Measurement of the frequency shift of a picosecond pulse from a mode-locked injection laser

The frequency shift (chirp) of a picosecond pulse emitted by an actively mode-locked cw injection laser with an external dispersive resonator was observed and measured. The chirp was δω = 0.5×10¹²sec^–1(δλ = 0.2nm at the 880 nm wavelength) for a 30 psec pulse and the frequency decreased during a pulse (down chirp). The measurements were made using dynamic spectrograms obtained using an image-converter streak camera operated in the linear horizontal scanning mode. Frequency sweeping within the pulse is attributed to the influence of changes in the injected carrier density on the refractive index of the active region and on the optical length of the laser resonator.