Optical waveguides – Optical fiber waveguide with cladding – With graded index core or cladding
Reexamination Certificate
2001-01-31
2003-01-28
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
With graded index core or cladding
C385S127000, C385S142000
Reexamination Certificate
active
06512873
ABSTRACT:
TECHNICAL FIELD
The invention relates to an optical body having modifiable light guiding properties, in particular an optical fiber, the light guiding properties of which can locally be changed by a suitable treatment, and to a method of producing an optical waveguiding body.
BACKGROUND
Several different methods of locally changing the light guiding properties such as the refractive index in an otherwise finished optical waveguide are previously known. Here can be mentioned:
1. Permanent increases of the refractive index in SiO
2
-based optical fibers doped with germanium oxide can be obtained by subjecting such fibers to ultraviolet radiation. This radiation should have wavelengths corresponding to the wavelengths of some absorption interval or intervals of germanium oxide(s). An example of the use of such changes of refractive index induced by ultraviolet light are fiber gratings, which are described e.g. in the document by Phillip St. Russel et al., “Fibre gratings”, Physics World, October 1993, pp. 41-46, and the published International patent application WO 94/00784. The produced gratings are for example utilized as filters. The grating structure of such gratings disappears, when they are exposed to temperatures higher tan 500-900° C.
2. The increase of the refractive index of glass material doped with germanium oxide can be reinforced, by making, before the treatment with ultraviolet light, hydrogen diffuse into the material, so called hydrogen sensitization. The refractive index of glass material doped with germanium oxides, phosphorous or phosphorous-aluminium can also be increased by making first hydrogen diffuse into the material in a treatment in a hydrogen gas atmosphere at a high pressure and by thereupon heating the material at not too high temperatures, such as in an interval above 500° C., see U.S. Pat. No. 5,500,031 for Atkins et al. corresponding to the published European patent application 0622343.
3. In the the Swedish patent application 9603406-1, filed Sep. 17, 1996, instead a chemical reaction induced by light is used to achieve such changes Then fluorine atoms are supposed to already initially exist in the core of the fiber, which atoms can be assumed to be bonded in the glass structure. To the fiber hydrogen atoms are added by making them diffuse into the fiber from a surrounding hydrogen gas atmosphere having a high pressure. The fiber is irradiated at selected areas with ultraviolet light in order to make the germanium atoms in the core together with hydrogen, which has diffused into the material, and the quarts material of the fiber form hydroxyl groups. The hydroxyl groups formed react with fluorine atoms to form hydrogen fluoride. Hydrogen fluoride is not bonded too strongly to the material but can by means of a suitable heating operation be made to diffuse from the core into the surrounding material. Thereby the concentration of fluorine in the core within the irradiated region is reduced, what increases the refractive index of the core within these regions. This method can be summarized by: Glass doped with fluorine is used in a fiber, which is first subjected to a hydrogen sensitization, thereupon to a UV-exposure and finally to a heating operation.
4. Dopants, for example germanium and fluorine atoms, which are initially arranged in a glass material, can diffuse in a heating operation, what can change the refractive index within portions exposed to heat. Germanium increases the refractive index of glass materials, whereas fluorine reduces the refractive index. Then, if germanium atoms for example exist within a limited region of an optical fiber, in which thus the refractive index is higher than in the surrounding material, the refractive index can be reduced in this limited region by a heating operation. Inversely the refractive index can by a heating operation be increased within a region having only fluorine atoms.
The methods according to 1. and 3. above are primarily used to produce grating structures, i.e. periodic changes of the refractive index in space, such as changes in the waveguiding core in an optical waveguide, for example changes of the refractive index of is the core in an optical fiber, which are periodic along the longitudinal axis of the fiber.
The method according to 2. above can be used to increase the refractive index of regions doped with germanium oxides, e.g. to produce waveguides in planar substrates and thus to allow the manufacture of integrated optical components.
The methods according to 1., 2. and 3. change the refractive index only within the regions, in which there initially is some concentration of germanium and/or of fluorine atoms. Surrounding, substantially undoped regions, such as for example the cladding of an optical fiber having a doped core region, are not generally noticeably influenced. The core in an optical waveguide maintains in these cases, during the processing, its extension, in particular its radius when the optical body considered is an optical fiber.
In the method according to 4. above very high temperatures must be used to produce a noticeable diffusion. In many typical waveguides, such as in conventional optical fibers intended for telecommunication, the increase of the refractive index in the portion, which is to form the very waveguide or the core of the waveguide, is produced by doping the glass material with germanium oxide GeO
2
when producing the waveguides. When thus a conventional optical fiber is exposed to a high temperature of the magnitude of order of 1600° C. during a not too short period of time, germanium atoms diffuse away out of the core region into the surrounding material, i.e. into the cladding. The difference between the refractive indices of a material doped with germanium and of a substantially undoped material, such as in the cladding, is proportional to the concentration of germanium atoms, what implies, that the diffusion gives a “smearing” of the refractive index, i.e. the core region is expanded and the refractive index thereof is reduced. The refractive index of a typical optical fiber before and after such a high temperature treatment, which also can be termed a core diffusion, is shown in the diagram of FIG.
7
.
For an optical fiber the numerical aperture thereof is given by
NA
=
n
1
2
-
n
2
2
,
where n
1
is the refractive index of the core of the optical fiber and n
2
is the refractive index of the cladding, surrounding the core of the fiber. By a high temperature heating operation thus the numerical aperture of the fiber can be reduced and in particular such a heating operation can be used in the end region of the fiber when connecting it to other fibers or components.
In the articles by H. Y. Tam, “Simple Fusion Splicing Technique for Reducing Splicing Loss Between Standard Singlemode Fibres and Erbium-doped Fibre”, Electronics Letters, Aug. 15, 1991, Vol. 27, No. 17, and J. S. Harper et al., “Tapers in Single-mode Optical Fibre by Controlled Core Diffusion”, Electronics Letters, Feb. 18, 1988, Vol. 24, No. 4 core diffusion is used to adapt mode fields when connecting optical fibers having different numerical apertures to each other or for adaption when connecting optical fibres having different diameters to each other. Core diffusion can also be used to adapt the mode fields in different “fiber-to-fiber-components”, see the articles by Kazuo Shiraishi et al., “Beam Expanding Fiber Using Thermal Diffusion of the Dopant”, J. Lightw. Teckin., Vol. 8, No. Aug. 8, 1991, and Kazuo Shiraishi et al., “Light-Propagation Characteristics in Thermally Diffused Expanded Core Fibers”, J. Lightw. Techn., Vol. 11, No. Oct. 10, 1993. Mode field adaption is also used in the methods described in U.S. Pat. Nos. 5,301,252, 5,142,603 and 5,381,503.
In the article by J. Kirchhof et al., “Diffusion behaviour of fluorine in silica glass”, J. of Non-Cryst. Solids 181, 1995, pp. 266-273, is among other things disclosed, that phosphorous atoms in a SiO
2
-material strongly favour the diffusion of fluorine atoms, i.e. that fluorine atoms in such a material obtain an increased
Acreo AB
Rahll Jerry T
Ullah Akm E.
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