"lossless" optical waveguide - loss floor in glass media

[jim bell]

 On Sunday, September 8, 2019, 03:05:57 PM PDT, coderman <coderman at protonmail.com> wrote:
 
 
 
‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
 On Sunday, September 8, 2019 9:27 PM, jim bell <jdb10987 at yahoo.com> wrote:
...

>>In order to evaluate this as a proposed idea, a physicist would consider:

>>1.  The loss of manufactured optical waveguides did indeed hit an unexplained 'floor' in the early 1980s, about 0.16 db/kilometers of loss.
2.  The manufacturers and users of such fibers have had a very powerful motivation to figure out how to lower their loss to well below 0.16 db/kilometers, for nearly 40 years.
3.   Nothing has yet been found, or it would have been employed.
4.   Photons do indeed possess an oscillating magnetic field.
5.   A nucleus of an isotope with 'spin' does indeed behave as magnetic dipole.
6.   Such a nucleus should be mechanically affected by the passage of light.
7.   Energy should be transferred from that light to the nucleus, and thus the atom, as the light passes.
8.   Removing most or all atoms with an electromagnetic 'spin' should remove this loss mechanism, in proportion to the amount of such isotopes remaining.

>>Do you have any other ideas as to how that loss is manifested?    


>i wonder if Rayleigh scattering is sufficient to account for the floor? unfortunately this is quite complicated in glass media; i don't know how to answer the question :/

>(e.g. the static shear modulus of glass multiplies the factors involved in determining a scattering coefficient...)


My understanding, since I first heard of Rayleigh scattering in the late 1970s, is that it explains (for example) why the Earth's sky is blue, rather than black.  (And Mars' sky is also blue, as well!).    Statistically, the number of molecules in a given volume of air varies, in proportion to the square root of the number of molecules ordinarily in that volume.  The number of molecules in a volume of space of 700 nanometers cubed (which I set to be the wavelength of red light) is:
PV=NRT, where Pressure is in atmospheres, volume in liters, N= the number of moles of molecules (6.02 E23), and T is temperature in Kelvin. R is the Gas Constant, whose value varies with the units normally employed.   https://en.wikipedia.org/wiki/Gas_constant     
| 0.0820573660809596... | L⋅atm⋅K−1⋅mol−1 |


So, N=(PV/RT)  N = ((1 atmosphere)*(((7E-7 meters)**3))(1000liters/cu.meter)) / (0.082 liters-atm/K/mol) * (273 degrees Kelvin))    =    1.532E(-17) moles of molecules.
Or, 1.532E(-17)  *  (6.02 E23)   =   9.22 * E6 molecules in (700 nanometers) cubed.  
The statistical variation is SQRT that value, divided by that value, or:   1/(SQRT 9.22 E6)  = 0.00033
This is the typical variation, as a proportion of the total,  in the number of gas molecules that are present in a volume of 700 nanometers cubed.
A ray of light passes through a myriad of similar regions of gas (air), and this difference in air density causes the light to disperse to some degree. 
But notice that a ray of blue light, wavelength 400 nanometer, should be affected more by passage through a given length of air, because the wavelength is shorter.  Thus, more blue light is dispersed than red light.  So, if you look up in the sky, away from the Sun, you see more blue light than red light.
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.  
 Jim Bell




best regards,
  
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