Showing papers on "Photoacoustic spectroscopy published in 2018"

Quartz-tuning-fork enhanced photothermal spectroscopy for ultra-high sensitive trace gas detection.

Abstract: A gas sensing method based on quartz-tuning-fork enhanced photothermal spectroscopy (QEPTS) is reported in this paper. Unlike usually used thermally sensitive elements, a sharply resonant quartz-tuning-fork with the capability of enhanced mechanical resonance was used to amplify the photothermal signal level. Acetylene (C2H2) detection was used to verify the QEPTS sensor performance. The measured results indicate a minimum detection limit (MDL) of 718 ppb and a normalized noise equivalent absorption coefficient (NNEA) of 7.63 × 10-9 cm-1W/√Hz. This performance demonstrates that QEPTS can be an ultra-high sensitive technique for gas detection and shows superiority when compared to usually used methods of tunable diode laser absorption spectroscopy (TDLAS) and quartz-enhanced photoacoustic spectroscopy (QEPAS). Furthermore, when compared to an optical detector, especially a costly mercury cadmium telluride (MCT) detector with cryogenic cooling used in TDLAS, a quartz-tuning-fork is much cheap and tiny. Besides, compared to the QEPAS technique, QEPTS is a non-contact measurement technique and therefore can be used for standoff and remote trace gas detection.

Recent advances in quartz enhanced photoacoustic sensing

Abstract: This review aims to discuss the latest advancements in quartz-enhanced photoacoustic spectroscopy (QEPAS) based trace-gas sensing. Starting from the QEPAS basic physical principles, the most used QEPAS configurations will be described. This is followed by a detailed theoretical analysis and experimental study regarding the influence of quartz tuning forks (QTFs) geometry on their optoacoustic transducer performance. Furthermore, an overview of the latest developments in QEPAS trace-gas sensor technology employing custom QTFs will be reported. Results obtained by exploiting novel micro-resonator configurations, capable of increasing the QEPAS signal-to-noise ratio by more than two orders of magnitude and the utilization of QTF overtone flexural modes for QEPAS based sensing will be presented. A comparison of the QEPAS performance of different spectrophone configurations is reported based upon signal-to-noise ratio. Finally, a novel QEPAS approach allowing simultaneous dual-gas detection will be described.

In vivo Microscopic Photoacoustic Spectroscopy for Non-Invasive Glucose Monitoring Invulnerable to Skin Secretion Products.

Abstract: Photoacoustic spectroscopy has been shown to be a promising tool for non-invasive blood glucose monitoring. However, the repeatability of such a method is susceptible to changes in skin condition, which is dependent on hand washing and drying due to the high absorption of infrared excitation light to the skin secretion products or water. In this paper, we present a method to meet the challenges of mid-infrared photoacoustic spectroscopy for non-invasive glucose monitoring. By obtaining the microscopic spatial information of skin during the spectroscopy measurement, the skin region where the infrared spectra is insensitive to skin condition can be locally selected, which enables reliable prediction of the blood glucose level from the photoacoustic spectroscopy signals. Our raster-scan imaging showed that the skin condition for in vivo spectroscopic glucose monitoring had significant inhomogeneities and large variability in the probing area where the signal was acquired. However, the selective localization of the probing led to a reduction in the effects of variability due to the skin secretion product. Looking forward, this technology has broader applications not only in continuous glucose monitoring for diabetic patient care, but in forensic science, the diagnosis of malfunctioning sweat pores, and the discrimination of tumors extracted via biopsy.

Ultrathin graphene diaphragm-based extrinsic Fabry-Perot interferometer for ultra-wideband fiber optic acoustic sensing.

Abstract: An ultra-wideband fiber optic acoustic sensor based on graphene diaphragm with a thickness of 10nm has been proposed and experimentally demonstrated. The two reflectors of the extrinsic Fabry-Perot interferometer is consist of fiber endface and graphene diaphragm, and the cavity is like a horn-shape. The radius of the effective area of the ultrathin graphene diaphragm is 1mm. Attributed to the strong van der Waals force between the diaphragm and the ceramic ferrule, the sensor head can be applied not only in the air but also underwater. Experimental results illustrate that ultra-wideband frequency response is from 5Hz to 0.8MHz, covering the range from infrasound to ultrasound. The noise-limited minimum detectable pressure level of 0.77Pa/Hz1/2@5Hz and 33.97μPa/Hz1/2@10kHz can be achieved, and the applied sound pressure is 114dB and 65.8dB, respectively. The fiber optic acoustic sensor may have a great potential in seismic wave monitoring, photoacoustic spectroscopy and photoacoustic imaging application due to its compact structure, simple manufacturing, and low cost.

