Space division multiplexing (SDM) can increase the spectral efficiency per fiber by multiplexing the signals in orthogonal LP modes in a few-mode fiber. In SDM systems, mode multiplexer/demultiplexer (MUX/DEMUX) selectively launches each LP mode. The Mode MUX/DEMUX should be integrated on a cost-effective platform, because the spatial parallelism lets network cost grow linearly with the number of multiplicities. However, it is difficult to build an integrated and low-cost system with reported fiber- and free-space optics devices. In this work, using a three-dimensional SiOx waveguide platform, we demonstrated an integrated mode MUX/DEMUX on a Si chip.
Fig. 1(a) shows a schematic of an integrated mode MUX/DEMUX for three modes. We used two layers of SiOx film with different refractive indices: 1.5 for the lower layer and 1.49 for the upper one. The interlayer clad thickness is 2.0 μm. The calculated electrical field patterns are shown in Fig. 1(a). At the fiber-chip interface (0 mm), the mode field patterns of the waveguides match the fiber-mode patterns with low losses. On the other hand, with changing the core width, the power of each mode converges in a different core (3 mm). The cores are separated (3.2 mm), and cores 3 and 4 are coupled into one single-mode port with a multi-mode interference coupler. Consequently, LP01, LP11a, and LP11b travel through single-mode ports 1, 3, and 2, respectively. The device has an area of 0.6 mm2, and the calculated loss and intermodal crosstalk are less than 2.3 dB and -11 dB, respectively. The fabrication process features the low-temperature formation of controlled-refractive-index SiOx films, which prevents thermal damage in the vertical stacking process, and is compatible with the silicon photonics platform [1, 2].
Figure 1(b) shows the measured near-field patterns (NFPs) of a fabricated device with different fiber-mode inputs. As designed, the LP01, LP11a, and LP11b modes were mainly demultiplexed to output ports 1, 3, and 2, respectively. The estimated intermodal crosstalk is less than -6.4 dB. The results show the feasibility of the integrated mode MUX/DEMUX. In future work, we plan to monolithically integrate it with Si/Ge active devices.