Patent
1986-01-07
1988-10-18
Lee, John
65 312, 65 331, 65 33, 350 9634, G02B 620
Patent
active
047782498
DESCRIPTION:
BRIEF SUMMARY
This invention relates to hollow optical fibres for low loss transmission of high energy radiation in the mid infra-red region, and is particularly concerned with such a fibre for transmitting energy from a carbon dioxide laser at a wavelength of 10.6 microns (.mu.m).
Although in recent years comparatively cheap and compact high powered gas lasers such as the CO.sub.2 laser particularly in the fields of medicine, communications, and industrial engineering, has been restricted by the absence of a suitable flexible transmission system. For example, the properties of laser energy at wavelengths in the region of 10.6 .mu.m in relation to biological tissue make the CO.sub.2 laser potentially very useful medically, particularly in the surgical field where the laser enables very precise and well controlled incisions to be made. However, the absence of a compact, flexible and manoeuvreable low loss transmission system has limited the surgical use of the CO.sub.2 laser mainly to direct line of sight operations. Some endoscopic surgery has been carried out using an articulated arm with mirrors mounted on mechanical joints, but this system is very restrictive and unwieldly. Clearly the provision of a flexible optical fibre suitable for surgical use and capable of low loss transmission of CO.sub.2 laser energy would really enable the surgical potential of the CO.sub.2 laser to be properly realized.
Conventional optical fibres made of glass cannot be used since most glasses, in common with many other inorganic materials, exhibit strong absorption characteristics at mid infra-red frequencies due to resonance effects in the molecular structure, and are therefore substantially opaque to such radiation. Optical fibres which do transmit in the mid infra-red range have, however, been made from certain alkali halides, for example thallous-bromo-iodide, commonly known as KRS5, but it is very difficult and expensive to manufacture suitable fibres from these materials due to their crystalline nature and the high level of purity which is necessary to achieve good transmission. Furthermore, KRS5 is chemically toxic, has poor chemical stability in terms of moisture resistance, and thermally degrades at comparatively low temperatures, which does not render it very suitable for surgical use.
To overcome this problem it has been proposed to make a mid infra-red transmitting optical fibre in the form of a flexible hollow core waveguide so that relatively low loss transmission of radiation in the relevant frequency range can be effected through the hollow core of the fibre. Various proposals for such hollow core optical fibre waveguides have been made, such as hollow circular metallic waveguides, dielectric coated hollow metallic waveguides, and hollow glass waveguides, but it is to hollow optical fibre waveguides of the last type which the present invention particularly relates.
The operation of a hollow core glass fibre waveguide is based on the fact that the absorption and dispersion characteristics of an oxide glass at mid infra-red frequencies, referred to as anomalous dispersion effects, cause the real part (n) of the complex refractive index (N=n-iK where K is the extinction or loss coefficient and is related to the attenuation or absorption coefficient) of the glass to become less than unity over a range of wavelengths (e.g. 7.5 to 9 .mu.m for silica based glasses), with the result that in this range radiation incident from the core on the inner wall of the hollow fibre at an angle greater than the critical angle will be substantially totally internally reflected.
The transmission loss of such a hollow optical fibre waveguide at any given wavelength will of course depend on the fibre geometry and the surface quality of the inner wall of the fibre, but will also depend on the values of n and K at the said wavelength. A zero K value will give zero attentuation of the internally reflected radiation, but in practice this is virtually impossible to achieve since it is a relatively high value of K which gives rise to the refractive in
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Cogent Limited
Lee John
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