Dispersion of the group velocity refractive index in GaAs double heterostructure lasers

The dispersion of the refractive index corresponding to the group velocity
n*_{1}
has been measured as a function of wavelength. It is obtained from the longitudinal mode spacing of GaAs buried heterostructure lasers at threshold. The dependence of
n*_{1}
on wavelength contains an approximately constant term due to the refractive index n
1
and a strongly dispersive component due to
-\lambda (\partialn_{1}/ \partial\lambda)
. For a given spectral bandwidth, the dispersion of
n*_{1}
causes a temporal broadening of a pulse as it passes through the medium. This dispersive effect is shown to contribute to the width of 0.65 ps long pulses obtained recently from mode locked semiconductor lasers. By reducing the length of the laser, the dispersive effect is reduced and it is suggested that pulses as short as 10
^-13
s should be obtainable from such mode locked semiconductor lasers.