  Lin et al. (Reports, 7 September 2018, p. 1004) reported a remarkable
proposal that employs a passive, strictly linear optical setup to perform
pattern classifications. But interpreting the multilayer diffractive setup as a
deep neural network and advocating it as an all-optical deep learning framework
are not well justified and represent a mischaracterization of the system by
overlooking its defining characteristics of perfect linearity and strict
passivity.
