A new M–Z interferometer all-optical switch (AOS) consisting of a single nonlinear fiber and a linear long-period fiber grating (LPFG) pair is investigated. The switching principle is based on shifting the transmission spectrum over half of the subspectrum width by the pump light. In this paper, two types of such devices are simulated. The first one concerns use of the usual silica fiber based on the Kerr effect, the switching power of which is 471 W, consisting of the experimental result3; the second one concerns the use of an erbium-doped fiber (EDF) based on the resonant nonlinearity, the switching power of which is reduced by less than 25 mW. The transmissions of the AOS in the EDF-LPFG pair for different parameters of LPFG are also discussed.