Spectral broadening of pulsating Al/x/Ga/1-x/As double heterostructure lasers

The longitudinal-mode spectrum of stripe-geometry double-heterostructure (DH) lasers exhibits a spectral broadening of about 0.8 A when the laser emits pulsations. The broadening results from a chirp to longer wavelengths which occurs during the approximately 0.4-ns-long pulse. The chirp is due to an increase of the refractive index resulting from the reduction in the carrier concentration during the pulse. Both plasma and band-to-band interactions affect the index change, with the latter mechanism giving the dominant contribution. A comparison of the wavelength shift obtained during the pulse with the current-dependent shift of the longitudinal modes at currents well below threshold confirms that the change in carrier concentration is responsible for the chirp.