Photothermal spectroscopy of CO2 in an intracavity mode-locked fiber laser configuration

Side-excitation light-induced thermoelastic spectroscopy

In this Letter, a side-excitation light-induced thermoelastic spectroscopy (SE-LITES) technique was developed for trace gas detection. A novel, to the best of our knowledge, custom quartz tuning fork (QTF) was used as a transducer for photon detection by the thermoelastic effect. The mechanical stress distribution on the QTF surface was analyzed to identify the optimum thermoelastic excitation approach. The electrode film on the QTF surface also works as a partially reflective layer to obtain a long optical absorption path inside the QTF body. With the long optical absorption length and the inner face excitation of the QTF, the thermoelastic effect was greatly enhanced. With an optimized modulation depth, a signal-to-noise ratio (SNR) improvement of more than one order of magnitude was achieved, compared to traditional LITES.

Flow-Enhanced Photothermal Spectroscopy

Photothermal spectroscopy (PTS) is a promising sensing technique for the measurement of gases and aerosols. PTS systems using a Fabry-Pérot interferometer (FPI) are considered particularly promising owing to their robustness and potential for miniaturization. However, limited information is available on viable procedures for signal improvement through parameter tuning. In our work, we use an FPI-based PTS configuration, in which the excitation laser irradiates the target collinearly to the flowing gas. We demonstrate that the generated thermal wave, and thus the signal intensity, is significantly affected by the ratio between excitation modulation frequency and gas flow velocity towards another. We provide an analytical model that predicts the signal intensity with particular considerations of these two parameter settings and validate the findings experimentally. The results reveal the existence of an optimal working regime, depending on the modulation frequency and flow velocity.

Sensitive mid-infrared photothermal gas detection enhanced by self-heterodyne harmonic amplification of a mode-locked fiber laser probe

In this work, a method of photothermal spectroscopic signal extraction is presented. The refractive index modulation readout is realized in a purely frequency detection-based approach, by demodulating the beatnotes of a mode-locked fiber laser operating at 1.56 µm. A unique and non-complex self-heterodyne harmonic amplification technique is employed, yielding an increase in the limit of detection by a factor of 22. The sensor's performance was evaluated by detecting nitric oxide at 5.26 µm, confirming the feasibility of separating the pump and probe sections of the device. The sensor reached a detection limit of 9.6 parts-per-billion by volume for an integration time of 136 s, with only a 20 cm-long laser-gas molecules interaction path length.

Hollow-core anti-resonant fiber based light-induced thermoelastic spectroscopy for gas sensing

In this paper, a hollow-core anti-resonant fiber (HC-ARF) based light-induced thermoelastic spectroscopy (LITES) sensor is reported. A custom-made silica-based HC-ARF with length of 75 cm was used as light medium and gas cell. Compared to a traditional multi-pass cell (MPC), the using of HC-ARF is advantageous for reducing the sensor size and easing the optical alignment. A quartz tuning fork (QTF) with a resonant frequency of 32766.20 Hz and quality factor of 12364.20 was adopted as the thermoelastic detector. Acetylene (C₂H₂) and carbon monoxide (CO) with absorption lines located at 6534.37 cm^-1 (1530.37 nm) and 6380.30 cm^-1 (1567.32 nm) were chosen as the target gas to verify such HC-ARF based LITES sensor performance. It was found that this HC-ARF based LITES sensor exhibits excellent linearity response to the analyte concentrations. The minimum detection limit (MDL) for C₂H₂ and CO detections were measured as 4.75 ppm and 1704 ppm, respectively. The MDL for such HC-ARF based LITES sensor can be further improved by using a HC-ARF with long length or choosing an absorption line with strong strength.

Acoustic microresonator based in-plane quartz-enhanced photoacoustic spectroscopy sensor with a line interaction mode

An acoustic microresonator (AmR) based in-plane quartz-enhanced photoacoustic spectroscopy (IP-QEPAS) sensor with a line interaction mode is proposed for what is believed to be the first time. The interaction area for the acoustic wave of the proposed AmR, with a slotted sidewall, is not limited to a point of the quartz tuning fork (QTF) prongs, but extends along the whole plane of the QTF prongs. Sixteen types of AmRs are designed to identify the best parameters. Water vapor (H₂O) is chosen as the analyte to verify the reported method. The results indicate that this AmR for IP-QEPAS with a line interaction mode not only provides a high signal level, but also reduces the thermal noise caused by the laser directly illuminating the QTF. Compared with standard IP-QEPAS without an AmR, the minimum detection limit (MDL) is improved by 4.11 times with the use of the technique proposed in this study.