Ultra-high sensitive fiber-optic Fabry-Perot cantilever enhanced resonant photoacoustic spectroscopy

Abstract: A new scheme of fiber-optic cantilever enhanced resonant photoacoustic spectroscopy (CERPAS), combining high sensitive fiber-optic Fabry-Perot cantilever microphone with resonant photoacoustic spectroscopy, is presented for trace gas detection. A fiber-optic cantilever microphone with high sensitivity at the frequency near 1.4 kHz is designed to match with a first-order longitudinal resonant photoacoustic cell, whose resonant frequency is 1402 Hz. For sensitivity improvement, an erbium-doped fiber amplified near-infrared laser, with the central wavelength of 1532.83 nm and the maximum output power of 1 W, is used as the light source for acoustic excitation. The trace acetylene detection experiment demonstrates that, with the wavelength modulation spectrum and second-harmonic detection methods, the gas detection limit is achieved to be 80 ppt, which is at least one order of magnitude improvement compared with other photoacoustic acetylene sensors reported so far.

Lock-in white-light-interferometry-based all-optical photoacoustic spectrometer.

Abstract: An all-optical photoacoustic spectroscopy based on lock-in white-light interferometry is proposed for trace gas detection. The cavity length of the fiber-optic Fabry–Perot cantilever microphone is demodulated by a high-speed white-light interferometer, whose spectral sampling is synchronously triggered by a phased locked signal. To improve the signal-to-noise ratio, the demodulated digital photoacoustic signal is further processed by a specially designed virtual lock-in amplifier. The designed photoacoustic spectrometer has been tested for trace acetylene (C2H2) detection in the near-infrared region. The normalized noise equivalent absorption coefficient for C2H2 is achieved to be 1.1×10−9 cm−1 W Hz−1/2.

Photoacoustic spectroscopy based multi-gas detection using high-sensitivity fiber-optic low-frequency acoustic sensor

Abstract: We demonstrate a high-sensitivity fiber-optic low-frequency acoustic sensor based on a thin Parylene-C diaphragm. The excellent diaphragm forming ability and good adhesion of Parylene-C make the sensor fabricate up to 9 mm in diameter. For the acoustic pressure of 50 mPa at the frequency of 30 Hz, acoustic testing demonstrates a signal to noise ratio of 37 dB, which is almost seven times higher than a conventional electric microphone. The low-frequency acoustic sensor, together with an infrared thermal radiation source, and a non-resonant cell, constitutes a photoacoustic detection system for multiple trace gases analysis. The detection limits of acetylene (C2H2), methane (CH4), ethane (C2H6), ethylene (C2H4), carbon monoxide (CO) and carbon dioxide (CO2) are achieved to be 0.11, 0.21, 0.13, 0.16, 0.15 and 0.48 parts-per-million, respectively. The detection system is applied to detect mixture of the six gases, and the average deviations of the six gases are not more than 5.0%.

Fiber-ring laser intracavity QEPAS gas sensor using a 7.2 kHz quartz tuning fork

Abstract: A novel trace gas sensor exploiting fiber-ring laser intracavity quartz-enhanced photoacoustic spectroscopy (FLI-QEPAS) is reported. The gas sensor system couples an erbium-doped fiber-ring laser with a custom-designed quartz tuning fork (QTF) operating at 7.2 kHz. The QTF is placed in the fiber cavity to fully utilize intracavity optical power for photoacoustic excitation. We obtained a short intracavity absorption length of 1.5 cm, significantly extending the linear dynamic range of the gas sensor to an order of 105. As a proof of principle, the fiber laser was tuned to the C2H2 absorption line at 1531.6 nm. Operating at the optimal gas pressure and modulation depth selected for wavelength modulation spectroscopy, a minimum detectable C2H2 concentration of 29 ppbv at 300-s integration time has been achieved.

Broadband cantilever-enhanced photoacoustic spectroscopy in the mid-IR using a supercontinuum.

