Bright pulses of light are unstable states in free-running semiconductor lasers. Stable bright solitons require an optical bistability---as predicted by mean-field theories such as the Complex Ginzburg Landau Equation (CGLE) or the Lugiato-Lefever Equation (LLE). However, this restriction is relaxed when two lasers are coupled to one another. Here, we identify a new state of light in a pair of semiconductor ring lasers with fast gain dynamics. Two racetrack (RT) quantum cascade lasers (QCLs) when coupled along their straight sections spontaneously produce a frequency comb over the hybridized modes of the coupled cavity. Waveform reconstruction measurements reveal the hybridized comb manifests itself as a pair of bright and dark pulses circulating the coupled cavity simultaneously. In addition, split-step integration of a pair of mutually forced CGLEs faithfully reproduces our experimental measurements, providing some insight on the formation of such states.
Active resonators based on semiconductor gain media encompass a large optical nonlinearity that arises from gain saturation and enables bright soliton generation. The ability to operate these resonators below the lasing threshold as tunable passive devices –– filters, modulators, phase shifters –– opens up an untapped potential of seamlessly integrated reconfigurable devices for both generation of multimode mid-infrared (4 – 12 μm) light and its manipulation.
Optical frequency combs (OFCs) stand as the cornerstone of modern optics, with
applications ranging from fundamental science to sensing and spectroscopy. Generation of
short optical soliton pulses in passive media such as optical fibers and microresonators has
been an established technique for stable OFC formation with a broad optical spectrum –
however these platforms are driven by an external optical signal and often rely on
additional bulky elements that increase the complexity of the system.
Here, we aim to overcome these difficulties by direct OFC generation in mid-infrared
semiconductor lasers, such as quantum and interband cascade lasers. After a general
introduction to such combs and their nonlinear dynamics, the soliton concept from
microresonator Kerr combs will be generalized to active media that are electrically-driven
and a new type of solitons in free-running semiconductor laser integrated on a chip will be
demonstrated.
Phase singularities are loci of darkness surrounded by light in a scalar field. We engineer an array of closely-spaced identical point singularities structured along the optic axis. The array is experimentally realized with a TiO2 metasurface under 760 nm narrowband illumination. We study possible application in blue-detuned neutral atom trap arrays, for which this field would enforce 3D confinement and a potential depth around 0.22 mK per watt of trapping power. The field is tolerant to around 10 nm changes in wavelength with a 0.11 degree angular bandwidth. Metasurface-enabled point singularity engineering may simplify and miniaturize the architecture required for super-resolution microscopes and dark traps.
Monolithic ring Quantum Cascade Lasers (QCLs) have recently emerged as a new platform for frequency comb generation in the mid-infrared with immediate applications in molecular gas spectroscopy and photonic generation of stable coherent sub-THz tones. In this talk I will show that depending on the way they are driven, ring QCLs can act as carrier generators, integrated intensity modulators, tunable filters, and on-chip optical amplifiers. The natural predisposition of these components to photonic integration opens a route to compact mid-infrared WDM transceivers for free space optical links and miniaturized 2D IR spectrometers.
Optical nonlinearities are known to coherently couple the amplitude and the phase of light, which can lead to the formation of perfectly periodic waveforms – known as frequency combs. Recently, self-starting frequency combs that do not rely on the emission of short pulses are appearing in numerous semiconductor laser types, among which is the quantum cascade laser. Here we discuss the role of a Bloch gain induced giant Kerr nonlinearity in Fabry-Pérot and ring cavity QCLs, paving the way towards electrically pumped Kerr combs.
Quantum cascade lasers and other semiconductor laser types constitute an attractive integrated platform for spectroscopic applications, as they emit self-starting Frequency Combs (FCs), unlike traditionally-used mode-locked lasers. Here, we explain self-starting FCs due to nonlinear effects arising from the laser gain itself, with particular attention on the coupling of the amplitude and phase of light, quantified by the Linewidth Enhancement Factor (LEF). We study both cavity geometries, Fabry-Perot and ring, reporting the conditions for stable comb formation and different methods of optimizing their performance. In analogy with Kerr microresonators, ring lasers show the formation of temporal localized soliton-like structures, indicating towards an untapped potential for discovering new states of light.
We use a prototypical model based on the Complex Ginzburg Landau Equation to study the dynamics of a multimode ring quantum cascade laser. We predict the existence and stability of different classes of localized structures in the system. In presence of coherent injection, we report the formation dissipative Temporal Solitons (TSs) which manifest multi-stability and coexistence with a stable CW solution. We show how these features allow for external manipulation of the spectral content of the optical frequency combs associated with TSs with a big impact on applications in the field of e.g. high precision spectroscopy and wireless communications.
