  The full scientific potential of the VLTI with its second generation
instruments MATISSE and GRAVITY require fringe tracking up to magnitudes K>14
with the UTs and K>10 with the ATs. The GRAVITY fringe tracker (FT) will be
limited to K~10.5 with UTs and K~7.5 with ATs, for fundamental conceptual
reasons: the flux of each telescope is distributed among 3 cophasing pairs and
then among 5 spectral channels for coherencing. To overcome this limit we
propose a new FT concept, called Hierarchical Fringe Tracker (HFT) that cophase
pairs of apertures with all the flux from two apertures and only one spectral
channel. When the pair is cophased, most of the flux is transmitted as if it
was produced by an unique single mode beam and then used to cophase pairs of
pairs and then pairs of groups. At the deeper level, the flux is used in an
optimized dispersed fringe device for coherencing. On the VLTI such a system
allows a gain of about 3 magnitudes over the GRAVITY FT. On interferometers
with more apertures such as CHARA (6 telescopes) or a future Planet Formation
Imager (12 to 20 telescopes), the HFT would be even more decisive, as its
performance does not decrease with the number of apertures. It would allow
building a PFI reaching a coherent magnitude H~10 with 16 apertures with
diameters smaller than 2 m. We present the HFT concept, the first steps of its
feasibility demonstration from computer simulations and the optical design of a
4 telescopes HFT prototype.
