiber according to the present invention comprises an inner coat with a radius r2 comprised between the central core and the recessed core to improve the fiber bandwidth. Preferably, the radius r2 is between 0

And 5 μm to achieve a compromise between improving fiber bandwidth and increasing manufacturing costs.

[0055] The curve in Figure 9 denoted "REF" is the reference curve characteristic of a gradient-weighted optical fiber with no recessed groove. It should be noted that 18 modes groups have a growing share of power. 18. group of modes is a group of modes, for which the effective refractive index is used as the minimum effective refractive index neff, min when calculating the numerical aperture of an optical fiber. A numerical aperture of 0.190 is then obtained.

[0056] In an example of an optical fiber according to the present invention, the optical fiber is composed of a central core, an inner skirt directly in contact with the latter, a recessed groove directly in contact with the inner cladding and the outer optical sheath.

[0057] The fiber according to the present invention may have a central core diameter and a numerical aperture typical of a multimode optical fiber to allow connection to a standard optical fiber. Thus, the optical fiber may have, for example, a center core diameter of 62.5 +/- 3 μm and a numerical aperture of 0.275 +/- 0.015. For example, the optical fiber may have a diameter of the middle core of 50 +/- 3 μm and a numerical aperture of 0.2 +/- 0.015.

[0058] The optical fiber according to the present invention can be produced by drawing from the final preforms.

[0059] The final preform can be produced by providing the basic preform with an outer overcladding layer (i.e. an overcladding process). The outer coating layer consists of doped or undoped natural or synthetic silica glass. There are several ways to provide an outer coat layer.

[0060] In the first example of the method, an outer coating layer can be provided by embedding and vitrifying natural or synthetic silica particles at the outer periphery of the primary preform under the influence of heat. Such a process is known, for example, from US patents 5,522,007; 5,194,714;

6,269,663; and 6,202,447.

[0061] In a further embodiment of the method, the basic preform can be

It is coated with a siliceous sheath tube that may or may not be doped. This t-shirt tube can then be adhered to the basic preform.

[0062] In yet another example of the method, the overcladding layer can be applied via the external gas deposition (OVD) method. Here, the soot layer is first deposited on the outer periphery of the basic preform, and then the soot layer is subjected to vitrification to form a glass.

[0063] The basic preforms may be produced by means of external vapor deposition techniques, such as external vapor deposition (OVD) and axial vapor deposition (VAD). Alternatively, the basic preforms can be produced via internal deposition techniques in which the glass layers are deposited on the inner surface of the substrate tube from doped or undoped silica glass, such as modified chemical vapor deposition (MCVD), chemical vapor phase chemical furnace (FCVD) ) and plasma chemical vapor deposition (PCVD).

[0064] In an embodiment, the basic preforms are produced by means of a PCVD process, which enables very precise adjustment of the shape of the gradient refractive index profile for the central core.

[0065] The recessed groove may be deposited on the inner surface of the substrate tube as part of a chemical vapor deposition process. A more typically recessed groove can be produced either (i) with a fluorine doped substrate tube as the starting point of an internal deposition process for embedding a central gradient refractive core or (ii) by crimping a fluorine-doped silicon middle-core core tube with a gradient refractive index light that can be produced by means of an external deposition process (eg OVD or VAD).

[0066] In yet another embodiment, the basic preform is produced via an internal deposition process using a fluorine doped substrate tube. The resulting tube containing embedded layers may be clad with one or more silica tubes with

In order to increase the thickness of the recessed groove or to form a recessed groove having a variable refractive index on its width, it is an admixture of fluorine. Although not required, one or more tapped tubes (e.g., fluorine-doped substrate tubes) may be adhered to the primary preform prior to the coating step. The clinging and sticking process can be repeated to build several layers of glass on the outside of the basic preform.

[0067] Of course, the present invention is not limited to the embodiments described by way of illustration. The optical fiber according to the invention can be installed in many transmission systems with good compatibility with other fiber optic systems.

[0068] The optical fiber according to the present invention is disclosed in more detail in the claims.

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