Abstract: This invention provides an improved optical amplifier, which comprises a rare earth doped optical fiber for amplifying the optical signal, at least one pump laser for providing a pump signal to the rare earth doped optical fiber, a wavelength division multiplexer for extracting a backward amplified spontaneous emission (ASE) signal in the C-band wavelength (1525-1565 nm) differ from the L-band wavelength (1565-1610 nm) containing the optical signal to be transmitted, a gain controller for controlling a power level of the pump signal based on a received power of the backward ASE signal.