It is proposed that a segmant of an Er doped fiber with a couple of fiber gratings on the ends may be used as a novel optical bistability device with low power and high speed. Operation and bistability threshold of the device are analysed in accordance with the distributed feedback couple mode theory and the nonlinear characteristic of the Er doped fiber. It is shown that a nanoseconds microwatts bistable operation of a centimeters sized device is realizable under nowaday technological conditions.