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 LP(01) mode staying in the central core that incorporates a biconical taper to suppress any remaining LP(11) modes. A typical fabricated demultiplexer, which is 18.5 mm long, shows a coupling ratio higher than ∼96% in the wavelength range of 1530-1570 nm for both couplers. The device shows negligible crosstalk to the LP(01)-mode channel, while the crosstalks to the LP(11)-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.