Spectral selectivity in capillary dye lasers

Abstract

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. Dye lasers have been a major player since mid 1960s with many attractive properties including a wide operating wavelength range, often spanning 50 to 100 nanometers, and can be reasonably efficient [1,2]. However, because of the difficulties in handling the dyes which can be poisonous or even carcinogenic, and because of their rapid degradation during operation due to photo-bleaching, dye lasers have been mostly replaced with solid state lasers. Recent advances in optofluidic systems have brought attentions back to dye lasers again [3], primarily because dyes can be recirculated in such systems in enclosed set up mitigating the disadvantages while benefiting directly from their most favorable properties. Miniaturizing liquid dye lasers into a microfluidic device has many potential advantages such as compactness, easy maintenance, safe laser operation, accurate spatial mode control, and low threshold energy [3][4][5][6][7] with a wide range of applications including chemical and biological sensing [8,9]. Microfluidic fiber lasers have also appeared in optical fiber platform resulting in dye fiber lasers [10][11][12][13][14][15][16][17]. Fiber lasers based on capillary tubes and photonic crystal fibers filled with a dye solution were studied in detail by Vasdekis et al. [10]. They reported a free spectral range (FSR) which was 300 times larger than what would be expected from a simple Fabry-Perot (FP) cavity analysis: they attributed this spectral selectivity to a Vernier resonant mechanism between two transversely propagating modes in the waveguide.In this paper, we explore the operation of a dye-filled fiber laser in the highly multi-modal regime, giving rise to multiple peaks in the laser emission spectrum, with a considerably larger spectral range (∼ 50 nm) compared with the two-mode operation regime reported by Vasdekis et al. We observe that a simple FP cavity analysis does not explain the FSR for the laser spectral modes, because it predicts spectral laser lines which are much more closely spaced than what we measure in the experiment. Here, we show that a ...