Horizontal Directional Coupler Formed With Waveguides of Different Heights for Mode-Division Multiplexing

We propose a horizontal directional coupler formed with two parallel waveguides of different heights. By breaking the symmetry in both the horizontal and the vertical direction, this directional coupler can be designed to allow us coupling between any two spatial modes of a few-mode waveguide, regardless of their symmetry properties, whereas a conventional directional coupler formed with planar waveguides of equal heights only allows us coupling between two spatial modes of the same symmetry in the vertical direction. As an example, we design and fabricate such a directional coupler with polymer material for the (de)multiplexing of the LP₀₁ and the LP_11b mode, which have a symmetric and an antisymmetric field distribution in the vertical direction, respectively. Our typical fabricated device shows a coupling ratio higher than 95% in the wavelength range from 1530 to 1560 nm. The insertion losses for the LP₀₁ and LP_11b modes are 9.6 and 12.8 dB, respectively. The performance of the device is weakly sensitive to temperature variations. The proposed directional coupler is easy to fabricate and can be used as a basic structure for the implementation of mode-controlling devices for mode-division-multiplexing applications.