Optical waveguides – Optical fiber waveguide with cladding
Patent
1996-08-02
1997-12-30
Bovernick, Rodney B.
Optical waveguides
Optical fiber waveguide with cladding
385124, 385127, 385128, G02B 602
Patent
active
057039860
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention concerns a monomode optical fiber.
An optical fiber is an optical waveguide consisting of a central part, referred to as an optical core, whose purpose is to guide the majority of the light waves, surrounded by a peripheral part, referred to as an optical sheath. In general, the optical sheath is surrounded by a protective covering made of synthetic material (a resin, for example) with a thickness of approximately 50 .mu.m.
The transmission wavelength currently chosen for optical fibers is 1.3 or 1.55 .mu.m. Indeed, it is at such wavelengths that it is possible to obtain a minimum attenuation in the transmission of light, inferior to 0.4 dB/km.
Thus, within the scope of the present invention, the fibers considered are intended to be used at a wavelength of 1.3 or 1.55 .mu.m, the most efficient for transmission.
Furthermore, it is well known that monomode optical fibers have a much larger bandwidth than multimode fibers, and it is for this reason that current and future developments in the field of optical fibers are focused on monomode optical fibers.
As a result, the present invention is particularly related to monomode optical fibers intended to be used at a wavelength of 1.3 or 1.55 .mu.m.
The monomode optical fibers currently used have a core diameter which varies from 8 to 10 .mu.m; the diameter of their optical sheath is 125 .mu.m, identical to that of multimode optical fibers. Because of the relatively significant diameter of the assembly made up of the optical core and sheath, the cost in silica to achieve the preform from which a standard monomode optical fiber is drawn is high. To reduce this cost, it has already been suggested to reduce the diameter of the core to approximately 2.5 to 3 .mu.m, while maintaining the diameter of the sheath at 125 .mu.m.
However, such a reduction in the diameter of the core does not lead to a sufficient reduction in cost.
Furthermore, since the diameter of the optical sheath remains unchanged, the transmission capacity of a fiber-optical cable containing such fibers is not increased.
SUMMARY OF THE INVENTION
One goal of the present invention is further to achieve a monomode optical fiber having a manufacturing cost inferior to that of known monomode optical fibers.
Another goal of the present invention is to decrease the dimensions of the monomode optical fibers so as to increase the capacity of fiber-optical cables.
For this purpose, the present invention proposes a monomode optical fiber comprising an optical core for guiding the majority of light waves, surrounded by an optical sheath, the difference between the maximum refraction coefficient of said core and that of said optical sheath being indicated as .DELTA.n and the radius of said core being indicated as a, characterized in that: the formula: ##EQU4## where k=2.pi./.lambda., .lambda. being (in .mu.m) the wavelength of the transmission, and where n.sub.g is the coefficient of said optical sheath, equal within 3.times.10.sup.-3 to that of pure silica, .mu.m, by the formula: ##EQU5## where ##EQU6##
By choosing the characteristic parameters of the optical fiber, i.e. a, .DELTA.n and .phi., in the manner indicated above, it is possible to achieve a monomode optical fiber with transmission wavelengths approaching 1.3 or 1.55 .mu.m, which has a reduced sheath diameter with respect to the fibers of prior art, while displaying an attenuation and a sensitivity to microcurvatures comparable to those of standard fibers.
It can therefore be observed that, according to the invention, it is not necessary for a monomode optical fiber to have an optical sheath with a diameter in the order of 125 .mu.m. Such a diameter was chosen for the original design of monomode fibers in order to obtain monomode and multimode fibers with the same dimensions (for multimode fibers, which have a core diameter of up to 85 .mu.m, the diameter of the optical sheath is close to 125 .mu.m so as to obtain the guidance required while preserving the flexibility of the fiber).
The invention thus
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Auge Jacques
Boniort Jean-Yves
Brehm Claude
Nouchi Pascale
ALCATEL N.V.
Bovernick Rodney B.
Sanghavi Hemang
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