ттттттт Original Message ттттттт

   On Sunday, September 8, 2019 11:42 PM, jim bell <jdb10987@yahoo.com>
   wrote:

   ...

   Silica fiber, being solid, has vastly less variation in the density of
   mass through the length of the fiber.  So, I wouldn't expect that
   Rayleigh scattering is a major component of optical fiber loss, except
   if you are considering short wavelengths and considering Si-29 content
   as the source of that scattering.

   i am finding this subject quite fascinating.  here's a paper on thermal
   conductivity in single isotope germanium crystals which makes me wonder
   about the effect on light permeability...

   [1]https://www.appi.keio.ac.jp/Itoh_group/publications/pdf/JTP00490.pdf

   """

   Isotopic disorder greatly decreases the thermal conductivityK(T) of
   dielectric single crystals if they are structurally perfect and
   sufficiently pure chemically. This effectwas predicted by
   Pomeranchuk1in 1942. The effect of isotopes on the thermal
   conduc-tivity of ordinary~non quantum! crystals has been studied
   experimentally for Ge,2LiF,3and recently for synthetic diamond.4The
   thermal conductivity of diamond enriched to99.9%12C is 50% higher than
   the thermal conductivity of natural diamond, where the12C content is
   equal to 98.9%, and at room temperature it is the highest of all known
   materials. Geballe and Hull2found that the thermal conductivity of
   germanium enriched to96%74Ge is approximately three times higher at the
   maximum than that of germanium with the natural isotopic composition.
   This increase in the thermal conductivity agrees qualitatively with the
   theory of Ref. 5, though it is much lower than the expected
   15-foldincrease calculated on the basis of the 15-fold decrease in the
   mean-squared deviation in the mass of the isotopes.

   ..."""

References

   1. https://www.appi.keio.ac.jp/Itoh_group/publications/pdf/JTP00490.pdf