An innovative quartz enhanced photoacoustic (QEPAS) gas sensing system operating in the THz spectral range and employing a custom quartz tuning fork (QTF) is described. The QTF dimensions are 3.3 cm 0.4 cm 0.8 cm, with the two prongs spaced by 800 mm. To test our sensor we used a quantum cascade laser as the light source and selected a methanol rotational absorption line at 131.054 cm1 (3.93 THz), with line-strength S ¼ 4.28 1021 cm mol1. The sensor was operated at 10 Torr pressure on the first flexion QTF resonance frequency of 4245 Hz. The corresponding Q-factor was 74 760. Stepwise concentration measurements were performed to verify the linearity of the QEPAS signal as a function of the methanol concentration. The achieved sensitivity of the system is 7 parts per million in 4 seconds, corresponding to a QEPAS normalized noise-equivalent absorption of 2 1010 W cm1 Hz1/2, comparable with the best result of mid-IR QEPAS systems.

We report on a novel intracavity quartz enhanced photoacoustic (I-QEPAS) gas sensing technique taking advantage from both the high Q-factor of standard tuning forks and the power build-up of a high-finesse optical resonator. This first prototype employs a distributed feedback quantum cascade laser operating at 4.3 μm. CO2 has been selected as gas target. Preliminary results demonstrate an improved sensitivity, close to the cavity enhancement factor (500) times the optical coupling efficiency (about 0.5), with respect to standard QEPAS technique. The detection limit was pulled from 7 ppm (obtained with standard QEPAS) down to 32 ppb, corresponding to normalized noise-equivalent absorption in the 10-9 W•cm-1•Hz-1/2 range.

A quartz-enhanced photoacoustic absorption spectroscopy (QEPAS)-based gas sensor was developed for methane (CH4) and nitrous-oxide (N2O) detection. The QEPAS-based sensor was installed in a mobile laboratory operated by Aerodyne Research, Inc. to perform atmospheric CH4 and N2O detection around two urban waste-disposal sites located in the northeastern part of the Greater Houston area, during DISCOVER-AQ, a NASA Earth Venture during September 2013. A continuous wave, thermoelectrically cooled, 158 mW distributed feedback quantum cascade laser emitting at 7.83 μm was used as the excitation source in the QEPAS gas sensor system. Compared to typical ambient atmospheric mixing ratios of CH4 and N2O of 1.8 ppmv and 323 ppbv, respectively, significant increases in mixing ratios were observed when the mobile laboratory was circling two waste-disposal sites in Harris County and when waste disposal trucks were encountered.

The impact of quantum cascade lasers (QCLs) intrinsically high sensitivity to external optical feedback intended for sensing applications such as in-line ablation rate measurements is experimentally demonstrated. We developed a QCL-based sensor to assess the voltage modulation at the laser terminals induced by fast displacement of the ablation front during the process. This work shows that the detection range of our diagnostic system is only limited by the emission wavelength of the QCL probe source and the capability to measure ablation rates as high as 160 nm/pulse was reported. This sensing technique can be employed with the whole class of quantum cascade lasers, whose emission spans from mid-IR to THz spectral region, thus enabling the extension of its applications to ultra-fast laser ablation processes.

We report on the experimental measurement of active region lattice (TL) and electronic temperatures (Te) in terahertz quantum cascade devices based on the phonon-photon-phonon scheme, by means of microprobe band-to-band photoluminescence spectroscopy. Three mesa devices, differing for doping region and number of quantum wells composing the active region, have been investigated. With device on, under band alignment for lasing condition, we measured a difference (Te – TL) ~ 40 K much smaller than the typical value (Te – TL ~ 100 K) reported for resonant-phonon THz QCLs.

