Showing papers on "Photoacoustic spectroscopy published in 2015"

THz quartz-enhanced photoacoustic sensor for H₂S trace gas detection.

Abstract: We report on a quartz-enhanced photoacoustic (QEPAS) gas sensing system for hydrogen sulphide (H₂S) detection. The system architecture is based on a custom quartz tuning fork (QTF) optoacoustic transducer with a novel geometry and a quantum cascade laser (QCL) emitting 1.1 mW at a frequency of 2.913 THz. The QTF operated on the first flexion resonance frequency of 2871 Hz, with a quality factor Q = 17,900 at 20 Torr. The tuning range of the available QCL allowed the excitation of a H₂S rotational absorption line with a line-strength as small as S = 1.13·10⁻²² cm/mol. The measured detection sensitivity is 30 ppm in 3 seconds and 13 ppm for a 30 seconds integration time, which corresponds to a minimum detectable absorption coefficient α(min) = 2.3·10⁻⁷ cm⁻¹ and a normalized noise-equivalent absorption NNEA = 4.4·10⁻¹⁰ W·cm⁻¹·Hz(-1/2), several times lower than the values previously reported for near-IR and mid-IR H₂S QEPAS sensors.

Enhanced near-infrared QEPAS sensor for sub-ppm level H2S detection by means of a fiber amplified 1582 nm DFB laser

Abstract: A power-boosted quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor is developed for sub-ppm H2S trace-gas detection in the near-infrared spectral region. The sensor is based on off-beam QEPAS with an erbium-doped fiber amplified 1582 nm distributed feedback (DFB) laser. The offset of the sensor floor noise caused by stray light and gas flow can be removed by an electrical modulation cancellation method, which lowers the noise to the theoretical thermal noise level. The sensor was optimized in terms of gas pressure and current modulation depth for H2S detection at 6320.6 cm−1. The linearity of the sensor response to the laser power and H2S concentration confirms that saturation does not occur. With ∼1.4 W optical excitation power and 67 s averaging time, a H2S detection sensitivity of 142 ppbv (parts per billion by volume) is achieved at atmospheric pressure and room temperature, which is the best value, reported in the literature so far for H2S QEPAS sensors. A side-by-side sensitivity comparison for different sensor systems is also reported.

High-speed intravascular spectroscopic photoacoustic imaging at 1000 A-lines per second with a 0.9-mm diameter catheter

Abstract: Intravascular spectroscopic photoacoustic technology can image atherosclerotic plaque composition with high sensitivity and specificity, which is critical for identifying vulnerable plaques. Here, we designed and engineered a catheter of 0.9 mm in diameter for intravascular photoacoustic (IVPA) imaging, smaller than the critical size of 1 mm required for clinical translation. Further, a quasifocusing photoacoustic excitation scheme was developed for the catheter, producing well-detectable IVPA signals from stents and lipids with a laser energy as low as ~30 μJ/pulse. As a result, this design enabled the use of a low-energy, high-repetition rate, ns-pulsed optical parametric oscillator laser for high-speed spectroscopic IVPA imaging at both the 1.2-μm and 1.7-μm spectral bands for lipid detection. Specifically, for each wavelength, a 1-kHz IVPA A-line rate was achieved, ~100-fold faster than previously reported IVPA systems offering a similar wavelength tuning range. Using the system, spectroscopic IVPA imaging of peri-adventitial adipose tissue from a porcine aorta segment was demonstrated. The significantly improved imaging speed, together with the reduced catheter size and multiwavelength spectroscopic imaging ability, suggests that the developed high-speed IVPA technology is of great potential to be further translated for in vivo applications.

