Molitor, Andreas (2015)
Investigations on Stokes polarization parameters of Vertical-Cavity Surface-Emitting Lasers: spatially, spectrally and time-resolved.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Their unique geometrical structure first suggested by Prof. Kenichi Iga in 1977, makes vertical-cavity surface-emitting lasers (VCSELs) compact, versatile, robust and reliable optoelectronic devices, which have already outperformed edge-emitting lasers (EELs) in terms of low threshold current and high wall-plug efficiency. Most relevantly, regarding application field entering and mass production, is their low cost manufacturing due to their property of enabling wafer-level testing. In addition to these benefits provided by their vertical design, VCSELs exhibit complex polarization behavior resulting from the circular symmetry of their resonator and from the isotropic gain of their active medium, mostly based on quantum well (QW) or quantum dots (QD). In principle, the polarization of the light emitted by these optoelectronic devices should reflect the selection rules of the quantum well (QW) optical transition symmetry and thus should exhibit a circularly polarized state of light. However, various symmetry breaking mechanisms, such as strain, thermal effects, electro-optically induced birefringence, dichroism or an applied magnetic field convert this circular polarization to an elliptically polarized emission, often with a large amount of linearly polarized light. Therefore, at a first glance, polarization resolved investigations, using a linear polarizer only, on intensity noise, wave chaos, transverse mode behavior and time-resolved measurements are legitimized, although being restricted to a projection on two orthogonal linear polarization components. But clearly these kinds of measurements do not reveal the entire knowledge of the polarization state, due to the conversion of an eventual circular component into a linear component.

In contrast to this rather simple, straightforward projection method the full information about the state of polarization is accessible only by determining the four Stokes polarization parameters (SPPs) S0, S1, S2 and S3. The remarkable fact about these four parameters is that they are directly measurable quantities, thus enabling the complete description of any state of polarization in mathematical terms, including unpolarized light as well as partially polarized light. Noteworthy about the Stokes formalism is the fact, that it has been invented already in 1852 by Sir George Gabriel Stokes. During this time, a crisis in optics appeared due to fundamental different approaches namely the wave- as well as the corpuscular-ansatz. The formulation of the SPPs supports the wave theory essentially, providing explanation of the famous Fresnel-Arago interference laws on the basis of the wave approach for the first time. These SPPs formulated in 1852, are even nowadays the appropriate formalism to describe the complex polarization behavior of VCSELs.

The aim of this work is to conduct comprehensive experimental investigations on spatially, spectrally and time-resolved SPPs of transverse single-mode and transverse multi-mode VCSELs. Furthermore, combined measurements such as spatio-spectrally as well as spectro-temporally resolved measurements will be also performed and compared among each other, demonstrating the consistency and reliability of these quantitative results. Moreover, a detailed analysis of each particular transverse mode will be performed, uncovering the physical mechanisms responsible for the state of polarization of the total emission of transverse single-mode as well as multi-mode VCSELs. In addition the time dependence of polarization states of the total emission on a microsecond timescale will be investigated. These overall results in combination with investigations on time-resolved SPPs measurements for each particular transverse mode will be a point of origin for future numerical simulations.

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