The MZI and NOR gate require lossless 3dB couplers to function properly. The MZI consists of two such couplers connected in series. NOR-gate-output1.png and mzi-output1.png seem to have 2 couplers in the layout, as required.
The optical circuits shown in
Vol. 54, No. 28 / October 1 2015 / Applied Optics
each have two or three lossless 3dB couplers connected in series or some combination of series and parallel.
The first coupler in NOR-gate-output.png does not appear to be a ossless 3dB coupler. The power does seem to be approximately evenly divided on the output waveguides, but there is a great deal of radiation loss, evident from all the light observed outside the waveguides.
The first coupler in mzi-output1.png seems to have similar trouble. MZI and NOR rely on total cancellation from interference between different waveguides. this will not be possible if there is so much radiation loss. I noticed the waveguides in this project are diffused. Diffused waveguides cannot tolerate much bending. Even a small curvature in the waveguide arc causes significant loss.
I recommend experiments with arc waveguides untill the minimum radius of curvature is determined. All designs for couplers should use either straight wqaveguides, or, if S bends or arcs are used, the radius of curvature should be equal or greater than as determined above to avoid radiation loss. Once that is achieved, adjustment should be made to couple 3dB to each output waveguide, to get suitable design for a lossless 3 dB coupler. With such devices, he MZI and NOR illustrated in Vol. 54, No. 28 / October 1 2015 Applied Optics will be possible by connecting the couplers as shown in that paper.