  We explore the spectral properties of a capillary dye laser in the highly
multimode regime. Our experiments indicate that the spectral behavior of the
laser does not conform with a simple Fabry-Perot analysis; rather, it is
strongly dictated by a Vernier resonant mechanism involving multiple modes,
which propagate with different group velocities. The laser operates over a very
broad spectral range and the Vernier effect gives rise to a free spectral range
which is orders of magnitude larger than that expected from a simple
Fabry-Perot mechanism. The presented theoretical calculations confirm the
experimental results. Propagating modes of the capillary fiber are calculated
using the finite element method (FEM) and it is shown that the optical
pathlengths resulting from simultaneous beatings of these modes are in close
agreement with the optical pathlengths directly extracted from the Fourier
Transform of the experimentally measured laser emission spectra.
