Hierarchical Fringe Tracking (HFT) is a fringe tracking concept optimizing the sensitivity
in optical long baseline by reducing to an absolute minimum the number of measurements
used to correct the OPD fluctuations. By nature, the performances of an HFT do not
decreases with the number of apertures of the interferometer and are set only by the
flux delivered by the individual telescopes. This a critical feature for future interferometers
with large number of apertures both for homodyne and heterodyne operation. Here we
report the design and first optical bench tests of integrated optics HFT chips for
a 4 telescopes interferometer such as the VLTI. These tests validate the HFT concept
and confirm previous estimates that we could track accurately fringes on the VLTI
up to nearly K similar to 15.9 with the UTs and K similar to 12.2 with the ATs with
a J+H+K fringe tracker with one HFT chip per band. This is typically 2.5 magnitudes
fainter than the best potential performance of the current ABCD fringe tracker in
the K band. An active longitudinal and transverse chromatic dispersion correction
allows the optimization of broad band fiber injections and instrumental contrast.
We also present a preliminary evaluation of the potential of such a gain of sensitivity
for the observations of AGNs with the VLTI.
