An optical system including an input pulse source for providing an input optical pulse the intensity of which is sufficiently high to result in stimulated Raman scattering; and a first optical component that spatially disperse the frequency components of the input optical pulse to produce a dispersed beam. A bandpass filter is disposed asymmetrically in the path of the dispersed beam to produce a modified dispersed beam; and a second optical component is disposed in the path of the modified dispersed beam to return the frequency components substantially to the spatially distribution of the input optical pulse.