The intensity noise of a laser source represents one of the key factors limiting the ultimate sensitivity in laser-based systems for sensing and telecommunication. For advanced applications based on interferometry, the availability of a shot-noise-limited local oscillator is even more important for the effective feasibility of high-precision measurements. This is particularly crucial in quantum optics applications based on homodyne detection schemes to measure non-classical light states, such as squeezed states. This work deeply investigates and analyzes the intensity noise features of the most widely used mid-infrared semiconductor heterostructured lasers: quantum cascade and interband cascade lasers. For this purpose, a comprehensive comparison of three different continuous-wave lasers operating at room temperature around 4.5 µm wavelength is presented. First, a thorough electro-optical characterization is given, highlighting the differences and the shared common characteristics of the tested devices. Then, a detailed intensity noise analysis is reported, identifying their different noise operations with a particular reference to shot-noise-limited operations. Finally, some perspectives towards advanced applications are discussed.

A low-cost single frequency laser emitting in the mid-infrared spectral region and dissipating minimal electrical power is a key ingredient for the next generation of portable gas sensors for high-volume applications involving chemical sensing of important greenhouse and pollutant gases. We propose here a Quantum Cascade Surface Emitting Laser (QCSEL), which we implement as a short linear cavity with high reflectivity coated end-mirrors to suppress any edge emission and use a buried semiconductor diffraction grating to extract the light from the surface. By wafer-level testing we investigate the cavity length scaling, extract mirror reflectivities larger than 0.9, and achieve a pulsed threshold power dissipation of 237 mW for an emission wavelength near 7.5 $\mu$m. Finally, we demonstrate single mode emission with a side-mode suppression ratio larger than 33 dB of a 248 $\mu$m short cavity mounted with the epitaxial layer up and operated in continuous wave at 20 $^\circ$C.

In many precision sensing applications, the final detection sensitivity is tightly related to the intensity noise of the laser source, which might represent the ultimate limit to the sensor performance. In this framework, we present here the intensity noise characterization of three different mid-infrared semiconductor devices (two quantum cascade lasers and one interband cascade laser). A fast homemade balanced detection system is used to measure the intensity noise of the emitted radiation over a broad Fourier-frequency range, facilitating the observation of shot-noise-limited radiation under specific measurement conditions and detection efficiency. This study allows for a direct performance comparison of the most widespread laser sources in mid-infrared sensing systems.

Laser & Photonics ReviewsVolume 18, Issue 8 2470049 Back CoverFree Access Quantum Cascade Surface Emitting Lasers (Laser Photonics Rev. 18(8)/2024) David Stark, David StarkSearch for more papers by this authorFilippos Kapsalidis, Filippos KapsalidisSearch for more papers by this authorSergej Markmann, Sergej MarkmannSearch for more papers by this authorMathieu Bertrand, Mathieu BertrandSearch for more papers by this authorBahareh Marzban, Bahareh MarzbanSearch for more papers by this authorEmilio Gini, Emilio GiniSearch for more papers by this authorMattias Beck, Mattias BeckSearch for more papers by this authorJérôme Faist, Jérôme FaistSearch for more papers by this author David Stark, David StarkSearch for more papers by this authorFilippos Kapsalidis, Filippos KapsalidisSearch for more papers by this authorSergej Markmann, Sergej MarkmannSearch for more papers by this authorMathieu Bertrand, Mathieu BertrandSearch for more papers by this authorBahareh Marzban, Bahareh MarzbanSearch for more papers by this authorEmilio Gini, Emilio GiniSearch for more papers by this authorMattias Beck, Mattias BeckSearch for more papers by this authorJérôme Faist, Jérôme FaistSearch for more papers by this author First published: 12 August 2024 https://doi.org/10.1002/lpor.202470049AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Graphical Abstract Quantum Cascade Surface Emitting Lasers In article number 2300663, David Stark, Jérôme Faist, and co-workers introduce the Quantum Cascade Surface Emitting Laser (QCSEL, pronounced "kjuxel"), a device concept targeting the mid-infrared equivalent of the Vertical Cavity Surface Emitting Laser (VCSEL). By leveraging miniaturization and wafer-level testing, the authors demonstrate the feasibility of producing low-cost and low-power consuming mid-infrared lasers in high volumes. These devices are proposed for the next generation of compact and portable gas sensing applications involving industrial process control, environmental monitoring, and medical diagnosis. Volume18, Issue8August 20242470049 RelatedInformation

