Mode converters based on cascaded long-period waveguide gratings.

We propose a mode-conversion platform based on cascaded long-period waveguide gratings. We demonstrate two specific devices by cascading a surface-corrugated grating and a sidewall-corrugated grating. One device allows conversion among the LP, LP, and LP modes in a cyclic manner. The maximum conversion efficiency of a typical experimental device fabricated with polymer materials is about 94% at 1540 nm. The other device allows conversion between the LP and LP modes. The maximum conversion efficiency of a typical experimental device is estimated to be higher than 90% at 1550 nm. The performance of both devices is polarization-insensitive. The proposed platform could be developed into various mode converters for mode-division-multiplexing systems and other mode-sensitive applications.