Photoacoustic Spectroscopy Signal Research Articles

The fiber ring laser intra-cavity gas sensor for C2H2 and CO2 detection based on photoacoustic spectroscopy

In this paper, a two-component gas sensing system in the resonant cavity of the fiber ring laser based on photoacoustic spectroscopy (PAS) is designed by embedding the PAS cell into the laser resonant cavity. A ring cavity erbium-doped fiber (EDF) laser is built in this system, combined with a longitudinal resonant PAS cell to detect the PAS signal, and a custom-designed wavelength modulator uses the fiber bragg grating (FBG) to paste the piezoelectric transducer (PZT) for scanning and modulating the gas absorption wavelength. The gas concentration is calculated by the second harmonic (2f) PAS signal. Based on time division multiplexing (TDM) technology, the acetylene (C2H2) and carbon dioxide (CO2) is detected respectively by the time-sharing operation of C-band and L-band EDFs for different wavelengths output. The time-sharing operation of the two wavelength laser is realized through optical switches, which effectively avoids the laser power loss and the instability of each wavelength caused by the mode competition phenomenon of multiple wavelengths. The detection performance of C2H2 and CO2 is implemented under normal temperature and pressure. The PAS signal intensity is further improved by optimizing the wavelength modulation amplitude and incident laser power. The linear fit (R2) and minimum detection limit (MDL) of C2H2 and CO2 are 0.99821, 728 ppbv, 0.9994 and 503 ppmv respectively. It shows that the two-component gas sensing system designed in this paper has a good linear response and high sensitivity, and plays an important role on the multi-component gas detection.

A Dual-Gas Sensor Using Photoacoustic Spectroscopy Based on a Single Acoustic Resonator

A dual-gas photoacoustic spectroscopy (PAS) sensor based on wavelength modulation spectroscopy (WMS) was developed and experimentally demonstrated. Distributed feedback (DFB) laser diodes, emitting at 1512 and 1653 nm, were utilized as the excitation sources for the simultaneous measurement of NH3 and CH4, respectively. The PAS signal was excited by modulating the DFB laser at the first longitudinal resonant frequency of a cylindrical acoustic resonator. Absorption lines for NH3 and CH4 were simultaneously recorded during one frequency scan of the DFB lasers without using any optical switch. The interference of NH3 and CH4 on each other was investigated for accurate detection. The limits of detection (LoDs) of the PAS sensor for NH3 and CH4 for an integration time of 100 s were determined to be 0.1 and 0.3 ppm, respectively. The present PAS sensor provides a new scheme for multi-gas analysis with the advantages of cost-effectiveness, a simple structure and multi-wavelength operation.

Two-component gas quartz-enhanced photoacoustic spectroscopy sensor based on time-division multiplexing of distributed-feedback laser driver current.

A two-component gas sensor in quartz-enhanced photoacoustic spectroscopy based on time-division multiplexing (TDM) technology of a distributed-feedback (DFB) laser driver current was proposed and experimentally demonstrated. The quartz tuning-fork-based photoacoustic spectroscopy (PAS) cell configuration with two optical collimators and two acoustic microresonators was designed to detect the second-harmonic (${2}f$2f) PAS signal. The two optical collimators guaranteed that the two laser beams would inject the PAS cell conveniently, providing higher power input than a 3 dB optical fiber coupler. Two-component gas sensing was achieved by the TDM of the DFB laser driver current. We used this two-component gas sensing technique to detect acetylene (${{\rm C}_2}{{\rm H}_2}$C2H2) at 1532.83 nm and methane (${{\rm CH}_4}$CH4) at 1653.722 nm. The ${{\rm C}_2}{{\rm H}_2}$C2H2 and ${{\rm CH}_4}$CH4 detection was achieved at a 2.4 s interval. The minimum detection limits of 1 ppmv for ${{\rm C}_2}{{\rm H}_2}$C2H2 and 13.14 ppmv for ${{\rm CH}_4}$CH4 were obtained, and the linear responses reached were 0.99968 and 0.99652 for ${{\rm C}_2}{{\rm H}_2}$C2H2 and ${{\rm CH}_4}$CH4, respectively. Moreover, the continuous monitoring of ${{\rm CH}_4}$CH4 and ${{\rm C}_2}{{\rm H}_2}$C2H2 for 40 min showed a good stability. The TDM technology of the DFB laser driver current would play an important role on the multi-component detection.

