Abstract: The present invention relates to a spectrometer module comprising an input, for receiving an incoming optical signal, a variable differential group delay (DGD) element, for applying a variable birefringence retardation to said incoming optical signal, and a detector unit for detecting the power of a signal exiting said variable DGD element, having a defined state of polarization. It also relates to a monitor module, a monitoring unit and a monitoring system, comprising such a spectrometer module for use in monitoring an optical network. Further, the invention relates to a spectrometer device, for spectrometry purposes, comprising a spectrometer module as stated above.