Short-lived species detection of nitrous acid by external-cavity quantum cascade laser based quartz-enhanced photoacoustic absorption spectroscopy

Abstract: Spectroscopic detection of short-lived gaseous nitrous acid (HONO) at 1254.85 cm−1 was realized by off-beam coupled quartz-enhanced photoacoustic spectroscopy (QEPAS) in conjunction with an external cavity quantum cascade lasers (EC-QCL). High sensitivity monitoring of HONO was performed within a very small gas-sample volume (of ∼40 mm3) allowing a significant reduction (of about 4 orders of magnitude) of air sampling residence time which is highly desired for accurate quantification of chemically reactive short-lived species. Calibration of the developed QEPAS-based HONO sensor was carried out by means of lab-generated HONO samples whose concentrations were determined by direct absorption spectroscopy involving a ∼109.5 m multipass cell and a distributed feedback QCL. A minimum detection limit (MDL) of 66 ppbv (1 σ) HONO was achieved at 70 mbar using a laser output power of 50 mW and 1 s integration time, which corresponded to a normalized noise equivalent absorption coefficient of 3.6 × 10−8 cm−1 W/Hz1/2. This MDL was down to 7 ppbv at the optimal integration time of 150 s. The corresponding 1σ minimum detected absorption coefficient is ∼1.1 × 10−7 cm−1 (MDL ∼ 3 ppbv) in 1 s and ∼1.1 × 10−8 cm−1 (MDL ∼ 330 pptv) in 150 s, respectively, with 1 W laser power.

High speed intravascular photoacoustic imaging with fast optical parametric oscillator laser at 1.7 μm

Abstract: Intravascular photoacoustic imaging at 1.7 μm spectral band has shown promising capabilities for lipid-rich vulnerable atherosclerotic plaque detection. In this work, we report a high speed catheter-based integrated intravascular photoacoustic/intravascular ultrasound (IVPA/IVUS) imaging system with a 500 Hz optical parametric oscillator laser at 1725 nm. A lipid-mimicking phantom and atherosclerotic rabbit abdominal aorta were imaged at 1 frame per second, which is two orders of magnitude faster than previously reported in IVPA imaging with the same wavelength. Clear photoacoustic signals by the absorption of lipid rich deposition demonstrated the ability of the system for high speed vulnerable atherosclerotic plaques detection.

Multi-quartz-enhanced photoacoustic spectroscopy

Abstract: A multi-quartz-enhanced photoacoustic spectroscopy (M-QEPAS) sensor system for trace gas detection is reported. Instead of a single quartz tuning fork (QTF) as used in QEPAS technique, a dual QTF sensor platform was adopted in M-QEPAS to increase the signal strength by the addition of the detected QEPAS signals. Water vapor was selected as the target analyte. M-QEPAS realized a 1.7 times signal enhancement as compared to the QEPAS method for the same operating conditions. A minimum detection limit of 23.9 ppmv was achieved for the M-QEPAS sensor, with a calculated normalized noise equivalent absorption coefficient of 5.95 × 10−8 cm−1W/√Hz. The M-QEPAS sensor performance can be further improved when more QTFs are employed or an acoustic micro-resonator architecture is used.

High finesse optical cavity coupled with a quartz-enhanced photoacoustic spectroscopic sensor

Abstract: An ultra-sensitive and selective quartz-enhanced photoacoustic spectroscopy (QEPAS) combined with a high-finesse cavity sensor platform is proposed as a novel method for trace gas sensing. We call this technique Intra-cavity QEPAS (I-QEPAS). In the proposed scheme, a single-mode continuous wave quantum cascade laser (QCL) is coupled into a bow-tie optical cavity. The cavity is locked to the QCL emission frequency by means of a feedback-locking loop that acts directly on a piezoelectric actuator mounted behind one of the cavity mirrors. A power enhancement factor of ∼240 was achieved, corresponding to an intracavity power of ∼0.72 W. CO2 was selected as the target gas to validate our sensor. For the P(42) CO2 absorption line, located at 2311.105 cm−1, a minimum detection limit of 300 parts per trillion by volume at a total gas pressure of 50 mbar was achieved with a 20 s integration time. This corresponds to a normalized noise equivalent absorption of 3.2 × 10−10 W cm−1 Hz−1/2, comparable with the best results reported for the QEPAS technique on much faster relaxing gases. A comparison with standard QEPAS performed under the same experimental conditions confirms that the I-QEPAS sensitivity scales with the intracavity laser power enhancement factor.

