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FP5

HARP Report Summary

Project ID: G1RD-CT-2000-00198
Funded under: FP5-GROWTH
Country: United Kingdom

157nm refractive index modification and FBG writing

The move to 157nm for the inscription of fibre Bragg gratings has been demonstrated. However, at this wavelength, there are some subtle influential effects than can affect the inscription process and which are to some extent, less important at longer wavelengths. The strong optical absorption that takes place in the VUV necessitates clean sample surfaces and highly transparent beam delivery systems. Absorption with very thin layers (nm’s) of organic material (e.g. residual fibre Jacket) or mono-layers of contaminants can cause significant attenuation of the beam.

However, in addition to ‘technical’ considerations, bulk and surface absorption becomes non trivial at 157nm in fused silica. With the precautions taken on sample preparation, it is important for the fibre cladding to be transparent in the VUV, i.e. ‘pure’ fused silica cladding has to be carefully tailored during manufacture to minimise absorption. Not only does the dose have to be high enough to modify the silica and thus manifest itself as a change in the refractive index, but, the fringe contrast needs to be preserved in the direction along the fibre. The spatial coherence should be maximised by operating in a configuration that minimises the beam divergence.

The successful inscription of fibre Bragg gratings in SMF28, HI980 and GeB co-doped fibre illustrates the possibility of utilising a 157nm VUV laser source. The results presented here, appear to be one of the first results relating to writing fibre Bragg gratings at 157nm. These results are therefore new and encouraging.

Contact

Jim FIERET, (R&D Group Leader)
Tel.: +44-186-5290400
Fax: +44-186-5290401
E-mail
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