Monomodal dispersion-shifted optical guide with large effective mode surface

The present invention relates generally to a single-mode optical guide, such as optical fiber, typically intended to be used as a long-distance transmission link. The invention finds application in particular in submarine connections. More specifically, the invention relates to a dispersion-shifted single-mode optical fiber with a large effective mode area.

A single-mode optical fiber called offset dispersion is such that at the transmission wavelength at which it is used, the chromatic dispersion of the transmitted wave is substantially zero. It can be shown that in silica, at the wavelength X equal to 1.27 µm, and only at this wavelength, the chromatic dispersion is zero. However, for a fiber intended to establish a long distance link, it is more advantageous to transmit at a wavelength λ = 1.55 μm since this then results in a minimum attenuation of light transmission. For λ = 1.55 μm, it is in return necessary to compensate for the non-zero chromatic dispersion of the silica in particular by an appropriate structure of the core and cladding of the optical fiber. The terminology “optical fiber with offset dispersion” is used to define a fiber in which the chromatic dispersion is made substantially zero by compensating for the intrinsic chromatic dispersion of the silica appearing for a wavelength different from 1.27 μm.

The effective surface of Seff mode of a fiber is defined by:<maths id="math0001" num=""><img file="EP0753771A2_D0001.tif" /></maths>    where Ψ (r) is a scalar function of field distribution in the fiber and r denotes the polar distance, in polar coordinates, of a point in a coordinate system transverse to and centered with respect to the fiber. It is shown that the quality of transmission in the fiber is all the better the larger the effective area of ​​Seff mode. Thus, to improve the quality of transmission in the fiber, which has the advantage of reducing the number of repeaters on the link and therefore the cost of installation, it is particularly advantageous to have the highest effective surface area. possible.

According to the prior art, two index profiles exist for the production of a single-mode fiber with offset dispersion and large effective mode area. These profiles are called respectively "trapezium profile + ring" and "profile with buried central area + ring". A profile defines an index level diagram in the core and sheath of the fiber as a function of the polar distance from the central point of the fiber. The level of index is a function only of this distance and does not depend on the angular coordinate, the central axis of the fiber defining an axis of revolution for each layer of the core and of the cladding.

The profile called "trapezoid + ring", described in French patent application No. 94-10615, defines a fiber comprising:<ul id="ul0001" list-style="dash" compact="compact"><li>a sheath having a predetermined refractive index, and</li><li>a core comprising, on the one hand, a central zone having a refractive index which is greater than the refractive index of the cladding and which decreases from a given polar distance value to reach the refractive index of the sheath and, on the other hand, a ring peripheral to the central area, which has a refractive index greater than the refractive index of the cladding and which is separated from this central zone by a niche zone of refractive index equal to the refractive index of the cladding.</li></ul>

This "trapezoid + ring" profile offers an effective Seff mode surface of up to 85 µm^2. The fibers defining such a profile nevertheless have the major drawback of being subject to losses by micro-bends.

The profile called "central buried zone + ring", described in the document entitled "Transmission Characteristics of a Coaxial Optical Fiber Line" published in the "Journal of lightwave technology, vol.11, n ° 11, November 1993, pages 1717- 1720 ", defines a fiber comprising:<ul id="ul0002" list-style="dash" compact="compact"><li>a sheath having a predetermined refractive index, and</li><li>a core comprising, on the one hand, a central zone having a refractive index lower than the refractive index of the sheath and, on the other hand, an annular zone which is peripheral to the central zone and which has an index of refraction greater than the refraction index of the sheath.</li></ul>

This profile with "buried central zone + ring" offers an effective surface of Seff mode up to 90 µm². The fibers using such a profile nevertheless do not appear optimal due to the significant intrinsic losses which are induced by the large index differences.

Thus, according to the prior art, the two types of fiber with offset dispersion and large effective mode area are not entirely satisfactory with regard to their behavior in terms of attenuation, whether it be losses by micro-bends or intrinsic losses. . The invention therefore aims to remedy these drawbacks by providing an optical fiber with offset dispersion and large effective mode area which has properties that are entirely satisfactory for all the attenuation parameters.