Piezotransistive transduction of femtoscale displacement for photoacoustic spectroscopy

Abstract: Measurement of femtoscale displacements in the ultrasonic frequency range is attractive for advanced material characterization and sensing, yet major challenges remain in their reliable transduction using non-optical modalities, which can dramatically reduce the size and complexity of the transducer assembly. Here we demonstrate femtoscale displacement transduction using an AlGaN/GaN heterojunction field effect transistor-integrated GaN microcantilever that utilizes piezoelectric polarization-induced changes in two-dimensional electron gas to transduce displacement with very high sensitivity. The piezotransistor demonstrated an ultra-high gauge factor of 8,700 while consuming an extremely low power of 1.36 nW, and transduced external excitation with a superior noise-limited resolution of 12.43 fm Hz(-1/2) and an outstanding responsivity of 170 nV fm(-1), which is comparable to the optical transduction limits. These extraordinary characteristics, which enabled unique detection of nanogram quantity of analytes using photoacoustic spectroscopy, can be readily exploited in realizing a multitude of novel sensing paradigms.

Opto-nanomechanical spectroscopic material characterization

Abstract: A hybrid approach combining mechanical force microscopy and infrared photoacoustic spectroscopy is used to characterize the morphological and compositional substructures of plant cell walls with a lateral resolution better than 20 nm.

Far- and near-field properties of gold nanoshells studied by photoacoustic and surface-enhanced Raman spectroscopies

Abstract: Gold nanoshells, with a silica core and different core and shell dimensions, have been extensively investigated. Optical far-field properties, namely extinction and absorption, have been separately determined by means of spectrophotometry and photoacoustic spectroscopy, respectively, in the 440–900 nm range. The enhancement factor for surface-enhanced Raman scattering, which is related to near-field effects, has been measured from 568 to 920 nm. The absorption contribution to extinction decreases as the overall diameter increases. Moreover, absorption and scattering display different spectral distributions, the latter being red shifted. The Surface Enhanced Raman Scattering enhancement profile, measured using thiobenzoic acid as a Raman probe, is further shifted to the red. The latter result suggests that the enhancement is dominated by the presence of hot spots, which are possibly related to the surface roughness of gold nanoshell particles.

Detection of dissolved gas in oil–insulated electrical apparatus by photoacoustic spectroscopy

Abstract: One of the most effective and convenient ways to determine the early stages of potential faults in oil-insulated electrical apparatus and to monitor the development is to routinely detect and analyze the concentrations and fluctuations of potential fault gases [1]–[5]. This method has been widely used in power-grid operations and is effective in preventing the occurrence of catastrophic faults. In the past, the technology to detect mixed gas by electrochemical sensors offered effective early warnings of potential faults in electrical apparatus. Today, gas chromatography is widely used to accurately detect the composition of various gases dissolved in oil. Fault development can accurately be diagnosed by using gas chromatography together with various other means such as the Duval Triangle and key gas methods. After extended use, these techniques have been found to have drawbacks, namely, the need to routinely replace or calibrate chromatographic columns and sensors because their properties change with use, and the consumption of calibration and carrier gases with gas chromatography use [6], [7]. Photoacoustic spectroscopy, developed in the 1970s, is a spectral detection and analysis technology derived from a combination of spectroscopy and calorimetry. The method is powerful in physical chemistry research that involves inorganic or organic compounds, semiconductors, metallic materials, and high-polymer materials. It has found widespread application in various disciplines, such as physics, chemistry, biology, medicine, and geology. The major advantages associated with photoacoustic spectroscopy are that it (1) is characterized by high monitoring sensitivity because it directly measures the energy absorbed by gases without background noise; (2) uses easily available air instead of high-purity inert gases as the carrier gas without consuming the to-be-measured gas; (3) does not require chromatographic columns and sensors, which are easily contaminated, undergo aging, and require frequent replacement; (4) does not require a calibration gas, raising the prospect of realizing true maintenance-free operation; and (5) displays a high level of versatility because it even detects the contents of high-concentration gases in the oil tank of on-load tap changers [8]–[12]. Therefore, photoacoustic spectroscopy can greatly enhance the on-line monitoring of electrical system maintenance owing to its high stability, low drift, and maintenance-free features. Considering these technical characteristics, we devised a test platform by combining the technology with the headspace degassing technique. Thus, we investigated the application of photoacoustic spectroscopy to detecting concentrations of gases dissolved in oil-insulated electrical apparatus.

