Analysis and Control of Optical Modes in VCSELs and VCSEL-Arrays

In this thesis, we studied the modal structure and the polarization of the optical field emitted by vertical-cavity surface-emitting lasers (VCSELs) emitting in the short- and long-wavelength domains (0.9-1.3μm). Our study concerns two types of components: single aperture VCSELs and coupled VCSEL-arrays. Light emitted from a laser cavity reflects a modal structure corresponding to specific spatial distributions of the optical field in the cavity, at given wavelengths. A polarization state is also associated with each spatial mode, manifesting the vectorial nature of the optical field concerned. Although, during this study, we used VCSELs working with similar fundamental principles whatever the wavelength is, in the details, the device design of short-wavelength VCSELs (∼960 nm) differed from the device design of VCSELs working at long-wavelength (1.3 μm). Consequently, each type of VCSELs displays different modal characteristics. First, we studied the spatial modes of short-wavelength (0.9 μm) VCSEL-arrays and compare the experimental results with modelling based on the coupled mode theory. Next, in order to extract the spatial mode distribution of the optical field together with the associated power, we proceeded to spatial coherence measurements for each VCSEL pairs within the array, accompanied by a theoretical study using the modal coherence theory developed by E. Wolf. This method was applied to 2×2, 3×3 and 4×4 VCSEL-arrays and allowed giving quantitative results about the modal structure including the spatial distribution and power. Consequently, it was possible to draw valuable information in order to improve the laser cavity structure so that it would be possible to produce controlled mono-mode emission. In a second part, we interned our attention to long-wavelength VCSELs. There, we studied experimentally the spatial mode and the polarization mode structure of wafer-fused long-wavelength (1.3 μm) VCSELs while comparing these results with a theoretical model implemented as a simulator by Tomasz Czyszanowski of Technical University of Lodz. Thanks to a spectral scan setup, it was possible to acquire the spatial distribution of each mode in the near- and far-field and consequently analyse accurately the behaviour of VCSEL-arrays working in multimode by spectrally separating each mode. As a conclusion, it was possible to show that multimode VCSEL-arrays behave similarly to single VCSELs having a similar size of the whole array but with a weak transverse gain-profile modulation. This weak modulation suppresses the local parasitic laser emissions (also called filamentation). Moreover, we noticed that as a result of the particular gain modulation the low-order modes are suppressed but several higher-modes not represented by a simple couple-mode model are present. In a third part, the polarization study of spatial modes of long-wavelength either single VCSELs or VCSEL-arrays shows that the polarization of the mode is linear and aligned in a preferential direction along the lattice axis, [110] or [1 ̄10] of the VCSEL wafer. We also observed that most of the VCSELs start emitting at threshold polarized in the [110] lattice direction. Based on a structural study of the tunnel junction embedded in these wafer-fused VCSELs, we assign this behaviour to birefringence or dichroism related to anisotropic epitaxial growth effects. As a rule, the VCSEL- arrays favourably feature a linear stable polarization unlike the single aperture VCSELs. This is probably related to the larger transverse extension of the modes in the VCSEL-arrays, requiring a larger index variation for inducing polarization switching. Finally to finish this study, we used the optical injection locking method on single VCSELs and VCSEL-arrays emitting in the long-wavelength range (1.3μm) in order to analyse and control the polarization mode and spatial mode lasing. We showed that it is possible to modulate the modes, i.e., by making work mode that does not and reinforce the power of working modes. The results of our studies provide guidelines for improving the mode control in VCSELs and VCSEL-arrays. The mode control can be obtained either passively by modifying the laser structure (cavity or mirror designs) or actively by dynamically changing some operating parameters (injection current, optical injection, active control of cavity shape, etc.).