An optical characterization device has a guiding body and a diffractive structure integrated with it. The guiding body delivers radiation to the diffractive structure at a preselected angle and the diffractive structure diffracts at least a portion of radiation guided to it to give spatial resolution according to wavelength. A position sensitive detector detects the position of the spatially resolved radiation and thus the radiation can be characterized according to wavelength. Embodiments can be used to monitor and adjust the wavelength of a radiation source, using a feedback connection. By using the integrated approach, a very small structure can be realized and the structure can be relatively simply fabricated and assembled. The guiding body might be either capable of supporting multimode propagation of the radiation or might be a single mode waveguide. Where it is capable of supporting multimode propagation, an input to the guiding body acts to select a mode for propagation of the radiation.