  The Planet Formation Imager (PFI) is a future world facility that will image
the process of planetary formation. It will have an angular resolution and
sensitivity sufficient to resolve sub-Hill sphere structures around newly
formed giant planets orbiting solar-type stars in nearby star formation
regions. We present one concept for this design consisting of twenty-seven or
more 4m telescopes with kilometric baselines feeding a mid-infrared
spectrograph where starlight is mixed with a frequency-comb laser. Fringe
tracking will be undertaken in H-band using a fiber-fed direct detection
interferometer, meaning that all beam transport is done by communications band
fibers. Although heterodyne interferometry typically has lower signal-to-noise
than direct detection interferometry, it has an advantage for imaging fields of
view with many resolution elements, because the signal in direct detection has
to be split many ways while the signal in heterodyne interferometry can be
amplified prior to combining every baseline pair. We compare the performance
and cost envelope of this design to a comparable direct-detection design.
