Height and width of the ridge forming the laser diode waveguide determine threshold current density and lateral mode stability. We measure the optical near‐field of the laser mode and simulate the two–dimensional mode distribution including waveguide losses and optical gain. The simulations show that weak guiding and not current spreading is the major cause for increased threshold current densities in weakly guided laser diodes. The near‐field measurements show fundamental and higher order modes for nominally identical ridge laser diodes. We demonstrate that asymmetric losses in the waveguide bias the lateral mode competition towards higher order modes for an intermediate range of index guiding. This asymmetric damping is specific for (Al,In)GaN laser diodes due to absorption introduced in the p ‐waveguide by magnesium doping. This competition of lateral modes can also be seen spectrally as two longitudinal mode combs of different optical gain. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The exciton is observed in (In,Al)GaN laser diodes as resonance in the optical gain spectra and in the spectra of the carrier induced change of the refractive index. The observed instability of the exciton with respect to the free electron-hole plasma with increasing carrier densities is accompanied by a blueshift of the exciton resonance due to the quantum confined Stark shift. The experiments confirm central points of many-body simulations of InGaN∕GaN quantum wells. The exciton becomes unstable near threshold and so lasing occurs from the electron–hole plasma.

Adapting the Hakki Paoli method to group III nitrides, we measure gain, differential gain, carrier-induced change of refractive index, carrier-induced phase shift, and the antiguiding factor. Our measurements also cover the low-carrier-density regime, in which spontaneous and piezoelectric fields and Coulomb interaction are only partially screened. This regime is most interesting as a comparison with existing theoretical simulations, including many-body effects.

Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text U. T. Schwarz, E. Sturm, V. Kümmler, A. Lell, and V. Härle, "Gain spectra and current-induced phase-shift in blue laser diodes," in Frontiers in Optics, OSA Technical Digest (CD) (Optica Publishing Group, 2003), paper WMM3. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Adapting the Hakki-Paoli method for blue laser diodes we measure gain spectra for (In/Al)GaN on SiC substrate laser diodes in the low carrier density regime. From the measured longitudinal mode spacing we calculate the effective refractive index and can exclude a mode spacing 'anomaly.' We demonstrate that the substrate has a considerable influence on the modal gain. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.