Position effects of acoustic micro-resonator in quartz enhanced photoacoustic spectroscopy

Abstract: a b s t r a c t The impact of acoustic micro-resonator (AmR) positions with respect to quartz tuning fork on signal amplitude, Q-factor and signal-to-noise ratio (SNR) of the quartz enhanced photoacoustic spectroscopy spectrophone was investigated. The replacement of the result plots' abscissas makes the highest signal amplitude and the lowest Q-factor for different AmRs appear at the two absolute positions, respectively. These positions are independent on the AmR geometrical parameters, which facilitates the assembly of the spectrophone. The noncoincidence between the positions of the two extreme values results in a flat peak of the SNR curve, which is different from previously reported results. The spectrophone designs for three different applications are discussed in detail.

Miniaturized photoacoustic trace gas sensing using a raman fiber amplifier

Abstract: This paper presents the development of a Raman fiber amplifier optical source with a maximum output power of 1.1 W centered around 1651 nm, and its application in miniaturized 3D printed photoacoustic spectroscopy (PAS) trace gas sensing of methane. The Raman amplifier has been constructed using 4.5 km of dispersion shifted fiber, a 1651 nm DFB seed laser and a commercial 4W EDFA pump. The suppression of stimulated Brillouin scattering (SBS) using a high frequency modulation of the seed laser is investigated for a range of frequencies, leading to an increase in optical output power of the amplifier and reduction of its noise content. The amplifier output was used as the source for a miniature PAS sensor by applying a second modulation to the seed laser at the resonant frequency of 15.2 kHz of the miniature 3D printed gas cell. For the targeted methane absorption line at 6057 cm-1 the sensor system performance and influence of the SBS suppression is characterized, leading to a detection limit (1σ) of 17 ppb methane for a signal acquisition time of 130 s, with a normalized noise equivalent absorption coefficient of 4.1•10-9 cm-1 W Hz-1/2 for the system.

Quartz-enhanced photoacoustic spectroscopy sensor for ethylene detection with a 3.32 μm distributed feedback laser diode

Abstract: An antimonide distributed feedback quantum wells diode laser operating at 3.32 μm at near room temperature in the continuous wave regime has been used to perform ethylene detection based on quartz enhanced photoacoustic spectroscopy. An absorption line centered at 3007.52 cm−1 was investigated and a normalized noise equivalent absorption coefficient (1σ) of 3.09 10−7 cm−1 W Hz−1/2 was obtained. The linearity and the stability of the detection have been evaluated. Biological samples’ respiration has been measured to validate the feasibility of the detection setup in an agronomic environment, especially on ripening apples.

Quartz Enhanced Photoacoustic Spectroscopy Based Trace Gas Sensors Using Different Quartz Tuning Forks

Abstract: A sensitive trace gas sensor platform based on quartz-enhanced photoacoustic spectroscopy (QEPAS) is reported. A 1.395 μm continuous wave (CW), distributed feedback pigtailed diode laser was used as the excitation source and H2O was selected as the target analyte. Two kinds of quartz tuning forks (QTFs) with a resonant frequency (f0) of 30.72 kHz and 38 kHz were employed for the first time as an acoustic wave transducer, respectively for QEPAS instead of a standard QTF with a f0 of 32.768 kHz. The QEPAS sensor performance using the three different QTFs was experimentally investigated and theoretically analyzed. A minimum detection limit of 5.9 ppmv and 4.3 ppmv was achieved for f0 of 32.768 kHz and 30.72 kHz, respectively.

Organosilicon phantom for photoacoustic imaging

Abstract: Photoacoustic imaging is an emerging technique. Although commercially available photoacoustic imaging systems currently exist, the technology is still in its infancy. Therefore, the design of stable phantoms is essential to achieve semiquantitative evaluation of the performance of a photoacoustic system and can help optimize the properties of contrast agents. We designed and developed a polydimethylsiloxane (PDMS) phantom with exceptionally fine geometry; the phantom was tested using photoacoustic experiments loaded with the standard indocyanine green dye and compared to an agar phantom pattern through polyethylene glycol-gold nanorods. The linearity of the photoacoustic signal with the nanoparticle number was assessed. The signal-tonoiseratio and contrast were employed as image quality parameters, and enhancements of up to 50 and up to 300%, respectively, were measured with the PDMS phantom with respect to the agar one. A tissue-mimicking (TM)-PDMS was prepared by adding TiO2 and India ink; photoacoustic tests were performed in order to compare the signal generated by the TM-PDMS and the biological tissue. The PDMS phantom can become a particularly promising tool in the field of photoacoustics for the evaluation of the performance of a PA system and as a model of the structure of vascularized soft tissues.

Measurement of Gas and Aerosol Phase Absorption Spectra across the Visible and Near-IR Using Supercontinuum Photoacoustic Spectroscopy.

Abstract: We demonstrate a method to measure the absorption spectra of gas and aerosol species across the visible and near-IR (500 to 840 nm) using a photoacoustic (PA) spectrometer and a pulsed supercontinuum laser source. Measurements of gas phase absorption spectra were demonstrated using H2O(g) as a function of relative humidity (RH). The measured absorption intensities and peak shapes were able to be quantified and compared to spectra calculated using the 2012 High Resolution Transmission (HITRAN2012) database. Size and mass selected nigrosin aerosol was used to measure absorption spectra across the visible and near-IR. Spectra were measured as a function of aerosol size/mass and show good agreement to Mie theory calculations. Lastly, we measured the broadband absorption spectrum of flame generated soot aerosol at 5% and 70% RH. For the high RH case, we are able to quantifiably separate the soot and water absorption contributions. For soot, we observe an enhancement in the mass specific absorption cross sectio...

Exploration and Practice in Photoacoustic Measurement for Glucose Concentration Based on Tunable Pulsed Laser Induced Ultrasound

Abstract: In this article, a tunable pulsed laser induced photoacoustic measurement setup of monitoring glucose concentration was established in the forward mode. In experiments, the time-resolved photoacoustic signal of glucose aqueous solution with different concentrations of 0-300 mg/dl were captured and averaged 512 times, and the photoacoustic peak-to-peak values were recorded using the wavelength scan in NIR region of 1300-2300 nm. The optimal characteristic wavelengths of glucose were determined via the difference spectral and the first derivative spectral algorithm, and correction models between peak-to-peak values of optimal wavelengths and concentration gradients were established using multivariate linear regression algorithm. Experimental results demonstrated that the profile and logarithm shape of time-resolved photoacoustic signal for glucose solutions were in good agreement with photoacoustic theories. The prediction effect of optimal wavelength of 1510 nm was best, its root-mean-square errors of corr...

Nonlinear photoacoustic spectroscopy of hemoglobin

Abstract: As light intensity increases in photoacoustic imaging, the saturation of optical absorption and the temperature dependence of the thermal expansion coefficient result in a measurable nonlinear dependence of the photoacoustic (PA) signal on the excitation pulse fluence. Here, under controlled conditions, we investigate the intensity-dependent photoacoustic signals from oxygenated and deoxygenated hemoglobin at varied optical wavelengths and molecular concentrations. The wavelength and concentration dependencies of the nonlinear PA spectrum are found to be significantly greater in oxygenated hemoglobin than in deoxygenated hemoglobin. These effects are further influenced by the hemoglobin concentration. These nonlinear phenomena provide insights into applications of photoacoustics, such as measurements of average inter-molecular distances on a nm scale or with a tuned selection of wavelengths, a more accurate quantitative PA tomography.

Modelling, verification, and calibration of a photoacoustics based continuous non-invasive blood glucose monitoring system.

