We report measurements of the transient multimode dynamics of vertical cavity surface emitting lasers excited by short ͑100-200 ps rise times, 1 ns duration͒ electrical pulses. Fast changes on the spatial distribution of the output power and strong mode competition are observed. Numerical simulations show that the observed dynamics are due to the partial overlap of the different transverse modes through spatial hole burning. © 1996 American Institute of Physics. ͓S0003-6951͑96͒04505-0͔Recent advances in semiconductor device technology has produced two-dimensional arrays of vertical cavity surface emitting lasers ͑VCSELs͒, that, with modulation bandwidths of the order of 10-15 GHz, 1 are suitable for optical data links and optical signal processing. However, when semiconductor lasers are modulated at gigahertz frequencies, transient effects can affect their performance. In particular, the presence of higher order modes in the cavity can cause rapid variations of the spectral, spatial, and noise characteristics of the laser beam. Recent theoretical calculations 2 predicted the importance of spatial hole burning effects in the dynamic evolution of transverse modes in VCSELs but, to our knowledge, no experimental studies on the subject have been reported. In this letter, we present spectrally and temporally resolved measurements of VCSELs under fast electrical excitation, which confirm the importance of mode competition and spatial hole burning in the dynamics in VCSELs, which are essentially different from the thoroughly studied dynamics in edge emitters. The transient response of edge emitters is the result of several longitudinal modes competing from the onset of laser operation.3 In contrast, the transient response measurements in VCSELs discussed here, show that the onset of a higher order spatial mode occurs after a delay dependent on prebias and pumping conditions. The presence of this higher order transverse mode is observed to produce a significant drop in the power of the TEM 00 mode due to the redistribution of the available gain between the modes.The lasers used in the measurements were gain guided GaAs/AlGaAs multiple quantum well VCSELs, with top and bottom mirrors consisting of step graded AlAs/AlGaAs distributed Bragg reflectors with 23 and 32 periods, respectively. 4 The active region diameter was chosen to be 8m to limit the number of transverse modes oscillating in the cavity. The VCSELs were prebiased through a bias tee and then electrically pumped using a fast pulse generator capable of providing square pulses of 0.8 -1 ns duration with 100-200 ps rise and fall times. Both the prebias and the peak currents were varied in order to characterize the VCSEL mode dynamics under different excitation conditions. Prebias currents were varied from 35% to 80% of the dc threshold value (I th ), while current pulse amplitudes ranging from 4 to 20 times I th were added to pump the VCSEL. The pulsed currents were measured using a fast current probe ͑rise and fall timesϽ100 ps͒ constructed specifically for this...

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We have developed a theory that explains the spectral distribution and the different time decay behaviors of the signals observed in picosecond excitation-correlation measurements of semiconductor materials. The results display good quantitative agreement with experiments on multiple quantum wells and show that band filling has an important role in determining the characteristics of the correlation signals. Two limit cases are found and mathematically characterized: the nondegenerate regime, where the signals are positive with exponential-like decays, and the degenerate regime, where band filling gives rise to negative signals. It is shown that the technique is a sensitive probe of the carrier dynamics not fully exploited in previous work.

Abstract-The multimode dynamics of vertical-cavity surfaceemitting lasers with different active-region diameters was measured under subnanosecond electrical excitation (800-ps pulse duration, 100-ps risetime). The dynamics is characterized by the delayed onset of higher order modes which have a turn-on delay that is dependent on the active-region diameter and the excitation parameters. A simple model that can be used to estimate this turn-on delay for large-area devices is presented. Polarizationresolved measurements show that, under this fast excitation condition, both orthogonal polarization states are isomorphic. The influence of the observed dynamics on the relative intensity noise of these devices is also discussed.