We demonstrate the generation of harmonic comb states in mid-infrared quantum cascade laser frequency combs by optical feedback injection from an external cavity. A Vernier-like method is presented to control the order of generated harmonic states up to the 6th order. In addition to increased repetition rates, such external cavity induced harmonic states also offer increased optical bandwidth and higher comb coherence. This scheme is easy to implement in spectroscopic systems based on quantum cascade laser frequency combs that can take advantage of these improved parameters.
We present a photothermal spectroscopy setup based on a broadband mid-infrared quantum cascade laser frequency comb (QCL-FC). In this PTS detection scheme, local refractive index changes of the gas sample due to absorption-induced local temperature changes are detected optically by a near-infrared heterodyne interferometer. Until now, this method has been demonstrated only with single-frequency lasers in the mid-infrared region, which limits its capability of targeting broadband absorption features. The QCL-FC used in this work covers the spectral range from 7.7 to 8.2 μm with a repetition rate of 9.9 GHz. A Fourier transform spectrometer modulates the intensity of QCL-FC, which excites photothermal effect of the gas sample in a Herriott multipass cell with optical path of 76 m. Spectroscopic measurements on nitrous oxide is performed as proof of concept. This technique combines the sensitivity of PTS detection and the broadband mid-infrared coverage of QCL-FC, which has a great potential to further promote the applications of QCL-FC in trace gas sensing.
In this work, we demonstrate a new sensing modality of a field-deployable mid-infrared quantum cascade laser dual-comb spectrometer with a raster-scan setup for collecting hyperspectral images of contaminated surfaces from a distance.
Trace-gas plume reconstruction and source localization using drone-assisted remote molecular dispersion spectroscopy was recorded at SPIE Photonics West held in San Francisco, California, United States 2022.
Mid-infrared semiconductor lasers have emerged as indispensable compact coherent sources for military and commercial applications. While much of the historical emphasis has been on maximizing the output power and/or spectral purity, a recent new focus has been on engineering these lasers to operate as optical frequency combs (OFCs) for broadband real-time spectroscopy. In particular, the combination of low-drive-power and broad gain bandwidth has made interband cascade laser (ICL) OFCs an attractive complement to quantum cascade laser OFCs operating at longer wavelengths. Moreover, ICL combs can potentially be incorporated into fully-integrated dual-comb spectrometers that employ fast, room-temperature IC photodetectors processed on the same chip. However, the high refractive index of the ICL’s GaSb substrate poses some challenges to the optical waveguiding. Because the modal index is considerably lower than that of the substrate, the optical field can penetrate the bottom cladding layer and leak into the GaSb, inducing wavelength-dependent interference that modifies the gain and group velocity dispersion (GVD) profiles. Even when the effect on lasing threshold is small, the comb properties can be adversely affected. Using the sub-threshold Fourier transform technique, we studied ICL combs with various ridge widths, substrate thicknesses, and center wavelengths. This allowed us to evaluate the effects of modal leakage on the GVD. We find that the resonant nature of the substrate modes induces oscillations, which affect both the spectral bandwidth and the phase-locking properties above threshold. Strategies to mitigate the GVD’s undesired and unpredictable spectral variation will be presented.
By exploiting the bi-functional operation capability of interband cascade laser (ICL) frequency combs, we have utilized the laser medium not only for comb generation, but also as a room-temperature photodetector with near-GHz bandwidth for multi-heterodyne beating of the comb lines. Our self-contained platform consuming less than 2 W of electrical power enables free-running room-temperature broadband dual-comb spectroscopy of 1,1 difluoroethane with ~3% standard deviation in 2 ms over 600 GHz of optical bandwidth around 3.6 µm. We will discuss progress toward the optimization of ICL combs for realizing compact, low-power chemical sensors operating in the MWIR.
Optical frequency combs have revolutionized the field of high resolution real-time molecular spectroscopy. Here, we demonstrate an electrically-driven optical frequency comb whose sub-picosecond pulses span more than 1 THz of spectral bandwidth centered near 3.3 mm. This is achieved by passively mode locking an interband cascade laser in a multi-contact architecture with gain and saturable absorber sections monolithically integrated on the same chip.