To this end, a single-mode optical guide comprising:<ul id="ul0003" list-style="dash" compact="compact"><li>an optical core, and</li><li>a sheath enveloping the optical core and defined by a predetermined refractive index,<ul id="ul0004" list-style="none" compact="compact"><li>the core comprising, on the one hand, a central zone having a refractive index lower than said predetermined refractive index and, on the other hand, an annular zone which is peripheral to said central zone and which has a refractive index greater than said index predetermined refraction,</li><li>is characterized, according to the invention, in that the core further comprises, between the peripheral annular zone and the sheath, at least one annular compound zone which consists of two successive annular zones having respectively lower and upper refractive indices said predetermined refractive index.</li></ul></li></ul>

It can be provided that in at least one of the central and annular zones, the refractive index is variable as a function of a polar distance defined relative to the center of the guide.

Other characteristics and advantages of the present invention will appear more clearly on reading the following description, with reference to the corresponding appended drawings, in which:<ul id="ul0005" list-style="dash" compact="compact"><li>Figure 1 shows a cross-sectional view of a single mode optical guide according to the invention;</li><li>Figure 2 is, according to a first variant of the invention, an index profile diagram according to the invention which is associated with the guide shown in Figure 1; and</li><li>FIG. 3 is, according to a second variant of the invention, an index profile diagram according to the invention which is associated with the guide shown in FIG. 1.</li></ul>

With reference to FIG. 1, a single-mode optical guide, or optical fiber, according to the invention, shows in cross section an optical sheath 1 enveloping an optical core 2. The optical core 2 consists of a circular central zone 21 enveloped by three successive concentric annular zones 22, 23 and 24. The optical fiber results from the pulling of material from a preform obtained by successive deposits of layers of suitably doped silica. According to a variant of the invention illustrated in FIG. 2 representing the level of index n as a function of the distance r in polar coordinates with respect to the center of the fiber, the central zone 21 has a constant refractive index lower than the sheath 1 refractive index and the first concentric annular zone 22, which is peripheral to the central zone, has a constant refractive index greater than the refractive index of sheath 1. Furthermore, between the annular peripheral zone 22 and the sheath 1, there is provided an annular compound zone consisting of the two successive annular concentric zones 23 and 24, called inner and outer, respectively having constant refractive indices lower and higher than the index sheath refraction 1. Thus, the invention differs from the prior art described in the aforementioned document "Transmission Characteristics of a Coaxial Optical Fiber Line" by this annular compound zone consisting of the two successive annular concentric zones interior and exterior 23 and 24 having respectively refractive indices lower and higher than the refractive index of the sheath 1. The physical property at the origin of the considerable advantage obtained by the invention is as follows. It is shown that the chromatic dispersion M of an optical fiber is a good approximation given by the sum of the material dispersion M1, equal to the dispersion obtained for a plane wave in a homogeneous medium of given index, and the guide dispersion M2, equal to the dispersion obtained when the dependence of the index values ​​as a function of the wavelength is neglected. It says:<maths id="math0002" num=""><math display="block"><mrow><mtext>M = M1 + M2.</mtext></mrow></math><img file="EP0753771A2_D0002.tif" /></maths>

It is further shown that:<maths id="math0003" num=""><math display="block"><mrow><msup><mrow><mtext>M1 = -λ / c. (∂</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><msup><mrow><mtext>n / ∂λ</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>), and</mtext></mrow></math><img file="EP0753771A2_D0003.tif" /></maths><maths id="math0004" num=""><math display="block"><mrow><msup><mrow><mtext>M2 = - [Δn / λc] .V (∂</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><msup><mrow><mtext>(VB) / ∂V</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>),</mtext></mrow></math><img file="EP0753771A2_D0004.tif" /></maths>    where λ is the wavelength of the light wave substantially equal to 1.55 µm, c is the speed of light, n is an index value variable, Δn is the difference in index between the index maximum of the core and the cladding index and V (∂²(VB) / ∂V²) is a term characterizing the guide dispersion, V being the normalized frequency and B the normalized effective index, the effective index being the index "seen" by the light wave propagating in the heart. To give the fiber zero chromatic dispersion, it is necessary to make the value taken by the sum zero (M1 + M2), and therefore to compensate the value M1 with a value M2 of the same absolute value and of opposite sign. In the aforementioned document "Transmission Characteristics of a Coaxial Optical Fiber Line", the optical fiber described is such that the value taken by V (∂²(VB) / ∂V²) in equality:<maths id="math0005" num=""><math display="block"><mrow><msup><mrow><mtext>M2 = - [Δn / λc] .V (∂</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><msup><mrow><mtext>(VB) / ∂V</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>),</mtext></mrow></math><img file="EP0753771A2_D0005.tif" /></maths> is low, which requires increasing Δn in order to compensate for the value of M1 with the value of M2. However, a significant difference in index between the core and the sheath of the optical fiber induces significant intrinsic losses.

