Fabrication tolerant and temperature tolerant mach zehnder interferometers

A Mach-Zender Interferometer (MZI) comprising:

• a first arm comprising:
• one or more first delay sections;
• and respective first undercuts under the one or more first delay sections, the one or more first delay sections having a total first length and a first width;
• a second arm comprising:
• one or more second delay sections;
• and respective second undercuts under the one or more second delay sections, the one or more second delay sections having a total second length and a second width,
• wherein the total first length is longer than the total second length, and the first width is less than the second width,
• wherein the first width and the second width are selected to be less than a threshold width, and
• wherein for widths above the threshold width, variations in index-to-width tolerance are anticorrelated with respective variations in index-to-thickness tolerance and index-to-temperature tolerance, and for respective widths below the threshold width the variations in the index-to-width tolerance are correlated with the respective variations in the index-to-thickness tolerance and the index-to-temperature tolerance.