Strong $\hbox{LP}_{01}$ and $\hbox{LP}_{11}$ Mutual Coupling Conversion in a Two-Mode Fiber Bragg Grating

A fiber Bragg grating is inscribed in a two-mode fiber using a 193-nm excimer laser and a phase mask. Strong LP 01 and LP 11 mutual coupling as large as -22.5 dB, which corresponds to a modal conversion efficiency of more than 99%, is experimentally achieved. The intermodal coupling mechanism is theoretically analyzed through a finite element method. This device can be used as a low-cost reflective LP 01 ↔ LP 11 converter.