An apparatus includes a spatial light modulator configured to receive an optical pulse train; and output a modulated optical pulse train; a non-linear optical system that receives the modulated optical pulse train and generates a non-linear optical signal; and a power detector that detects a power of the generated non-linear optical signal. A control system outputs a signal to the spatial light modulator to cause it to modulate the optical pulse train by modulating the spectral phase of the optical pulse at wavelengths within a current wavelength range subset and maintain the spectral phase of the optical pulse constant at wavelengths outside the current wavelength range subset; and based on the detected power, extracts values of the spectral phase for the optical pulse at wavelengths within the current wavelength range subset, the values extracted being those that compress the optical pulses.