FR2782392A1 - Shifted dispersion single mode fibre optic index doping technique; has central triangular, rectangular or trapezoidal profile and outer sheath region with specific inner doped region and higher doped outer region sections - Google Patents

Abstract

The fibre optic cable construction has a central core and an outer sleeve with a doped index profile. The core of the cable has a trapezoidal, triangular or rectangular index profile (n1). The outer ring has a first doped index region (n2). There is a second index level in the outer section with an index level (n3) over a region above that of the first outer ring doping region.

Description

l

  OFFSET MONOMODE FIBER OPTICAL INCLUDING A RING

OUTSIDE

  The present invention relates to a single mode optical fiber with offset dispersion.

  The so-called dispersion-shifted fibers (DSF) are such that at the transmission wavelength at which they are used. which is generally different from the wavelength of 1.3 Ltm for which the dispersion of the silica is substantially zero, the chromatic dispersion of the transmitted wave is substantially zero; that is to say that the chromatic dispersion of the silica, which is not zero, is compensated - hence the use of the term shifted - by an increase in the difference in index An between the core of the fiber and the optical cladding . This index difference makes it possible to offset the wavelength for which the chromatic dispersion is zero; it is obtained by the introduction of dopants into the fiber, during the manufacture of the latter, for example by a process of MCVD known per se, and which is not described in more detail here. A typical value for the difference in index between the cladding and the core of the fiber is 24.10-3; the increase in the index in silica can be obtained by using germanium as dopant. The terms sheath and core are known to those skilled in the art; conventionally the part which extends to a diameter is called a sheath

  of 125 im. The heart corresponds to the part where approximately 70% of the light energy propagates.

  These single-mode fibers must also preferably have characteristics which correspond to the requirements of cable manufacturers and system manufacturers: on the one hand, small mode diameters, and good "cableability", resulting in a bending capacity of the fiber, and low attenuation; on the other hand, a mode diameter as large as possible, a large effective area, an appropriate value of the wavelength Xo of zero dispersion. The use of these fibers for transmission systems with wavelength multiplexing makes the constraints even more severe. It would be preferable to use fiber having an X0 value greater than 1565 nm or 1585 nm; it is thus possible to avoid the use of dispersion compensated fiber (DCF or "dispersion compensating fiber"), which has the drawback of promoting non-linear effects, of being highly attenuating and difficult to cable. However, existing fibers with such values of -0

  have a weak effective surface, or a strong attenuation around 1550 nm.

  3) Many index profiles have been proposed for such monomode optical fibers with offset dispersion. The index profile is generally qualified as a function of the shape of the curve representing the index as a function of the radius of the fiber. We thus speak of index profile in "rung", in "trapezoid" or in "triangle" for curves representative of the variation of the index as a function of the radius which have respective forms of rung, trapezoid or triangle. These curves are generally representative of the theoretical or target profile of the fiber, the

  fiber manufacturing constraints which can lead to a significantly different profile.

  The first offset dispersion optical fibers were of the index hopping, triangle, aussian or AC power type. The characteristics of these fibers are no longer considered to be sufficient, in particular because of a small mode diameter and their sensitivity to curvatures, even though these fibers have attenuation values of less than 0.20 dB / km at 1,550 nm. A new family of fibers has therefore been developed: fibers with an index profile in Gaussian with

pedestal, or "dual shape core".

  Thus, patent application EP-A-0 789 257 describes a fiber with a Gaussian index profile with pedestal. The profile of this fiber is shown in Figure 1. This fiber has a

  mode diameter of 9 S.m, and a wavelength. 0 of the order of 1582 nm.

  Patent US-A-5 278 931 describes an optical fiber having an improved sensitivity to curvature, with a core region having an index higher than that of the rest of the fiber, and a reduced diffusion tail at the interface between the sheath core. This document suggests the presence

  optional of a ring around the part of the heart having an index higher than that of the sheath.

  US-A-4,641,917 contains a similar teaching, suggesting the presence of a higher index ring appropriately chosen to lower the cut-off wavelength or to

  obtain a fiber less sensitive to bending.

  These various known solutions do not make it possible, while retaining the characteristics adapted to good cableability, to obtain a value of wavelength −0 high, or which can

be freely chosen.

  The invention proposes a solution which makes it possible to maintain a large effective area Aef, typically beyond 70! Tm2; this limits non-linear effects. The fiber according to the invention also makes it possible to obtain a low attenuation, preferably less than 0.2 dB / km. It also makes it possible to obtain, for all these parameters, a wavelength ko of zero chromatic dispersion advantageously greater than 1585 nm; which makes it possible to avoid or limit, if the fiber is used for transmissions in multiplexing of wavelengths, the four-wave mixture

  and the use of dispersion compensated fibers.

  In other words, the invention proposes a method for increasing the value X0 of the wavelength for which the chromatic dispersion is canceled out in a single-mode optical fiber with offset dispersion, having a cladding with a given index (ns) , a fiber core with a Gaussian index profile with pedestal, and a mode diameter advantageously greater than 8 mt, comprising the addition in the index profile of the fiber core of an external annular part of index

greater than that of the sheath.

  The invention thus relates to a single-mode optical fiber with offset dispersion, having - a cladding with a given index, - a fiber core having a Gaussian index profile with pedestal, characterized in that the index profile of the fiber has an outer index ring

superior Li that of the sheath.

  Advantageously, the index of said ring is greater than or equal to that of the pedestal.

  In one embodiment, said ring has a thickness of between 0.3 and 0.8 times

  the radius of the part of the heart in Gaussian.

  Preferably, the index of said pedestal is greater than that of the sheath.

