Monomode optical fiber with optimized dispersion shift for high speeds

[0002] The present invention relates to a dispersion-shifted single-mode optical fiber suitable for being used in wavelength division multiplexed transmission systems having high data rates.

[0003] Single-mode fibers must have characteristics that correspond to the requirements both of cable-makers and of system designers: firstly they must have good “cablability”, i.e. putting the fiber in a cable must give rise to only very low additional attenuation; and secondly they must have large effective areas to avoid non-linear effects, and suitable values for the zero-dispersion wavelength λ₀.

[0004] The use of such fibers in wavelength division multiplexed transmission systems using RZ, NRZ, or soliton-type pulses involves further constraints, particularly with increasing number of transmitted channels, data rate per channel, and post-amplifier power, and with decreasing spacing between channels. It is preferable to have a fiber that has non-zero chromatic dispersion at the transmission wavelengths so as to avoid non-linear effects, and in particular four-wave mixing (FWM) in the presence of a plurality of channels. Non-zero dispersion-shifted fibers (NZ-DSFs) are therefore used that have zero chromatic dispersion wavelengths λ₀that lie outside the ranges of the channels of the multiplexes so as to avoid the problems caused by four-wave mixing.

[0005] In addition, the use of such fibers for wavelength division multiplexed systems involves looking for a shallow chromatic dispersion gradient so as to retain propagation characteristics that are similar for the various channels.

[0006] Numerous index profiles have been proposed for such dispersion-shifted single-mode optical fibers. The index profile is generally described as a function of the appearance of the curve representing the refractive index as a function of the radius of the fiber. Conventionally, distance r to the center of the fiber is plotted along the x-axis, and index, defined by its difference relative to the index of the cladding of the fiber, is plotted up the y-axis in relative value (Δn) or in percentage (Δn %=100×Δn/n).

[0007] The index profile is thus said to be “stepped”, “trapezium-shaped” or “triangular” for curves representing the variation of refractive index as a function of radius that are respectively stepped, trapezium-shaped, or triangular.

[0008] The article entitled “Transmission characteristics of a coaxial optical fiber line”, Journal of Lightwave Technology, vol. 11 No. 11, November 1993, describes a reference profile of the “central trough” type.

[0009] One of the fiber profiles proposed in that article has a zero chromatic dispersion wavelength in the vicinity of 1450 nm with an effective area greater than 60 μm². Those two characteristics make it possible to satisfy the requirements in terms of non-linear effects and of four-wave mixing. However, a fiber having such a profile cannot be used in high data rate telecommunications systems because it has bending losses that are very high, and in practice considerably higher than those required by all specifications.

[0010] Document EP-0 724 171 is also known, and it proposes a plurality of dispersion-shifted single-mode optical fiber profiles that can be used in high data rate telecommunications systems. Those fibers make it possible to convey a plurality of channels in the vicinity of 1550 nm while avoiding non-linear effects by means of an effective area that is greater than 60 μm².

[0011] Unfortunately, because their zero chromatic dispersion wavelength lies in the vicinity of 1530 nm or of 1560 nm, those fibers have chromatic dispersion that is very low at the transmission wavelengths of the window situated around 1550 nm. As a result, the four-wave mixing phenomenon is not avoided in such fibers.

[0012] Finally, the article entitled “Maximum effective area for non-zero dispersion-shifted fiber” (OFC'98 —Feb. 22 to 27, 1998 —pp 303-304) describes a dispersion-shifted single-mode optical fiber having an effective area greater than 60 μm², low losses, and a zero chromatic dispersion wavelength in the vicinity of 1490 nm. There too, at the transmission wavelengths, the chromatic dispersion is too low to avoid four-wave

[0013] An object of the present invention is thus to develop a dispersion-shifted optical fiber that is optimized for high data rates, i.e. that has a large effective area, low losses, and a zero chromatic dispersion wavelength that is distinct from 1550 nm, making it possible to avoid the four-wave mixing phenomenon.

[0014] To this end, the present invention provides a dispersion-shifted single-mode optical fiber having an effective area greater than or equal to 60 μm²;

[0015] characterized in that it has a zero chromatic dispersion wavelength λ₀lying in the range 1400 nm to 1480 nm, and bending losses of less than 0.05 dB at 1550 nm for a winding of 100 turns of the fiber around a radius of 30 mm.

