Glass manufacturing – Processes – Forming product or preform from molten glass
Patent
1990-10-30
1992-08-18
Jones, W. Gary
Glass manufacturing
Processes
Forming product or preform from molten glass
65 29, 65DIG16, 65 43, 385 51, C03B 2320
Patent
active
051395509
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to optical couplers formed from optic fibres composed of fluoride glass. Fluoride type glasses, normally containing fluorozirconate (ZrF4) and optic fibres formed form such glasses are disclosed in Br Telecom Technol J Vol 5 No 2 April 1987 "Progress in Fluoride Fibres for Optical Communications"--P W France et al and in a paper entitled "Properties of Fluorozirconate Fibres for Applications in the 0.5 to 4.5 Micrometer Region" by P W France et al which was presented at the SPIE Conference at San Diego in August 1987. Such fluoride fibres are the subject of European Patent Application No 85304280.2 (Publication No 170380). These three documents are incorporated herein by reference and it is to be understood that the term "fluoride glass" includes but is not limited to all the fluoride-containing glasses described in the above papers and all the fluoride glasses having anions that are fluoride ions (F).
The main advantage of optic fibres composed of fluoride glass is their increased bandwidth which results in transmission in the far infra-red portion of the spectrum. Although such optic fibres are likely to have future applications in telecommunications, their most important use presently is in optical fibre sensors. For long term stability of the measurements of such sensors there is a need to monitor input power to the sensor for use as a reference, and this is only possible using a directional coupler of some description. There is also a need for optical couplers for fluoride optic fibres in order to enable the power of one laser to be split between a number of sensors, particularly in view of the high cost of the lasers used in such applications.
It is known to form optical couplers from two or more optic fibres by disposing the optic fibres across one another so as to bring their claddings into contact, and tensioning and heating the optic fibres so as to fuse together at least the contacting portions of their claddings. Hitherto, such a method has been applied to conventional silica optic fibres which have a relatively wide usable viscosity range between their glass transmission temperature Tg and their crystallisation temperature Tx which enables the fibre claddings to be fused to an appropriate degree without difficulty. However typical fluoride glasses such as ZBLAN glass can be drawn only over a relatively narrow temperature range as disclosed in FIG. 3 of the Br Telecom Technol J article referred to above. We have found that even when it is attempted to fuse optic fibres of fluoride glass within this narrow temperature range of typically 20.degree. C., considerable difficulties arise. We believe that such difficulties arise from the crystallisation of the glass which is induced by oxygen in the air even at temperatures significantly below the crystallisation temperature Tx.
The present invention provides a method of forming an optical coupler from two or more optic fibres comprising disposing the optic fibres across one another so as to bring their claddings into contact, and tensioning and heating the optic fibres so as to fuse together at least the contacting portions of their claddings, characterised in that the optic fibres are composed of a fluoride glass and are heated in an oxygen-free atmosphere to a temperature corresponding to a glass viscosity in the range 100 Pa.s to 100 kPa.s.
By excluding oxygen and heating the fibres to a temperature corresponding to a viscosity in the above range, we have unexpectedly found that optical couplers of fluoride glass may be produced with a reproducible coupling ratio, despite the rapid variation in viscosity with temperature, typically corresponding to a temperature range from 300.degree. C. to 340.degree. C. over the above viscosity range. Preferably the temperature employed corresponds to a glass viscosity in the range 1 kPa.s to 10 kPa.s. For a ZBLAN glass of composition Zn F.sub.4 --Ba F.sub.2 --LaF.sub.3 --AlF.sub.3 --NaF--PbF.sub.2 the preferred temperature range is 323.degree. C. plus or minus 3.degree. C., more pr
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Nicholls Simon T.
Scott Michael
British Telecommunications public limited company
Bruckner John J.
Jones W. Gary
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