Abstract: This paper examines the use of photoacoustic spectroscopy (PAS) at an excitation wavelength of 905 nm for making continuous non-invasive blood glucose measurements. The theoretical background of the measurement technique is verified through simulation. An apparatus is fabricated for performing photoacoustic measurements in vitro on glucose solutions and in vivo on human subjects. The amplitude of the photoacoustic signals measured from glucose solutions is observed to increase with the solution concentration, while photoacoustic amplitude obtained from in vivo measurements follows the blood glucose concentration of the subjects, indicating a direct proportionality between the two quantities. A linear calibration method is applied separately on measurements obtained from each individual in order to estimate the blood glucose concentration. The estimated glucose values are compared to reference glucose concentrations measured using a standard glucose meter. A plot of 196 measurement pairs taken over 30 normal subjects on a Clarke error grid gives a point distribution of 82.65% and 17.35% over zones A and B of the grid with a mean absolute relative deviation (MARD) of 11.78% and a mean absolute difference (MAD) of 15.27 mg/dl (0.85 mmol/l). The results obtained are better than or comparable to those obtained using photoacoustic spectroscopy based methods or other non-invasive measurement techniques available. The accuracy levels obtained are also comparable to commercially available continuous glucose monitoring systems.

Research on the detection of SF6 decomposition products based on non-resonant photoacoustic spectroscopy

Abstract: Under partial discharge (PD), a gas-insulated switchgear (GIS) will produce SF6 decomposition products. If the concentrations of these products are higher than the standard values, this indicates that the GIS equipment is faulty. Photoacoustic spectroscopy techniques show a high sensitivity and good stability and, furthermore, they can enable on-line monitoring, making them widely used in the detection of SF6 decomposition products in GIS equipment. The basic principles of photoacoustic spectroscopy were introduced and a set of devices for the online detection of SF6 decomposition based on non-resonant photoacoustics was designed and realized. According to the infrared spectral characteristics of the target gases, the appropriate narrowband filters and infrared radiation light source were selected. This detection system is able to continuously detect CO, SO2 and CF4. Calibration experiments with different concentrations of CO, SO2 and CF4 were carried out and the results show that the photoacoustic signal and concentration have a perfect linear relationship and the detection limits for CO, SO2 and CF4 are 5.9116, 8.2824 and 5.5226 ppm. Relative errors of the single measurements of CO, SO2 and CF4 are all less than 10% and the system exhibits excellent stability in a continuous 12 hour measurement.

Remote mid-infrared photoacoustic spectroscopy with a quantum cascade laser.

Abstract: We demonstrate non-contact remote photoacoustic spectroscopy in the mid-infrared region. A room-temperature-operated pulsed external-cavity quantum cascade laser is used to excite photoacoustic waves within a semitransparent sample. The ultrasonic waves are detected remotely on the opposite side of the sample using a fiber-optic Mach-Zehnder interferometer, thereby avoiding problems associated with acoustic attenuation in air. We present the theoretical background of the proposed technique and demonstrate measurements on a thin polystyrene film. The obtained absorption spectrum in the region of 1030-1230 cm(-1) is compared to a spectrum obtained by attenuated total reflection, showing reasonable agreement.

Optical-resolution photoacoustic imaging through thick tissue with a thin capillary as a dual optical-in acoustic-out waveguide

Abstract: We demonstrate the ability to guide high-frequency photoacoustic waves through thick tissue with a water-filled silica-capillary (150 μm inner diameter and 30 mm long). An optical-resolution photoacoustic image of a 30 μm diameter absorbing nylon thread was obtained by guiding the acoustic waves in the capillary through a 3 cm thick fat layer. The transmission loss through the capillary was about −20 dB, much lower than the −120 dB acoustic attenuation through the fat layer. The overwhelming acoustic attenuation of high-frequency acoustic waves by biological tissue can therefore be avoided by the use of a small footprint capillary acoustic waveguide for remote detection. We finally demonstrate that the capillary can be used as a dual optical-in acoustic-out waveguide, paving the way for the development of minimally invasive optical-resolution photoacoustic endoscopes free of any acoustic or optical elements at their imaging tip.