Abstract: We demonstrate cantilever-enhanced photoacoustic spectroscopy in the mid-infrared using a supercontinuum source. The approach is broadband and allows for higher photoacoustic signal intensity and an enhanced signal-to-noise ratio as compared to systems employing conventional black body radiation sources. Using this technique, we perform spectroscopic measurements of the full ro-vibrational band structure of water vapor at 1900 nm and methane at 3300 nm with relative signal enhancement factors of 70 and 19, respectively, when compared to measurements that use the black body radiation source. Our results offer a novel perspective for photoacoustic detection opening the door to sensitive broadband analyzers in the mid-infrared spectral region.

Optical frequency comb photoacoustic spectroscopy

Abstract: We report the first photoacoustic detection scheme using an optical frequency comb-optical frequency comb photoacoustic spectroscopy (OFC-PAS). OFC-PAS combines the broad spectral coverage and the high resolution of OFCs with the small sample volume of cantilever-enhanced PA detection. In OFC-PAS, a Fourier transform spectrometer (FTS) is used to modulate the intensity of the exciting comb source at a frequency determined by its scanning speed. One of the FTS outputs is directed to the PA cell and the other is measured simultaneously with a photodiode and used to normalize the PA signal. The cantilever-enhanced PA detector operates in a non-resonant mode, enabling detection of a broadband frequency response. The broadband and the high-resolution capabilities of OFC-PAS are demonstrated by measuring the rovibrational spectra of the fundamental C-H stretch band of CH4, with no instrumental line shape distortions, at total pressures of 1000 mbar, 650 mbar, and 400 mbar. In this first demonstration, a spectral resolution two orders of magnitude better than previously reported with broadband PAS is obtained, limited by the pressure broadening. A limit of detection of 0.8 ppm of methane in N2 is accomplished in a single interferogram measurement (200 s measurement time, 1000 MHz spectral resolution, 1000 mbar total pressure) for an exciting power spectral density of 42 μW/cm-1. A normalized noise equivalent absorption of 8 × 10-10 W cm-1 Hz-1/2 is obtained, which is only a factor of three higher than the best reported with PAS based on continuous wave lasers. A wide dynamic range of up to four orders of magnitude and a very good linearity (limited by the Beer-Lambert law) over two orders of magnitude are realized. OFC-PAS extends the capability of optical sensors for multispecies trace gas analysis in small sample volumes with high resolution and selectivity.

Nitrous oxide quartz-enhanced photoacoustic detection employing a broadband distributed-feedback quantum cascade laser array

Abstract: We present a gas sensing system based on quartz-enhanced photoacoustic spectroscopy (QEPAS) employing a monolithic distributed-feedback quantum cascade laser (QCL) array operated in a pulsed mode as a light source. The array consists of 32 quantum cascade lasers emitting in a spectral range from 1190 cm−1 to 1340 cm−1. The optoacoustic detection module was composed of a custom quartz tuning fork with a prong spacing of 1 mm, coupled with two micro-resonator tubes to enhance the signal-to-noise ratio. The QEPAS sensor was validated by detecting the absorption of the P- and R-branches of nitrous oxide. The measurements were performed by switching the array QCLs in sequence while tuning their operating temperature to retrieve the fine structure of the two N2O branches. A sensor calibration was performed, demonstrating a linear responsivity for N2O:N2 concentrations from 1000 down to 200 parts-per-million. With a 10 s lock-in integration time, a detection sensitivity of less than 60 parts-per-billion was achieved permitting the monitoring of nitrous oxide at global atmospheric levels.

On the accuracy of aerosol photoacoustic spectrometer calibrations using absorption by ozone

Abstract: . In recent years, photoacoustic spectroscopy has emerged as an invaluable tool for the accurate measurement of light absorption by atmospheric aerosol. Photoacoustic instruments require calibration, which can be achieved by measuring the photoacoustic signal generated by known quantities of gaseous ozone. Recent work has questioned the validity of this approach at short visible wavelengths (404 nm), indicating systematic calibration errors of the order of a factor of 2. We revisit this result and test the validity of the ozone calibration method using a suite of multipass photoacoustic cells operating at wavelengths 405, 514 and 658 nm. Using aerosolised nigrosin with mobility-selected diameters in the range 250–425 nm, we demonstrate excellent agreement between measured and modelled ensemble absorption cross sections at all wavelengths, thus demonstrating the validity of the ozone-based calibration method for aerosol photoacoustic spectroscopy at visible wavelengths.

