Photoacoustic spectroscopy (PAS) is a powerful and non-destructive optical analysis technique that can be used to quantitatively analyze the composition of gases, liquids, and solids. With the continuous innovation of modern laser techniques, various new laser light sources have emerged that play an important role in promoting the development of photoacoustic spectroscopy based on laser light sources. Moreover, novel signal processing algorithms and detection strategies have been reported. In theory, PAS is essentially established with wavelength independence, high resolution, and high sensitivity. These unique characteristics make them widely used in environmental science, solid-state physics, industrial process control, biomedicine, and other fields. However, the thermal noise caused by the absorption of the incident laser by the windows or inner wall of the photoacoustic (PA) cell (particularly when the high-power laser source is used as the signal excitation light source), and the electrical noise of electronic devices (such as acoustic signal detectors) are still key technical issues that limit the detection sensitivity of PA-spectroscopy-based gas sensors. To resolve the background noise problem in these technical issues, a differential resonance photoacoustic gas detection method that fully utilizes the phase-dependent characteristics of the PA resonance cavity is proposed.
Considering the problem of noise limitation in PAS system sensitivity, resonance enhancement detection strategies are usually adopted to achieve effective suppression of the background noise of the PA system. Typically, cylindrical resonant PA cells, Helmholtz resonators, spherical resonators, and quartz tuning forks are used. In the field of signal processing algorithms, differential detection is an effective method for eliminating background noise interference, improving signal quality, and improving the spectral signal-to-noise ratio (SNR), and has good application value in various signal processing. Therefore, in this study, a high-sensitivity PA gas detection technique is developed by combining resonance PAS characteristics based on the differential detection principle. To demonstrate the proposed technique, a cylindrical PA cell with double resonant cavities is designed, and relevant theoretical and experimental studies are conducted for sensitive sensing gas detection. A differential-resonance PAS gas sensor system is integrated by using a near-infrared diode laser near 1391.6 nm and a double PA cell. To further improve the detection sensitivity, a wavelength modulation spectroscopy second-harmonic (WMS-2F) detection method is employed. Moreover, the Allan variance analysis algorithm is used to evaluate the system sensitivity and stability.
To evaluate the gas-sensing technique, ambient water vapor (H2O) is analyzed. The potential crosstalk effect between the double-resonance cavities is investigated using theoretical simulations (Fig.1) and experimentally confirmed. Differential detection is applied for measuring background noise and H2O PA spectral signals. The calculated results indicate that the standard deviations of the background noise can be improved by approximately 1.9 times (Fig.4) by utilizing the phase-dependent characteristics of the two resonance cavities (Fig.5), and the PA spectral signal amplitude can also be significantly enhanced (Fig.7). Moreover, a detection limit of ~3.0×10-6 is obtained for ambient H2O concentration measurements under the optimal averaging time of 115 s without using differential detection (Fig.8). After using the differential algorithm, the system stability is further improved, the optimal stability time is increased to more than 200 s, and the corresponding detection sensitivity is improved to 2.0×10-6 (Fig.8).
This study proposes a high-sensitivity gas detection technique based on resonant PAS with differential detection principle. Allan variance analysis indicates that high-sensitivity detection of several 10-6 level H2O concentrations can be achieved using a low-power near-infrared (NIR) diode laser. Compared to the traditional single-channel PA detection mode, the results prove that the proposed differential resonant PAS detection technique can effectively improve the system stability and detection sensitivity, and the optimal signal average time can be doubled.
The technology for retrieving aerosol extinction coefficients from LiDAR is mature. However, further progress is required to retrieve the vertical distribution of aerosol mass concentration. In addition, accuracy evaluation of aerosol mass concentration from LiDAR is challenging owing to the lack of standard vertical PM2.5 mass concentration. Therefore, in this study, a PM2.5 mass concentration retrieval algorithm was developed by integrating real-time temperature, relative humidity, and extinction coefficient profiles. The PM2.5 mass concentration at four heights of the Shenzhen Meteorological Gradient Observation Tower was used as the standard value to evaluate the accuracy of the model under different weather conditions and seasons.
