Transverse mode dynamics in vertical cavity surface emitting lasers excited by fast electrical pulses

Abstract

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...

Picosecond emission dynamics of vertical-cavity surface-emitting lasers: Spatial, spectral, and polarization-resolved characterization

“…Modeling results concerning the turn-on behavior also indicate that the laser starts to emit in low order modes due to their lower threshold, compared with higher order modes [15]. Spectrally, however, not spatially resolved experiments performed by Buccafusca et al [7], [8], [19] agree very well with the observed behavior. The set of excited modes during the first relaxation oscillation peak depends on pumping conditions, and there is little change in the relative intensity increase among the modes, indicating that during this time the threshold behavior of each mode dominates the evolution of the near-field intensity [8].…”

“…The observation, analysis, and understanding of the spatio-temporal dynamics on picosecond timescales, associated with the multiple transverse modes, is thus not only promising for device optimization, but also allows us to gain physical insight into the underlying mechanisms and laser-matter interaction. Up to now, only spatially integrated [7], [8] and one-dimensional cuts [9] of the time-resolved dynamics of the near-field intensity exist, precluding investigations of the dynamics of the full 2-D-spatial interactions.…”

Statistics of transverse mode turn-on dynamics in VCSELs

“…The carrier density slope must vanish at the origin when there is azimuthal symmetry and, when the injection current is supplied in a region near the center of the device, a vanishing slope of the carrier density can be assumed at the boundary of the active region [22]. The resulting time evolution of the carrier density components is given by (9) and the time evolution of the optical intensities by (10) In the above equations, new parameters appear (11) which accounts for the driving current of the component, resulting from the overlapping with the current density profile.…”

“…On the other hand, it is known that spatial hole burning (SHB) and carrier diffusion play key roles in transverse mode selection and, in turn, these processes depend upon the laser drive current [8], [9]. Experiments have been carried out recently showing such effects [10]. Consequently, the excitation of transverse modes when the laser is subject to large excursions in the bias current will be strongly influenced by SHB effects.…”

Dynamics of phase-locking random lasers

“…The mode-locked state may be achieved after a certain transient [1,2], by different approaches: in fibers just a small Kerr nonlinearity is needed to obtain mode locking [3], while in laser cavities nonlinear coupling may be obtained by active or passive modulation [4]. The ML is modeled by a nonlinear Schrodinger equation whose stable solutions are solitonlike.…”