Semiconductor laser frequency comb sources such as type-I quantum well cascade diode lasers, interband cascade lasers and quantum cascade lasers have recently shown tremendous potential as spectroscopic sources for chemical sensing. By simultaneously providing broadband coverage and high-spectral resolution in the mid-IR spectral region, they enable detection of large molecules with broadband absorption spectra and small molecules with well-resolved spectral lines. There is a strong interest in technologies that can provide sensitive spectroscopic detection of chemicals using a compact integrated photonics systems, and semiconductor sources offer unique opportunity for system integration. In this paper I will demonstrate results from a dual comb spectroscopy (DCS) systems based on semiconductor sources that effectively down-convert mid-IR spectra to the RF domain where one can perform spectroscopic signal detection followed by chemical concentration retrieval algorithms. We utilize phase and timing correction algorithms to allow for coherent averaging of data generated by free-running lasers over extended time-scales. Examples of high-resolution spectroscopic detection of small and large molecules in a gas phase will be presented. Current limitations and future directions towards fully integrated photonics DCS systems will be discussed.
We present a chip-scale spectroscopic methane sensor, incorporating a tunable laser, sensor waveguides, and methane reference cell, assembled as a compact silicon photonic integrated circuit. The sensor incorporates an InP-based semiconductor optical amplifier/photodetector array, flip-chip soldered onto a silicon photonic substrate using highprecision waveguide-to-waveguide interfaces. The InP chip provides gain for a hybrid external cavity laser operating at 1650 nm. The sensor features a 20-cm-long TM-mode evanescent-field waveguide as the sensing element and is compatible with high-volume wafer-scale silicon photonics manufacturing and assembly processes. This sensor can be an enabling platform for economical methane and more general distributed environmental trace-gas monitoring.
We present a portable optical spectrometer for fugitive emissions monitoring of methane (CH4). The sensor operation is based on tunable diode laser absorption spectroscopy (TDLAS), using a 5 cm open path design, and targets the 2ν3 R(4) CH4 transition at 6057.1 cm-1 (1651 nm) to avoid cross-talk with common interfering atmospheric constituents. Sensitivity analysis indicates a normalized precision of 2.0 ppmv·Hz-1/2, corresponding to a noise-equivalent absorbance (NEA) of 4.4×10-6 Hz-1/2 and minimum detectible absorption (MDA) coefficient of αmin = 8.8×10-7 cm-1·Hz-1/2. Our TDLAS sensor is deployed at the Methane Emissions Technology Evaluation Center (METEC) at Colorado State University (CSU) for initial demonstration of single-sensor based source localization and quantification of CH4 fugitive emissions. The TDLAS sensor is concurrently deployed with a customized chemi-resistive metal-oxide (MOX) sensor for accuracy benchmarking, demonstrating good visual correlation of the concentration time-series. Initial angle-ofarrival (AOA) results will be shown, and development towards source magnitude estimation will be described.
Due to its high toxicity, monitoring of hydrogen sulfide (H2S) concentration is essential in many industrial sites (such as natural gas extraction sites, petroleum refineries, geothermal power plants, or waste water treatment facilities), which require sub-parts-per-million sensitivities. We report on a quantum cascade laser-based spectroscopic system for detection of H2S in the midinfrared at ∼7.2 μm. We present a sensor design utilizing Herriott multipass cell and a wavelength modulation spectroscopy to achieve a detection limit of 140 parts per billion for 1-s integration time.
KEYWORDS: Spectroscopy, Signal to noise ratio, Quantum cascade lasers, Absorption, Methane, Spectral resolution, Optical engineering, Signal detection, Digital filtering, Sensors
While midinfrared radiation can be used to identify and quantify numerous chemical species, contemporary broadband midinfrared spectroscopic systems are often hindered by large footprints, moving parts, and high power consumption. In this work, we demonstrate multiheterodyne spectroscopy (MHS) using interband cascade lasers, which combines broadband spectral coverage with high spectral resolution and energy-efficient operation. The lasers generate up to 30 mW of continuous-wave optical power while consuming <0.5 W of electrical power. A computational phase and timing correction algorithm is used to obtain kHz linewidths of the multiheterodyne beat notes and up to 30 dB improvement in signal-to-noise ratio. The versatility of the multiheterodyne technique is demonstrated by performing both rapidly swept absorption and dispersion spectroscopic assessments of low-pressure ethylene (C2H4) acquired by extracting a single beat note from the multiheterodyne signal, as well as broadband MHS of methane (CH4) acquired with all available beat notes with microsecond temporal resolution and an instantaneous optical bandwidth of ∼240 GHz. The technology shows excellent potential for portable and high-resolution solid-state spectroscopic chemical sensors operating in the midinfrared.