We compare the electrical power dependence of the lattice temperature and the electronic temperature of GaAs/AlxGa1-xAs THz quantum cascade lasers (QCLs) with different active region schemes, as extracted by the analysis of microprobe band-to-band photoluminescence experiments. Thermalized non-equilibrium distributions are found in all classes of QCLs. While in the case of bound-to-continuum structures all subbands share the same temperature, the upper laser level of active regions based on the resonant-phonon scheme heats up by ΔT ~ 100 K with respect to lower energy levels. The comparison among samples with different Al mole fractions show that the use of smaller x values leads to larger electronic temperatures.

We report on a spectroscopic technique named intracavity quartz-enhanced photoacoustic spectroscopy (I-QEPAS) employed for sensitive trace-gas detection in the mid-infrared spectral region. It is based on a combination of QEPAS with a buildup optical cavity. The sensor includes a distributed feedback quantum cascade laser emitting at 4.33 lm. We achieved a laser optical power buildup factor of 500, which corresponds to an intracavity laser power of 0.75 W. CO2 has been selected as the target molecule for the I-QEPAS demonstration. We achieved a detection sensitivity of 300 parts per trillion for 4 s integration time, corresponding to a noise equivalent absorption coefficient of 1.4108 cm1 and a normalized noise-equivalent absorption of 3.21010 W cm1 Hz1/2.

A quartz-enhanced photoacoustic spectroscopy sensor system was developed for the sensitive detection of hydrogen peroxide (H2O2) using its absorption transitions in the v6 fundamental band at 7.73 lm. The recent availability of distributed-feedback quantum cascade lasers provides convenient access to a strong H2O2 absorption line located at 1295.55 cm1. Sensor calibration was performed by means of a water bubbler that generated titrated average H2O2 vapor concentrations. A minimum detection limit of 12 parts per billion (ppb) corresponding to a normalized noise equivalent absorption coefficient of 4.6109 cm1W/Hz1/2 was achieved with an averaging time of 100 s.

We report on single mode optical transmission of hollow core glass waveguides (HWG) coupled with an external cavity mid-IR quantum cascade lasers (QCLs). The QCL mode results perfectly matched to the hybrid HE11 waveguide mode and the higher losses TE-like modes have efficiently suppressed by the deposited inner dielectric coating. Optical losses down to 0.44 dB/m and output beam divergence of ~5 mrad were measured. Using a HGW fiber with internal core size of 300 µm we obtained single mode laser transmission at 10.54 µm and successful employed it in a quartz enhanced photoacoustic gas sensor setup.

An innovative spectroscopic system based on an external cavity quantum cascade laser (EC-QCL) coupled with a mid-infrared (mid-IR) fiber and quartz enhanced photoacoustic spectroscopy (QEPAS) is described. SF6 has been selected as a target gas in demonstration of the system for trace gas sensing. Single mode laser delivery through the prongs of the quartz tuning fork has been obtained employing a hollow waveguide fiber with inner silver–silver iodine (Ag–AgI) coatings and internal core diameter of 300 μm. A detailed design and realization of the QCL fiber coupling and output collimator system allowed almost practically all (99.4 %) of the laser beam to be transmitted through the spectrophone module. The achieved sensitivity of the system is 50 parts per trillion in 1 s, corresponding to a record for QEPAS normalized noise-equivalent absorption of 2.7 × 10−10 W cm−1 Hz−1/2.

A novel spectroscopic technique based on modulation spectroscopy is described. We demonstrated three applications: measurement of temperature differences in a gas, detection of broadband absorbing species and isotope ratio measurements

We report on the optical coupling between hollow core waveguides and external cavity mid-IR quantum cascade lasers (QCLs). Waveguides with 1000 μm bore size and lengths ranging from 2 to 14 cm, with metallic (Ag)/dielectric (AgI or polystyrene) circular cross-section internal coatings, have been employed. Our results show that the QCL mode is perfectly matched to the hybrid HE11 waveguide mode, demonstrating that the internal dielectric coating thickness is effective to suppress the higher losses TE-like modes. Optical losses down to 0.44 dB/m at 5.27 μm were measured in Ag/polystyrene-coated waveguide with an almost unitary coupling efficiency.

