This invention provides a method for measuring and monitoring the state of polarization (SOP) of a polarization maintaining (PM) fiber and the like using a narrowband fiber Bragg grating (FBG) written on the same. The PM fiber therefore 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 will generally reflect 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. After having adjusted the PM fiber to a predetermined SOP, it is possible to accurately measure the optical properties of a second grating, or of another optical device, according to the predetermined SOP. The method of the present invention can also be advantageously used for accurate measurement of reflectivity.