Optical amplifier with a spectral gain monitor using a volume phase grating

An optical amplifier system for amplifying an input wavelength division multiplexed (WDM) optical signal with a first optical coupler to extract a portion of the power of the input signal, an erbium-doped fiber amplifier to generate an output signal and a second optical coupler to extract a portion of the power of the output signal. A spectral monitoring unit having a volume phase grating separates the extracted input and output signals into spectral components. A photo-detector array of the spectral monitoring unit determines the power level of the spectral components. The system further includes a controller operative to control the operation of the amplifier in response to the power levels of the spectral components.