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 length-apodized long-period grating, where π-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-LP_11a and an LP₀₁-LP_11b mode converter with a sidewall grating and a surface grating formed along a polymer channel waveguide, respectively. The fabricated LP₀₁-LP_11a and LP₀₁-LP_11b mode converters provide a conversion efficiency higher than 99% over a bandwidth of ~120 nm and ~150 nm, respectively, or a conversion efficiency higher than 90% over a bandwidth of ~180 nm and ~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 grating-based mode-coupling devices for a wide range of applications.