Mode multiplexer based on integrated horizontal and vertical polymer waveguide couplers

We demonstrate a mode demultiplexer with two cascaded few-mode polymer waveguide directional couplers fabricated on the same substrate along the horizontal and vertical directions, respectively. The three waveguides that form the two couplers have the same core size. The horizontal and vertical couplers are designed to provide complete power transfer for the LP_11a and LP_11b modes, with the LP₀₁ mode staying in the central core that incorporates a biconical taper to suppress any remaining LP₁₁ modes. A typical fabricated demultiplexer, which is 18.5mm long, shows a coupling ratio higher than ∼96% in the wavelength range of 1530-1570nm for both couplers. The device shows negligible crosstalk to the LP₀₁-mode channel, while the crosstalks to the LP₁₁-mode channels are lower than 15.6 and 13.4 dB for the TE and TM polarizations, respectively. The device can be considered polarization insensitive. The propagation losses for the three modes are about 2.0 dB/cm. This device could find applications in mode-division-multiplexing systems.