A coherent optical spectrum analyzer for monitoring a spectrum of a fiber link is provided. The coherent optical spectrum analyzer comprises an input connectable to the fiber link, the input being connected to a first input of a coherent detector having at least two input, the first and a second input, and an output. The coherent optical spectrum analyzer further comprises a local oscillator having an output connected to the second input of the coherent detector, wherein the output of the coherent detector is connected to a first input of a processing unit, the processing unit also being connected to an input of the local oscillator, the processing unit being configured for analyzing information from the coherent detector. The local oscillator comprises a semiconductor laser tuned by temperature to a specific wavelength and swept by changing a bias current, the local oscillator being controlled by the processing unit.