Low index contrast imaging fibers
J. M. Stone, H. A. C. Wood, K. Harrington, and T. A. Birks
University of Bath / University of Edinburgh
The title of this paper means that we have made imaging fibres that use glass with a lower concentration of impurities (dopants) in the light-guiding regions (‘cores’) than is found in typical imaging fibres. Different concentrations of impurities give different refractive indices in the glass (a refractive index is a number that determines how light travels through that particular material). It is this difference in refractive indices that is responsible for confining the light within optical fibres.
Imaging fibres are usually highly doped in order to confine the light inside the thousands of light-guiding spots that make up the transmitted image. If it is not well confined, then the resulting image will be blurred. We describe using a lower concentration of dopants in a smoother distribution, contrary to current commercial imaging fibres, in order to make them cheaper.
Imaging fibres are expensive because the materials used to build them are not common to other areas of optical fibre fabrication. In a clinical environment for use in patients, this equipment also needs costly sterilisation between uses. A low cost imaging fibre is desirable because it could be used and then disposed of. An obvious way to reduce the cost would be to use a cheaper material to make the fibre.
Imaging fibres have a high number of light-guiding cores packed together in a small space. If the light inside is not well confined, it will easily leak to neighbouring cores and blur the output image. The best way to confine the light this is by using glass that has the highest refractive index contrast possible.
Increasing the refractive index by increasing dopant concentration pushes up the cost. Glass that has been mass-produced for telecommunications uses a lower refractive index contrast than that used for imaging fibres, and is typically cheaper. An imaging fibre made with this glass would therefore allow it to be both low cost and single-use. However, in order to create a good image with such a fibre, the problems with weaker light confinement (which decreases image quality) would need to be compensated for with clever fibre design.
We would like to design an imaging fibre that uses mass-produced telecommunications materials, and does not compromise imaging quality.
We made imaging fibres where the cores have greater variation in size between neighbours. This means that even though light is less confined to the cores than in a typical imaging fibre, the materials used means that the light is less able to successfully spread to its neighbours because of the size mismatch.
We fabricated different designs of fibres and tested them in imaging systems to compare their imaging qualities.
We have developed imaging fibres that can use materials from telecommunications glass. We have shown that these imaging fibres perform and image well when compared to commercially available fibres.
This paper was published in Optics Letters, Vol. 42, Issue 8, pp. 1484-1487