Quartz tuning forks resonance frequency matching for laser spectroscopy sensing

In this paper, we report on the performance of quartz tuning fork (QTF) based laser spectroscopy sensing employing multiple QTFs. To avoid that resonance frequency mismatching of the QTFs degrades the sensor performance, two types of resonance frequency matching method are here proposed. A system based on the coupling of two sensing modules, one based on quartz-enhanced photoacoustic spectroscopy (QEPAS) and one on light-induced thermoelastic spectroscopy (LITES) technique, was realized to validate the proposed methods. Each module employed a different QTF (QTF1 and QTF2, respectively). Operating temperature or pressure of QTF2 were regulated to match the resonance frequency of QTF1, which operated at 25.0 °C and atmospheric pressure. Without regulation, the difference between QTF1 and QTF2 resonance frequencies was 2.42 Hz and the superposition coefficient η was only 54.7%. When the temperature regulation was carried out, at a QTF2 operating temperature of 67.5 °C, an optimal η value of 95.0% was obtained. For the pressure regulation approach, if operating QTF2 at pressure of 500 Torr, η reached a value of 97.2%. The obtained results show that the proposed two methods are effective in resonance frequency matching of QTFs for gas sensing systems.

H-shaped acoustic micro-resonator-based quartz-enhanced photoacoustic spectroscopy

An H-shaped acoustic micro-resonator (AmR)-based quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor is demonstrated for the first time. The H-shaped AmR has the advantages of easy optical alignment, high utilization of laser energy, and reduction in optical noise. The parameter of the H-shaped AmR is designed based on the standing wave enhancement characteristic. The performance of the H-shaped AmR-based QEPAS sensor system and bare quartz tuning fork (QTF)-based sensor system are measured under the same conditions by choosing water vapor (H₂O) as the target gas. Compared with the QEAPS sensor based on a bare QTF, the detection sensitivity of the optimal H-shaped AmR-based QEPAS sensor exhibits a 17.2 times enhancement.

Highly Sensitive Sphere-Tube Coupled Photoacoustic Cell Suitable for Detection of a Variety of Trace Gases: NO2 as an Example

The concentration of trace gases in the atmospheric environment is extremely low, but it has a great impact on the living environment of organisms. Photoacoustic spectroscopy has attracted extensive attention in the field of trace gas detection because of its high sensitivity, good selectivity, and fast response. As the core of a photoacoustic detection setup, the photoacoustic cell has a significant impact on detection performance. To improve detection sensitivity, a sphere-tube coupled photoacoustic cell (STPAC) was developed, which was mainly composed of a diffuse-reflective sphere and an acoustic resonance tube. Modulated light was reflected multiple times in the sphere to increase optical path, and photoacoustic (PA) signals were further amplified by the tube. Based on STPAC, a PA gas detection setup was built with a laser diode (LD) at 450 nm as the light source. The experimental results showed that the minimum detection limit (noise equivalent concentration, NEC) of NO₂ was ~0.7 parts per billion (ppb). Compared with the T-type PA cell (TPAC) in which the modulated light passed through the sphere, the signal-to-noise ratio of STPAC was increased by an order of magnitude at the same concentration of the NO₂ sample.

A Review of Antiresonant Hollow-Core Fiber-Assisted Spectroscopy of Gases

Antiresonant Hollow-Core Fibers (ARHCFs), thanks to the excellent capability of guiding light in an air core with low loss over a very broad spectral range, have attracted significant attention of researchers worldwide who especially focus their work on laser-based spectroscopy of gaseous substances. It was shown that the ARHCFs can be used as low-volume, non-complex, and versatile gas absorption cells forming the sensing path length in the sensor, thus serving as a promising alternative to commonly used bulk optics-based configurations. The ARHCF-aided sensors proved to deliver high sensitivity and long-term stability, which justifies their suitability for this particular application. In this review, the recent progress in laser-based gas sensors aided with ARHCFs combined with various laser-based spectroscopy techniques is discussed and summarized.

Multi-pass quartz-enhanced photoacoustic spectroscopy-based trace gas sensing

A multi-pass quartz-enhanced photoacoustic spectroscopy (MP-QEPAS)-based trace gas sensor is reported. In MP-QEPAS, a multi-pass laser beam pattern through the prong spacing of a quartz tuning fork (QTF) is obtained by means of two right-angle prisms. A large QTF with the prong length of 17 mm and prong spacing of 0.8 mm was employed to increase the passage of multi-pass time and ease the alignment of the beam reflection pattern through the QTF. This multi-pass configuration allows the laser beam to pass through the QTF prong spacing six times. Water vapor (H₂O) was chosen as target gas to investigate the performance of the MP-QEPAS sensor. Compared to a conventional QEPAS measurement, the MP-QEPAS technique provided an enhancement of signal level of ∼3.2 times.

