We show that an all-optical optical regenerator can improve the bit-error ratio (BER) of a signal passing through it only if the regenerator has different power transfer functions for the logical ones and logical zeros. A regenerator that operates with a single transfer function-which constitute most of optical regenerators reported in the literature-cannot improve the BER, but can only reduce the BER degradation when, for example, placed before optical amplifiers. Of all the optical

more »

... s reported to date, only the one proposed by Mamyshev, based on filtering a self-phase modulated signal, has different transfer functions for the logical ones and the logical zeros. This makes the Mamyshev scheme a superior candidate for ultrahigh-speed all-optical regeneration.