Optical waveguides – Optical fiber waveguide with cladding
Patent
1998-03-09
2000-08-15
Lee, John D.
Optical waveguides
Optical fiber waveguide with cladding
385 37, 385122, G02B 616
Patent
active
061048526
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to an optical waveguide that has a photosensitive refractive index, and has particular but not exclusive application to optical fibres.
BACKGROUND
It is well known that a germanium doped silica fibre exhibits photosensitivity, so that the refractive index of the core of the fibre changes when exposed to blue-green light, as demonstrated by Hill et al "Photosensitivity in Optical Waveguides; Application to Reflection Filter Fabrication" Applied Physics Letters Vol. 32 No. 10, 647 (1978). It was later shown that even stronger effects occurred if the core was exposed to ultra-violet radiation close to the absorption peak of a germania-related defect at a wavelength of 240 nm. Reference is directed to G. Meltz et al "Formation of Bragg Gratings in Optical Fibres by Transverse Holographic Method" Opt. Lett. Vol. 14, No. 15 823 (1989). The photosensitive phenomenon is not restricted to germania alone; cerium, europium and erbium: germanium have all shown varying degrees of sensitivity in a silica host optical fibre, but none has been as sensitive of germania. Germanium-boron codoping has also proved highly successful producing large index modulations of the core, of the order of 10.sup.-3 and reference is directed to Y. Duval et al, "Correlation between Ultra-violet-induced Refractive Index Change and Photo-luminance is Gedoped Fibre" Applied Physics Letters, Vol. 61, No. 25, 2955 (1992).
Furthermore, it has been reported that the photosensitivity can be enhanced by hot hydrogen treatment of optical fibres. Reference is directed to G. Meltz et al, "Bragg Grating Formation and Germanio Silicate Fibre Photosensitivity" International Workshop of Photo Induced Self-Orgisation Effects in Optical Fibres SPIE Vol. 1516, p185 (1991). Conventionally, optical fibres are formed by taking a glass tube and exposing the interior thereof to a dopant gas, so as to form a dopant deposit on the interior surface thereof. Thereafter, the glass tube is heated and sintered so as to collapse its interior with the result that the dopant forms a core region through the centre. The effect of the dopant is to raise the refractive index of the central or core region and leave a surrounding cladding region of the lower refractive index. The resulting, collapsed, glass tube is then drawn to produce a fine optical fibre, of reduced diameter.about.120 .mu.m, with a core surrounded by cladding. In a conventional manner, the difference .DELTA.n between the refractive indices of the cladding n.sub.1 and the core n.sub.2 causes light to be guided along the core.
In conventional photosensitive optical fibres, i.e. fibres which have a photosensitive core, it is possible to record so-called refractive index Bragg gratings in the fibres and for a general review, reference is directed to "Photosensitive Optical Fibres: Devices and Applications" Kashyap et al, Optical Fibre Technology 1, 17-34 (1994). In a method described in EP-A-0 668 514, the cladding is rendered photosensitive as well as the core , so that the refractive index grating is recorded in both the core and, to an extent, in the cladding. Also, reference is directed to "Optical fiber design for strong gratings photoimprinting with radiation mode suppression" E. Delevaque et al, Conference on Fiber Communication, Technical Digest Series, Vol 8, No 6, pp 343-346, which discloses an optical fibre with a photosensitive core and a photosensitive intermediate region between the core and the cladding. A refractive index grating is written into the core and the intermediate region, which results in suppression of cladding modes. Photosensitive regions around the fibre core have also been used hitherto for mode matching, as described in U.S. Pat. No. 5,416,863.
Refractive index gratings produced in optical fibres according to these prior recording methods can be used as narrow band reflective filters. One use of the reflective filter is to provide a fibre grating laser, as will now be explained.
It is known that when the core of a silica optical fib
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British Telecommunications public limited company
Lee John D.
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