NO2 trace gas monitoring in air using off-beam quartz enhanced photoacoustic spectroscopy (QEPAS) and interference studies towards CO2, H2O and acoustic noise

Abstract: We present the development and characterization as well as comprehensive interference studies of a photoacoustic NO2 trace gas detection system. The system is based on an off-beam quartz enhanced photoacoustic scheme (off-beam QEPAS) and signal generation was initiated by amplitude modulating a low-cost diode laser emitting at 450 nm. The QEPAS sensor element features double-resonant amplification, still it is only ∼ 5 × 5 × 2.5 mm in size. The individual and combined resonance characteristics were investigated and specified to 52 dB amplification, adding up 15 dB acoustic- and 37 dB mechanical-resonance amplification. The linearity of the photoacoustic signal dependency on the analyte concentration was verified from 200 ppbV to 100 ppmV NO2 in synthetic air. The detection limit (3σ) was determined to 1.8 ppbV using a lock-in time constant of 10 s and an averaging time of 20 s. The normalized noise equivalent absorption coefficient was specified to 2.5·10−8 W cm−1 Hz−0.5. The stability of the signal was investigated over time and a slight drift by 1‰ was observed after 30 min without temperature stabilizing the photoacoustic cell (PAC). Noise analysis was performed by means of Allan deviation and the inverse dependency of response time and precision of the system on the lock-in time constant was outlined. We performed interference analyses towards N2, O2, CO2, H2O and acoustic noise, respectively. Although neither spectral interferences nor losses due to slow NO2 VT-relaxation were observed, O2 was identified to cause a 15% signal drop due to VVNO2-O2-relaxation. Changing H2O concentrations were found to cause acoustic detuning, which cannot be compensated by adjusting the frequency of modulation, because of the double-resonant feature of the PAC. However, alternative approaches of compensation were discussed. Finally, we carried out heavy traffic noise simulations and determined the QEPAS setup to be 46 times less susceptible towards ambient noise compared to standard microphone-based photoacoustic setups.

Fiber-amplifier-enhanced resonant photoacoustic sensor for sub-ppb level acetylene detection

Abstract: A near-infrared fiber-amplifier-enhanced resonant photoacoustic spectroscopy (PAS) sensor is developed for sub-ppb level acetylene (C2H2) detection. The photoacoustic excitation source is composed of a telecommunication distributed feedback (DFB) laser and an erbium-doped fiber amplifier (EDFA). The weak photoacoustic second-harmonic signal is demodulated by a field-programmable gate array (FPGA) based lock-in amplifier. For sensitivity improvement, wavelength modulation spectrum and wavelet denoising methods are employed. The PAS sensor is optimized in terms of resonant frequency and current modulation depth for C2H2 detection at the wavelength of 1532.83 nm. The linearity of the sensor response to the laser power and C2H2 concentration confirms that saturation does not occur with the excitation power less than 1 W. With 1 W excitation power and 60 s averaging time, the detection limit (1δ) is achieved to be 0.37 ppb, which is the best value compared with other photoacoustic C2H2 sensors ever reported so far.

HCN ppt-level detection based on a QEPAS sensor with amplified laser and a miniaturized 3D-printed photoacoustic detection channel.

Abstract: Ultra-high sensitive and stable detection of hydrogen cyanide (HCN) based on a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor was realized using an erbium-doped fiber amplifier (EDFA) as well as a miniaturized 3D-printed photoacoustic detection channel (PADC) for the first time. A HCN molecule absorption line located at 6536.46 cm−1 was selected which was in the range of the EDFA emission spectrum. The detection sensitivity of the reported EDFA-QEPAS sensor was enhanced significantly due to the high available EDFA excitation laser power. A 3D printing technique was used to develop the compact PADC, resulting in a size of 29 × 15 × 8 mm3 and a mass of ~5 g in order to improve the sensor stability and implement sensor applications requiring a compact size and light weight. At atmospheric pressure, room temperature and a 1 s acquisition time, a minimum detection limit (MDL) of 29 parts per billion (ppb) was achieved, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 1.08 × 10−8 cm−1W/Hz-1/2. The long-term performance and the stability of the HCN EDFA-QEPAS sensor system were investigated using an Allan deviation analysis. It indicated that the MDL can be improved to 220 parts per trillion (ppt) with an integration time of 300 s, which demonstrated this compact, integrated and miniaturized 3D-printed PADC based sensor had an excellent stability.

