Inverse design and demonstration of a compact on-chip narrowband three-channel wavelength demultiplexer

In wavelength division multiplexing (WDM) schemes, splitters must be used to combine and separate different wavelengths. Conventional splitters are fairly large with footprints in hundreds to thousands of square microns, and experimentally-demonstrated MMI-based and inverse-designed ultra-compact splitters operate with only two channels and large channel spacing ($>$100 nm). Here we inverse design and experimentally demonstrate a three-channel wavelength demultiplexer with 40 nm spacing (1500 nm, 1540 nm, and 1580 nm) with a footprint of 24.75 $\mu\mathrm{m}^2$. The splitter has a simulated peak insertion loss of -1.55 dB with under -15 dB crosstalk and a measured peak insertion loss of -2.29 dB with under -10.7 dB crosstalk.