Fig. 1. Relative losses of the 5-m-length fiber. Inset shows the cross section of our multi-resonant fiber.

Fig. 2. Experimental configurations constructed for (a) M² and (b) S² measurements, respectively. SMF, single-mode fiber; FUT, AS-PBGF under test; L, L₁, and L₂, aspherical lens; M₁ and M₂, reflective mirror; BQA, beam quality analysis; ASE source, amplified spontaneous emission source; PBS, polarizing beam splitter; CF, collecting fiber; OSA, optical spectrum analyzer.

Fig. 3. Measured M² results under different wavelengths. (a) The U-shaped curve covering the third PBG and (b) partial magnification of the results in the SAE region.

Fig. 4. S² data analysis within the NB and LAE regions with a 5-m-length fiber when the coiled diameter Φ = 80 cm.

Fig. 5. Theoretical calculations of PBGs distribution and beam quality factor M² based on the measured structural parameters.

Fig. 6. Theoretical calculations of FM profile and wavefront distribution at 870 nm, 1030 nm, and 1140 nm, respectively.

Fig. 7. Beam propagation property of the two representative structures. (a) The “mixed-cell” structure reported in Ref. [18], (b) the “diamond-shaped” scheme proposed in Ref. [17], and (c) calculation results of M² in their respective PBGs.