We demonstrate efficient and compact 1xN wavelength-demultiplexing by using objective-first inverse design algorithm. Ultra-high device performances were achieved for the certain designs of 1x2, 1x3, and 1x4 demultiplexers with very small footprints at the orders of a few microns. The presented 1x2, 1x3, and 1x4 devices operate at the wavelength sets of 1.31μm, 1.55 μm; 1.31 μm, 1.47 μm, 1.55 μm, and; 1.31 μm, 1.39 μm, 1.47 μm, 1.55 μm, respectively. The transmission efficiencies at the corresponding target wavelengths of each vertically or horizontally aligned channels were obtained mostly to be near-unity together with very small crosstalk ranges of around 0.01% - 1.2%. The inverse design approach allowing the implementation of more than four output channels together with the novel functionalities will pave the way for compact and manufacturable 1xN couplers, which is of ultimate significance for integrated photonics.