Direct time-domain measurement of optical spectra of ultrashort pulses

The standard method for measuring optical spectra using an optical spectrum analyzer or monochromator uses a diffraction grating to disperse the optical signal in the spatial domain. In this paper we propose and implement an alternative approach which uses temporal dispersion to resolve different spectral components of ultrashort optical pulses at different instants in time. The method is very simple, requiring only a length of optical fiber, a fast photodiode and a sampling oscilloscope. The method resolves only the pulsed component of the spectrum and is particularly useful when one wishes to discriminate the pulsed spectral components from a spectrum containing both pulsed and continuous wave (cw) components