Kirk C. Brown

Date of Award

3-6-2006

Document Type

Thesis

Degree Name

Master of Science in Applied Physics

Department

Department of Engineering Physics

First Advisor

Timothy H. Russell, PhD

Abstract

Many active methods of scaling laser brightness have been demonstrated in recent years. The goal of this research was to demonstrate the feasibility of passively combining multiple laser beams using Stimulated Brillouin Scattering (SBS) in a long multimode optical fiber. This method of combination employed a “Gatling gun” fiber array that allowed several collimated beams to be focused by a lens into an optical fiber. The retroreflected Stokes beam is passed through the center of the beam combiner for analysis. In addition to experimental methodology and equipment used, the theoretical and historical background of SBS in optical fibers is provided. The ability to combine four single-mode off-axis input beams using a fiber array and a long multimode optical fiber was demonstrated. The Stokes beam generated by the four off-axis input beams was found to be near Gaussian with an M2 value of ≤ 1.08. The reflected Stokes beam intensity was erratic with no discernible period. The maximum average reflected power was approximately 50 mW with a conversion efficiency of 3.5% and a slope efficiency of 11.4%. The peak reflected power was measured to be over 400 mW with a conversion efficiency of 42%. The threshold required to generate SBS in this configuration was quadruple the predicted power required to generate SBS using an on-axis pump beam. The deleterious effects of the addition of an off-axis side channel was demonstrated. When a Stokes beam was created using an on-axis pump beam, the result was a stable Stokes beam. The addition of just one side channel caused the Stokes beam to become erratic and fluctuate in intensity with no discernible period. A hypothesis is provided and compared to current theory. The success of this thesis suggests the possibility of a new approach to pumping a fiber Brillouin laser which could reduce the threshold, increase conversion efficiency, and maintain excellent beam quality.

AFIT Designator

AFIT-GAP-ENP-06-01

DTIC Accession Number

ADA450062

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