The authors demonstrate a microfluidic dye laser using a liquid core optical ring resonator (LCORR). The LCORR is made of a fused silica capillary with a wall thickness of a few microns. The circular cross section of the capillary forms a ring resonator that supports whispering gallery modes (WGMs) and provides an optical feedback for lasers. Due to the high Q factor of the WGM (107), a low lasing threshold is achieved (1μJ∕mm2). In addition, they show that the laser can be coupled out via a tapered fiber in touch with the LCORR, thus providing a mechanism for easy laser delivery.

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We develop a versatile integrated opto-fluidic ring resonator (OFRR) dye laser that can be operated regardless of the refractive index (RI) of the liquid. The OFRR is a micro-sized glass capillary with a wall thickness of a few micrometers. When the liquid in the core has an RI lower than that of the capillary wall (n=1.45), the capillary circular cross-section forms the ring resonator and supports the whispering gallery modes (WGMs) that interact evanescently with the gain medium in the core. When the core RI is higher than that of the wall, the WGMs exist at the core/wall interface. In both cases, the WGMs can have extremely high Q-factor (>109), providing excellent optical feedback for low-threshold lasing. In this paper, we analyze the OFRR laser for various core RI's and then we demonstrate the R6G laser when the dye is in ethanol (n=1.36), chloroform (n=1.445), and quinoline (n=1.626). The lasing threshold of 25 nJ/mm(2) is achieved, two to three orders of magnitude lower than the previous work in microfluidic lasers. We further show that the laser emission can be efficiently out-coupled via an optical waveguide in touch with the OFRR for both high and low RI liquid core, allowing for easy guiding and delivery of the laser light.

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