Comments on Shakir's “zero-area optical pulse processing by degenerate four-wave mixing”

We disagree with one of the contentions of Shakir's paper, namely that a zero-area pulse can be generated by an unstable phase conjugator. We find, on the contrary, that any unstable conjugator would, if the pumps were not depleted, generate a conjugate wave of infinite area. We show by counterexample that the response generated by Shakir's technique is noncausal if the conjugator is unstable.