Optical waveguides – With optical coupler – Input/output coupler
Patent
1992-11-03
1994-12-27
Bovernick, Rodney B.
Optical waveguides
With optical coupler
Input/output coupler
385 36, 359566, G02B 600
Patent
active
053772881
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods of forming refractive index gratings in optical waveguides of particular, but not exclusive, application to forming such gratings in optical fibre waveguides.
2. Related Art
Optical waveguide gratings have many applications, for example as passive wavelength filters in wavelength multiplexed telecommunications and sensor systems and as frequency selective elements in active fibre devices.
One approach to making an optical waveguide grating is to form an external grating interacting with the evanescent field of the waveguide, for example by etching a grating close to the nearly exposed core of an optical fibre. A second approach, with which the present application is concerned, is to form a refractive index grating within the core of the waveguide. A standing wave is set up using two interfering beams derived from a single-frequency laser which, if sufficiently intense, writes a refractive index grating into the waveguide core over a time period in the order of minutes.
WO86/01303 published on the Feb. 27, 1986 describes a method of forming such a grating in an optical fibre waveguide by illuminating it transversely with a standing wave interference pattern set up by two suitable angled ultraviolet beams derived from a single coherent source. The two ultraviolet beams are produced by directing the source UV beam onto a beamsplitter which produces a pair of subsidiary beams which are reflected by a pair of mirrors to form a standing interference pattern in the region of the optical fibre. The grating spacing is controllable by varying the angle of incidence of the interfering subsidiary beams.
The writing wavelength is chosen to be one which efficiently modifies the refractive index of the medium. The grating so formed will be effective at longer wavelengths at which it may not be possible to write a grating.
This prior art apparatus for forming such gratings has several optical elements which must be maintained accurately in position relative to the UV source and optical fibre for the several minutes of exposure time needed to form the grating. This may be adequate when production of the gratings is made in carefully prepared and controlled surroundings. However, the applicant has identified a need for such gratings that necessitate the formation of gratings in less controlled surroundings.
BRIEF SUMMARY OF THE INVENTION
According to the present invention a method of forming a refractive index grating in an optical waveguide comprises positioning the optical waveguide against a first face of a block of refractive material and directing a coherent beam of optical radiation at a first wavelength at the block so that a standing wave field is formed within the optical waveguide by a first portion of the beam propagating through the block directly to the first face and a second portion reflected at a second face of the block, which field is maintained until a refractive index grating reflective at a second wavelength longer than the first is formed.
Gratings reflective in the regions of 1.3 .mu.m and 1.5 .mu.m have particular application to current silica based optical communication systems employing silica telecommunication fibres. It is also applicable to other wavelengths for example the formation of gratings operative in the 2.7 .mu.m window of fluoride fibres written with visible light, for example.
The block is conveniently a right triangular or rectangular prism which are readily available.
The position of the fibre relative to the prism can be maintained by simply fixing the fibre to the prism face leaving only the source of optical radiation to be fixed relative to the prism. The alignment problems associated with the prior art sidewriting technique are thereby greatly reduced.
The optical waveguide may be an optical fibre or a planar waveguide such as a germania doped planar waveguides formed by flame hydrolysis deposition.
The second face of the block may reflect the beam by total internal reflection, or if thi
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Armitage Jonathan R.
Kashyap Raman
Bovernick Rodney B.
British Telecommunications public limited company
Sanghavi Hemang
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