Accuracy Enhancement for Noninvasive Glucose Estimation Using Dual-Wavelength Photoacoustic Measurements and Kernel-Based Calibration

Abstract: Frequent monitoring of blood glucose levels is an essential part of diabetes care, but the invasiveness of current devices deters regular measurement. Noninvasive measurement techniques are painless to implement and rely on changes in sample properties to estimate glucose concentration. However, such methods are affected by the presence of different biomolecules, resulting in an increased estimation error and necessitating calibration to obtain accurate glucose concentration estimates. The use of photoacoustic spectroscopy for continuous noninvasive glucose monitoring is studied through measurements on different sample media. In vitro photoacoustic measurements taken from aqueous glucose solutions, solutions of glucose and hemoglobin, and whole blood samples at multiple excitation wavelengths show amplitude and area-based signal features to rise with the increase in sample glucose concentration. The calibration of photoacoustic measurements from glucose solutions using Gaussian kernel-based regression results in a root mean square error (RMSE), mean absolute difference (MAD), and mean absolute relative difference (MARD) of 7.64 mg/dl, 5.23 mg/dl, and 2.07%, respectively. Kernel-based calibration also performs well on solutions of glucose and hemoglobin, and whole blood samples, resulting in lower estimation errors than that of previous efforts and with glucose estimates being in the acceptable zones of a Clarke error grid (CEG). It allows for individual calibration of photoacoustic measurements in vivo , resulting in an RMSE, MAD, and MARD of 19.46 mg/dl, 10.79 mg/dl, and 7.01%, respectively, with 89.80% of the estimates being within Zone A of the CEG. The improvement in estimation accuracy with dual-wavelength photoacoustic measurements and kernel-based calibration would enable continuous noninvasive glucose monitoring, facilitating improved diabetic care.

Cloud Computing-Based Non-Invasive Glucose Monitoring for Diabetic Care

Abstract: Near infrared photoacoustic spectroscopy is utilized for the development of a continuous non-invasive glucose monitoring system for diabetics. A portable embedded system for taking photoacoustic measurements on tissues to estimate glucose concentration is implemented using field programmable gate array (FPGA). The back-end architecture for high-speed data acquisition and de-noising of photoacoustic measurements operates at 274.823 MHz on a Xilinx Virtex-II Pro FPGA. The glucose measurement technique is verified in vitro on glucose solutions and in vivo on tissues, with photoacoustic signal amplitude varying linearly with sample glucose concentration. A kernel-based regression algorithm using multiple features of the photoacoustic signal is used to estimate glucose concentration from photoacoustic measurements. The calibration algorithm provides a superior performance over previous efforts with a mean absolute relative difference of 8.84% and Clarke Error Grid distribution of 92.86% and 7.14% over Zones A and B of the grid. A cloud computing platform for automated monitoring of blood glucose levels is proposed to enable individuals with diabetes to connect with doctors and caretakers. The developed system is connected to the cloud service using a mobile device, which facilitates implementation of computationally intensive calibration tasks and the storage and analysis of measurement data for treatment and monitoring.

A novel photoacoustic spectroscopy gas sensor using a low cost polyvinylidene fluoride film

Abstract: A proof-of-concept gas sensor based on innovative photoacoustic spectrophone was introduced for purpose of developing a low cost and simple configuration photoacoustic spectroscopy gas sensor and demonstrated, in which the photoacoustic signal was detected with a low cost polyvinylidene fluoride (PVDF) film being used as transducer, instead of using conventional microphone or quartz tuning fork In such a PVDF-based photoacoustic spectroscopy (PVDF-PAS) approach, the PVDF film plays a role both as a cantilever used in cantilever-enhanced photoacoustic spectroscopy and as a piezoelectric quartz tuning fork used in quartz-enhanced photo-acoustic spectroscopy Feasibility of the introduced PVDF-PAS for trace gas sensing was demonstrated by measuring H2O vapor with a minimum detection limit of 40 ppmv using a lock-in time constant of 30 ms, this corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 24 × 10−7 cm-1W/Hz1/2 The use of polyvinylidene fluoride film as transducer for photoacoustic signal sensing offers good flexibility, water resistance, chemical stability and low cost for applications under harsh or/and specific environmental conditions

Heterodyne interferometric signal retrieval in photoacoustic spectroscopy.

