Controlled excitation and regulation of LP11a and LP11b modes based on long period gratings in a slightly elliptical core two-mode fiber

Controlled excitation and regulation of LP11a and LP11b modes based on long-period fiber gratings (LPFGs) written in two-mode elliptical core fibers by CO2 laser are investigated theoretically and experimentally. The fabrication of LPFGs in a slightly elliptical core two-mode fiber (SEC-TMF) is demonstrated, which realizes the conversion of LP11a/b modes separately or simultaneously by changing the CO2 laser exposure direction in relation to the fiber axis with high efficiency of more than 98%. Furthermore, a method for regulating the mode conversion efficiency of LP11a/b modes is proposed by applying transverse loads in different directions on the LPFGs to introduce additional refractive index modulation. The feasibility of the method is verified by experimental results and theoretical analysis. Via applying a transverse load at a specific angle on the dual-wavelength resonant LPFG written in the SEC-TMF, the conversion efficiency of LP11b and LP11a is regulated from 97.4% and 71.3% to 33.1% and 98.3%, respectively. This method achieves secondary regulation of mode conversion by applying transverse load, which has the advantages of flexibility and reconfigurability.