  This work aims at investigating the optical transmission system needed for
such lightweight sail, taking into account the physical constraints of such
unprecedented link and focusing on the optimal scheme for the optical signal
emission. In particular, the optical signal is distributed to several emitters
on the sail. The light diffraction resulting from the pattern of the emitters
acting coherently determines the characteristics of the whole beam transmitted
by the sail and of the received signal on the Earth. The performance of the
digital communication system using pulse position modulation (PPM) can be
assessed and channel coding schemes are proposed. We are using the paradigm for
which the entire sail communication system is described as a Tree-of-light: the
detectors, CPU, memory and laser transmitter are the central unit, representing
the trunk of the tree. The branches of the tree are the waveguides, directed to
the sail surface. By means of multimode splitters, the signal is further
distributed via the petioles to the emitters, the leaves, realized by grating
couplers (GCs), on which this work is more focused.
