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(a) Interferogram generated by a FTIR when measuring the subthreshold spectrum of a HR-coated comb device. Individual bursts in the interferogram are labelled sequentially. The resolution is 0.11 cm −1 . (b) Measurement of the GVD and (c) gain of HR-coated QCL-comb as a function of current.
Source publication
An efficient mid-infrared frequency comb source is of great interest to high speed, high resolution spectroscopy and metrology. Here we demonstrate a mid-IR quantum cascade laser frequency comb with a high power output and narrow beatnote linewidth at room temperature. The active region was designed with a strong-coupling between the injector and t...
Citations
... As the comb is significantly broadened in the fifth and sixth harmonic states, the range of the exported spectra is increased to capture all modes. the method of device-level dispersion management [16,17] and microwave injection [23]. Notice that optical modes that are at least 3 dB above noise floor and with expected spacing are counted when determining the spectral bandwidth. ...
We demonstrate the implementation of external cavity optical feedback to improve coherence and promote generation of harmonic states by a mid-infrared quantum cascade laser frequency comb. In particular, we present a Vernier-like scheme to realize harmonic comb states that increase the repetition rate of the comb by a factor of up to 6 and broaden spectral coverages from 46 cm⁻¹ to 92 cm⁻¹. Intermode beatnote and dual comb characterization indicate that the coherence of the comb has greatly improved for sub-optimal devices when the comb is operated in these harmonic states. This approach to control the generation of harmonic states and improve comb performance can be readily incorporated to various sensing systems and has great potential in spectroscopic measurements that require high repetition rates and/or broad optical bandwidth.
... QCL-based dual-comb developments by other research groups have been successful at extending the spectral coverage of the lasers, extending the spectral coverage of the lasers, increasing the spectral resolution of the system, as well as stabilizing and calibrating the frequency axis. 49,50 Already QCL frequency combs with more than 100 cm −1 coverage have been documented, 51 and recently, a QCL-based dual-comb spectrometer with a spectral resolution of ∼30 MHz over a 55 cm −1 wavelength range was demonstrated. 44 System configuration updates to increase the sensitivity are already in progress, such as incorporating a multi-pass cell to enable point sensing capabilities to the system, 52 adding motorized control of the beam-expanding optics to allow for optimization of the return signal at various distances, and adding a third single-mode laser as a frequency reference. ...
Detection of airborne chemical releases in densely populated urban environments requires precise sensors with high temporal and spatial resolution capable of covering large areas. For this purpose, we present a mobile mid-infrared quantum cascade laser dual-comb spectrometer for identification and quantification of chemical plumes. Field tests with the remote sensor were conducted during daytime in the downtown Boston area over a five day period during which chemical releases were simulated by intermittently emitting non-toxic substances. Open-air sensing was performed with retroreflectors positioned at up to 230 m distance and with sensitivities in the ppm m range for one second of averaging time. The field campaign demonstrates a step toward a semiconductor dual-comb spectroscopic sensor in the mid-infrared fingerprint region, suitable for long-term deployments. These types of sensors will be valuable complements to existing optical sensors for urban hazardous gas leak monitoring, air quality assessments, and localization of clandestine chemical production.
... Conventional two-section MLLs with saturable absorbers are strongly dependent on amplitude modulation (AM) to modify/ shape comb spectra. In contrast, frequency modulation (FM) is usually observed in fast gain media, e.g., QD lasers and quantum cascade lasers (QCLs) [27][28][29]. Fast gain dynamics normally lead to spatial hole burning and phase modulation [30][31][32], which can effectively trigger phase locked comb generation. Although both AM and FM locking techniques have their own repetition rates, AM locked lasers produce narrow pulses, while FM locked lasers generate quasi-continuous signals. ...
Quantum dot lasers on silicon have gained significant interest over the past decade due to their great potential as an on-chip silicon photonic light source. Here, we demonstrate multi-wavelength injection locking of InAs/GaAs quantum dot Fabry–Perot (FP) lasers both on GaAs and silicon substrates by optical self-injection via an external cavity. The number of locked laser modes can be adjusted from a single peak to multiple peaks by tuning wavelength dependent phase and mode spacing of back-injected light through a Lyot filter. The multi-wavelength injection locked laser modes exhibit average optical linewidth of ∼ 20 kHz , which are narrowed by approximately three orders of magnitude from their free-running condition. Furthermore, multi-wavelength self-injection locking via an external cavity exhibits flat-top optical spectral properties with approximately 30 stably locked channels under stable operation over time, where the frequency detuning is less than 700 MHz within 40 min. Particularly, FP lasers by direct epitaxial growth on silicon substrates are self-injection locked as a flat-top comb source with tunable free spectral range from approximately 25 to 700 GHz. The reported results emphasize the great potential of multi-wavelength injection locked lasers as tunable on-chip multi-wavelength light sources.