The influence of meteorological factors on the vertical distribution of aerosol mass concentration is extremely complex, particularly under precipitation conditions where the LiDAR signal attenuation is severe. Therefore, in this study, only the effects of temperature and relative humidity on the vertical distribution of aerosols under non-precipitation weather conditions were investigated. In practical applications, sample data are initially preprocessed, including the outlier handling (triple standard deviation removal), rainy day data, and missing value removal. The extinction coefficient at the lowest height of the LiDAR, ground temperature, relative humidity from the microwave radiometer, and PM2.5 mass concentration near the ground were substituted into an exponential model. The data from 2500 h were subsequently used for model fitting. The model parameters were automatically determined based on the minimum mean square error. Thus, the extinction coefficient, temperature, and relative humidity profiles at a specific height could be selected to calculate the PM2.5 mass concentration at the corresponding height. To investigate the accuracy of the PM2.5 mass concentration inversion, comparisons were conducted between PM2.5 mass concentrations at four heights (70, 120, 220, and 335 m) on the Shenzhen Meteorological Gradient Observation Tower.
By comparing different weather conditions, the correlation coefficients between the simulated and measured values at the four heights are over 0.68 (Figs.3 and 4). The maximum mean absolute error (MAE) and root mean square error (RMSE) are 6.88 μg/m3 and 18.56 μg/m3, respectively, appearing at a height of 335 m on sunny days. In different seasons, the correlation coefficients at the four heights range from 0.78?0.93, 0.71?0.81, 0.73?0.80, and 0.63?0.75, respectively (Table 4). The PM2.5 mass concentration spatiotemporal distribution and transport process on July 29, 2022, was selected as a case study for analysis (Fig.6
This study established a multivariate PM2.5 mass concentration fitting model based on an exponential model combining temperature, relative humidity, and extinction profiles. The optimal parameters were selected based on the minimum mean-square deviation index, and the output was validated. Compared to the linear and exponential basic models, the accuracy of the multivariate fitting model has been improved, with correlation coefficients at all four heights above 0.80. The minimum MAE and RMSE are approximately 4 μg/m3 and 7 μg/m3, respectively. Under clear and cloudy weather conditions, the correlation coefficients at four altitudes exceed 0.68, and the MAE and RMSE are below 7 μg/m3 and 19 μg/m3,respectively. The simulation results spanning different seasons demonstrate that the average mass concentration of PM2.5 in Shenzhen is below 35 μg/m3. The simulated PM2.5 mass concentration exhibited seasonal variation patterns. In addition, the simulation results for spring, summer, and autumn are better than those for winter. This may be due to the uncertainty caused by the relatively high aerosol mass concentrations in winter. Considering the uncertainty caused by the LiDAR and microwave radiometer measurement processes, the validation results of the proposed multivariate model performed well within an acceptable range.
The femtosecond optical frequency comb (FOFC) comprises a series of ultra-short laser pulses with the same temporal separation in the time domain and discrete, equidistant, and stable phase-related frequency components in the frequency domain. The FOFC can accurately measure the absolute frequency of an atomic clock and serve as a natural time-frequency reference. Currently, the most stable and compact light source is the mode-locked erbium-doped fiber laser with a central wavelength of 1.55 μm, typically employing highly nonlinear fibers to broaden the spectrum across the entire transparent range of silica fiber (350?2400 nm). However, the output power of the erbium-doped fiber FOFC is generally in the range of a few hundred milliwatts. Therefore, increasing the output power of the FOFC remains a crucial challenge. The mid-infrared FOFC holds significant application value in next-generation spectroscopy, as it can be used to detect gases such as carbon dioxide and ammonia and extend the FOFC wavelength to the molecular fingerprint spectrum range (3?20 μm) through nonlinear crystals. This spectrum range is vital for chemical composition analysis, making the development of high-power mid-infrared FOFCs a pressing need.
This system comprises an erbium-doped fiber FOFC, a super-continuum converter, a double-cladding thulium-doped fiber amplifier system, and a transmission diffraction grating pulse compressor. Initially, the erbium-doped fiber FOFC utilizes a highly nonlinear fiber with normal dispersion for frequency broadening. Additionally, a self-pump amplifier composed of thulium-doped fiber generates a femtosecond seed with a central wavelength of 1925 nm. This seed is injected into a chirped pulse amplification system comprising a 55 m long highly nonlinear fiber with normal dispersion, a three-stage thulium-doped fiber amplifier, and a transmission diffraction grating pulse compressor. To characterize the noise of the high-power mid-infrared FOFC, we analyze the relative intensity noise and the phase noise of the pulse train using a signal source analyzer. Moreover, we co-couple the super-continuum laser generated by the high-power mid-infrared FOFC in the fluorotellurite fiber with a 1064 nm iodine-stabilized Nd∶YAG laser to detect the beat signal and verify the performance of the high-power mid-infrared FOFC.
