Ultra-broadband mode converters based on length-apodized long-period waveguide gratings

We propose an ultra-broadband mode converter based on the structure of a lengthapodized long-period grating, where p-phase shifts are introduced at strategic locations of the grating profile. Using a 3-section length-apodized grating structure, we design and fabricate an LP01-LP11a and an LP01-LP11b mode converter with a sidewall grating and a surface grating formed along a polymer channel waveguide, respectively. The fabricated LP01-LP11a and LP01-LP11b mode converters provide a conversion efficiency higher than 99% over a bandwidth of similar to 120 nm and similar to 150 nm, respectively, or a conversion efficiency higher than 90% over a bandwidth of similar to 180 nm and similar to 300 nm, respectively. The transmission characteristics of these devices are weakly sensitive to polarization and temperature variations. These mode converters can find applications in ultra-broadband mode-division-multiplexing transmission systems based on few-mode fibers and the design principle can be applied to general gratingbased mode-coupling devices for a wide range of applications. (C) 2017 Optical Society of America