A crucial element for the next generation of portable gas sensors for high-volume applications, especially involving chemical sensing of important greenhouse and pollutant gases, is the development of a low-cost, low-power consuming, single-frequency laser operating in the mid-infrared spectral range. In this regard, we propose the implementation of a Quantum Cascade Surface Emitting Laser (QCSEL). Our design involves a linear microcavity with high reflectivity coated end-mirrors and a buried semiconductor diffraction grating to extract the light from the surface.

The mid-infrared (MIR) spectral region (2-20 μm) is the molecular “fingerprint” region for many important organic and inorganic molecules [1], [2]. Miniaturized optical gas sensors based on MIR absorption spectroscopy are highly attractive for many applications such as industrial process control, environmental monitoring and medical diagnosis [3]. To enable low-cost and portable MIR gas sensors, compact and low power consuming single-mode light sources operating in the range of interest are highly desirable. $\mu \mathrm{m}$

The ultrafast gain recovery dynamics [1] observed in quantum cascade lasers (QCLs) fundamentally restricts the formation of intracavity pulses. On the other hand, this picosecond gain response makes the QCL uniquely suited for microwave modulation of its pump current. Here, we leverage on this property and generate short optical pulses (~ 30 ps) with up to Watt level peak power. Lasing on a single longitudinal mode is achieved via optical injection seeding. We characterize the generated optical pulses in both frequency and time domain using a spectrometer in combination with an optical sampling method. The obtained results are interpreted in the framework of laser rate equations.

Frequency comb lasers with fast gain recovery times naturally favor the emission of frequency mod-ulated periodic signals, which are useful for multiple applications in spectroscopy and communications. Models and phase measurements predict an ultrashort strong intensity spike at the instantaneous frequency discontinuity of the cavity cycle. Here we experimentally study the ultrashort spike of fast-gain frequency modulated combs through direct upconversion sampling, and measure a width below that of a transform-limited pulse. Specifi-cally, we demonstrate a mid-infrared quantum cascade laser which inherently lases in a frequency modulated comb and produces a spike with full-width at half-maximum below 600 fs, which is below the Fourier-limit derived from the corresponding spectrum. We believe these ultrashort spikes can be highly beneficial for sub-bandwidth time domain measurements. Using mean-field theory based simulations, we confirm the occurrence of such features as well as further optimize our system for going even further below the Fourier-limit.

We demonstrate an asynchronous optical sampling technique for the temporal characterization of frequency modulated combs. On the basis of a mid-infrared quantum cascade laser frequency comb, we measure both its instantaneous intensity and optical frequency.

Phase modulation is demonstrated in a quantum Stark effect modulator designed to operate in the mid-infrared at wavelength around 10 µm. Both phase and amplitude modulation are simultaneously resolved through the measurement of the heterodyne signal arising from the beating of a quantum cascade laser with a highly stabilized frequency comb. The highest measured phase shift is more than 5 degrees with an associated intensity modulation of 5 %. The experimental results are in full agreement with our model in which the complex susceptibility is precisely described considering the linear voltage dependent Stark shift of the optical resonance.

