Optical waveguides – Optical fiber waveguide with cladding – With graded index core or cladding
Patent
1997-01-23
1999-09-21
Sanghavi, Hemang
Optical waveguides
Optical fiber waveguide with cladding
With graded index core or cladding
385126, G02B 622
Patent
active
059564482
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention refers to a single-mode optical waveguide of the type used in optical communication systems, particularly in the 1550 nanometer spectral region.
BACKGROUND OF THE INVENTION
Optical waveguides or optical fibers have been widely used in communications, having become the preferred wide band communication media. Due to providing an increase in transmission capacity, the single-mode optical waveguide has received special attention.
During tha manufacturing phase of an optical waveguide, especially those made out of silicate glass and, more specifically, during the transformation of the preform, the matrix glass is submitted to softening temperatures over 2000.degree. C., tensed and rapidly cooled generating, radially, internal stresses that give rise to structural defects that increase the waveguide optical loss (Ainslie, B. el al., 25, vol, 12, no. 2 British Telecom Technology, 1984)
The levels of internal stress on the optical waveguide increase with the stress applied during the drawing of said waveguide, the region of greatest internal stresses being the region between the core of the waveguide and the adjacent optical cladding region, since the thermal expansion coefficient and viscosity in these regions differ, originating stresses between these regions. Furthermore, these stress levels depend on the concentration of the dopants used in the glass composition of the optical waveguide, such as P.sub.2 O.sub.5, F and GeO.sub.2, during the construction of the refractive index profile.
It is known that when a certain region of an optical waveguide preform, with a lower viscosity than that of the cladding glass, is submitted to a drawing process, this region is put under compression. One of the known effects of submitting an optical waveguide to compression is the refractive index variation: the higher the level of compression, the higher the increase in refractive index (photoelastic effect).
For a given refractive index profile it is possible to control the viscosity and the thermal expansion index and, consequently, the internal of each of the core and cladding regions, through the adequate use of the materials during the chemical deposition step. This adequate combination of materials involves, for example, the control of the level of thermal expansion which will compensate or simply minimize the above-mentioned radial compression. This control of the thermal expansion can be obtained by means of a controlled incorporation of dopants, such as GeO.sub.2, F and P.sub.2 O.sub.5, together with the matrix glass SiO.sub.2.
In a known solution, utilizing waveguides with a step-index profile, a decrease of the residual stresses after the manufacture of the optical waveguide is obtained during the chemical processing of the waveguide, controlling the use of fluorine and germanium in the core and the cladding of the waveguide, with SiO.sub.2. This composition makes it possible to build structures where the viscosity is the same along the entire cross section of the waveguide, strongly reducing the stress problem. However, this methodology is only applicable to some optical waveguide manufacturing techniques.
Further to the problem of controlling internal stresses, other parameters relevant to the operation of optical waveguides must be observed, such as, low attenuation and chromatic dispersion, the latter associated to the form of the refractive index profile of the core.
In the efforts to broaden the single-mode optical waveguide bandwidth, the reduction in the chromatic dispersion of these optical waveguides at the operation region of the optical sources is of great importance.
Although the wavelength of 1330 nanometers presents, almost naturally, a low chromatic dispersion, the point of minimal attenuation of optical waveguides is at the spectral region of 1550 nanometers. Hence, solutions have been developed for obtaining optical waveguides presenting low chromatic dispersion at this wavelength. In order to obtain a chromatic dispersion near zero at this region, a
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Ninslie et al, "Monomode optical fibers with graded-index cores for low dispersion at 1.55 um", British Telecom Technology, vol. 2, No. 2, pp. 25-34, Apr. 1984.
Bhagavatula et al, "Bend-Optimized dispersion-shifted segmented core designs for specialized 1550-nm operation", Conference on Optical Fiber Communication, pp. 94-96, Feb. 1985.
da Silva Braga Robinson Luiz
Smolka Francisco Martim
Algar S.A. Empreendimentos e Participacoes
Sanghavi Hemang
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