An optical monitor device and method are presented aimed at providing low-loss monitoring of an optical signal confined in an optical guiding structure. An optical resonator structure is placed spatially adjacent to the guiding structure and is optically coupled to the guiding structure, and an optical detector is optically coupled to the resonator structure. The coupling between these elements is such that the coupling of light from the guiding structure to the optical resonator is smaller then the coupling of light from the resonator to the optical detector.