We will report here on the design and realization of optoacoustic sensors based on an external cavity QCL laser source emitting at 10,54 μm, fiber-coupled with a QEPAS spectrophone module. SF6 has been selected as the target gas. Single mode laser delivery through the prongs of the quartz tuning fork has been realized using a hollow waveguide fiber with internal core size of 300 μm. The achieved sensitivity of the system was 50 part per trillion in 1 s corresponding to a record for QEPAS normalized noise-equivalent absorption of 2,7•10-10 W•cm-1•Hz-1/2. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

The application of an innovative spectroscopic balancing technique to measure the isotope 18O/16O ratio in water vapor is reported. Quartz enhanced photoacoustic spectroscopy has been employed as the absorption sensing technique. Two isotope absorption lines with the same quantum numbers, with very close lower energy levels, have been selected to limit the sensitivity to temperature variations and guarantee identical broadening as well as relaxation properties. The sensitivity in measuring the deviation from a standard sample δ18O is 1.4‰, in 200 sec of integration time.

Geometrical parameters of micro-resonator for a quartz enhanced photoacoustic spectroscopy sensor are optimized to perform sensitive and background-free spectroscopic measurements using mid-IR quantum cascade laser (QCL) excitation sources. Such an optimized configuration is applied to nitric oxide (NO) detection at 1900.08 cm−1 (5.26 μm) with a widely tunable, mode-hop-free external cavity QCL. For a selected NO absorption line that is free from H2O and CO2 interference, a NO detection sensitivity of 4.9 parts per billion by volume is achieved with a 1-s averaging time and 66 mW optical excitation power. This NO detection limit is determined at an optimal gas pressure of 210 Torr and 2.5% of water vapor concentration. Water is added to the analyzed mixture in order to improve the NO vibrational-translational relaxation process.

We measured the lattice and subband electronic temperatures of terahertz quantum cascade devices based on the optical phonon-scattering assisted active region scheme. While the electronic temperature of the injector state (j = 4) significantly increases by ΔT = Te 4 – TL ~40 K, in analogy with the reported values in resonant phonon scheme (ΔT ~70-110 K), both the laser levels (j = 2,3) remain much colder with respect to the latter (by a factor of 3-5) and share the same electronic temperature of the ground level (j = 1). The electronic population ratio n2/n1 shows that the optical phonon scattering efficiently depopulates the lower laser level (j = 2) up to an electronic temperature Te ~180 K.

A sensitive spectroscopic sensor based on a hollow-core fiber-coupled quantum cascade laser (QCL) emitting at 10.54 μm and quartz enhanced photoacoustic spectroscopy (QEPAS) technique is reported. The design and realization of mid-IR fiber and coupler optics has ensured single-mode QCL beam delivery to the QEPAS sensor. The collimation optics was designed to produce a laser beam of significantly reduced beam size and waist so as to prevent illumination of the quartz tuning fork and microresonator tubes. SF6 was selected as the target gas. A minimum detection sensitivity of 50 parts per trillion in 1 s was achieved with a QCL power of 18 mW, corresponding to a normalized noise-equivalent absorption of 2.7×10^−10   W⋅cm^−1/Hz^1/2.

We demonstrated a sensing technique for in-line ablation rate detection using a quantum cascade laser (QCL) under external optical feedback. The design of the QCL-based diagnostic system allowed to monitor the voltage modulation at the laser terminals induced by fast dynamics in the ablation process. Real-time detection of the ablation front velocity as well as in-situ investigations of the surface temperature were provided. Experimental results on fast ablation rates per pulse correlate well with the theoretical prediction. The detection range was demonstrated to be limited only by the QCL-probe emission wavelength, which is scalable up to the THz spectral region.