Abstract: A new heterodyne interferometric method for optical signal detection in photoacoustic or photothermal spectroscopy is demonstrated and characterized. It relies on using one laser beam for the photoacoustic excitation of the gas sample that creates refractive index changes along the beam path, while another laser beam is used to measure these changes. A heterodyne-based detection of path-length changes is presented that does not require the interferometer to be balanced or stabilized, which significantly simplifies the optical design. We discuss advantages of this new approach to photoacoustic signal detection and the new sensing arrangements that it enables. An open-path photoacoustic spectroscopy of carbon dioxide at 2003 nm and a novel sensing configuration that enables three-dimensional spatial gas distribution measurement are experimentally demonstrated.

Photoacoustic response induced by nanoparticle-mediated photothermal bubbles beyond the thermal expansion for potential theranostics

Abstract: Photoacoustic responses induced by laser-excited photothermal bubbles (PTBs) in colloidal gold solutions are relevant to the theranostics quality in biomedical applications. Confined to the complexity of nonstationary, multiscale events, and multiphysical parameters of PTBs, systematic studies of the photoacoustic effects remain obscure. Photoacoustic effects mediated by PTB dynamics and a physical mechanism are studied based on a proof-of-principle multimodal platform integrating side-scattering imaging, time-resolved optical response, and acoustic detection. Results show excitation energy, nanoparticle (NP) size, and NP concentration have strong influence on photoacoustic responses. Under the characteristic enhancement regime, the photoacoustic signal amplitude increases linearly with excitation energy and increases quadratically with the NP diameter. As for the effects of the NP concentration (characterized by absorption coefficient), a higher photoacoustic signal amplitude is generally induced by a dense NP distribution. However, with an increase in the NP size, the shielding effect of NP swarm prevents the increase of photoacoustic responses. This study presents experimental evidence of some key physical phenomena governing the PTB-induced photoacoustic signal generation in gold NP suspensions, which may help enrich theranostic approaches in clinical applications by rationalizing operation parameters.

Broadband cantilever-enhanced photoacoustic spectroscopy in the mid-IR using supercontinuum

Abstract: We demonstrate cantilever-enhanced photoacoustic spectroscopy in the mid-infrared using a supercontinuum source. The approach is broadband, compact, and allows for higher photoacoustic signal intensity and enhanced signal-to-noise ratio as compared to systems employing conventional back body radiation sources. Using this technique, we perform spectroscopic measurements of the full ro-vibrational band structure of water vapor at 1900 nm and methane at 3300 nm with relative signal enhancement factors of 70 and 19, respectively, when compared to measurements that use a black body radiation source. Our results offer novel perspective for photoacoustic detection opening the door to compact and sensitive broadband analyzers in the mid-infrared spectral region.

Photoacoustic Spectroscopy Investigation of Zinc Oxide/Diatom Frustules Hybrid Powders

Abstract: Photoacoustic spectroscopy investigation was carried out on ZnO nanoparticles grown at 80 °C on porous surface of diatomite (DE) by sol–gel technique, using zinc acetate dihydrate as ceramic precursor and triethanolamine to mediate the surface growth of the nanoparticles. Absorption and scattering characteristics of hybrid ZnO/DE powder in the UV–Vis range were inferred by photoacoustic spectroscopy, and results were analyzed based on Helander’s theory. In particular, we discussed in detail the procedure to calculate the absorption and scattering spectra of the hybrid powder showing how from the scattering coefficient in the 300–450 nm range it is possible to obtain information on the size distribution of the ZnO nanospheres. By applying photoacoustic spectroscopy for different modulation frequencies, we showed that is also possible to perform a size distribution depth profile of the ZnO aggregates, opening the way to interesting developments in this research field.

Ultrafast laser-scanning optical resolution photoacoustic microscopy at up to 2 million A-lines per second

Abstract: The imaging speed of optical resolution photoacoustic microscopy (OR-PAM) using pulsed excitation is fundamentally limited by the range ambiguity condition, which defines the maximum laser pulse repetition frequency (PRF). To operate at this theoretical upper limit and maximize acquisition speed, a custom-built fiber laser capable of operating at a PRF of up to 2 MHz was combined with a fast laser scanning optical OR-PAM system based on a stationary fiber-optic ultrasound sensor. A large area (10 mm × 10 mm) of the mouse ear was imaged within 8 s, when acquiring 16 million A-lines and operating the laser at a PRF of 2 MHz. This corresponds to a factor of four improvement in imaging speed compared to the fastest OR-PAM system previously reported. The ability to operate at high-imaging frame rates also allows the capture of hemodynamic events such as blood flow. It is considered that this system offers opportunities for high throughput imaging and visualizing dynamic physiological events using OR-PAM.

