  The Planet Formation Imager (PFI) is a project for a very large optical
interferometer intended to obtain images of the planet formation process at
scales as small as the Hill sphere of giant exoplanets. Its main science
instruments will work in the thermal infrared but it will be cophased in the
near infrared, where it requires also some capacity for scientific imaging. PFI
imaging and resolution specifications imply an array of 12 to 20 apertures and
baselines up to a few kilometers cophased at near infrared coherent magnitudes
as large as 10. This paper discusses various cophasing architectures and the
corresponding minimum diameter of individual apertures, which is the dominant
element of PFI cost estimates. From a global analysis of the possible
combinations of pairwise fringe sensors, we show that conventional approaches
used in current interferometers imply the use of prohibitively large telescopes
and we indicate the innovative strategies that would allow building PFI with
affordable apertures smaller than 2 m in diameter. The approach with the best
potential appears to be Hierarchical Fringe Tracking based on "two beams
spatial filters" that cophase pairs of neighboring telescopes with all the
efficiency of a two telescopes fringe tracker and transmit most of the flux as
if it was produced by an unique single mode aperture to cophase pairs of pairs
and then pairs of groups of apertures. We consider also the adaptation to PFI
of more conventional approaches such as a combination of GRAVITY like fringe
trackers or single or multiple chains of 2T fringe trackers.
