The Vertical Integrated Spectrometer (VIS) focal plane: orders of magnitude improvement in mm-wave Line Intensity Mapping (LIM) with an inline waveguide-to-on-chip transition

Scalable, densely packed spectrometer arrays (integral field units, or IFUs) would enable intensity mapping sensitive enough for precision measurements of cosmology and astrophysics in a new redshift regime. This scalability requires a new architecture, with spectrometers oriented parallel with incident light. The key technology required is an inline waveguide-to-coplanar-waveguide transition. We present a millimeter-wave IFU concept that includes this new transition with on-chip kinetic-inductance-based spectrometers. The transition was designed with efficient Bayesian optimization and made robust to manufacturing tolerance with a novel optimization algorithm. It has >95% coupling over an octave bandwidth for 95% of randomly sampled transitions within tolerance. The design approach is of general interest to many instrument design problems; a Python implementation of the algorithm alongside an extension that automates a widely-used mesh-based EM solver are made publicly available. We also present the status of our efforts to fabricate transition prototypes in the 85--170 GHz band.