  Previously proposed designs of integrated photonic devices have used the
intuitive brute force approach or optimization methods that employ parameter
search algorithms. However, a small parameter space and poor exploitation of
the underlying physics have limited device performance, functionality, and
footprint. In this paper, we propose efficient and compact 2D 1xN
in-plane-incidence wavelength demultiplexers by using recently developed
objective-first inverse design algorithm. Output ports in the presented 1xN
photonic devices are located along the transverse to the input channel.
Ultra-high device performance was achieved for the specific designs of 1x2,
1x4, and 1x6 wavelength (de)multiplexers with small footprints 2.80 um x 2.80
um, 2.80 um x 4.60 um, 2.80 um x 6.95 um, respectively. We used two approaches
to binarization-level-set and binarization-cost-to obtain silicon wavelength
demultiplexer considering fabrication constraints. For instance, the
transmission efficiency of binarization-cost 1x2 demultiplexer was -0.30 dB for
1.31 um and -0.54 dB at 1.55 um while crosstalk at the operating wavelengths
are negligibly small, i.e., -17.80 and -15.29 dB, respectively. Moreover, for
the binarization-cost 1x4 demultiplexer, the transmission efficiency values
were approximately -1.90 dB at 1.31, 1.39, 1.47, and 1.55 um as the crosstalk
was approximately -13 dB. Furthermore, the objective-first algorithm was used
to employ our demultiplexers as multiplexers which means the ports that were
once used as inputs in demultiplexers are designed to be used as outputs. The
inverse design approach that allows for the implementation of more than six
output channels together with the proposed functionalities can help develop
compact and manufacturable 2D 1xN couplers.