Semiconductor lasers are compact, electrically pumped sources of coherent light. If designed properly they emit frequency combs, which nowadays find application in various areas such as telecommunications, metrology and high-precision spectroscopy for chemical sensing and medical diagnostics. Laser design and optimization require profound knowledge of the working principles at play and powerful techniques to infer their characteristics. One of these characteristic quantities is the linewidth enhancement factor (LEF). It originates from a theoretical description of the linewidth broadening beyond the Shawlow-Townes-limit, but is recently moving in the spotlight of frequency comb research to understand dynamic processes like modulation response, comb formation and even soliton generation. However, as of yet the experimental investigation of the LEF was limited to single-mode laser operation or measurements below to the lasing threshold. This is insufficient, since the LEF changes drastically with the laser bias and is dependent on the wavelength.
In this work we present a novel technique which enables the spectrally-resolved measurement of the LEF of an arbitrary laser source regardless of the operation state or bias. It relies on the RF modulation of the driving bias and “Shifted Wave Interference Fourier Transform Spectroscopy” (SWIFTS) – a phase-sensitive measurement scheme. When investigating a laser frequency comb, a single-shot measurement reveals the spectral dependence of the LEF over the whole comb spectrum.
Extensive simulations utilizing a numerical spatiotemporal model based on the Maxwell–Bloch formalism were performed to explore the success of this approach. The technique was vastly tested by simulating both, single-mode lasers and laser frequency combs. A comparison to the theoretical model shows good agreement.
The experimental demonstration of the technique is performed on a quantum cascade laser (QCL) frequency comb. The resulting LEF values are comparable to recent publications and follow the spectral shape predicted by the theoretical model.
Optical nonlinearities are known to coherently couple the amplitude and the phase of light, which can lead to the formation of perfectly periodic waveforms – known as frequency combs. Recently, self-starting frequency combs that do not rely on the emission of short pulses are appearing in numerous semiconductor laser types, among which is the quantum cascade laser. This novel type of combs is gaining vast attention from researchers due to their self-starting nature and compactness, making them an ideal platform for further development of spectroscopic applications. Their spontaneous formation was explained through an interplay of phenomenological nonlinearity and dispersion in the laser active region, although the actual physical processes remained unclear until now. Here we show that Bloch gain – a phenomenon described by Bloch and Zener in the 1930s – plays an essential role in their formation. We demonstrate that a Bloch gain contribution is present in any quantum cascade laser and becomes particularly dominant under saturation.
Bloch gain in QCLs with ultrafast gain recovery induces a giant Kerr nonlinearity, which is two orders of magnitude larger than the bulk values. The resonant Kerr nonlinearity provides coherent coupling between the amplitude and the phase of the laser field, which serves as a locking mechanism for frequency comb operation. We show that in Fabry-Pérot QCLs this results in frequency-modulated combs with a linear frequency chirp. In ring cavity QCLs, the Bloch gain is able to induce a single-mode instability by tuning the laser in the phase turbulence regime. This can lead to the formation of locked spatial patterns that are related to dissipative Kerr solitons, paving the way towards electrically pumped Kerr combs.
The recent generalised theory of frequency comb generation in externally pumped cavities with and without population inversion suggested an intimate link between quantum cascade lasers (QCLs) and Kerr resonators. In this talk we overview recent experimental developments in chip-scale ring cavity QCLs with and without output coupling ports, that allow operation in self-pumped and externally pumped configurations, and their ability to support cavity solitons.
Metasurfaces are a promising platform to exceed their traditional counterparts not only in compactness but also for functionality. However, current designs are limited when trying to implement multiple, non-paraxial functions with a single metasurface as they are bound to either a small angular range or to low efficiencies.
Here, we present a new non-local metasurface design that enables the implementation of multiple, independent functions with a large difference in deflection angle. We further demonstrate the capabilities of this approach for advanced control of light emission systems by creating a wavelength-tunable external cavity laser with holographic output based on such metasurface.
An amplitude-modulated optical frequency comb generated by a passively mode-locked InGaAs double quantum well semiconductor laser is optically injected into a laser emitting a single optical mode continuous wave output in solitary operation. Optical frequency comb generation in the injected laser is experimentally demonstrated and regimes of injection locking are analyzed.
Optical self-injection stabilization of a passively mode-locked InGaAs double quantum well semiconductor laser is reported. Time-domain, frequency-domain and spectral domain measurement results in dependence on feedback strength and feedback delay demonstrate pulse width and higher harmonic mode-locking control. Traveling-wave equation simulations allow to explain the measured sequence of harmonic mode-locking orders in dependence on the feedback strength and delay.
The transition from frequency-modulated to amplitude-modulated frequency comb operation is studied experimentally in a multi-section InAs/InGaAs quantum dot frequency comb laser. Temporally and spectrally-resolved intensity and phase as well as time and frequency domain analysis allow to identify frequency-modulated and amplitude-modulated comb operation regimes in dependence on the laser gain current and absorber reverse bias voltage.
It is a well-established truth that spatial hole burning (SHB) in a standing-wave cavity is an essential single-mode instability mechanism for multimode operation of quantum cascade lasers (QCLs). We discovered recently that another instability mechanism–phase turbulence–is capable of triggering an onset of previously unseen types of frequency combs in traveling-wave ring cavity QCLs in absence of SHB. This new regime of laser operation reveals a connection with Kerr combs and paves the way to manipulation and engineering of comb states in QCLs.
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