Fig. 1. Schematic diagram of the fiber mode selection coupler. Output of the single-mode fiber and the few-mode fiber, when the fundamental mode laser is separately launched into (a) the single-mode fiber, and (b) the few-mode fiber

Fig. 2. Mode effective index curves for the LP₀₁ mode in SMF and the LP₁₁ mode in FMF as functions of core radius, at the wavelength of 532 nm

Fig. 3. Simulation of periodic transformation between the LP₀₁ mode in the single-mode fiber and the LP₁₁ mode in the few-mode fiber, in the coupling region of MSC

Fig. 4. Spectrum diagram of the output of the single-mode fiber (black curve) and the output of the low-mode fiber (red curve) when the wide-spectrum laser is injected from the single-mode fiber input of the MSC under the matching conditions. At the working wavelength of 532 nm, most of the power output is from the few-mode fiber end

Fig. 5. Schematic diagram of the FED imaging system based on the fiber MSC

Fig. 6. Optical field distribution of the first two order eigenmodes supported by the LFM output end of MSC. (a) LP₀₁ mode; (b) LP₁₁ mode

Fig. 7. Optical field distribution of solid spot (purple dashed line), hollow spot (blue dashed line), and equivalent spot (red solid line) in the FED imaging system based on the fiber MSC with different weighting coefficients, obtained at the output end of MSC. (a) r =0.5；(b) r =0.7；(c) r =1

Fig. 8. Imaging result of gold particles using the FED system based on the fiber MSC. (a) Solid spot scanning; (b) Donut-shaped spot scanning; (c) Image of gold particle samples obtained by weighted subtraction of Figs. (a,b). Figures (a1-c1) and (a2-c2) are enlarged views of two different areas in Figs. (a-c), respectively

Fig. 9. Imaging result of nano-scale wire pairs using the FED system based on fiber MSC. (a) Solid spot scanning; (b) Donut-shaped spot scanning; (c) Image of nano-scale wire pairs obtained by weighted subtraction of Figs. (a, b)