End-to-end simulation of Hierarchical Fringe Tracking

We discuss a new generation fringe tracker (FT) that implements a Hierarchical Fringe Tracker (HFT) architecture with a very broad band near infrared spectral coverage from 1.1 to 2.2 mu m in the J, H and K bands. The goal is to approach the absolute maximum fringe tracking sensitivity in optical long baseline interferometry, first on the VLTI, and to show that an HFT has performances independent from the number of apertures, a key characteristic for larger interferometers from CHARA to the VLTI with more UTs or combining all UTs and ATs to future very large interferometers. This paper describes the development in progress of an end-to-end simulator of such a system based on our first laboratory tests of prototype HFT. This simulator already allowed us to define a new optimization for the integrated optics HFT chips, to discuss a set of operating parameters for our new generation fringe tracker and to confirm that it is applicable to an indefinite number of apertures and should approach or even exceed a limiting sensitivity on the VLTI of K similar to 16, which is a gain of at least 3 magnitudes over the expected performance of the current GRAVITY FT in the context of the ongoing GRAVITY+ VLTI upgrade.