The development of compact optical sensors for trace chemical species in the gas phase is of interest for a large number of applications. Among them, environmental monitoring of hazardous compounds is boosting the development of portable, cost-effective and robust trace-gas sensors for the prevention of damage caused by inhalation of toxic substances. Hydrogen Sulfide (H2S) is an extremely toxic and irritating gas, appearing in many context of productive and social life like petroleum refinery and geothermal activity. The concentration limits set by the U.S. Occupational Safe and Health Administration (OSHA) are 20 parts per million (ppm) for long lasting exposure, and a peak limit of 50 ppm for no more than 10 minutes. Inhalation of concentrations of 500-1000 ppm will cause rapid unconsciousness and death through respiratory paralysis and asphyxiation.

Quartz-enhanced photo-acoustic spectroscopy (QEPAS) is one of the most robust and sensitive trace-gas detection techniques, which in the mid-IR range offers the advantage of high sensitivity, compactness and fast time-response. One of the main features of the photoacoustic techniques is that no optical detection is re-quired. Thus, the use of the QEPAS technique in THz range would allow to avoid the use of low-noise but expensive, bulky and cryogenic bolometers. The results obtained in the development of QEPAS sensors for trace gas detection of several chemical species, employing mid-IR and THz laser sources will be reviewed. Nor-malized noise equivalent absorption coefficients (NNEA) down to 10-10 cm-1W/Hz½ and part per trillion concentration detection ranges have been attained.

The development and performance of a continuous wave (CW), thermoelectrically cooled (TEC) quantum cascade laser (EC-QCL) based sensor for quantitative measurements of nitric oxide (NO) concentrations in exhaled breath will be reported. Human breath contains ~ 400 different chemical species, usually at ultra low concentration levels, which can serve as biomarkers for the identification and monitoring of human diseases or wellness states. By monitoring exhaled NO concentration levels, a fast non-invasive diagnostic method for treatment of patients with asthma and chronic obstructive pulmonary disease (COPD) is feasible. The NO concentration measurements are performed with a 2f wavelength modulation based quartz enhanced photoacoustic spectroscopy (QEPAS) technique, which is very suitable for real time breath measurements, due to the fast gas exchange inside a compact QEPAS gas cell (<5 mm3 typical volume). In order to target the optimal interference free NO R (6.5) absorption doublet at 1900.08 cm-1 (λ~5.263 μm) a Daylight Solutions Inc. widely tunable, mode-hop free 100 mW EC-QCL was used. The sensor reference channel includes a 10 cm long reference cell, filled with a 0.5% NO in N2 at 150 Torr, which is used for line-locking purpose. A minimum detection limit (1σ) for the EC-QCL based line locked NO sensor is ~5 ppbv with a 1 sec update time by a custom built control QCL compatible electronics unit.

A quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor system was developed for the sensitive detection of hydrogen peroxide (H2O2) using its absorption transitions in the v6 fundamental band near 7.73 μm. The recent availability of distributed-feedback quantum cascade lasers (DFB-QCLs) provides convenient access to a strong H2O2 absorption line located at 1295.55 cm-1. Sensor calibration was performed by means of a water bubbler that generated titrated average vapor concentrations. A minimum detection limit of 75 parts per billion (ppb) was achieved at a pressure of 80 torr for a 1 sec data acquisition time. The long-term repeatability and stability of the sensor was investigated by measuring time-varying H2O2 mixtures for ~2 hrs. An Allan deviation analysis was performed to investigate the long-term performance of the QEPAS sensor system, indicating the feasibility of a minimum detection limit of 12 ppb using the optimum data averaging time of 100 sec.