  Advantageously, the part of the fiber between said pedestal and said ring has a thickness of between 0.3 and 0.8 times the radius of the part of the heart in gaussian direction. The fiber can also have an annular part between said external ring and the sheath. index less than or equal to the index of said outer ring and the index of the sheath. The fiber according to the invention is generally and preferably such that the value of the wavelength for which the chromatic dispersion is canceled out is advantageously greater than 1565 nm,

  preferably greater than 1585 nm.

  Advantageously, said fiber has an effective surface area greater than 70 μm 2.

  Preferably, it has an attenuation of less than 0.2 dB / km for a value of

wavelength of 1550 nm.

  The invention therefore proposes to add a ring on the existing fiber profiles, of the Gaussian type with pedestal. It keeps a low attenuation, an effective surface

  important, while obtaining high kX0 values, typically greater than 1565 nm.

  The invention generally makes it possible to obtain for the fiber a wavelength ko of at least 1565 nm and the following characteristics: - an effective surface greater than 70 p, m2; and

  - an attenuation of less than 0.2 dB / km for a wavelength value of 1550 nm.

  Other characteristics and advantages of the invention will appear on reading the description

  which follows of various embodiments of the invention, given by way of example and with reference to the appended drawings which show: - Figure 1 a schematic representation of the set point profile of a single mode fiber with known offset dispersion; - Figure 2 a schematic representation of the index profile of a fiber according to the invention

  Figure I has already been commented on.

Example

  The invention proposes to add a ring in a Gaussian index profile with pedestal.

  Figure 2 shows a schematic representation of a fiber according to the invention. The fiber of FIG. 2 has a fiber core with a part of Gaussian, for radii less than a, in which the index varies between n i = I 1.10- 'and ni = 1.5.10-3. a, is between 2.5 and 4 gm. Around this gaussian, the fiber has a pedestal, of index or substantially constant, between the values of

  rays ai and a3; a3 is two to three times ai.

  Around this pedestal, the invention has a ring; the latter is separated from the pedestal by an annular part, between the spokes a3 and a4, of index n4 lower than that of the pedestal, and for example of index lower than that of the sheath ns; a value of the order of 0.5.al for a4 - a-, tprovides good results. More generally, a value between 0.3 and 0.8 times aI is appropriate. We could also

  choose an annular part of index n4 greater than or equal to that of the sheath ns.

  The ring is then provided according to the invention, which has an index n3 greater than that of the sheath ns and that of the annular part n4. This ring has for example an index n3 of 4.10-3, and extends over the radii between a4 and a5, with a5 - a4 having a value of the order of 0.5.a. More

  generally a value of a5 - a4 between 0.3 and 0.8 times al is appropriate.

  Around this ring extends the sheath with index ns; one can also provide just before the sheath, in particular if this facilitates the manufacture of the fiber, a second annular part of index n5 lower than that of the sheath ns, and of index n5 lower than the index n3 of the ring . This index n5 can

  be greater or less than the index n4 between the pedestal and the ring.

  The invention improves the transmission characteristics of the fibers of the prior art; because it allows however to increase the value X0 for which the chromatic dispersion is zero: for comparison, one can consider an optical fiber having a profile of classical index in Gaussian with pedestal, like that of figure 7 of EP- A-0 789 257. Such a fiber has the following characteristics ko = 1582 nm; mode diameter 10.1 µm;

  cut-off wavelength: 1690 nm.

  In comparison, according to the invention, the addition of a ring as on the profile of FIG. 2 makes it possible to reduce the attenuation by 0.01 dB / km at 1550 nm, to reduce the cut-off wavelength and

  to increase the effective area by 10.m2.

  In the example presented, the indices n4 and n5 are equal to the index ns of silica and equal to each other. The invention can also be implemented with values of the indices n4 and n5 different

  from each other, and greater than or equal to the index of the sheath nS.

  Of course, the present invention is not limited to the examples and embodiments described and shown, but it is susceptible of numerous variants accessible to the person skilled in the art.

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Claims (10)

  1.- Single-mode optical fiber with offset dispersion, having - a cladding with a given index (ns), - a fiber core having a Gaussian index profile with pedestal, characterized in that the index profile of the fiber has an outer index ring   (n3) higher than that of the sheath (ns).   2. Fiber according to claim I such that the index (n3) of said ring is greater than or equal to that (n2) of the pedestal.   3.- Fiber according to one of claims I or 2 such that the index (n2) of said pedestal is   greater than that (ns) of the sheath.   4.- fiber according to one of claims I to 3 such that said ring has a thickness (a5 -   a4) between 0.3 and 0.8 times the radius (al) of the part of the heart in Gaussian.   5. Fiber according to one of claims 1 to 4 such that the index (n4) between said pedestal and said   ring is less than or equal to that (ns) of the sheath.   6. Fiber according to one of claims I to 5 such that the part of the fiber between said pedestal   and said ring has a thickness (a4 - a3) of between 0.3 and 0.8 times the radius (al) of the part from the heart in Gaussian.   7. Fiber according to one of claims 1 to 6 as it has between said outer ring and   the sheath an annular part of index (nc) less than or equal to the index (n3) of said outer ring and the sheath index (ns).   8.- Fiber according to one of claims I to 7 such that the value of the wavelength for   which the chromatic dispersion is canceled is greater than 1565 nm.   9. Fiber according to claim 8 such that the value of the wavelength for which the   chromatic dispersion vanishes is greater than 1585 nm.   10.- Fiber according to one of claims 1 to 9 as it has an effective surface more than 70 gm2.   I 1I.- Fiber according to one of claims I to 10 such that it has a lower attenuation   at 0.2 dB / km for a wavelength value of 1550 nm.