Fiber wedge end-face coupler for capillary whispering-gallery-mode resonators

Objective For the detection requirements of the immobilization status of the inner surface modifiers in capillary microreactors, the sensing detection by exciting the optical whispering gallery mode (WGM) of the capillary microreactor can achieve highly sensitive, unmarked, and directly quantitative real-time detection of the immobilization status of the inner surface modifiers in capillary microreactors. Therefore, the study of mode excitation in capillary microreactor WGM is crucial. The near-field coupling devices commonly used for coupled resonant excitation of capillary microcavity WGM resonance are mainly prisms and fiber cones. However, the prism coupling system is complex and the devices are large and difficult to integrate and miniaturize. Fiber cone devices are fragile and have poor robustness. To meet the practical application requirements of capillary microreactor inner wall condition detection, a coupled resonant system with certain robustness and a simple system that can be integrated and miniaturized is required. To this end, an optical fiber wedge end coupler that excites the WGM resonance of a microtubular cavity is proposed. Methods A resonant model of fiber wedge end-face coupled with capillary microcavity was established (Fig.1), by theoretically studying the principle of coupling the wedge end-face to the microcavity swiftly, simulating the optical field distribution and coupling resonance of the wedge end-face (Fig.2), and analyzing the spectral characteristics of the coupling resonance of the fiber wedge end-face to the microcavity (Fig.3). The wedge-shaped end fiber (Fig.7) with 75° polishing angle was prepared by grinding (Fig.6), and the experimental system (Tab.1) was built for the corresponding experimental verification. The proposed WGM coupled resonance of wedge-shaped end surfaces and capillary microcavities is investigated by theoretical analysis, simulation, and experimental verification. Results and Discussions Simulation of the coupling of the fiber wedge end-face to the microtubular cavity is conducted using RSoft software. The fiber wedge end-face can be successfully coupled (Fig.2) to excite WGM resonance by choosing a suitable angle for phase matching and mode field overlap with the capillary microcavity. The transmission spectrum obtained from the coupled structure shows a Lorentz concave valley (Fig.4), which is consistent with the theoretical analysis. In the experiments, different coupling resonance states can be obtained by adjusting the coupling spacing (Fig.9) using two wedge-terminated fibers with polishing angle of 75°. The Lorentz concave-valley type coupled resonance spectrum (Fig.10) is collected, which is consistent with the theoretical study and simulation analysis. The free spectral range near the wavelength of the resonance spectrum 1 563.074 nm is about 1.734 nm and the Q value is about 6.15 × 103. The proposed fiber wedge end coupler can effectively excite the WGM mode of the capillary microcavity. Conclusions A fiber wedge end coupler for excitation of microtubular cavity WGM resonance is proposed. The coupler has good robustness, the coupling resonance structure is stable, and the system is simple and easy to be integrated into a small size. A suitable polishing angle is selected for the end-face of the wedge fiber to meet the phase-matching condition of WGM. By adjusting the appropriate coupling spacing, the wedge-shaped fiber end surfaces are formed to overlap with the microtubular cavity mode field. Resonance between the fiber optic waveguide mode at the wedge end-face and the capillary WGM mode occurs for effective energy exchange, which in turn effectively excites the capillary microcavity WGM. The coupling structure was simulated and analyzed using RSoft, and the system was built. Experimental tests were performed using an optical fiber wedge end coupler with a polishing angle of 75° and a quartz capillary with an outer diameter of 370 μm and a wall thickness of 60 μm. The experiment was successfully coupled to excite the WGM resonance with a free spectral range of about 1.734 nm near the resonance wavelength of 1 563.074 nm and a Q value of about 6.15 × 103. The proposed fiber optic wedge end coupler can be effectively used for mode excitation of capillary microreactor WGM resonance, which is an important guideline for capillary microreactor biochemical assay applications. © 2023 Chinese Society of Astronautics. All rights reserved.