The development and performance of a continuous wave (CW), thermoelectrically cooled (TEC) external cavity quantum cascade laser (EC-QCL) based sensor for quantitative measurements of nitric oxide (NO) concentrations in exhaled breath will be reported. Human breath contains ~ 400 different chemical species, usually at ultra low concentration levels, which can serve as biomarkers for the identification and monitoring of human diseases or wellness states. By monitoring exhaled NO concentration levels, a fast non-invasive diagnostic method for treatment of patients with asthma and chronic obstructive pulmonary disease (COPD) is feasible. The NO concentration measurements are performed with a 2f wavelength modulation based quartz enhanced photoacoustic spectroscopy (QEPAS) technique, which is very suitable for real time breath measurements, due to the fast gas exchange inside a compact QEPAS gas cell (<5 mm3 typical volume). In order to target the optimal interference free NO R (6.5) absorption doublet at 1900.08 cm-1 (λ~5.263 μm) a Daylight Solutions Inc. widely tunable, mode-hop free 100 mW EC-QCL was used. The sensor reference channel includes a 10 cm long reference cell, filled with a 0.5% NO in N2 at 150 Torr, which is used for line-locking purpose. A minimum detection limit (1σ) for the EC-QCL based line locked NO sensor is ~5 ppbv with a 1 sec update time by a custom built control QCL compatible electronics unit.

A detailed review on the development of quartz-enhanced photoacoustic sensors (QEPAS) for the sensitive and selective quantification of molecular trace gas species with resolved spectroscopic features is reported. The basis of the QEPAS technique, the technology available to support this field in terms of key components, such as light sources and quartz-tuning forks and the recent developments in detection methods and performance limitations will be discussed. Furthermore, different experimental QEPAS methods such as: on-beam and off-beam QEPAS, quartz-enhanced evanescent wave photoacoustic detection, modulation-cancellation approach and mid-IR single mode fiber-coupled sensor systems will be reviewed and analysed. A QEPAS sensor operating in the THz range, employing a custom-made quartz-tuning fork and a THz quantum cascade laser will be also described. Finally, we evaluated data reported during the past decade and draw relevant and useful conclusions from this analysis.

Measurements characterizing spatial mode filtering of mid-infrared (mid-IR) laser beams using hollow core fiber optics are presented. The mode filtering depends strongly on the fiber diameter, with effective mode filtering demonstrated with bore diameters of d = 200 μm and 300 μm. In addition to mode filtering, beam profile measurements also demonstrate the strong dependence of the mode quality on the fiber coupling conditions. As predicted, optimal coupling is achieved using relatively slow optics that produce focused spots that nearly fill the fiber diameter. Examples of the utility of using hollow fibers for mode-filtering to improve molecular spectroscopy experiments are also discussed.

Solid state gas sensors based on metal or metal oxide nanostructures are promising for real-time detection of low levels of nitrogen oxides (NOx) owing to their low cost, high sensitivity and availability of a variety of metal oxides with different gas sensing characteristics.1

The detection and quantification of trace chemical species in the gas phase is of great interest in a wide range of applications such as environmental monitoring, industrial process control and medical diagnostics. In combination quantum cascade laser (QCLs), quartz enhanced photoacoustic spectroscopy (QEPAS) offers the advantage of high sensitivity and fast time-response. Very efficient QCL-based QEPAS sensors have been demonstrated for trace detection of several chemical species, such as NH3, NO, CO2, N2O, CO, CH2O, etc. Small size sensors with simple optical alignment has been realized employing fiber-coupled system between near IR laser source and QEPAS spectrophones, thus the possibility to extend this approach also for mid-IR light sources, considering the small size of the QEPAS module, may allow compact integration also with QCLs.

We report on the application of an innovative spectroscopic balancing technique to measure isotopologue abundance quantification. We employ quartz enhanced photoacoustic spectroscopy in a 2f wavelength modulation mode as an absorption sensing technique and water vapor as a test analyte. Isotope absorption lines with very close lower energy levels and with the same quantum numbers have been selected to limit the sensitivity to temperature variations and guarantee identical broadening relaxation properties. A detection sensitivity in measuring the deviation from a standard sample δ18O of 1.4‰, in 200 sec of integration time was achieved.