Ppbv-Level Ethane Detection Using Quartz-Enhanced Photoacoustic Spectroscopy with a Continuous-Wave, Room Temperature Interband Cascade Laser.

Abstract: A ppbv-level quartz-enhanced photoacoustic spectroscopy (QEPAS)-based ethane (C2H6) sensor was demonstrated by using a 3.3 μm continuous-wave (CW), distributed feedback (DFB) interband cascade laser (ICL). The ICL was employed for targeting a strong C2H6 absorption line located at 2996.88 cm−1 in its fundamental absorption band. Wavelength modulation spectroscopy (WMS) combined with the second harmonic (2f) detection technique was utilized to increase the signal-to-noise ratio (SNR) and simplify data acquisition and processing. Gas pressure and laser frequency modulation depth were optimized to be 100 Torr and 0.106 cm−1, respectively, for maximizing the 2f signal amplitude. Performance of the QEPAS sensor was evaluated using specially prepared C2H6 samples. A detection limit of 11 parts per billion in volume (ppbv) was obtained with a 1-s integration time based on an Allan-Werle variance analysis, and the detection precision can be further improved to ~1.5 ppbv by increasing the integration time up to 230 s.

Quantitative photoacoustic imaging of two-photon absorption.

Abstract: Photoacoustic tomography (PAT) is a hybrid imaging modality where we intend to reconstruct optical properties of heterogeneous media from measured ultrasound signals generated by the photoacoustic effect. In recent years, there have been considerable interests in using PAT to image two-photon absorption, in addition to the usual single-photon absorption, inside diffusive media. We present a mathematical model for quantitative image reconstruction in two-photon photoacoustic tomography (TP-PAT). We propose a computational strategy for the reconstruction of the optical absorption coefficients and provide some numerical evidences based on synthetic photoacoustic acoustic data to demonstrate the feasibility of quantitative reconstructions in TP-PAT.

Modeling photoacoustic imaging with a scanning focused detector using Monte Carlo simulation of energy deposition.

Abstract: Photoacoustic imaging using a focused, scanning detector in combination with a pulsed light source is a common technique to visualize light-absorbing structures in biological tissue. In the acoustic resolution mode, where the imaging resolution is given by the properties of the transducer, there are various challenges related to the choice of sensors and the optimization of the illumination. These are addressed by linking a Monte Carlo simulation of energy deposition to a time-domain model of acoustic propagation and detection. In this model, the spatial and electrical impulse responses of the focused transducer are combined with a model of acoustic attenuation in a single response matrix, which is used to calculate detector signals from a volumetric distribution of absorbed energy density. Using the radial symmetry of the detector, the calculation yields a single signal in less than a second on a standard personal computer. Various simulation results are shown, comparing different illumination geometries and demonstrating spectral imaging. Finally, simulation results and experimental images of an optically characterized phantom are compared, validating the accuracy of the model. The proposed method will facilitate the design of photoacoustic imaging devices and will be used as an accurate forward model for iterative reconstruction techniques.

Thermophysical and Optical Properties of L-Tartaric Acid Crystal

Abstract: Thermal properties play a vital role particularly for the materials which are used in high power laser irradiation. Photoacoustic technique is very much feasible to findout the thermal transport properties of solid materials. In the present work, nonlinear optical (NLO) single crystal of L-tartaric acid (LTA) was grown by Sankaranarayanan-Ramasamy (SR) method for dimensions of 140 mm in length and 11.5 mm in diameter. The grown LTA crystal has been subjected to UV-Vis-NIR spectral study to analyze the optical transmittance and absorbance characteristics. The thermal characterization studies were performed using photoacoustic spectrometer (PAS). Thermal characterization involves measurement of thermal parameters such as thermal diffusivity, thermal effusivity, thermal conductivity and specific heat capacity. The experimental results of photoacoustic spectrometer show that the thermal diffusivity of LTA is higher than the reported values of a few other well known NLO materials.