A method for measuring and monitoring the state of polarization (SOP) of a polarization maintaining (PM) fiber using a narrowband fiber Bragg grating (FBG) written on the same is provided. The PM fiber comprises a first narrowband reference FBG which is used as a reference to measure and monitor the SOP of the PM fiber. Due to the birefringence properties of the PM fiber, the reference FBG typically reflects two narrowband spectra, each having a central wavelength; one in the slow axis and one in the fast axis. By measuring the intensity of the reflected spectra in each axis and by tuning the fiber with a polarization controller, it is possible to adjust the fiber to a predetermined SOP. Accurate measures of optical properties (e.g. reflectivity) of optical devices (e.g. a FBG grating), according to the predetermined SOP, are possible.