In quantum cascade laser frequency combs, the intensity distribution of the optical spectrum can be split into two well-separated lobes of longitudinal modes that, even when far apart, have a common phase relation and preserve equal frequency separation. The temporal dynamics of two lasers emitting at 4.4 and 8.1 µm operating in this bilobed regime are here investigated. The laser intensity shows a peculiar temporal behavior associated with the spectral features whereby, every half a round-trip, the total emitted power switches from one lobe to the other, with a perfect temporal anti-correlation. The anti-correlation between the lobes is also observed in the intensity noise figure of the emission. This coherent phenomenon arises from gain nonlinearities induced by spatial hole burning and the extremely fast gain dynamics typical of quantum cascade lasers.

We present low threshold quantum cascade surface emitting lasers (QCSELs) emitting at wavelengths of 4.5 micrometers or 8 micrometers. To extract the light vertically from the InP-based buried heterostructure laser a second order InGaAs/InP grating is used. Both ridge facets are formed by dry-etching followed by coating a dielectric-metal film. Due to the high reflectivity of the facets, the cavity can be shortened well below 500 micrometers reducing the threshold power to several hundred milliwatts. The proposed device concept allows large-scale fabrication and wafer-level characterization. The results are an important step towards low-cost and low-power consuming quantum cascade lasers for portable MIR gas sensors.

Semiconductor lasers with extremely low threshold power require a combination of small volume active region with high-quality-factor cavities. For ridge lasers with highly reflective coatings, an ultra-low threshold demands significantly suppressing the diffraction loss at the facets of the laser. Here, we demonstrate that introducing a subwavelength aperture in the metallic highly reflective coating of a laser can correct the phase front, thereby counter-intuitively enhancing both its modal reflectivity and transmissivity at the same time. Theoretical and experimental results manifest a decreasing in the mirror loss by over 40% and an increasing in the transmissivity by 10 4 . Implementing this method on a small-cavity quantum cascade laser, room-temperature continuous-wave lasing operation at 4.5 μm wavelength with an electrical consumption power of only 143 mW is achieved. Our work suggests possibilities for future portable applications and can be implemented in a broad range of optoelectronic systems.

In this work, we assessed the response and and prospects of controlling the QCL comb state when the source laser geometry has been optimized for external microwave modulation.

In this work, we demonstrate control over the time-domain state quantum cascade laser output state using microwave modulation. We demonstrate narrow, pulse-like features with a full-with at half-maximum of 558 fs when isolated, which corresponds to the expected Fourier-transform limited pulse-width.

We demonstrate that mid-infrared quantum cascade laser frequency combs are highly suitable as high-accuracy frequency references. We fully stabilize the comb and exploit it for comb-calibrated spectroscopy, achieving 100-kHz frequency accuracy at 7.7 μm.

We demonstrate the manipulation of the quantum cascade laser output state using microwave-injection. In the spectral domain, the optical bandwidth can be doubled, whereas in the time-domain, we observe narrow, approximately 1 ps wide features.

A laser exhibits both controllable gain and loss and, under proper design conditions, is an ideal non-Hermitian system allowing the direct observation and engineering of spectral singularities such as exceptional points (EPs). A dual section distributed feedback (DFB) quantum cascade laser (QCL) is a prototype of such a system, allowing the controlled coupling of a ladder of cavity Fabry-Perot (FP) modes to a quarter wave shifted DFB mode. Tuning the coupling strength and the gain difference between these two set of modes enables probing the regimes from weak coupling to strong coupling and the robust observation of exceptional point singularities. At these exceptional points, the laser exhibits a sequence of lasing and coherent prefect absorption dynamics1,2 when pumped above transparency. Additionally, the pumping scheme allows the deliberate lifting of the exceptional point degeneracy. These results show that dual section QCL is a perfect platform to study exceptional points because the coupling parameter and system loss can be tuned in a single device.

In this work, we demonstrate the generation of 630 fs, 4.5 W pulses from a mid-infrared quantum cascade laser by gain modulation induced spectral broadening and external pulse compression. Such sources open new pathways for broadband supercontinuum generation in the mid-infrared.

A frequency comb in a quantum cascade laser can appear with two distinct spectral lobes. These two spectral features are switching in time and show strong anticorrelations similarly to parametric phenomena.