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 LP11a and LP11b 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.
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