Determine the causes of the discrepancy between HFT chip theory and measurements

Determine the causes of the difference between the theoretical behavior and the laboratory measurements of the 10-output and 12-output hierarchical fringe-tracking chips, in order to optimize the new HFT chip.

Background

The paper presents laboratory tests of second-generation hierarchical fringe-tracking photonic chips designed for four telescopes and optimized with a phase delay of $\theta=0$. The theoretical design predicts specific output-intensity relationships, including equal intensities for the $A12$ and $B12$ outputs, a constant $C^-$ output, and efficient transfer of flux into the second stage through the $C^+$ path.

The 12-output chip does not exhibit the expected flux division or phasing, while the 10-output chip more closely follows the theoretical behavior but still shows a central-output flux lower than expected and a nonconstant $C^-12$ signal. The authors therefore leave unresolved the physical or fabrication-related causes of the discrepancy between the theoretical design and the measured intensities.