... This is why the majority of mid-IR QCL combs have been developed for the = 7-9 µm range. 6,17,18 . Attempts to reach shorter wavelengths have required more complex waveguide engineering 1,3,4 or the deposition of chirped (Gires-Turnois interferometer, GTI) mirrors, 2,19,20 which complicates the device fabrication and may potentially introduce reliability issues. ...
... A variety of approaches have been developed in the last decade to generate frequency combs across the mid-IR that include mode-locked Tm-doped fiber [3] and Cr:ZnS solid-state lasers [4], comb sources based on differencefrequency generation (DFG) [5][6][7][8][9][10][11][12][13][14][15], intrapulse DFG (IDFG) [16][17][18][19][20][21][22][23][24][25], optical parametric oscillators (OPOs) [26][27][28], supercontinuum (SC) generation in fibers and waveguides [29][30][31][32][33][34], quantum cascade lasers (QCL) [35,36], interband cascade lasers (ICL) [37], microresonators (µ-res) [38][39][40][41][42], and electrooptic modulation (EOM) [43]. ...
Coherent laser beams in the 3 to 20 {\mu}m region of the spectrum are most applicable for chemical sensing by addressing the strongest vibrational absorption resonances of the media. Broadband frequency combs in this spectral range are of special interest since they can be used as a powerful tool for molecular spectroscopy offering dramatic gains in speed, sensitivity, precision, and massive parallelism of data collection. Here we show that a frequency comb realized through subharmonic generation in an optical parametric oscillator (OPO) based on orientation-patterned gallium phosphide (OP-GaP) pumped by a Kerr-lens mode-locked 2.35-{\mu}m laser can reach a continuous wavelength span of 3-12 {\mu}m, thus covering most of the molecular 'signature' region. The key to achieving such a broad spectrum is to use a low-dispersion cavity entailing all gold-coated mirrors, minimally dispersive and optically thin intracavity elements, and a specially designed pump injector. The system features a smooth ultra-broadband spectral output that is phase coherent to the pump laser comb, 245-mW output power with high (>20%) optical conversion efficiency, and a possibility to reach close to unity conversion from a mode-locked drive, thanks to the non-dissipative downconversion processes and photon recycling.
... Drawback of this system is its system complexity, therefore difficult to build as a robust and compact module for practical applications. In addition, a various type of tunable QCL are demonstrated [31,143,144]. Such devices, however, requires high levels of processing instruments as well as high environmental stability. ...
Mitt-Infrarot-Technologie (mid-IR) ist ein äußerst leistungsfähiges Werkzeug für die Anwendung in der Molekülspektroskopie, da die Schwingungsmoden vieler Moleküle in diesem Wellenlängenbereich liegen. Der Quantenkaskadenlaser mit externem Resonator (EC-QCL) kann alle Bereiche dieses Spektrums abdecken. Das Hauptanliegen dieser Arbeit ist die Verbesserung der Leistung des EC-QCL im Hinblick auf die Breite des Wellenlängen-Durchstimmbereichs und die Laserleistung. Theoretische Untersuchungen bestätigen zunächst, dass der QCL die Schlüsselrolle bei EC-Systemen einnimmt: Die Effizienz des EC wird bestimmt durch die Effizienz des QCL und die Güte der Antireflex-Schicht (ARC) der Laserfacette. Die Breite des Durchstimmbereichs wird bestimmt durch das Gain-Spektrum des QCL. Im Rahmen dieser Arbeit wurden die QCL in unserer Gruppe hergestellt und vom QCL-Wachstum selbst bis hin zur Facettenbeschichtung optimiert. Eine der größten Herausforderungen in der Herstellung des EC-Systems ist die Reduktion des Reflexionsvermögens innerhalb der Facetten des Laserchips. Dafür haben wir ein neues ARC-Konzept entwickelt und auf dem beschichteten Substrat demonstriert, dass innerhalb des gesamten, sehr breiten Wellenlängenbereichs von 7–12 μm die Reflexion auf unter 1% reduziert wird. Das Beschichtungsmodell wurde außerdem auf „broad-gain“-QCL-Facetten angewendet, wodurch die Reflexion auf 0,75% über den gesamten Emissions-Wellenlängenbereich reduziert werden konnte. Ein weiterer Schwerpunkt dieser Arbeit ist die Entwicklung und Konstruktion von EC-Lasersystemen. Es wurden zwei kompakte Laser vom Littrow-Typ entwickelt, die von 920 cm-1 bis 1190 cm-1 durchstimmbar sind und die eine Pulsleitung von 0.45 W erreichen. Außerdem wurde eine neue optische Konfiguration des EC-Systems vorgeschlagen um eine höhere Ausgangsleistung zu erzielen. Dieser „Intra-cavity Out-coupling Laser“ erreicht eine Pulsleistung von 1 W und den gleichen Emissionbereich wie die beiden Littrow-Laser.
