  We report initial observations of coherent optical transition radiation
interferometry (COTRI) patterns generated by microbunched electrons from
laser-driven plasma accelerators (LPAs). These are revealed in the angular
distribution patterns obtained by a CCD camera with the optics focused at
infinity, or the far-field, viewing a Wartski two-foil interferometer. The beam
divergences deduced by comparison to results from an analytical model are
sub-mrad, and they are smaller than the ensemble vertical beam divergences
measured at the downstream screen of the electron spectrometer. The transverse
sizes of the beamlet images were obtained with focus at the object, or near
field, and were in the few-micron regime as reported by LaBerge et al. The
enhancements in intensity are significant relative to incoherent optical
transition radiation (OTR) enabling multiple cameras to view each shot. We
present two-foil interferometry effects coherently enhanced in both the 100-TW
LPA at 215 MeV energy at Helmholtz-Zentrum Dresden-Rossendorf and the PW LPA at
1.0-GeV energy at the University of Texas-Austin. A transverse emittance
estimate is reported for a microbunched beamlet example generated within the
plasma bubble.