[0016] The fiber of the invention is a dispersion-shifted single-mode fiber having low bending losses and a large effective area, as well as chromatic dispersion that, for the wavelengths situated in the transmission window around 1550 nm, is both low enough not to give rise to information losses during transmission, and also high enough to avoid the four-wave mixing phenomenon over the entire transmission window. In addition, the chromatic dispersion gradient of the fiber of the invention is shallow.

[0017] Advantageously, the fiber is single-mode in fiber.

[0018] For high data rate systems, it is also useful for the fiber to provide single-mode propagation of the channels of the multiplex. ITU-T G 650 gives a definition of the cutoff wavelength in cable. The theoretical cutoff wavelength of the fiber is generally greater by several hundred nanometers than the cutoff wavelength in cable. The propagation in an optical fiber can be single-mode even if the theoretical cutoff wavelength is greater than the wavelength of the signals used: beyond a distance of a few meters or a few tens of meters, which is short compared with the propagation distances in optical-fiber transmission systems, the secondary modes disappear because of attenuation that is too great. The propagation in the transmission system is then single-mode.

[0019] In a preferred embodiment of the invention, the wavelength λ₀for which chromatic dispersion is zero lies in the range 1450 nm to 1480 nm, and preferably in the vicinity of 1475 nm.

[0020] Advantageously, the fiber has chromatic dispersion lying, in absolute terms, in the range 1 ps/nm.km to 6 ps/nm.km for a wavelength of 1550 nm.

[0021] The fiber has a chromatic dispersion gradient lying, in absolute terms, in the range 0.045 ps/nm².km to 0.09 ps/nm².km, and preferably in the range 0.045 ps/nm².km to 0.075 ps/nm².km for wavelengths of 1550 nm. These values for the chromatic dispersion gradient guarantee, in the range in which the fiber is used, that the chromatic dispersion remains substantially constant. The fiber is thus suitable for use under wavelength division multiplexing, and has chromatic dispersion values that are substantially equal over the band of the multiplex.

[0022] Preferably, the attenuation of the fiber is less than or equal to 0.23 dB/km at 1550 nm. Such an attenuation value guarantees that transmission losses are limited.

[0023] In a first embodiment of the invention, the fiber has an index profile comprising a central portion of index n₁less than the index n_Sof the cladding of the fiber, an inner ring of index n₂greater than the index of the cladding and extending around said central portion, an outer ring of index n₄greater than the index of the cladding and extending around said inner ring, and, between said inner ring and said outer ring, an annular portion of index n₃less than or equal to the indices of said inner ring and of said outer ring.

[0024] In which case, the difference Δn₁between the index of the central portion of the fiber and the index of the cladding preferably lies within a range of −3.8×10⁻³±10%.

[0025] The radius a₁of the central portion of the fiber advantageously lies within a range of 1.84 μm±5%.

[0026] In an embodiment, the difference Δn₂between the index of the inner ring of the fiber and the index of the cladding lies within a range of 11.35×10⁻³±10%.

[0027] It is possible to make provision for the ratio a₁/a₂between the radius a₂of the central portion of the fiber and the radius a₂of the inner ring to lie within a range of 0.47+5%.

[0028] In another embodiment, the difference Δn₃between the index of the annular portion and the index of the cladding lies within a range of −5.7×10⁻³35 10%.

[0029] In addition, the thickness a₃−a₂of the annular portion advantageously lies within a range of 3.54 μm±5%.

[0030] Preferably, the difference Δn₄between the index of the outer ring and the index of the cladding lies within a range of 5.55×10⁻³±10%.

[0031] In addition, the thickness a₄−a₃of the outer ring lies within a range of 1.46 μm±10%.

[0032] In a second embodiment of the invention, the fiber has an index profile comprising a central portion of maximum index n₁greater than the index n_Sof the optical cladding, a ring of maximum index n₄greater than the index of the optical cladding and surrounding said central portion, and an intermediate zone of index n₂less than n₁, and less than n₄, between said central portion and said ring.

[0033] In a preferred implementation of the second embodiment, the difference Δn₁between n₁, and n_Slies in the range 6×10⁻³to 15×10−3.

[0034] The difference Δn₄between n₄and n_Sis less than 10×10⁻³.

