We report the demonstration of low order distributed feedback ÍDFBÍ optofluidic dye lasers with reduced threshold. The laser chips were realized in polydimethylsiloxane using replica molding with two masters. A comparison between first, second, and third order DFB dye lasers was performed, while the second order DFB dye laser exhibited the lowest pump threshold of 78 nJ/pulse. Compared to previous reports on higher order Bragg grating structures, the pump threshold in this work is approximately 30-fold lower than the state of the art due to the reduction in the cavity losses and the more efficient pumping configuration. © 2009 American Institute of Physics. ÍDOI: 10.1063/1.3079799ÍIn recent years, there has been rapid progress in developing optofluidic technology which has enabled a broad spectrum of novel optical toolboxes for integrated optics and lab-on-a-chip applications. [1][2][3] To this end, optically pumped microfluidic dye lasers are of particular interest, since they exhibit the advantages of laser emission, combined with cost-effective processing and a wide choice of emission wavelengths.Several resonator configurations have been demonstrated for microfluidic dye lasers such as the Fabry-Perot cavity, 4 microdroplet, 5 capillary tube, 6 and photonic crystal fiber, 7 while the distributed feedback ÍDFBÍ resonators were the more efficient ones which enable narrow linewidth single mode lasing operation and easy fabrication. 8,9 A DFB microfluidic dye laser was first demonstrated by Balslev and Kristensen 8 who used a 130th Brag grating in a multimode waveguide with a threshold of 20 J / pulse. Then a pure single mode optofluidic dye laser with significant lower pump threshold of 3.2 J / pulse was achieved by Li et al. 9 However, microfluidic dye lasers still rely on bulky pumping laser systems due to their high thresholds, thus placing a restriction on their practicality. Hence, further reduction in the lasing threshold is highly desirable especially for compact diode pumping, as recently demonstrated for solid state organic semiconductor lasers. 10 In this letter, we present low order liquid core DFB microfluidic dye lasers with reduced lasing threshold. Our strategy to lower the pump threshold was based on reducing the out of plane diffraction losses by employing low order diffraction gratings and increasing the pumping efficiency by a longitudinal pumping geometry. The threshold gain g th for a DFB laser can be expressed aswhere a is the propagation loss coefficient in the cavity due to optical scattering, absorption, and out of plane diffraction and a m is the loss coefficient due to the finite reflection from the grating, also referred to as mirror loss coefficient. 11 By decreasing the grating order, both propagation loss a and mirror loss a m can be decreased due to the reduced out of plane diffraction and enhanced in-plane coupling, respectively. By overcoming the previous technical limitation, 9 we developed a two-step replica molding process that allowed fabricating low order DFB structures wi...