An apparatus for monitoring fiber-fault in an optical networks, such as a passive optical networks, suitable for monitoring at least one fiber path in connection. The apparatus includes at least one fiber grating, having an individual central wavelength and coupling with the fiber networks; at least one reflection unit; and at least one optical gain medium unit, which is coupled between the fiber grating and the reflection unit, so as to produce at least one optical signal. The optical signal is amplified by allowing the optical signal to transmit to-and-fro between the fiber grating and the reflection unit, and then inducing excitation. The optical signal corresponding to the reflection unit has a corresponding spectrum for use in monitoring the optical network.