[0035] Other characteristics and advantages of the invention will appear on reading the following description of embodiments of the invention given by way of example and with reference to the figures, in which:

[0036] FIG. 1 is a diagram showing the index profile of a first embodiment of a fiber of the invention; and

[0037] FIG. 2 is a diagram showing the index profile of a second embodiment of a fiber of the invention.

[0038] The invention proposes a dispersion-shifted single-mode optical fiber having a large effective area, typically larger than 60 μm², or even larger than 70 μm², low bending losses, and a zero dispersion wavelength λ₀lying in the range 1400 nm to 1480 nm. A λ₀value lying in the range 1450 nm to 1480 nm, e.g. in the vicinity of 1475 nm, is appropriate for wavelength division multiplexed transmissions.

[0039] Advantageously, the cutoff wavelength λ_Cis chosen such that the fiber of the invention is single-mode in theory, or at least single-mode in fiber, i.e. in the definition given in ITU-T G.650 of the fiber λ_C.

[0040] If, in addition, provision is made for the sensitivity to microbending to be less than or comparable with that of the fiber sold under reference G652 by the Applicant, then the fiber is guaranteed to have good cablability. Its cablability is particularly good if the attenuation caused by bending as measured conventionally over 100 turns of a fiber wound on a reel having a radius of 30 mm is less than 0.5 dB, as proposed in Recommendation ITU-T G.650, or, even better, less than 0.05 dB.

[0041] FIG. 1 shows a diagram of an index profile 10 which can be used to obtain the characteristics of the invention, in a first embodiment. The index profile shown in FIG. 1 is an index profile having a central trough with an outer ring. Starting from the center of the fiber and going towards the cladding, the profile has a central portion 11 of substantially constant index to a radius a₁The index n₁of this central portion is less than or equal to the index n_Sof the silica of the cladding represented by the portion 15 of the profile 10. In the embodiment shown in FIG. 1, the difference Δn₁between the index n₁and the index n_Sof the cladding is equal to −3.8×10⁻³, and the radius a₁is equal to 1.84 μm. Index values Δn₁in a range of ±10% around said value are appropriate; the value of the radius a₁may vary without any problem by ±5% about the proposed value.

[0042] Around the central portion 11 of index lower than that of the cladding 15, the fiber of the invention has an inner ring 12 of index n₂greater than the index of the cladding, from radius a, to radius a₂. In the embodiment shown in FIG. 1, Δn₂is equal to 11.35×10⁻³and the inner ring 12 extends over a thickness of 2.04 μm. More generally, values for Δn₂lying in a range of ±10% around said value are suitable, and the thickness of the inner ring 12 is preferably such that the ratio a₁/a₂can vary by ±5% relative to the reference value of 1.84/3.88=0.47 proposed in the example.

[0043] Around the inner ring 12, the fiber has an outer ring 14 of index n₄greater than that of the cladding 15, which ring is separated from the inner ring 12 by an annular portion of index n₃less than or equal to the index of the cladding 15. In the embodiment shown in FIG. 1, the annular portion 13 has an index Δn₃of −5.7×10⁻³from radius a₂to radius a₃. As in this example, the index Δn₃of the annular portion 13 of the fiber of the invention is preferably less than or equal to the index Δn₁of the central portion 11 of the fiber. Advantageously, it varies over a range of ±10% around the value −5.7×10⁻³proposed in the example of the figure. The thickness a₃−a₂between the inner ring 12 and the outer ring 14 is preferably about 3.45 μm; variations in a range of ±5% are acceptable.

[0044] In the example shown in FIG. 1, the outer ring 14 has an index Δn₄of 5.55×10⁻³; values within a range of ±10% around this index are suitable. The thickness a₄−a₃of the outer ring 14 preferably lies within a range of ±10% around the value of 1.46 μm given in the example of the figure.

[0045] Within the range of the channels of a wavelength multiplex, i.e. typically in the range 1500 nm to 1600 nm, this choice of index profile guarantees a shallow chromatic dispersion gradient and a chromatic dispersion that makes it possible to avoid four-wave mixing; it is thus possible to reduce the distance between repeaters in a transmission system that uses the optical fiber of the invention as the transmission medium, and to reduce the problem of chromatic dispersion compensation.

[0046] In the example of FIG. 1, the effective area is 66 μm²at 1550 nm, and 63 μm²at 1480 nm, the dispersion gradient is 0.057 ps/nm².km, at 1550 nm. The chromatic dispersion is equal to 4.6 ps/nm.km at 1550 nm. It is zero for a value of 1475 nm.

