Abstract: Techniques and devices based on a wave-guiding element which has a spatial grating pattern that is an oscillatory variation along its optic axis. The wave-guiding element is configured to receive an input optical signal and to produce an output optical signal by reflection within a Bragg reflection band produced by the spatial grating pattern so as to produce time delays of different reflected spectral components as a nonlinear function of spatial positions along said optic axis at which the different reflected spectral components are respectively reflected. Such a wave-guiding element may be a nonlinearly chirped fiber grating A control unit may be engaged to the wave-guiding element and is operable to change a property of the spatial grating pattern along the optic axis to tune at least relative time delays of the different reflected spectral components nonlinearly with respect to wavelength.