Compact Three-Dimensional Polymer Waveguide Mode Multiplexer

We propose a compact three-dimensional waveguide mode (de)multiplexer based on the configuration of two collocated asymmetrical directional couplers, formed with two dissimilar single-mode cores placed alongside a central rectangular three-mode core in the horizontal and vertical directions, respectively. The waveguides are designed to allow the LP_11a and LP_11b modes of the central core to completely couple to the LP₀₁ modes of the two side cores, respectively, with the LP01 mode staying in the central core, which is tapered down at one end to strip off any remaining LP₁₁ modes. We fabricate the device with polymer materials by the conventional microfabrication process. A typical fabricated device, which has a total length of 9.0 mm, shows coupling ratios varying from 91% to 99% for the two LP₁₁ modes, and crosstalks among the modes in the side cores varying from -23.2 to -14.6 dB in the wavelength range 1530-1570 nm (the C-band). The crosstalks in the central core are negligible. The insertion loss of the device with fiber leads is about 10 dB and the mode-dependent loss is about ±1 dB. The performance of the device is weakly sensitive to the polarization state of light and insensitive to temperature variations. This device can be used in broadband mode-division-multiplexing transmission systems based on few-mode fibers.