Polycrystalline silver-halidefiberswith AgBr cores and AgCl claddings have
shown to have excellent crystalline IRfiber properties. There are several extrinsic
absorption bands for Ag-halidefibers, for example, 3 and 6.3μm due to residual
water ions. The attenuation of Ag-halidefibers is normally less than 1 dB/m in the
5-to-12μm spectral band and somewhat higher in the 12-to-18μm spectral band.
At 10.6μm the loss is 0.3–0.5 dB/m and can be as low as 0.2 dB/m at this
wavelength.
3.13 Optical Fiber Bundles.
To achieve greater throughput withflexible glass or plasticfibers, multiplefibers
are often arranged in a bundle [ 5 , 54 – 56 ]. Eachfiber acts as an independent
waveguide that enables light to be carried over long distances with minimal
attenuation. A typical largefiber bundle array can consist of a few thousand to 100
thousand individualfibers, with an overall bundle diameter of <1 mm and an
individualfiber diameter between 2 and 20μm. Suchfiber bundles are mainly used
for illumination purposes. In addition to flexibility, fiber bundles have other
potential advantages in illumination systems:
- Illuminate multiple locations with one source by splitting the bundle into two or
more branches - Merge light from several sources into a single output
- Integrate different specialtyfibers into one bundle
For bundles with a large number offibers, the arrangement of individualfibers
inside the bundle normally occurs randomly during the manufacturing process [ 5 ].
However, for certain biomedical applications, it is required to arrange a smaller
number offibers in specific patterns. Opticalfibers can be bundled together in an
aligned fashion such that the orientations of thefibers at both ends are identical.
Such afiber bundle is called acoherentfiber bundleor anorderedfiber bundle.
Here the term“coherent”refers to the correlation between the spatial arrangement
of thefibers at both ends and does not refer to the correlation of light signals. As a
result of the matchedfiber arrangements on both ends of the bundle, any pattern of
illumination incident at the input end of the bundle is maintained when it ultimately
emerges from the output end.
Figure3.15illustrates illumination and coherentfiber bundle configurations [ 5 ].
As shown in Fig.3.15a, the opticalfibers are oriented randomly on both ends in a
bundle configuration that contains hundreds or thousands offibers. Figure3.15b
shows one possibility of an orderedfiber arrangement in a bundle. In this hexagonal
packaging scheme, the centralfiber can be used for illuminating a target tissue area
and the surroundingfibers can be used forfluorescent, reflected, or scattered light
returning from the tissue. The discussions in Chap. 7 give further examples offiber
arrangements in optical probes.
3.12 Middle-Infrared Fibers 83