A quartz enhanced photo-acoustic sensor employing a single-mode quantum cascade laser emitting at 3.93 Terahertz (THz) is reported. A custom tuning fork with a 1 mm spatial separation between the prongs allows the focusing of the THz laser beam between them, while preventing the prongs illumination. A methanol transition with line-strength of 4.28 × 10^−21 cm has been selected as target spectroscopic line. At a laser optical power of ∼ 40 μW, we reach a sensitivity of 7 parts per million in 4s integration time, corresponding to a 1σ normalized noise-equivalent absorption of 2 × 10^−10 cm^−1W/Hz½.

A novel technique based on modulation spectroscopy with two excitation sources and quartz enhanced photoacoustic is described. We demonstrated two applications: measurement of temperature in a gas mixture and detection of broadband absorbing chemical species.

An innovative quartz enhanced photoacoustic (QEPAS) gas sensing system operating in the THz spectral range and employing a custom quartz tuning fork (QTF) will be described. The QTF dimensions are 3.3 cm x 0.4 cm x 0.8 cm, with the two prongs spaced by ~ 800 μm. To test our sensor we used a quantum cascade laser as light source and selected a methanol rotational absorption line at 131.054 cm−1 (~3.93 THz), with line-strength S = 4.28•10-21 cm/mol. The sensor was operated at 10 Torr pressure on the first flexion QTF resonance frequency of 4245 Hz. The corresponding Q factor was 74760. Stepwise concentration measurements were performed to verify the linearity of the QEPAS signal as a function of the methanol concentration. The achieved sensitivity of the system is 7 parts per million in 4 seconds, corresponding to a QEPAS normalized noise-equivalent absorption of 2•10-10 W•cm-1•Hz-1/2, comparable with the best result of mid-IR QEPAS systems.

We report on the first demonstration of a quartz enhanced photo-acoustic (QEPAS) sensor in the Terahertz (THz) range. The sensor is based on a QCL emitting at 3.93 THz and a customized quartz tuning fork. For methanol detection we reached a normalized noise-equivalent absorption of 2×10-10 cm-1W/Hz½ comparable with the best result of mid-IR QEPAS.

We report on an innovative quartz enhanced photoacoustic (QEPAS) gas sensor operating in the THz spectral range, employing a custom quartz tuning fork (QTF) with the two prongs spaced by ~800 μm. To test our sensor we employed a quantum cascade laser light source and selected a methanol rotational absorption line falling at 131.054 cm-1 (~3.93 THz), with line-strength S = 4.28•10-21 cm. The sensor operated at 10 Torr pressure on the QTF first flexion resonance frequency at 4245 Hz. We achieved a QEPAS normalized noise-equivalent absorption of 2•10-10 W·cm-1•Hz-1/2 comparable with the best result of mid-IR QEPAS systems.

This talk will focus on recent advances in the development of sensors based on infrared (IR) semiconductor lasers for the detection, quantification, and monitoring of trace gas species as well as their applications to medical diagnostics, environmental monitoring, industrial process control, and security. The development of compact trace gas sensors, in particular based on quantum cascade (QC) and interband cascade (IC) lasers, permits the targeting of strong fundamental rotational-vibrational transitions in the mid-IR, that are one to two orders of magnitude more intense than overtone transitions in the near-IR [1].

A compact widely-tunable fiber-coupled sensor for trace gas detection of hydrogen sulfide (H2S) in the mid infrared is reported. The sensor is based on an external-cavity quantum cascade laser (EC-QCL) tunable between 7.6 and 8.3 μm wavelengths coupled into a single-mode hollow-core waveguide. Quartz-enhanced photoacoustic spectroscopy has been selected as detecting technique. The fiber coupling system converts the astigmatic beam exiting the laser into a TEM00 mode. During a full laser scan, we observed no misalignment between the optical beam and the tuning fork, thus making our system applicable for multi-gas or broad absorber detections. The sensor has been tested on N2:H2S gas mixtures. The minimum detectable H2S concentration is 450 ppb in ~3 s integration time, which is the best value till now reported in literature for H2S optical sensors.