Fig. 1. Four electrode arc heating fiber structure block diagramr

Fig. 2. Influence of included Angleθon temperature field

Fig. 3. Influence of distance Dx on temperature field

Fig. 4. Influence of distance Dy on temperature field

Fig. 5. Effect of voltage v on temperature field

Fig. 6. Temperature distribution of four electrode rod arc discharge

Fig. 7. Four electrode rod fiber heating and its temperature

Fig. 8. Four electrode rod fiber heating and its temperature

Fig. 9. Plasma hot melt torsional processing system with wide constant temperature zone for arc discharge with four electrodes

Fig. 10. Helical long period fiber grating

Fig. 11. Fiber cross section and 3D refractive index profile

Fig. 12. Different fiber HLPFG fabricated based on a four-electrode arc system

Fig. 13. HLPFG with different periods fabricated based on four-electrode arc system

Fig. 14. Side view of three spiral fiber samples with different pitch （a） Section of single-mode fiber； （b） single-mode fiber helical structure with a period of 450 µm； （c） Single-mode fiber helical structure with a period of 900 µm； （d） Cross-sectional diagram of eccentric fiber； （e） eccentric fiber helical structure with a period of 450 µm； （f） eccentric fiber helical structure with a period of 900 µm； （g） partial dual-core fiber section diagram； （h） partial two-core fiber helical structure with a period of 450µ m； （i） partial dual-core fiber helical structure with a period of 900 µm.