Photoacoustic Characterization of Randomly Oriented Silver Nanowire Films

Abstract: In this work, the photoacoustic characterization in the UV/Vis range of randomly oriented silver nanowire films deposited onto either a quartz or polymeric substrate is presented. This study was performed for a set of films differing in both metallic nanowire dimensions, as well as metal content. Samples were prepared starting from suspensions of Ag nanowires in isopropanol (IPA) $$(25~\mathrm{mg}{\cdot }\mathrm{ml}^{-1})$$ , differing in both the length and diameter of the nanowires. The obtained films were characterized by scanning electron micrography (SEM) images; thus, the metal filling factor was retrieved with MATLAB software based on a visual method. Following the morphological characterization, both spectrophotometry and the photoacoustic spectroscopy (PAS) technique were employed to investigate in detail the absorbance spectra of silver nanowire films, in order to evidence their peculiar properties in the UV/Vis spectral range. Specifically, this photothermal technique is particularly useful to investigate a film that may exhibit relevant scattering phenomena, as for metallic nanowire films. The obtained experimental results show that the choice of the metal filling factor may affect the absorbance spectra of the resulting mesh.

Near-IR telecommunication diode laser based double-pass QEPAS sensor for atmospheric CO2 detection

Abstract: A novel double pass quartz enhanced photoacoustic spectroscopy (QEPAS) sensor for atmospheric CO2 detection is developed by use of a 1.5 μm telecommunication diode laser. A low-cost high-reflection concave mirror is positioned behind a traditional QEPAS based acoustic detection module to enhance the absorption optical path and improve the detection sensitivity. The obtained minimum detection limit is 29 ppmV for 1 s averaging time which corresponds to a normalized noise equivalent absorption coefficient (NNEA, 1σ) of . With a 247 s averaging time, it demonstrates an ultimate detectable sensitivity of 1.74 ppmV.

Application of quantum cascade lasers to trace gas detection

Abstract: The potential of Quantum Cascade Laser technology has been recently harnessed in industry, medicine and military to create a range of original infrared gas sensors. These sensors have opened up many new applications due to compact size, excellent sensitivity, robust construction and low power requirements. They rely on infrared absorption spectroscopy to determine identity and quantity of gases. The measurement of these gases has relied on different technologies including multi-pass spectroscopy, photoacoustic spectroscopy, cavity ring down spectroscopy, and their various modifications. In this review paper some technologies are described in terms of its advantages/disadvantages in many application. The results of own works about methane, ammonia, nitric oxide, nitrous oxide, and carbonyl sulfide detection are presented as well.

Characterisation of a PVCP-based tissue-mimicking phantom for quantitative photoacoustic imaging

Abstract: Photoacoustic imaging can provide high resolution images of tissue structure, pathology and function. As these images can be obtained at multiple wavelengths, quantitatively accurate, spatially resolved, estimates for chromophore concentration, for example, may be obtainable. Such a capability would find a wide range of clinical and pre-clinical applications. However, despite a growing body of theoretical papers on how this might be achieved, there is a noticeable lack of studies providing validated evidence that it can be achieved experimentally, either in vitro or in vivo. Well-defined, versatile and stable phantom materials are essential to assess the accuracy, robustness and applicability of multispectral Quantitative Photoacoustic Imaging (qPAI) algorithms in experimental scenarios. This study assesses the potential of polyvinyl chloride plastisol (PVCP) as a phantom material for qPAI, building on previous work that focussed on using PVCP for quality control. Parameters that might be controlled or tuned to assess the performance of qPAI algorithms were studied: broadband acoustic properties, multiwavelength optical properties with added absorbers and scatterers, and photoacoustic effciency. The optical and acoustic properties of PVCP can be tuned to be broadly representative of soft tissue. The Gruneisen parameter is larger than expected in tissue, which is an advantage as it increases the signal-to-noise ratio of the photoacoustic measurements. Interestingly, when the absorption was altered by adding absorbers, the absorption spectra measured using high peak power nanosecond-pulsed sources (typical in photoacoustics) were repeatably different from the ones measured using the low power source in the spectrophotometer, indicative of photochemical reactions taking place.