Majority of chemical species of interest in security and safety applications (e.g. explosives) have complex molecular structures that produce unresolved rotational-vibrational spectroscopic signatures in the mid-infrared. This requires spectroscopic techniques that can provide broadband coverage in the mid-IR region to target broadband absorbers and high resolution to address small molecules that exhibit well-resolved spectral lines. On the other hand, many broadband mid-IR absorbers exhibit well-resolved rotational components in the THz spectral region. Thus, development of spectroscopic sensing technologies that can address both spectral regions is of great importance. Here we demonstrate recent progress towards broadband high-resolution spectroscopic sensing applications with Fabry-Perot quantum cascade lasers (QCLs) and frequency combs using multi-heterodyne spectroscopy (MHS) techniques.
In this paper, we will present spectroscopic sensing of large and small molecules in the mid-IR region using QCLs operating at ~8.5µm. An example high-resolution, broadband MHS of ammonia (small molecule) and isobutane (broadband absorber) at atmospheric pressure in the 1165-1190 cm^-1 range will be discussed. We have developed a balanced MHS system for mitigation of the laser intensity fluctuations. Absorption spectroscopy as well as dispersion spectroscopy with minimum fractional absorption down to ~10^-4/Hz1/2 and fast spectral acquisition capabilities down to 10 µs/spectrum range will be demonstrated. In order to mitigate the shortcomings of the limited chemical selectivity in the mid-IR, THz QCL based spectrometer is currently under development to provide spectral de-congestion and thus significantly improve chemical identification. Preliminary characterization of the performance of THZ QCL combs for the THz QCL-MHS will be presented.
Interband cascade lasers (ICLs) have proven to be efficient semiconductor sources of coherent mid -infrared (mid-IR) radiation. Single mode distributed-feedback (DFB) ICLs are excellent high-resolution spectroscopic sources for targeting important molecular species in the mid-IR fingerprint region, but are limited to a narrow spectral tuning range. Recent developments in multi-heterodyne spectroscopy with multi-mode Fabry-Perot (FP) lasers have enabled significant progress towards broadband high-resolution spectroscopic sensing applications in the mid-infrared. Here, we characterize the mode structure and tuning properties of multi-mode FP-ICLs for the purpose of evaluating the feasibility of ICL-based multiheterodyne spectroscopy.
Quantitative laser spectroscopic measurements of complex molecules that have a broad absorption spectra require broadly tunable laser sources operating preferably in the mid-infrared molecular fingerprint region. In this paper a novel broadband mid-infrared laser source comprising of an array of single-mode distributed feedback quantum cascade lasers was used to target a broadband absorption feature of benzene (C6H6), a toxic and carcinogenic atmospheric pollutant.
The DFB-QCL array is a monolithic semiconductor device with no opto-mechanical components, which eliminates issues with mechanical vibrations. The DFB-QCLs array used in this work provides spectral coverage from 1022.5 cm-1 to 1053.3 cm-1, which is sufficient to access the absorption feature of benzene at 1038 cm-1 (9.64 μm).
A sensor prototype based on a 76 m multipass cell (AMAC-76LW, Aerodyne Research) and a dispersive DFB-QCL array beam combiner was developed and tested. The Allan deviation analysis of the retrieved benzene concentration data yields a short-term precision of 100 ppbv/Hz1/2 and a minimum detectable concentration of 12 ppbv for 200 s averaging time. The system was also tested by sampling atmospheric air as well as vapors of different chemical products that contained traces of benzene.
