Scattering losses in glass arise from microscopic variations in the material density, from compositional fluctuations and from structural inhomogeneties or defects occurring during fiber manufacture.
I. Linear Scattering mechanism:
It causes the transfer of some or all of the optical power contained within one propagating mode to be transferred linearly into a different mode.
This process tends to a result in attenuation of the transmitted light as the transfer may be to a leaky or radiation mode which does not continue to propagate within the fibre core, but is radiated from the fibre. With all linear process there is no change of frequency on scattering.
Linear scattering may be categorized as:
a. Rayleigh scattering
b. Mie scattering
a. Rayleigh scattering:
For glass fibers the foremost type of scattering is Rayleigh scattering. With this process, atoms or other particles within the fiber absorb the light signal and instantly re-emits the light in another direction.
In this way Rayleigh scattering appears very much like absorption but it absorbs and redirects the light so quickly that is considered scattering.
- The Rayleigh scattering formula is given by:
$$γ_R = \frac{8π^3}{3λ^4} n^8 p^2 β_c kT_F$$
Where γ_R is Rayleigh scattering coefficient.
λ is optical wavelength and η is the refractive index of the medium
p is average photoelastic coefficient
$β_c$ is isothermal compressibility at a fictive temperature $T_F$
k is Boltzmann’s constant
b. Mie scattering:
Imperfections caused due to inhomogenities at the core- cladding interface which causes scattering of light.
The scattering created by such inhomogenities is mainly in the forward direction and is called Mie scattering.
It can be reduced by removing imperfections of glass at the time of manufacture, increasing the relative refractive index of the core and the cladding.
II. Non Linear Scattering Losses:
This scattering cause disproportionate attenuation, usually at high optical power levels.
This non linear scattering causes the optical power from one mode to be transferred in either the forward or backward direction to the same, or other modes, at a different frequency.
Non Linear scattering may be categorized as:
a. Stimulated Brillouin scattering (SBS)
b. Stimulated Raman scattering (SRS)
a. Stimulated Brillouin scattering (SBS):
In SBS strong optical signal generates acoustic waves. These waves produce variations in refractive index.
It causes lightwaves to scatter in backward direction towards transmitter called as backward scatter wave which affects the forward signal leading to depletion in signal power.
b. Stimulated Raman scattering (SRS):
SRS is transferring of energy from short wavelengths to neighbouring high wavelength channels.
If two input signal with equal power are transmitted than the former will lose its own energy and the latter will gain this energy, this limits the performance of the system.
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