  Linear optical elements are pivotal instruments in the manipulation of
classical and quantum states of light. The vast progress in integrated quantum
photonic technology enables the implementation of large numbers of such
elements on chip while providing interferometric stability. As a trade-off
these structures face the intrinsic challenge of characterizing their optical
transformation as individual optical elements are not directly accessible. Thus
the unitary transformation needs to be reconstructed from a dataset generated
with having access to the input and output ports of the device only. Here we
present a novel approach to unitary reconstruction that significantly improves
upon existing approaches. We compare its performance to several approaches via
numerical simulations for networks up to 14 modes. We show that an adapted
version of our approach allows to recover all mode-dependent losses and to
obtain highest reconstruction fidelities under such conditions.
