Direct characterization of linear-optical networks
  We introduce an efficient method for fully characterizing multimode linear-optical networks. Our approach requires only a standard laser source and intensity measurements to directly and uniquely determine all moduli and non-trivial phases of the matrix describing a network. We experimentally demonstrate the characterization of a $6{\times}6$ fiber-optic network and independently verify the results via nonclassical two-photon interference. 
Saleh Rahimi-Keshari, Matthew A. Broome, Robert Fickler, Alessandro   Fedrizzi, Timothy C. Ralph, and Andrew G. White
quant-ph