Wavelength modulation technique-based photoacoustic spectroscopy for multipoint gas sensing.

A multipoint gas sensing scheme based on photoacoustic spectroscopy was proposed. Multiple photoacoustic spectroscopy (PAS) gas cells (resonant frequency f0=5.0 kHz) were connected in series for the multipoint gas sensing with wavelength modulation technique. The PAS signal was excited by modulating the tunable distributed feedback laser diode wavelength at f0/2 using a changing driving current. The gas concentration of each gas cell was obtained by the PAS signal, which was demodulated by the lock-in amplifier. A multipoint PAS experiment to detect the water vapor at 1368.597nm was implemented to verify the scheme we presented. With the three PAS gas cells, the linear response to the water vapor concentration of our sensors achieved 0.9978, 0.99591, and 0.99617, and their minimum detection limits were 479, 662, and 630ppb, respectively.

Fiber-ring laser-based intracavity photoacoustic spectroscopy for trace gas sensing.

We demonstrated a novel trace gas sensing method based on fiber-ring laser intracavity photoacoustic spectroscopy. This spectroscopic technique is a merging of photoacoustic spectroscopy (PAS) with a fiber-ring cavity for sensitive and all-fiber gas detection. A transmission-type PAS gas cell (resonant frequency f0=2.68 kHz) was placed inside the fiber-ring laser to fully utilize the intracavity laser power. The PAS signal was excited by modulating the laser wavelength at f0/2 using a custom-made fiber Bragg grating-based modulator. We used this spectroscopic technique to detect acetylene (C2H2) at 1531.6nm as a proof of principle. With a low Q-factor (4.9) of the PAS cell, our sensor achieved a good linear response (R2=0.996) to C2H2 concentration and a minimum detection limit of 390ppbv at 2-s response time.

Analysis Accuracy Improvement of Gas Second Harmonic Photoacoustic Spectroscopy Based on Independent Component Analysis

In this study, a principle of a second harmonic detection for tracking gas trace component is discussed, and the resultant demodulation and distortion recovery methods are expounded accordingly. A line- tunable gas photoacoustic spectroscopy signal can be regarded as the weighted sum of feature absorptions using an analysis of characteristics, and a sample from gas photoacoustic spectroscopy signal based on second harmonic detection is selected to make an analysis accuracy improvement using independent component analysis. The relevant tests showed that the technique had advantages. The demodulation technique loses the least information over the analysis of linear approximation or combination, and it can be used to analyze any gas phase sample whose signal profile has a “sharp” peak.

Photo-acoustic spectroscopy of polymer with infrared free electron laser

We have been proposing a new photo acoustic spectroscopy (PAS) method with an infrared free electron laser (IR-FEL). The IR-FEL is a continuous tunable laser in the lasing wavelength in an finger-print wavelength region with high peak power. The IR-FEL resonantly induce the specific molecular vibration of the samples, resulting casue the photo acoustic pheneomena with high sensitivity. Thus, the new PAS method with the IR-FEL is high sensitive analysis method applicable to low absorbance measurement. In this study we show the feasibility of application of the MIR-FEL to the photon source of the PAS method by measuring the PAS spectrum of polymers such as a poly ethylene terephthalate (PET) film. In addition, we construct an imaging system of the FEL-PAS to make a map of the PAS signal with a high spatial resolution that is a diffraction limit of the IR-FEL.

Photothermal characterization of thin-layer chromatography plates

The potential of photothermal beam deflection spectrometry (PTDS), photoacoustic spectroscopy (PAS) and photothermal radiometry (PTR) for the characterization of thin-layer chromatography (TLC) plates with respect to the surface and in-depth distribution of different compounds inside the sorbent was investigated. Photothermal measurements of TLC chromatograms of the Camag III test dye mixture demonstrate that accurate and comparable values for thermal diffusivity of spots in chromatograms and of the sorbent can be obtained by PTDS and PAS. PAS has been demonstrated to be the most reliable and least time-consuming among the three investigated techniques. Differences observed between the PAS signal and the phase-frequency scans obtained from the spots of the same compound on different TLC plates could be due to the irreproducibility of chromatogram development or the inhomogeneousness in different TLC plates.