Fig. 1. Position of the coupling spot on the end cap-fiber interface.

Fig. 2. Light incident on the fiber end face (half cross section).

Fig. 3. Relationship between the normalized backscattering power and the offset of the spot center in the (a) outer cladding, (b) inner cladding, and (c) core.

Fig. 4. Relationship between the normalized backscattering power and (a) L0, (b) L1, (c) σ, and (d) dspot.

Fig. 5. Measurement result of the etched-cladding roughness.

Fig. 6. Schematic of the experimental setup.

Fig. 7. Comparison of the test results and simulation results of the verification experiment.

Fig. 8. Schematic of the FFOS.

Fig. 9. Homemade FFOS: (a) model structure, (b) physical structure.

Fig. 10. Test results of the FFOS: (a) power and efficiency, (b) long-time stability.