KEYWORDS: Optical networks, Simulation of CCA and DLA aggregates, Laser spectroscopy, Optical sensing, Remote sensing, Tunable lasers, Gases, Digital signal processing, Wavelength division multiplexing, Absorption spectroscopy
We propose an optical gas sensing network directly overlaid onto optical access networks, hybrid TDM/WDM-PONs. Centralized remote gas monitoring is demonstrated using three different sensing technologies: Chirp Laser Dispersion Spectroscopy (CLaDS), Direct Laser Absorption Spectroscopy (DLAS) and tunable diode laser absorption spectroscopy (TDLS). DLAS performs fast threshold detection while CLaDS provides quantitative information about the gas. Additionally, TDLS utilizes a cost-effective solution for multiple gases detection. The results confirm that centralized remote gas sensing can be realized in optical communication networks using standard single-mode fiber (SMF), which provides a real time, low cost, and maintenance-free solution.
W. Chen, T. Wu, W. Zhao, G. Wysocki, X. Cui, C. Lengignon, R. Maamary, E. Fertein, C. Coeur, A. Cassez, Y. Wang, W. Zhang, X. Gao, W. Liu, F. Dong, G. Zha, Xu Zheng, T. Wang
Chemically reactive short-lived species play a crucial role in tropospheric processes affecting regional air quality and
global climate change. Contrary to long-lived species (such as greenhouse gases), fast, accurate and precise monitoring
changes in concentration of atmospheric short-lived species represents a real challenge due to their short life time (~1 s
for OH radical) and very low concentration in the atmosphere (down to 106 molecules/cm3, corresponding to 0.1 pptv at
standard temperature and pressure).
We report on our recent progress in instrumentation developments for spectroscopic sensing of trace reactive species.
Modern photonic sources such as quantum cascade laser (QCL), distributed feedback (DFB) diode laser, light emitting
diode (LED), difference-frequency generation (DFG) parametric source are implemented in conjunction with highsensitivity
spectroscopic measurement techniques for : (1) nitrous acid (HONO) monitoring by QCL-based long optical
pathlength absorption spectroscopy and LED-based IBBCEAS (incoherent broadband cavity-enhanced absorption
spectroscopy); (2) DFB laser-based hydroxyl free radical (OH) detection using WM-OA-ICOS (wavelength modulation
off-axis integrated cavity output spectroscopy) and FRS (Faraday rotation spectroscopy), respectively; (3) nitrate radical
(NO3) and nitrogen dioxide (NO2) simultaneous measurements with IBBCEAS approach.
Applications in field observation and in smog chamber study will be presented.
A novel heterodyne-enhanced Faraday rotation spectroscopic (H-FRS) system for trace gas detection of nitric oxide
(NO) is demonstrated. The system is based on a quantum cascade laser emitting at ~5.2 μm and a mercury cadmium
telluride photodetector (both thermoelectrically cooled). The heterodyne detection is performed at 30MHz, where
the laser relative intensity noise is significantly smaller than at low frequencies. With an implementation of active
interferometer stabilization technique, the current system shows total noise level that is only 5.4 times above the
fundamental shot-noise limit and the Faraday rotation angle sensitivity of 2.6 × 10-8 rad/√Hz. The NO detection
limit of 30.7 ppb-v/√Hz was achieved for the R(8.5)e NO transition using 100 Gauss magnetic field and 0.15 m
optical path length.
Academic and industrial researchers require ultra-low power, compact laser
based trace-gas sensor systems for the most demanding environmental and space-borne
applications. Here the latest results from research projects addressing these applications
will be discussed: 1) an ultra-compact CO2 sensor based on a continuous wave quantum
cascade laser, 2) an ultra-sensitive Faraday rotation spectrometer for O2 detection, 3) a fully
ruggedized compact and low-power laser spectrometer, and 4) a novel non-paraxial nonthin
multipass cell. Preliminary tests and projection for performance of future sensors based
on this technology is presented.
A new spectroscopic technique for remote molecular detection is presented. Chirped Laser Dispersion Spectroscopy
(CLaDS) uses a two-color dynamic interferometric heterodyne detection to measure optical dispersion caused by
molecular transitions. The dispersion sensing is based on measurement of instantaneous frequency of an optical
heterodyne beatnote which provides high immunity to optical power fluctuations. Thus CLaDS is well suited to long
distance remote sensing and open-path monitoring. In this work we present CLaDS experimental setup for remote
sensing of nitric oxide using 5.2 μm quantum cascade laser. System performance as well as advantages and limitations
are discussed.
