Electro-Optic Sampling of a Free-Running Terahertz Quantum-Cascade-Laser Frequency Comb

Markmann, Sergej; Stark, David; Singleton, Matthew; Beck, Mattias; Faist, Jérôme; Scalari, Giacomo

doi:10.1103/physrevapplied.19.064063

Cited by 3 publications

(1 citation statement)

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“…This is in good agreement with the various measurement and simulation results of ridge waveguide THz QCL combs presented in refs. [34][35][36][37]. The situation is different in mid-IR QCL combs, where ultrafast gain saturation and gain asymmetry (arising from non-parabolicity and Bloch gain) [22] play a major role in driving the laser dynamics.…”

Section: Linear Chirp and Flatter Spectrummentioning

confidence: 99%

Frequency‐Modulated Combs via Field‐Enhancing Tapered Waveguides

Senica,

Dikopoltsev,

Forrer

et al. 2023

Laser & Photonics Reviews

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Frequency‐modulated (FM) combs feature flat intensity spectra with a linear frequency chirp, useful for metrology and sensing applications. Generating FM combs in semiconductor lasers generally requires a fast saturable gain, usually limited by the intrinsic gain medium properties. Here, it is shown how a spatial modulation of the laser gain medium can enhance the gain saturation dynamics and nonlinearities to generate self‐starting FM combs. This is demonstrated with tapered planarized terahertz (THz) quantum cascade lasers (QCLs). While simple ridge THz QCLs typically generate combs presenting a mixture of amplitude and frequency modulation, the on‐chip field enhancement resulting from extreme spatial confinement leads to an ultrafast saturable gain regime, generating a pure FM comb with a flatter intensity spectrum and a clear linear frequency chirp. The observed linear frequency chirp is reproduced using a spatially inhomogeneous mean‐field theory model, which confirms the crucial role of field enhancement. In addition, the modified spatial temperature distribution within the waveguide results in an improved high‐temperature comb operation, up to a heat sink temperature of 115 K, with comb bandwidths of 600 GHz at 90 K. The spatial inhomogeneity leads as well to very intense radio frequency (RF) beatnotes up to ‐30 dBm and facilitates dynamic switching between various harmonic states in the same device.

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Section: Linear Chirp and Flatter Spectrummentioning

confidence: 99%

Frequency‐Modulated Combs via Field‐Enhancing Tapered Waveguides

Senica,

Dikopoltsev,

Forrer

et al. 2023

Laser & Photonics Reviews

Self Cite

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Unified theory for frequency combs in ring and Fabry–Perot quantum cascade lasers: An order-parameter equation approach

Silvestri,

Brambilla,

Bardella

et al. 2024

APL Photonics

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We present a unified model to describe the dynamics of optical frequency combs in quantum cascade lasers (QCLs), incorporating both ring and Fabry–Pérot (FP) cavity configurations. The model derives a modified complex Ginzburg–Landau equation (CGLE), leveraging an order parameter approach, and is capable of capturing the dynamics of both configurations, thus enabling a comparative analysis. This result demonstrates that FP QCLs, in addition to ring QCLs, belong to the same universality class of physical systems described by the CGLE, which includes, among others, systems in the fields of superconductivity and hydrodynamics. In the modified CGLE, a nonlinear integral term appears that is associated with the coupling between counterpropagating fields in the FP cavity and whose suppression yields the ring model, which is known to be properly described by a conventional CGLE. We show that this crucial term holds a key role in inhibiting the formation of harmonic frequency combs (HFCs), associated with multi-peaked localized structures, due to its anti-patterning effect. We provide support for a comprehensive campaign of numerical simulations in which we observe a higher occurrence of HFCs in the ring configuration compared to the FP case. Furthermore, the simulations demonstrate the model’s capability to reproduce experimental observations, including the coexistence of amplitude and frequency modulation, linear chirp, and typical dynamic scenarios observed in QCLs. Finally, we perform a linear stability analysis of the single-mode solution for the ring case, confirming its consistency with numerical simulations and highlighting its predictive power regarding the formation of harmonic combs.

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