Long-period gratings on lithium-niobate waveguides: phase-matching turning points for ultra-broadband mode conversion

We analyze a polymer long-period grating (LPG) formed on the surface of a z -cut lithium-niobate (LN) few-mode waveguide for TM 00 –TM 10 mode conversion. We identify and explore the turning points along the phase-matching curves of the LPGs for the achievement of ultra-broadband mode conversion. We fabricate a number of gratings with annealed proton-exchanged LN waveguides and our experimental results agree well with the theoretical results. A typical fabricated grating operating near the turning point provides a mode extinction ratio larger than 20 dB from 1550 nm to a wavelength longer than 1700 nm, and another one provides a mode extinction ratio larger than 10 dB from 1464 nm to a wavelength longer than 1700 nm. Our gratings show good thermal stability. The structure of the LPG on LN waveguide could be applied to the realization of ultra-broadband high-speed mode switches for mode-multiplexing applications.