Title page for ETD etd-51697-144745

Type of Document Master's Thesis
Author Velayudhan, Nirmalkumar
Author's Email Address nirmalv@qualcomm.com
URN etd-51697-144745
Title Analysis of Thermally Diffused Single Mode Optical Fiber Couplers
Degree Master of Science
Department Electrical Engineering
Advisory Committee
Advisor Name Title
Murphy, Kent A.
Wang, Anbo
Claus, Richard O. Committee Chair
  • Optical Couplers
  • Dopant Diffusion
  • Modeling and Analysis
Date of Defense 1994-12-22
Availability unrestricted
The phenomenon of dopant diffusion as a viable means of coupler fabrication is

investigated. It is well known that the diffusion of dopants can improve the uniformity of

multimode star couplers manufactured by the fused biconical taper technique. The

theoretical basis for the same phenomenon in a single mode coupler is developed, on the

basis of the theory of diffusion and the Gaussian approximation for circular fibers.

A novel technique to manufacture and design single mode optical fiber couplers

with a minimization of the manufacturing complexity is demonstrated. Traditionally fused

biconical tapered couplers have been manufactured by twisting, fusing and elongating

optical fibers at elevated temperatures. Usually, high temperature oxy-hydrogen flames are

used for such purposes and some degree of skill is needed for a human operator. The

complexity of control procedures for automation of the process is greatly increased by the

fact that the tapering process is an integral part of the feedback loop. This can be eliminated

if a constant tension is maintained on the fibers in the heating process while heat is applied

uniformly from a source such as a platinum wire furnace. Since the refractive index

differentials responsible for the guiding phenomenon at optical frequencies are directly

dependent on concentration of dopants like fluorine and germania, radial diffusion of such

dopants causes the fiber cores that are heated in a platinum wire furnace to come closer

together. Such proximity leads to the phenomenon of evanescent field interaction or

coupling of optical power from one arm of the coupler to the other.

The time evolution of the coupling process can be predicted in theory. While initial

results are promising, the ability to automate the manufacture of couplers will be

successful only after greater control over the variables is established. It is the intention of

this work to understand the physics behind the mechanism as well as to prove the

feasibility of modeling real world phenomena under controlled conditions.

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