  Motivated by the recent observation of periodic filter characteristics of an
oval-shaped micro-cavity, we study the possible interference of multiple beams
in the far field of a laser-illuminated quadrupolar glass fiber. From numerical
ray-tracing simulations of the experimental situation we obtain the
interference-relevant length-difference spectrum and compare it with data
extracted from the experimental filter results. Our analysis reveals that
different polygonal cavity modes being refractively output-coupled in the
high-curvature region of the fiber contribute to the observed far-field
interference.
