Fig. 1. Geometric illustration of beam propagation in optical fiber. (a) Laser output diagram of flat-end fiber; (b) laser output diagram of spherical-tip fiber; (c1)‒(c4) microscope images and (d1)‒(d4) SEM images of spherical-tip fiber with radii of 70, 80, 90, and 100 μm

Fig. 2. Experiment flowchart. (a) Schematic diagram of the spherical-tip fiber SERS probe preparation; (b1)‒(b3) SEM images of the spherical-tip fiber SERS probe; (c) schematic diagram of Raman signal detection

Fig. 3. Simulation results of spherical-tip fiber SERS probe. (a) SEM image of Ag NPs distribution; (b) schematic diagram of simulation model; (c) electric field simulation result of spherical-tip fiber; (d) electric field distribution on the end face; (e) relationship between the radius and the maximum electric field intensity of the fiber end face

Fig. 4. Optimization of Raman characteristics of spherical-tip fiber SERS probe, where R6G is the probe molecule. (a1)(a2) Different evaporation times; (b1)(b2) different radii; (c1)(c2) different numbers of cycles

Fig. 5. Raman signal detection results. (a) Data processing results under different R6G concentrations; (b) detection limit for R6G; (c) stability test; (d) multi-molecule detection for R6G, MG, and CV; (e) detection of uric acid