A basic wireless laser spectroscopic sensor network for monitoring of trace-gases will be presented. The prototype lowpower
sensor nodes targeting carbon dioxide are based on tunable diode laser absorption spectroscopy and operate using
a 2 μm VCSEL and a 3.5 m Herriott multi-pass cell. The sensor system, which employs real-time wireless
communications, is controlled by custom electronics and can be operated autonomously. The sensor core electronics
performs molecular concentration measurements using wavelength modulation spectroscopy with an active laser
frequency locking to the target transition. The operating sensor node consumes approximately 300 mW of electrical
power and can work autonomously for up to 100 hours when powered by a 10.5 Ah Lithium-ion polymer battery.
Environmentally controlled long term (12 hours) stability tests show sensor node detection limit of ~0.286 ppm with 1
second integration time and the ultimate minimum detectable fractional absorption of 1.5x10-6 is obtained after 3500
seconds averaging time. The sensor node performance results and preliminary tests in a basic network configuration are
discussed.
We demonstrate room temperature lasing of quantum cascade lasers (QCLs) operating at 3.45 and 3.55 μm under pulsed
and continuous wave (CW) operations, respectively. To the best of our knowledge those are the shortest wavelengths
ever achieved at room temperature from QCLs with strain balanced GaInAs/AlInAs material on InP substrate. With the
back facet high reflection coated, a maximum output power of 60 mW was obtained at 10 °C under CW operation. A
tuning range of 124 cm-1 was obtained in a pulsed mode in external cavity configuration.
QC laser active regions with multiple transitions from strongly coupled upper laser states to lower laser states were
designed to achieve broad gain spectra as well as high performance. Two broad gain QC laser designs in the 7-9 μm
wavelength region were demonstrated based on the continuum-to-bound design. The first embodiment of one design at
~8 μm enables external cavity tuning over 190 cm-1 (7.5 - 8.8 μm) in pulsed mode operation at 0°C. We also
demonstrated a QC laser structure based on a continuum-to-continuum active region in the 4-5 μm wavelength region
with a gain spectrum of ~ 430 cm-1. External cavity tuning over 340 cm-1 (4.4 - 5.2 μm) was achieved with this design in
pulsed mode operation at 15°C. In spite of the broad gain spectrum, a low threshold current density (1.6 kA/cm2), large
slope efficiency (4.5 W/A), good temperature performance (T0=160 K), high peak power (up to 5 W) and high wall plug
efficiency (WPE, up to 20%) were achieved for ridge lasers with as cleaved facets, in pulsed mode operation at 295 K,
demonstrating that it is possible to make a wide gain spectrum compatible with high power and efficiency performance.
We describe our novel instrumentation architectures for infrared laser spectrometers. Compact, power
efficient, low noise modules allow for optimized implementation of cell phone sized sensors using
VCSELs, diode, and quantum cascade laser sources. These sensors can consume as little as 0.3W with full
laser temperature (<0.001K/Hz1/2) and current control (<2ppm/Hz1/2 noise), photodiode preamplification
(<2pA/Hz1/2 noise floor, 1MΩ transimpedance), and digital lock-in amplification with 3 independent
channels. We have implemented sensors based on laser absorption spectroscopy, photoacoustic
spectroscopy, and Faraday rotation spectroscopy using the openPHOTONS systems, with performance
rivaling standalone laboratory measurement instrumentation. Additionally, as openPHOTONS is an open
source software repository, this instrumentation can be quickly adapted to new optical configurations
and applications. Such modules allow the development of flexible sensors, whether implementing
closed path spectrometers, open path perimeter monitoring, or remote backscatter based sensors. This
work is also the enabling technology for wireless sensor networks (WSN) of precision sensors, a
desirable sensing paradigm for long term, wide area, precision, temporally and spatially resolved
studies. This approach can complement existing remote sensing and mapping technologies including
satellite observations and sparse networks of flux towers.