Balanced-detection interferometric cavity-assisted photothermal spectroscopy employing an all-fiber-coupled probe laser configuration

The interferometric cavity-assisted photothermal spectroscopy (ICAPS) method has been proven highly suitable for sensitive and compact gas detection by application of an optical cavity as transducer for photothermal spectroscopy. This work reports on the implementation of an overall fiber-coupled probe laser configuration detecting the reflectance of the individual interferometers in a balanced-detection ICAPS system. The layout greatly improves the overall sensor system robustness. Two identical 1 mm path length cavities were used for balanced detection, enabling sensor operation close to the fundamental limit of shot noise by efficiently cancelling excess noise. A quantum cascade laser served as a mid-infrared excitation source to induce refractive index changes in the sample, and a near-infrared fiber laser served as probe source to monitor the photo-induced refractive index variations. The metrological figures of merit for the sensor were investigated by SO₂ detection. For the targeted absorption band centered at 1380.93 cm^-1, a 3 ppbv minimum detection limit was achieved with a 1 s integration time, corresponding to a normalized noise equivalent absorption of 4.5 × 10^-9 cm^-1 W Hz^-1/2.

Quartz-enhanced photoacoustic-photothermal spectroscopy for trace gas sensing

A trace gas detection technique of quartz-enhanced photoacoustic-photothermal spectroscopy (QEPA-PTS) is demonstrated. Different from quartz-enhanced photoacoustic spectroscopy (QEPAS) or quartz-enhanced photothermal spectroscopy (QEPTS), which detected only one single kind of signal, QEPA-PTS was realized by adding the photoacoustic and photothermal signals generated from two quartz tuning forks (QTFs), respectively. Water vapor (H₂O) with a volume concentration of 1.01% was selected as the analyte gas to investigate the QEPA-PTS sensor performance. Compared to QEPAS and QEPTS, an enhanced signal level was achieved for this QEPA-PTS system. Further improvement of such a technique was proposed.

Mid-infrared photoacoustic gas monitoring driven by a gas-filled hollow-core fiber laser

Development of novel mid-infrared (MIR) lasers could ultimately boost emerging detection technologies towards innovative spectroscopic and imaging solutions. Photoacoustic (PA) modality has been heralded for years as one of the most powerful detection tools enabling high signal-to-noise ratio analysis. Here, we demonstrate a novel, compact and sensitive MIR-PA system for carbon dioxide (CO₂) monitoring at its strongest absorption band by combining a gas-filled fiber laser and PA technology. Specifically, the PA signals were excited by a custom-made hydrogen (H₂) based MIR Raman fiber laser source with a pulse energy of ⁓ 18 μJ, quantum efficiency of ⁓ 80% and peak power of ⁓ 3.9 kW. A CO₂ detection limit of 605 ppbv was attained from the Allan deviation. This work constitutes an alternative method for advanced high-sensitivity gas detection.

Trace gas sensing based on multi-quartz-enhanced photothermal spectroscopy

A multi-quartz-enhanced photothermal spectroscopy (M-QEPTS) based trace gas detection method is reported for the first time. Different from traditional QEPTS sensor employing a single quartz tuning fork (QTF) as a photothermal detector, two QTFs were used in M-QEPTS to increase the signal amplitude by adding the generated piezoelectric signals. The coating film of the QTFs was removed in order to improve the laser absorption and transmission. Acetylene (C2H2) was chosen as the target analyte. Wavelength modulation spectroscopy (WMS) and 2nd harmonic detection were utilized for the concentration detection. Limit of detection (LoD) of 0.97 ppm was achieved with a 1 second integration time for the M-QEPTS sensor, which realized a 1.51 times signal enhancement compared to a traditional QEPTS sensor employing a single QTF. By using an Allan deviation analysis approach, LoD of 0.19 ppm for an optimum integration time of 200 s was obtained.

State-of-the-Art Laser Gas Sensing Technologies

The increasing desire to detect and monitor in different fields [...]

A Review of Photothermal Detection Techniques for Gas Sensing Applications

Photothermal spectroscopy (PTS) is a technique used for determining the composition of liquids, solids and gases. In PTS, the sample is illuminated with a radiation source, and the thermal response of the analyte (e.g., refractive index) is analyzed to gain information about its content. Recent advances in this unique method of detecting gaseous samples show that photothermal gas spectroscopy can be an interesting alternative to commonly used absorption techniques. Moreover, if designed properly, sensors using PTS detection technique can not only reach sensitivities comparable with other, more complex techniques, but can significantly simplify the design of the sensor. In this review, recent developments in photothermal spectroscopy of gases will be summarized and discussed.

Ultra-high sensitive light-induced thermoelastic spectroscopy sensor with a high Q-factor quartz tuning fork and a multipass cell

An ultra-high sensitive light-induced thermoelastic spectroscopy (LITES) sensor based on a resonant high Q-factor quartz turning fork (QTF) and a Herriot multipass cell was demonstrated for the first time, to the best of our knowledge. The performance of LITES and widely used tunable diode laser absorption spectroscopy (TDLAS) were experimentally investigated and compared at the same conditions. Carbon monoxide (CO) was chosen as the analyte to verify the reported sensors' performance. With a minimum detection limit (MDL) of 470 ppb for 60 ms integration time and a noise equivalent absorption (NEA) coefficient of 2.0×10^-7  cm^-1 Hz^-1/2, and a MDL of 17 ppb with an optimum integration time of 800 s, the reported LITES sensor showed a superior sensing capability compared with a TDLAS sensor and a conventional quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor.