High purity fiber synthesis

Question 1

What is the most prominent constituent of optical fibers?

Answer 1

~ silica

Question 2

What drove the development of vapor phase reaction fabrication methods?

Answer 2

~ the requirement for ultra high purity (low optical absorption/scattering)

Question 3

What is the outside vapor deposition method? (OVD)

Answer 3

~ oxygen and silicon tetrachloride reactant vapors are metered through nozzles

~ these nozzles in turn are surrounded by nozzles that supply methane and oxygen that combust to form a flame

~ this provides thermal energy to facilitate the rxn: SiCl4 + O2 –> SiO2 + 2Cl2

~ the SiO2 formed deposits as a ~200nm amorphous soot on the outside of a rotating ceramic rod (bait rod)

~ ceramic core has a higher CTE so slips out easily after finished assembly to form a preform, the central hole disappears when a fiber is drawn from the preform

~

Question 4

How is water avoided? Why?

Answer 4

~ water is a combustion product

~ OH- in the glass network is detrimental to performance

~ water is eliminated as it combines with the chlorine product: 2H2O + 2Cl –> 4HCl + O2

Question 5

How does soot deposition occur?

Answer 5

~ aided by thermophoresis: the tendency of a particle suspended in a temp gradient to move from a high temp (flame) to a low temp (bait rod surface)

~ the gas on the high temp side, with greater kinetic energy, applies a net force pushing soot particles in the direction of the bait rod

Question 6

What is the result of soot deposition?

Answer 6

~ results in a porous translucent mass that is then sintered (1000-1600C), eliminating porosity, densifying it into a preform

Question 7

What happens after the deposition of soot?

Answer 7

~ it is electrically heated with flowing Cl2 introduced to the center hole of the preform (after the bait rod is removed) to remove any residual water

Question 8

What is GeO2 used for?

Answer 8

~ used as the dopant to vary the index of refraction (higher the GeO2 content, the higher the index of refraction)

~ either for graded index or to deposit cladding material on top of the core