4-input photonic kernel-nulling for the VLTI

Kernel-nulling is a beam combination architecture for a nulling interferometer optimized for high-contrast companion detection at high angular resolutions. This novel approach is able to suppress the photon noise from stellar light by coherent destructive interference, similar to a classical nuller, and in addition produces a pair of raw nulled outputs that possess the same response to instrumental/environmental perturbations while still producing distinct response to off-axis light. The difference between a pair of raw outputs will result in a kernel-null observable which is robust to piston induced phase error terms to second order, removing the dominant source of noise in classical nulling. We present initial laboratory characterization results of a four-input kernel-nuller photonic device at near-infrared wavelengths. The necessary routing, beam splitting, and recombination is performed within a planar silicon nitride (SiN) photonic chip fabricated using photo-lithography. The four input beams are recombined to create one bright output and three nulled outputs. The linear combination of a pair of these nulled outputs creates one kernel-null. This device is designed to explore the possible circuitry for the VIKiNG visitor instrument being commissioned for the VLTI. We also present simulation results of how kernel-nulling can be used at the VLTI for the astrophysical detection of high-contrast companions.