  Space-division multiplexing (SDM) is one of the key enabling technologies to
increase the capacity of fiber communication systems. However, implementing
SDM-based systems using multimode fiber has been challenging with the need for
compact, low-cost, and scalable mode de/multiplexer (DE/MUX). Here we present a
novel integrated mode MUX for few-mode fibers (FMFs) which can launch up to
eight spatial and polarization channels. The new design is composed of a
two-dimensional multimode grating coupler (MMGC), highly compact mode size
converters (MSCs), and adiabatic directional couplers (ADCs). Eight data lanes
in FMFs can be selectively launched with integrated optical phase shifters.
Experimental results reveal efficient chip-to-fiber coupling with peak
efficiencies of -3.8 dB, -5.5 dB, -3.6 dB, and -4.1 dB for LP01, LP11a, LP11b,
and LP21b modes, respectively. Meanwhile, the proposed design can efficiently
couple all the degenerate LP modes in a two-mode FMF, allowing signal
descrambling in the demultiplexer. Thanks to the use of integrated
subwavelength Mikaelian lens for mode-independent field size conversion with
loss $\leq$0.25 dB, the total footprint of the MMGC and MSCs is only 35x35
$\mu$m$^{2}$. The proposed design shows great potential for densely integrated
photonic circuits in future SDM applications.