Faraday Rotation Spectroscopy (FRS) is a polarization based spectroscopic technique which can provide higher
sensitivity concentration measurements of paramagnetic gases and free radicals than direct absorption spectroscopic
techniques. We have developed sensor systems which require only 0.2W to perform TDLAS (tunable diode laser
absorption spectroscopy), and can additionally be quickly duty cycled, enabling operation in wireless sensor
networks of laser-based trace gas sensors We adapted our integrated TDLAS electronics to perform FRS in a
compact and more sensitive system for quantification of molecular oxygen (O2) using a 762.3nm VCSEL in the A
band. Using an AC magnetic field, we demonstrate detector noise dominated performance, achieving 2.1×10-6/Hz1/2
equivalent detectable fractional absorption and a minimum detection limit of 462 ppmv O2 in 1 second in a 15cm
path. At longer paths and integration times, such a sensor will enable oxygen measurements at biotic respiration
levels (<1ppmv) to measure CO2 - O2 exchange for mapping natural exchange of greenhouse gases. Potential
improvement of detection limits by increasing various system performance parameters is described.
The ν1+ν3 combination band of uranium hexafluoride (UF6) is targeted to perform analytical enrichment measurements
using laser absorption spectroscopy. A high performance widely tunable EC-QCL sources emitting radiation at 7.74 μm
(1291 cm-1) is employed as an UF6-LAS optical source to measure the unresolved rotational-vibrational spectral
structure of several tens of wavenumbers (cm-1). A preliminary spectroscopic measurement based on a direct laser
absorption spectroscopy of methane (CH4) as an appropriate UF6 analyte simulant, was demonstrated.
We combine laser processing and the technique of a scanning near-field optical microscope (SNOM) for realization of
laser-patterning on a nanoscale, laser ablation near the fiber tip, and micro-analysis of solid surface samples by laserinduced
breakdown spectroscopy (LIBS). We describe an universal SNOM-like setup allowing to produce near-field
laser patterns by laser heating and laser ablation, laser-induced breakdown spectroscopy, and atomic force microscope
(AFM) topography investigation with the same optical fiber tip, which is used as near-field emitter or as probe. With
solid Si and Al samples, three laser processing regimes were observed with increasing laser pulse energy: (1) cone
formation (only for Si, smallest features with 500 nm width and 100 nm height), (2) formation of small craters (smallest
features with 450 nm width and 250 nm depth), and (3) crater formation with a width of more than 2 μm with emission
of evaluable plasma emission line spectra.
Nitric oxide detection with Faraday rotation spectroscopy offers excellent
sensitivity and high specificity together with outstanding long term system performance. Development of a
transportable, cryogen-free, prototype field instrument based on mode-hop-free external cavity quantum
cascade laser targeting the optimum NO Q(3/2) transition at 1875.8 cm-1 is reported . The system shows a
minimum detection limit of 5.4 ppb with a 1sec. lock-in time constant. Continuous, unattended NO
monitoring with >1 hour white noise limited averaging times is reported.
Recent advances in the development of sensors based on infrared quantum cascade lasers for the detection of trace gas
species is reported. Several selected examples of applications in environmental and industrial process monitoring as well
as in medical diagnostics using quartz enhanced photoacoustic spectroscopy and laser absorption spectroscopy will be
described.
Exhaled nitric oxide (NO) is an important biomarker in asthma and other respiratory disorders. The optical performance of a NO/CO2 sensor employing integrated cavity output spectroscopy (ICOS) with a quantum cascade laser operating at 5.22 µm capable of real-time NO and CO2 measurements in a single breath cycle is reported. A NO noise-equivalent concentration of 0.4 ppb within a 1-sec integration time is achieved. The off-axis ICOS sensor performance is compared to a chemiluminescent NO analyzer and a nondispersive infrared (NDIR) CO2 absorption capnograph. Differences between the gas analyzers are assessed by the Bland-Altman method to estimate the expected variability between the gas sensors. The off-axis ICOS sensor measurements are in good agreement with the data acquired with the two commercial gas analyzers. This work demonstrates the performance characteristics and merits of mid-infrared spectroscopy for exhaled breath analysis.
Recent achievements in laser-induced surface patterning obtained in our group are summarized. Here, we have employed both a SNOM-type setup and two-dimensional lattices of SiO2 microspheres formed by self-assembly processes. With the SNOM-type setup we have
demonstrated nanoscale photochemical and photothermal etching, mainly of Si in Cl2 atmosphere. With 2D lattices of microspheres a large number of single features can be generated by a single or a few laser shots. Among the examples presented is the surface patterning by ablation, etching, deposition, and surface modification.
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