Spectral selectivity in optical fiber capillary dye lasers.

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 to a simple Fabry-Perot (FP) 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 FP mechanism. The theoretical calculations presented confirm the experimental results. Propagating modes of the capillary fiber are calculated using the finite-element method and it is shown that the optical path lengths resulting from simultaneous beatings of these modes are in close agreement with the optical path lengths directly extracted from the Fourier transform of the experimentally measured laser emission spectra.