Comparison between optical gain spectra of in _xGa _1-xN/In _0.02Ga _0.98N laser diodes emitting at 404 nm and 470 nm

We investigate the gain formation properties of In _1-xGa _1-xN laser diodes emitting at 404 nm and 470 nm (sample A and B, respectively). Employing the Hakki-Paoli method, their optical gain spectra were measured at 298 K and 333 K. Since spatial fluctuations of the well width and In composition are considerable for long-wavelength laser diodes, broadening of the optical gain and luminescence spectra are clearly enhanced in sample B when compared with sample A. Moreover, population inversion occurs in at least two discrete energy levels for the gain spectra of sample B. It was found that only the modal gain of the higher gain peak can overcome the mirror loss, therefore the lower one does not contribute to lasing action. We also estimate internal loss of A and B to be 25 cm ^-1 and 30 cm ^-1, respectively. The maximum gain value of A increases linearly with increasing injected current even at 333 K, while that of B grows sub-linearly.