Optical waveguides – Optical fiber waveguide with cladding – With graded index core or cladding
Patent
1996-05-02
1997-08-19
Palmer, Phan T. H.
Optical waveguides
Optical fiber waveguide with cladding
With graded index core or cladding
385126, G02B 618
Patent
active
056596499
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a dispersion-shifted monomode optical fiber.
BACKGROUND OF THE INVENTION
So-called "dispersion-shifted" monomode optical fibers are such that at the transmission wavelength at which they are used, which is generally other than 1.3 .mu.m (the wavelength at which the dispersion of silica is substantially zero), the chromatic dispersion of the transmitted wave is substantially zero, i.e. the non-zero chromatic dispersion of the silica is compensated (hence the use of the term "offset") in particular by an increase in the index difference .DELTA.n between the core and the optical cladding.
The transmission wavelength presently selected for line fibers, i.e. fibers designed to perform long distance transmission, e.g. for transoceanic connections, is substantially equal to 1.55 .mu.m. It is at this wavelength that it is possible to obtain minimum transmission attenuation of light, of the order of 0.2 dB/km.
Thus, in the context of the present invention, the fibers under consideration are designed to be used at a wavelength of 1.55 .mu.m since that is the most efficient for transmission.
Also, it is well known that the bandwidth of monomode optical fibers is much greater than that of multimode fibers, which is why present and future developments of lines for long distance transmission concentrate on monomode optical fibers.
Consequently, the present invention applies most particularly to dispersion-shifted monomode optical fibers designed to be used at a wavelength substantially equal to 1.55 .mu.m.
More precisely, the invention relates to such optical fibers in which curvature losses do not exceed 0.005 dB/m when the radius of curvature is 30 mm. It is well known that such a limitation on curvature losses is necessary to ensure that the optical fiber operates under proper transmission conditions.
At present, numerous dispersion-shifted monomode optical fiber profiles are being studied and they are widely described in the literature.
The simplest known profiles referred to as "step", "trapezium", or "triangle", are such that the refractive index in the core varies as a function of distance from the axis of the fiber, so that when shown as a function of said distance the index appears as a curve constituting respectively a rectangle, a trapezium, or a triangle, while the index in the optical cladding surrounding the core is constant and less than that of the core.
A "pedestal" profile is also known in which the central portion forming the "inner" core of the optical fiber is surrounded successively by an "outer" core of refractive index lower than that of the inner core, and then by optical cladding of index lower than that of the outer core.
Also known is a profile referred to as being of the "trapezium and central ring" type which is shown very diagrammatically in FIG. 1, where there can be seen the curve representing the refractive index n in the fiber as a function of distance d from the axis of the fiber. In that profile, the core C comprises: index varies in such a manner as to give the curve the form of a trapezium, and in the limit, of a triangle or of a rectangle; surrounding the central portion 10; and (0<h<1), which is constant for example, greater than n.sub.s, and less than n.sub.s +.DELTA.n.
The layer 12 is surrounded by a cladding layer G of index equal to n.sub.s.
In practice, the term "trapezium" when used for the central portion 10 of the core C covers the limiting shapes of a triangle and of a rectangle.
Finally, as described in an article entitled "Transmission characteristics of a coaxial optical fiber line", published in Journal of Lightwave Technology, Vol. 11, No. 11, November 1993, a profile is known of the "buried central hollow" type which is shown very diagrammatically in FIG. 2, where there can be seen the curve of refractive index n in the optical fiber as a function of distance d from the axis of the fiber. In that profile, the core C' comprises a central portion 20 of minimum index n.sub.s +h.DELTA.n (h<0) surrounded by a layer 21 of inde
REFERENCES:
patent: 4406518 (1983-09-01), Matsumura et al.
patent: 4755022 (1988-07-01), Ohashi et al.
G. Mahlke et al, "Lichtwellenleiterkabel" Siemens AG, 1986 Berlin Munchen pp. 49-51.
S. Mahmoud et al, "Transmission characteristics of a coaxial optical fiber line", Journal of Lightwave Technology, vol. 11, Nov. 1993, NY US, pp. 1717-1720.
F. Ruhl, "Cutoff decomposition on multiply clad fibers", Journal of Lightwave Technology, vol. 4, No. 11, Nov. 1986, New York, US, pp. 1651-1654.
V.A. Bhagavatuala et al, "Bend-Optimized Dispersion-Shifted Single-Mode Designs", Journal of Lightwave Technology, vol. LT-3, No. 5, 1985 New York, pp. 954-957 no month.
Vengsarkar A.M. et al, "Dispersion-compensating sigle-mode fibers: efficient designs for first-and second-order compensation", Optics Letters, vol. 18, No. 11 1 Jun. 1993, pp. 924-926, XP000304592 no month, no year.
Audouin Olivier
Hamaide Jean-Pierre
Nouchi Pascale
Sansonetti Pierre
Alcatel Fibres Optiques
Palmer Phan T. H.
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