Low dissipation integrated frequency combs are ideal candidates to realize miniaturized spectrometers without moving parts and hence are of great interest for integrated photonics.
After reviewing frequency comb generation in interband cascade lasers (ICLs), the nonlinear dynamics and performance limiting mechanisms of ICLs will be discussed. A newly developed k-space resolved non-equilibrium carrier transport model combined with experimental studies enables us to explore different loss mechanisms, as well as to explore the reasons, why passive mode-locking of ICLs for short pulse generation remains challenging.
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.
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.
Mid-infrared optical sensors integrating plasmonic waveguides and quantum cascade optoelectronics are an emerging field of research leading to promising results in chemical sensing, environmental monitoring, and biomedical diagnosis. In this work, we investigate TiO2 as waveguiding material for mid-infrared surface plasmon polariton waveguides and show its potential for integrated sensors. Simulations reveal suitable TiO2 dimensions and diffraction grating couplers for ~4.3 µm light. Following these theoretical considerations, we fabricated such devices monolithically integrated with quantum cascade detectors (QCDs) and present their characterization. We further discuss their application in innovative biosensing experiments including glucose detection.
Frequency combs are ideal candidates to build chip integrated spectrometers without moving parts. I will give an overview on comb generation in the mid-infrared using interband cascade lasers.
We demonstrate that ICLs naturally show the same frequency comb characteristics as QCL frequency combs and shed light into previous experiments. To generate the comb, we utilize the intrinsic fast time dynamics of the laser gain medium to enable phase-locking via four-wave mixing that is due to anti-phase oscillations of the population inversion. The observed comb state, the frequency modulated state, is fundamentally different to traditional mode-locking, where short pulses are generated.
As a main characterization technique, we use the linear RF phase measurement technique shifted wave interference Fourier transform spectroscopy (SWIFTS). A detailed comparison between SWIFTS and the intensity autocorrelation for the pulse shape characterization will be presented. A more intuitive picture of the synchronization states in frequency combs is provided by the analogy to coupled clocks, which reveals a illustrative understanding of how these lasers can be switched to the pulsed regime. Using this knowledge we demonstrate the active mode-locked mid-infrared ICLs with picosecond pulse emisson.
A key feature of ICLs is that the very same layer structure can also be used as sensitive photodetectors. The fact that ICLs utilize fast carrier transport via intersubband scattering is a great advantage for the high frequency response of the on-chip photodetectors. Combined with the low power requirements of the laser this makes ICL technology an ideal platform to realize future miniaturized dual-comb spectrometers for hand-held and battery driven devices.
[1] B. Schwarz et al. Optica 6, 890 (2019)
[2] J. Hillbrand et al. Optica 6, 1334 (2019)
[3] H. Lotfi et al. APL 109, 151111 (2016)
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.
Radio-frequency (RF) modulation of FP-QCLs has been demonstrated to be a crucial tool for the control and manipulation of frequency comb states and internal synchronization mechanisms. Coherent injection locking of the repetition frequency, switching from the intrinsic anti-phase synchronized FM-state to the in-phase synchronized AM-state and spectral broadening are just a few examples of recent findings linked to RF injection as a means of control mechanism.
We present the observation of controlled lateral mode switching in a two section, RF-modulation optimized, 12μm broad ridge FP-QCL. The mode switching is induced by strong RF modulation close to the free-running laser beatnote at the repetition frequency. It is shown that the anti-phase FM-comb state intrinsic to most free-running FP-QCLs, exceeding a certain ridge width, favors a higher order dual lobe state which can be observed in the far-field. Injecting an RF signal with a power of 35 dBm, 65 MHz above the free-running laser beatnote, the resulting comb state is shown to favor the fundamental, mode. The resulting single lobe far-field distribution was measured experimentally with a slow MCT detector mounted on a rotational stage. The lateral mode switching from the first higher order- to the fundamental mode can be observed over the entire laser operation current bias range of the gain section. 2D mode simulations yielding the fundamental and higher order mode group refractive indices show good agreement with the measured results. Furthermore RF modulation below the free-running laser beatnote shows the excitement of a second, higher order lateral mode, where three lobes are observed in the far-field.
The results presented show the potential for reliable lateral mode control via strong RF modulation close to the repetition frequency of broader ridge FP-QCLs. This opens up the possibility for high power comb operation scaling with the ridge width, while still maintaining a single lobe far-field
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.
In this work we monolithically integrate a quantum cascade laser (QCL) and detector (QCD) addressing the same wavelengths lambda=1550-1650 cm-1 for liquid spectroscopy. QCL and QCD are combined using a 50-100 µm-long dielectric-loaded surface-plasmon-polariton (DLSPP) waveguide, which typically guides >>90% of the mode outside of the cavity. We show the analysis of the protein bovine serum albumin (BSA) and its denaturation process between 25°C-90°C in real time in a microfluidic cell (60 µl) for 20-60 mg/ml BSA-concentrations. To further test the sensor-robustness, we directly submerge it into a beaker and detect H2O up to 35%-40%, solved in isopropyl alcohol.
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