In February 2022, Nakano laboratory’s paper, “Scalable and Robust Photonic Integrated Unitary Converter Based on Multiplane Light Conversion” was published in the journal of Physical Review Applied.
An optical unitary converter (OUC) that can convert a set of N mutually orthogonal optical modes into another set of arbitrary N orthogonal modes is expected to be a key device in diverse applications, including optical communication, deep learning, and quantum computing. While various types of OUC have been demonstrated on photonic integration platforms, the sensitivity against a slight deviation in waveguide dimensions has been the crucial issue in scaling N. Here, we demonstrate that an OUC based on the concept of multiplane light conversion (MPLC) shows outstanding robustness against waveguide deviations. Moreover, it becomes more and more insensitive to fabrication errors as we increase N, which is in clear contrast to the conventional OUC architecture, composed of 2×2 Mach-Zehnder interferometers. The physical origin behind this unique robustness and scalability is studied by considering a generalized OUC configuration. As a result, we reveal that the number of coupled modes in each stage plays an essential role in determining the sensitivity of the entire OUC. The maximal robustness is attained when all-to-all-coupled interferometers are employed, which are naturally implemented in a MPLC-based OUC.