In the context of the invention, the value taken by V (∂²(VB) / ∂V²) is high enough not to resort to a significant difference in Δn index between the core and sheath of the fiber. This results in much smaller intrinsic losses.

By way of nonlimiting example, an embodiment of an optical guide according to the invention can be given. In this embodiment, as illustrated in FIG. 2, the notations Δn have been chosen._i and r_i to respectively designate an index variation relative to the index of the sheath 1, and a radius measured at an end point of each of the zones forming the core 2, i designating the index identifying each of the zones 21, 22 , 23 and 24. The production gives:<maths id="math0006" num=""><math display="block"><mrow><msub><mrow><mtext>Δn</mtext></mrow><mrow><mtext>21</mtext></mrow></msub><msup><mrow><mtext>=-5,5.10</mtext></mrow><mrow><mtext>-3</mtext></mrow></msup><msub><mrow><mtext>, Δn</mtext></mrow><mrow><mtext>22</mtext></mrow></msub><msup><mrow><mtext>=11.10</mtext></mrow><mrow><mtext>-3</mtext></mrow></msup><msub><mrow><mtext>, Δn</mtext></mrow><mrow><mtext>23</mtext></mrow></msub><msup><mrow><mtext>=-5,5.10</mtext></mrow><mrow><mtext>-3</mtext></mrow></msup><msub><mrow><mtext>, Δn</mtext></mrow><mrow><mtext>24</mtext></mrow></msub><msup><mrow><mtext>=5,5.10</mtext></mrow><mrow><mtext>-3</mtext></mrow></msup><mtext>,</mtext></mrow></math><img file="EP0753771A2_D0006.tif" /></maths> and<maths id="math0007" num=""><math display="block"><mrow><msub><mrow><mtext>r</mtext></mrow><mrow><mtext>21</mtext></mrow></msub><msub><mrow><mtext>= 2.1 µm, r</mtext></mrow><mrow><mtext>22</mtext></mrow></msub><msub><mrow><mtext>= 4.2 µm, r</mtext></mrow><mrow><mtext>23</mtext></mrow></msub><msub><mrow><mtext>= 7.06 µm and r</mtext></mrow><mrow><mtext>24</mtext></mrow></msub><mtext>= 8.4 µm</mtext></mrow></math><img file="EP0753771A2_D0007.tif" /></maths>

The index profile proposed by the invention gives the optical fiber an effective mode surface substantially equal to 85 μm².

In a variant of FIG. 2, the index profile shown in FIG. 3 is such that in each of the two zones 22 and 24, the refractive index n is variable, in a triangle, as a function of the polar distance r while remaining greater than the refraction index of the sheath 1. This variant is not limiting, and the invention provides that the refractive index in at least one zone 21, 22, 23 and 24 can be variable as a function of the polar distance r according to a given geometry such as triangle, rhombus , ogive curve, etc. The following characteristics are nevertheless observed. In zones 21 and 23, the refractive index of the core remains higher than the refractive index of the sheath 1. In zones 22 and 24, the refractive index of the core remains lower than the refractive index of the sheath 1.

Although the above description is limited to considering a single annular compound zone consisting of the two successive annular concentric zones 23 and 24 having respectively refractive indices lower and greater than the refractive index of the sheath 1, several successive annular compound zones may each be provided, each consisting of two inner and outer annular concentric zones respectively having refractive indices lower and greater than the refractive index of the sheath 1.

Another important advantage resulting from the invention is the low spectral slope of chromatic dispersion obtained, for example equal to 0.065 ps / nm²/ km in the example given above. This means that the chromatic dispersion M fluctuates little as a function of the wavelength λ used, which offers the possibility of multiplexing in wavelength, typically at wavelengths close to λ = 1.55 μm.