Christian Degen

Transverse modes in oxide confined VCSELs: Influence of pump profile, spatial hole burning, and thermal effects

We present experimental studies on the transverse mode emission behaviour of oxide-confined Vertical Cavity Surface Emitting Lasers (VCSELs). VCSELs with aperture diameters of 6um and 11um exhibit a wide variety of emission patterns from low order Hermite- Gaussian modes to high order Laguerre-Gaussian modes. We obtain detailed information about the spatial gain distribution by recording spontaneous emission intensity profiles during lasing operation. We conclude from these profiles, that the spatial carrier distribution is primarily governed by the in uence of pump induced current spreading and is only secondarily in uenced by further effects such as spatial hole burning, and thermal gradients in the laser. The combination of these mechanisms causes a strong tendency towards the emission of high order transverse modes.

Influence of anisotropies on transverse modes in oxide-confined VCSELs

We present a comprehensive fully vectorial model for the cavity eigenmodes of oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with the details of their complex structure. It includes device-inherent symmetry-breaking mechanisms like noncircular geometries and material anisotropies related to the elasto-optic and electro-optic effect. The latter is accounted for in the model starting from the material and doping profiles. We compare these theoretical results with experimental findings of spectrally and polarization-resolved transverse mode nearfields of oxide-confined VCSELs with two different aperture diameters. Within a parametric study of the influence of aperture anisotropies, we are able to calculate frequencies and gains of all transverse mode families, their polarization dependence and their spatial mode profiles which are in good agreement with the experimental findings.

Polarization-switching influence on the intensity noise of vertical-cavity surface-emitting lasers

We experimentally investigate the influence of polarization switching on the intensity noise of transverse single-mode vertical-cavity surface-emitting lasers. We show that a polarization switch always leads to strong noise in the two individual orthogonal polarization modes over a large range of pump current in the vicinity of the switch, whereas the effect on the total emission noise strongly depends on the strength of the coupling between total intensity noise and polarization fluctuations, which is mainly determined by the amount of birefringence. Thus our results demonstrate that the occurrence of a polarization switch can have a strong deteriorating effect on the noise behavior, not only if polarization selection is present, but also on the total emission noise, and therefore has to be considered in future investigations and low-noise applications.

Thermally induced local gain suppression in vertical-cavity surface-emitting lasers

Joule heating is one of the dominant mechanisms determining the transverse mode formation in vertical-cavity surface-emitting lasers at high injection currents. We give experimental evidence that in this operation regime, strong heating results in local gain suppression in the center of the laser, which overbalances the confining effect of thermal lensing, and thus favors the formation of high order modes. From our investigations of small aperture devices, we conclude that efficient heat removal is crucial for achieving single mode emission at high injection currents.

Amplitude-squeezed emission from a transverse single-mode vertical-cavity surface-emitting laser with weakly anticorrelated polarization modes.

We discuss the polarization-resolved intensity noise characteristics of a transverse single-mode vertical-cavity surface-emitting laser (VCSEL). Measurements by a sensitivity-enhanced lock-in amplifier detection scheme yield only an imperfect anticorrelation between the strong and the weak orthogonally polarized fundamental modes. Yet the total emission shows amplitude squeezing of 0.4 dB below the shot-noise level. Unlike for transverse two-mode emission, a perfect anticorrelation between the two polarization modes is not necessary for generation of amplitude-squeezed emission in a transverse single-mode VCSEL, in agreement with theoretical predictions.

Surface-emitting second-harmonic generation in AlGaAs/GaAs waveguides

We investigate the impact of waveguide properties on the performance of surface-emitting second-harmonic generation (SESHG) devices. Using multi-layer AlGaAs/GaAs ridge waveguides that are optimized for SESHG at fundamental wavelengths λω = 1060 and 1550 nm, we achieve nonlinear cross-sections among the highest reported so far. In these devices, characteristics of the guided fundamental wave strongly determine the SESHG performance: multiple reflections in the longitudinal direction affect the conversion efficiency while higher order lateral waveguide modes modify the SESHG farfield. Both effects are significant for applications of SESHG in integrated photonic circuits.

Generation of intensity squeezed light with VCSELs: theory and experiment

We present comprehensive investigations on the polarization resolved quantum noise behavior of vertical cavity surface emitting lasers (VCSELs) with various cavity designs. These experiments are accompanied by detailed studies using a semiclassical approach, first, to calculate and model theoretically the amplitude fluctuations and correlations of a two orthogonal polarization-mode semiconductor laser as realized by a VCSEL, and second, to explore theoretically the squeezing potential of VCSELs. First, we find that the possible amount of squeezing with VCSELs should be superior as compared to edge emitting lasers for ideal single mode operation and under comparable pumping conditions. Second, we demonstrate that there exists the possibility of squeezing also in the two-mode regime besides sub-shot noise emission in the well known ideal single-mode operation regime configuring the recently published first experimental demonstration of the generation of amplitude squeezed light with VCSELs. Third, we demonstrate for the first time the generation of amplitude squeezed light by a lateral and polarization single mode VCSEL with the achievement of a maximum squeezing of 0.9 dB. Finally the future trends, the applications and the limitations of these non-classical states of light with respect to metrology applications are discussed.