Measurement and Interpretation of Dynamic Spectrograms of Picosecond Light Pulses

A dynamic spectrogram depicts intensity as a function of frequency and time simultaneously, subject to the classical uncertainty relation δωδt≈2π. For an optical pulse having the Fourier transform |g(ω)|eiφ(ω), high-resolution spectroscopy gives |g(ω)|2, while the dynamic spectrogram gives knowledge of ∂φ(ω)/∂ω permitting reconstruction of many features of the amplitude and phase modulations of the original pulse. Linear, parabolic, and two sinusoidal spectrogram shapes are interpreted theoretically. An instrument for measuring dynamic spectrograms is described. It involves the use of a spectrometer with ultrafast time response and has been used in an antisymmetric mode to measure frequency sweep rate as a function of wavelength for picosecond laser pulses. The change in the dynamic spectrogram of a pulse brought about by linear pulse compression and the resultant change in pulse envelope shape have been computed for a typical picosecond pulse.