[0047] The attenuation in the fiber is about 0.23 dB/km. This value makes it possible to transmit over long lengths, and typically guarantees a length of 100 km between the repeaters of a transmission system.

[0048] With the index profile of FIG. 1, it is also possible to obtain bending losses of less than 10⁻⁴dB for 100 turns of fiber about a reel having a radius of 30 mm, i.e. values of about 10⁻⁶dB/m, for wavelengths in the vicinity of 1550 nm. The sensitivity to microbending is half that of the above-mentioned fiber G652, for wavelengths in the vicinity of 1550 nm.

[0049] The fiber of FIG. 1 can be manufactured by the person skilled in the art by means of known techniques, such as modified chemical vapor deposition (MCVD) or other techniques commonly used to manufacture optical fibers.

[0050] The profile of FIG. 1 constitutes an example making it possible to implement the invention. Other profiles may make it possible to reach the gradient and dispersion values proposed in the invention. By way of additional examples, the parameters of other profiles are given below, as are the values of the characteristics obtained. These profiles are analogous in appearance to the profile of FIG. 1, but they have different numerical values.

[0051] Table 1 gives possible index and radius values for these other profiles. The radii are given in micrometers, and the indices in Δn (relative values). Each row of the table corresponds to a possible profile.

[0052] These various profiles make it possible to obtain the characteristics indicated in Table 2 on the corresponding rows.

[0053] Table 2 gives the theoretical cutoff wavelength λ_Cin nm, the zero chromatic dispersion wavelength λ₀in nm, the dispersion gradient at 1550 nm dC/dλ in ps/nm².km, the chromatic dispersion at 1550 nm DC in ps/nm.km, the mode diameter W₀₂at 1550 nm in μm, the effective area A_effat 1550 nm in μm², the sensitivity to bending S_C(i.e. bending losses) for 100 turns on a radius of 30 mm in dB, and the sensitivity to microbending at 1550 nm Sμ_C. This sensitivity to microbending is given in proportion to that of the above-mentioned fiber G 652.

[0054] The values in Table 2 show that the profiles defined in Table 1 make it possible to obtain the characteristics of the invention. As in the example shown in FIG. 1, variations around the reference values proposed in Table 1 remain possible; variations in the indices Δn of ±1O%, in the radius a₁of ±10%, in the ratio a₂/a₁of ±10%, in the thickness a₃−a₂of ±10%, and in the thickness a₄−a₃of ±10% give suitable results.

[0055] FIG. 2 is a diagram showing an index profile 20 which can be used to obtain the characteristics of the invention, in a second embodiment. The index profile of FIG. 2 is an index profile of the “trapezium-and-ring” type. Starting from the center of the fiber and going towards the cladding, this profile has a central portion 21 in the form of a trapezium of small radius a₁and of large radius a₂. The maximum index n₁of this central portion 21 is greater than or equal to the index n_Sof the silica of the cladding represented by the portion 24 of the profile 20.

[0056] Around said central portion 21 of index n₂greater than that of the cladding 24, the fiber of the invention has a ring 23 of index n₃greater than that of the cladding, and less than n₁, from radius a₃to radius a₄.

[0057] Between the central portion 21 and the ring 23, the fiber has an annular portion 22 of index n₂less than n₁and less than n₃.

[0058] The type of profile shown in FIG. 2 offers the same advantages as that shown in FIG. 1.

[0059] The fiber shown in FIG. 2 can be manufactured by the person skilled in the art by means of known techniques, such as MCVD or other techniques in common use for manufacturing optical fibers.

[0060] By way of examples, possible parameters are given below for the profile of FIG. 2 making it possible to obtain a fiber of the invention, as are the values of the characteristics obtained.

[0061] Table 3 gives the possible radius and index values for the profiles. The radii are given in micrometers, and the indices are given in An. Each row of the table corresponds to a possible profile.

[0062] The various profiles make it possible to obtain the characteristics indicated in Table 4 on the corresponding rows (same headings as in Table 2).

[0063] The values of Table 4 show that the profiles defined in Table 3 make it possible to obtain the characteristics of the invention.

[0064] Naturally, the present invention is not limited to the embodiments described and shown, but rather numerous variants of it are accessible to the person skilled in the art. In particular it is possible to obtain the characteristics of the invention with families of profiles other than those described with reference to FIGS. 1 and2.