In the seminar we will present a description of interference phenomenon in grating- based spectrometers due to applying ultra fast pulses of light. When a grating- based spectrometer is illuminated with a coherent source of pulsed light, short pulses traverse the instrument. Depending on the spectral width of the input pulses and free spectral range (FSR) of the spectrometer, two situations can be distinguished. If the spectral width of the pulse spectrum is smaller than the FSR of the device the description of the interference may be done in terms of superposition of overlapping pulses. In the case, when the spectral width of the pulse spectrum is larger than the FSR, pulses do not overlap inside the device. A quantitative model which will be presented in the seminar makes possible the explanation of the occurring interference in terms of a superposition of monochromatic components that describe a pulse. Accordingly, the temporal and spatial overlap between pulses is not a necessary condition for the occurrence of interference. The model was experimentally tested using an arrayed waveguide grating (AWG) especially designed and fabricated for testing the non-overlapping pulses regime. The quantitative model agrees well with those tests.