KONEČNÝ, A. Časoprostorová dynamika a koherentní řízení frekvenčních hřebenů kvantových kaskádových laserů [online]. Brno: Vysoké učení technické v Brně. Fakulta strojního inženýrství. 2021.

Posudek vedoucího

Detz, Hermann

The thesis is focused on a deeper understanding of the frequency comb formation in quantum cascade lasers. The device behavior is described based by formulating a master equation that governs the optical gain, losses as well as the carrier dynamics within the active region that eventually give rise to the formation of comb states. This work is immediately relevant to the communities around intersubband lasers and infrared spectroscopy for multiple reasons: * This is a topic at the forefront of QCL research. Frequency combs in these devices were only discovered and engineered very recently. * The option to drive such laser devices into comb states in a controlled way enables dual-comb spectroscopy techniques, integrated on a single chip, that will be the basis of next generation spectroscopy systems, ultimately replacing FTIRs by miniaturized systems without moving mechanical parts. * The development of such spectroscopy techniques enables a multitude of bio-chemical, medical and environmental sensing applications. The thesis first outlines the present state of spectroscopic systems and describe the basic operational principle of quantum cascade laser. Based on this, frequency combs are introduced step by step, first starting with their spontaneous formation and then describing options to induce them using external signals via injection locking. A thorough description of the model then describes the interplay of gain dynamics and dispersion within the laser cavity that enable the different regimes of multi-mode, single-mode and comb operation, as well as the strategy for the numerical implementation, leading to a discretized master equation. This first leads to a one-dimensional model, which is then extended by a waveguide mode solver to provide two-dimensional mode profiles. Taking this model as a basis, the thesis explores first self-starting combs, as well as the option for injection locking. The simulation results provide a first glimpse into the locking mechanisms of spontaneously formed and injection locked combs and contribute to a general understanding of this quickly evolving field. The model may in future also serve to explain frequency combs in other semiconductor lasers.

Posudek oponenta

Souza, Patricia Lustoza de

Optical frequency combs have major impact in metrology, spectroscopy and fundamental physics. In particular in the field of spectroscopy they have reached maturity and have already outperformed conventional Fourier techniques in terms of speed, sensitivity and resolution. On the other hand, spectroscopic experiments using optical frequency combs require a sophisticated set-up with various optical elements and electronic equipment, especially for the mid-infrared range. A major step towards simple, fast and precise spectroscopy of molecules in this frequency range was the generation of mid-infrared frequency combs using quantum cascade lasers (QCLs) in 2012 [1]. Short after, in 2014, molecules` spectroscopy using two frequency combs was demonstrated using the compact QCLs [2]. It is then clear that this is a new field of research with great potential for many pratical applications in the future. Although the concepts involved in optical frquency combs generation and their use in spectroscopy in the mid-infrared range have already been demonstrated, there are many theoretical issues which remain unclear and should be addressed for the technique to reach the market. More recently, in 2019, theoretical issues related to which are indeed the requirements and limitations of a QCL for optical frequency combs generation have been reported [3]. The choice of the subject of the work presented by Bc. Aleš Konečný, `Spatio-temporal dynamics and coherent control of quantum cascade laser frequency combs‘, as his Master`s thesis, is therefore, both relevant and timely. Different parameters have been shown to affect phase locking in QCLs and, therefore, the generation of optical frequency combs in QCLs. The main ones are: spatial hole burning, group velocity dispersion and Kerr nonlinearity. Bc. Aleš Konečný has used a simulation tool developed at TU-Wien to evaluate the role played by pumping current, diffusion coefficient, group velocity dispersion (GVD) and line width enhancement factor (LEF) in generating the optical frequency combs in QCLs. The waveguiding model used in the simulation tool is based on a spatio-temporally resolved Maxwell-Bloch equations. Particular emphasis has been given to optical injection locking, a topic of much current interest. It is the first time such sistematic analysis is reported. In section 3.1, interesting results on the interplay between LEF and GVD on the generation of frequency combs are presented and it should be noted that the theoretical analysis shines light into the origin of the observed dispersion, distinguishing between gain and waveguide induced dispersion. This is important for a full understanding of the frequency comb generation mechanisms. What I found particularly interesting are the wave guiding simulations shown in section 3.2 using different waveguide geometries, which can be immeadiatly applied to real device structures. The group velocity dispersion can be determined and used to better model the frequency comb generation, contributing to the design of devices with optimized performance. Thus, the work developed by Bc. Aleš Konečný contributes both to the fundamental understanding of the mechanisms behind frequency combs` generation and to their implementation in real structures. Finally, a major core of the work presented is the optical injection locking, described in section 3.3, which is, in fact, the key for the use of optical frequency combs in spectroscopy in the mid-infrared range. The results presented have shown how intricate the locking mechanism can be. Being at its early stages, it becomes clear that this field of research is broader than maybe previously anticipated. There is no doubt that the simulations developed by Bc. Aleš Konečný give a robust contribution to this filed, which is in its infancy. [1] Hugi, A., Villares, G., Blaser, S., et al. Mid-infrared frequency comb based on a quantum cascade laser. Nature, 492 (7428), 2012, pp. 229–233. doi:10.1038/nature11620. [2] Villares, G., Hugi, A., Blaser, S., and Faist, J. Dual-comb spectroscopy based on quantum-cascade-laser frequency combs. Nature Communications, 5 (5192), 2014. doi:10.1038/ncomms6192. [3] Opačak, N. and Schwarz, B. Theory of Frequency-Modulated Combs in Lasers with Spatial Hole Burning, Dispersion, and Kerr Nonlinearity. Phys. Rev. Lett., 123, 2019, p. 243902. doi:10.1103/PhysRevLett.123.243902.

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