... Moreover, these devices were shown to operate with fixed phases a linear frequency chirp characteristic of a mainly frequency-modulated output [9]. Recently, average powers of greater than 1 W and frequency coverage over 100 cm −1 have been demonstrated [10,11], making these devices indispensable tools for mid-infrared spectroscopy [12,13]. ...
The effect of spatial hole burning on mode proliferation has been shown both theoretically and experimentally to be important for frequency comb operation in quantum cascade lasers (QCLs). It is therefore of great interest to investigate the frequency comb operation from integrated ring cavities where this effect is absent. Here we report a mid-infrared quantum cascade laser based on a buried heterostructure ring cavity that exhibits unidirectional lasing with a frequency comb spectrum displaying a predominantly ${{\rm sech}^2}$ s e c h 2 profile over a bandwidth of $\sim{10}{^{ - 1}}$ ∼ 10 − 1 . A single beat note with 3 dB linewidth of $ \lt {900}\;{\rm Hz}$ < 900 H z was observed over the last 85% of the whole current dynamic range. To further verify the frequency comb operation, a multiheterodyne experiment was carried out using a standard Fabry–Pérot QCL comb as a reference laser. This measurement allowed the retrieval of the spectral phases, indicating an operation with strong amplitude modulation.
... Mid-IR spectroscopy is one of the most commonly used methods for characterizing optical devices, 1 with widespread applications in sensing gases such as carbon dioxide (CO 2 ) and sulfur oxide (SO 2 ). 2 A thermal emitter is an integral part of such a sensor and one of its key requirements is to generate a narrowband and pure mid-IR signal. The conventional choices for mid-IR sources include interband quantum cascade lasers, 3,4 which are excellent narrowband sources but suffer from high power consumption and high fabrication cost. Alternatively, a narrowband source can be obtained by tailoring the broadband spectrum of a blackbody radiator using IR filters. ...
A conductor-backed dielectric metasurface thermal emitter at mid-IR frequencies with narrowband emissivity is experimentally demonstrated. The metasurface emitter consists of a high permittivity silicon resonator on top of a ground plane, whose resonant mechanism is explained using image theory. The resonator, placed close to a copper ground plane, is designed to produce a magnetic resonance, resulting in a low-profile device with a single emission peak in its subwavelength frequency range. The thermal emitter is next fabricated using common CMOS processes. Frequency dependent optical constants of plasma-enhanced chemical vapor deposited films of Si, SiO2, and evaporated Cu are also reported in the mid-IR range. Narrowband thermal emission is successfully obtained at around 7.22μm (41.5 THz), which corresponds to the absorption band of SO2. The Q-factor of about 37 is achieved with a peak emissivity of 0.65, which is significantly higher compared to the reported Q-factors of state-of-the-art plasmonic resonators.
... Under engineered dispersion and gain profiles, this multimode operation is able to be locked into comb modes with welldefined phase relation via a train of four-wave mixing (FWM) enabled by a third-order χ (3) nonlinearity. 3 With the significant development in power efficiency 4,5 and dispersion management 6,7 , frequency comb sources based on mid-IR QCLs have emerged as appealing comb sources in the mid-IR wavelength range with stable phase relation 8, . Based on the same nonlinear effects, QCL frequency comb's operating frequency was also extended to THz range. ...
... Since the repetition frequency f rep,2 for the harmonic comb is so large and cannot be measured directed with the spectrum analyser, we will use Eqs. (1)(2)(3)(4)(5)(6) to estimate the tuning rate for f rep,2 and f ceo,2. In the experiment, the bias on the DFB device is fixed while the tuning FP current to tune the heterodyne signal. ...
... Although the procedure can be applied to any free-running DCS system, e.g. such based on electro-optic modulators, optically pumped microresonators, or fiber lasers, the main motivation for this work is linked to the recent developments in chip-scale frequency combs based on electrically-pumped semiconductor sources such as quantum cascade lasers (QCLs) [9,[15][16][17][18][19] or interband cascade lasers (ICLs) [20][21][22]. These sources have reached a state of development where they can be considered as the basis for miniaturized, battery-operated, portable spectrometers with small footprints [23]. ...
Dual-comb spectroscopy is a rapidly developing spectroscopic technique that does not require any opto-mechanical moving parts and enables broadband and high-resolution measurements with microsecond time resolution. However, for high sensitivity measurements and extended averaging times, high mutual coherence of the comb-sources is essential. To date, most dual-comb systems employ coherent averaging schemes that require additional electro-optical components, which increase system complexity and cost. More recently, computational phase correction approaches that enables coherent averaging of spectra generated by free-running systems have gained increasing interest. Here, we propose such an all-computational solution that is compatible with real-time data acquisition architectures for free-running systems. The efficacy of our coherent averaging algorithm is demonstrated using dual-comb spectrometers based on quantum cascade lasers, interband cascade lasers, mode-locked lasers, and optically-pumped microresonators.
