© 2005 OSA/FIO 2005
FTuA2
Waveguide Mode Dynamics of InGaN Laser Diodes Ulrich T. Schwarz Naturwissenschaftliche Fakultät II – Physik, Universität Regensburg, Universitätsstr. 31, D-93053 Regensburg, Germany Tel.: +49-941 943 2113; Fax.: +49-941 943 2754; e-mail:
[email protected]
Abstract: We use a scaning near-field microscope (SNOM) in combination with a time resolved detection scheme to measure the evolution of the near-field and far-field of InGaN laser diode waveguide modes. We observe lateral mode competition, filamentation, and beam steering. From intensity distributions measured simultaneously at different propagation distances from the LD facet we reconstruct the time dependent phase distribution of the waveguide mode. 2005 Optical Society of America
OCIS codes: (140.2020, 300.6470) Laser Diodes, Mode Dynamics, Near-field
For the last years research on InGaN laser diodes (LD) pivots around cw characteristics, the most important of which are lifetime, optical output power, and quality of the far-field. For applications in data communication and optical data storage (DVD, blu-ray disk) the dynamic properties relative intensity noise (RIN), frequency chirp, and beam pointing stability are as important. These dynamic properties of laser diodes are determined by the differential gain and by the coupling of gain and carrier induced change of refractive index, i.e. by the antiguiding factor or alpha factor. Using the Hakki-Paoli method, we measure optical gain, differential gain, and carrier induced change of the refractive index for InGaN LDs, all as a function of carrier density and wavelength [1,2]. The resulting alpha factor of α=4 at laser threshold [3] is relatively high or a heterostructure LD. A strong tendency for blue LDs for filamentation can thus be expected. We use a selft-built scanning near-field microscopy (SNOM) in combination with a time-resolved detection scheme for a direct observation of mode competition, filamentation, and beam steering [4-6]. With the same setup we can measure the far-field and the propagation from near-field to far-field. From the simulataneous measurements of the intensity at different distances z to the LD facet one can reconstruct the near-field phase distribution [5,6]. For both gain guided and ridge LDs the near-field phase shows a positive curvature which is in agreement with filamentation. The phase also exhibits a tilt of the order of π over the facet in agreement with the tilting of the beam pointing direction. Variation of the far-field and tilting of the phase are obviously synchronous, as the phase is being derived from the propagating beam. It is intersting to note that the intensity distribution of the near-field changes to a lesser degree during beam steering, as compared to the phase. Beam pointing instability seems thus to be linked to an instability of the phase distribution in the waveguide. [1] U. T. Schwarz, E. Sturm, W. Wegscheider, V. Kümmler, A. Lell, and V. Härle, "Gain spectra and current-induced change of refractice index in (In/Al)GaN diode lasers," Phys. Status Solidi A 200, 143 (2003). [2] U. T. Schwarz, E. Sturm, W. Wegscheider, V. Kümmler, A. Lell, and V. Härle, "Excitonic signature in gain and carrier induced change of refractive index spectra of (In,Al)GaN quantum well lasers," Appl. Phys. Lett. 85, 1475 (2004). [3] U. T. Schwarz, E. Sturm, W. Wegscheider, V. Kümmler, A. Lell, and V. Härle, "Optical gain, carrier-induced phase shift, and linewidth enhancement factor in InGaN quantum well lasers," Appl. Phys. Lett. 83, 4095 (2003). [4] U. T. Schwarz, M. Pindl, E. Sturm, M. Furitsch, A. Leber, S. Miller, A. Lell, and V. Härle, "Influence of ridge geometry on lateral mode stability of (Al,In)GaN laser diodes," Phys. Status Solidi A 202, 261 (2005). [5] U. T. Schwarz and M. Pindl, "Near-field and far-field dynamics of (Al,In)GaN laser diodes," SPIE proc. 5738 (2005). [6] U T. Schwarz, M. Pindl, W. Wegscheider, C. Eichler, F. Scholz, M. Furitsch, A. Leber, S. Miller, A. Lell, and V. Härle, "Near-field and far-field dynamics of (Al,In)GaN laser diodes," Appl. Phys. Lett. 86, 161112 (2005).
Fig. 1. Near-field intensity distributions of a 2.5 µm ridge LD at I=2Ith. a) to h) show the evolution of the near-field at the beginning of the laser pulse. Consecutive frames are 10 ns apart. Width of each frame is 5 µm.
