Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
1999-08-11
2001-05-08
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S037000, C385S142000, C065S091000
Reexamination Certificate
active
06229945
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical devices which include refractive index modulation, e.g. reflection gratings.
2. Related Art
Reflection gratings are often implemented as waveguides which have a path region and/or a confining region with a modulated refracted index. The waveguiding structure is often in the form of a fibre. The modulation preferably takes the form of alternate regions of higher and lower refractive index. When radiation traverses the modulation, it is selectively reflected. The period of the refractive index modulation is usually equal to the wavelength to be reflected or to a multiple or sub-multiple of said wavelength. Thus periods in the range 250 to 600 nm preferentially reflect selected wavelengths within the range 800-1650 nm.
Reflection gratings have many applications in optical signalling. For example a reflection grating can be associated with a fibre laser in order to narrow the lasing bandwidth. When the refractive index bands are not perpendicular to the fibre axis, the grating can be used for the selective removal of unwanted wavelengths. In addition to reflection gratings, refractive index modulation has other applications, e.g. to achieve phase matching in waveguides, to control spot size and/or shape in waveguides and for storing information.
Refractive index modulations is conveniently produced by an optical process in which a photosensitive glass is exposed to radiation which causes an adequate change in its refractive index. The radiation has higher and lower intensities corresponding to the intended pattern of modulation of the refractive index of the glass. In many commonly used embodiments, the mutual interference of two beams of radiation produces the variation of intensity appropriate for reflection gratings. In the case of information storage, the pattern of radiation relates to the data to be stored.
Silica/germania glasses are widely used in optical telecommunications and it has been noticed that these glasses have an optical absorption band extending approximately over the wavelength range 225-275 nm and exposure to radiation within this band increases the refractive index of the silica/germania composition. The peak of the band occurs at a wavelength which is close to 240 nm. It has, therefore, been proposed to produce refractive index modulation, e.g. to make reflection gratings, by exposing silica/germania glass compositions to radiation within the wavelength band 225-275 nm. Radiation close to 240 nm is particularly suitable. High powers of radiation, e.g. above 1 mW continuous, are needed to produce adequate changes in the refractive index and writing times of a few minutes to a few hours are appropriate.
WO86/01303 describes the writing of phase gratings in optical fibres or waveguides by the application of intense beams of ultraviolet light. It is stated that the grating is produced in the core of a wave guide and that the core is preferably a germanium-doped silica or glass filament.
SUMMARY OF THE INVENTION
The sensitivity of the glass is important, and this invention is based upon the unexpected discovery that glasses which contain B
2
O
3
are particularly sensitive to radiation, e.g. radiation close to 240 nm, and that these glasses are well adapted to carry the necessary refractive index modulation. Preferably the glass contains at least one of SiO
2
and GeO
2
as well as the B
2
O
3
.
Compositions consisting essentially of GeO
2
and B
2
O
3
preferably containing at least 2 mole % of each component, are suitable for thin film optical devices which are capable of storing data in the form of refractive index modulation. Compositions consisting essentially of SiO
2
and B
2
O
3
, preferably containing at least 2 mole % of each component, are particularly suitable for carrying the refractive index modulation wherein said modulation constitutes a reflection waveguide located in the confining region of an optical waveguide. Glass consisting essentially of SiO
2
and GeO
2
would be particularly suitable for use as the path region of said waveguide.
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Electronics Letters, vol. 29, No. 1, Jan. 7, 1993, pp. 45-47, Williams et al, “Enhanced UV Photosensitivity in Boron Codoped Germanosilicate Fibres”.
SPIE, Vo. 1516, International Workshop on Photoinduced Self-Organization Effects in Optical Fiber (1991), “Bragg Grating Formation and Germanosilicate Fiber Photsensitivity”, G. Meltz et al, pp. 185-199.
Patent Abstracts of Japan, Publ. No. JP55040477, 21-03-80, Production of Diffraction Grating, Abstract.
Electronics Letters, vol. 27, No. 21, Oct. 10, 1991, Stevenage, Herts., G.B., pp. 1945-1947, “Formation of Moire Grating in Core of Germanosilicate Fibre By Transverse Holographic Double Exposure Method”.
Database WPIL, Week 8739, 1987, Derwent Publications Ltd, & JP A 62 189 407 (Agency of Ind. Sci. Tech.), Aug. 19, 1987, Light Waveguide Manufacture Heat Treat Polish Glass Coating Silicon Baseplate Thermal Oxidation Film.
Soviet Inventions Illustrated, Section Ch. Week 8309, Apr. 13, 1983.
Ainslie Benjamin J
Armitage Jonathan R
Kashyap Raman
Maxwell Graeme D
Williams Douglas L
Bovernick Rodney
British Telecommunications public limited company
Kim Ellen E.
Nixon & Vanderhye P.C.
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