The 1.55 μm femtosecond laser output from the erbium-doped fiber femtosecond optical frequency comb is symmetrically broadened to the spectral range of 1100?2200 nm by the highly nonlinear fiber with normal dispersion (Fig.2). The resultant super-continuum laser is injected into the self-pump pre-amplifier to obtain a femtosecond seed with a central wavelength of 1925 nm and an average power of 50 mW [as indicated by the dashed line in Fig.3(a)]. This seed is then broadened to hundreds of picoseconds through the normal dispersion fiber and amplified by the three-stage double-cladding thulium-doped fiber amplifier to yield a picosecond pulse with a central wavelength of 2000 nm and an average power of 36.07 W. After compression, a femtosecond pulse with an average power of 22.72 W and a pulse width of 240 fs is obtained [Fig.3(b)]. The integral values of relative intensity noise and timing jitter are 1.16% and 472 fs, respectively (integral range of 10 Hz?1 MHz) (Figs.4 and 5). The super-continuum laser (Fig.6) generated by the high-power mid-infrared FOFC and the 1064 nm laser produce a beat signal with a signal-to-noise ratio of 40 dB, meeting the counting requirements of the counter (Fig.8).
We demonstrate a high-power FOFC based on an erbium-doped FOFC, generating a 2 μm femtosecond seed through a highly nonlinear fiber with normal dispersion and self-pump pre-amplifier. The highly nonlinear optical fiber with normal dispersion effectively overcomes noise sensitivity issues associated with nonlinear dynamics of abnormal dispersion, such as soliton self-frequency shift and Raman soliton, during super-continuum generation. The femtosecond pulse, obtained with an average power of 22.72 W and a pulse width of 240 fs, marks a significant advancement in developing high-power mid-infrared FOFCs. This development contributes to the spectroscopic analysis of molecular structures and dynamics and facilitates the expansion of optical frequency combs into the molecular fingerprint spectrum range (3?20 μm).
A laser scanning projector can deflect a laser beam quickly and accurately, and the path of the laser spot can be shaped into a pattern, thereby facilitating processing and assembly. To ensure the precise calibration of the projection system, several cooperative targets should be scanned to solve the coordinate transformation equations between the world and projector frames. Typically, two calibration methods are employed: One involves manipulation of the laser to scan the cooperative target area, identify points on the edge of the target, and determine the center of the circle through least-squares fitting. This approach requires numerous scanning points, leading to an ineffective calibration process. The alternative approach utilizes binocular cameras for simultaneous multipoint positioning, which results in a reduced calibration time. However, identifying cooperative targets requires significant arithmetic resources. In addition, limited by the performance of the camera, this method is restricted to the calibration distance, and its ability to adapt to ambient light is poor. The correlated double-sampling (CDS) technique can be adopted to enhance the anti-interference ability of the system. Furthermore, a discontinuous scanning method based on the bisection principle is proposed. This technique can precisely identify the boundaries of cooperative targets as well as considerably reduce the number of scanning points, thereby guaranteeing the precision of cooperative target localization.
CDS was investigated to realize band-pass filtering and enhance the adaptability of the detection system to ambient light. Subsequently, an integral sampling circuit was designed to reduce the effects of high-frequency noise. The silicon photodiode operates in a zero-bias state and can produce an output signal proportional to the incident light intensity. According to the abovementioned features, a change in the signal light can be detected under different lighting conditions. TINA-TI was used for circuit simulation (Fig.3), and a circuit prototype (Fig.5) was constructed to verify the performance of the detection module. The control program is written to a 32 bit microcontroller to realize integrated functions, such as the output of control signals, signal acquisition, and information transmission. The designed printed circuit board was placed in the light-exit window of a laser scanning projector (Fig. 9). When the scanning point is within the high-reflection area of the target, the CDS module can stably detect the reflected signal. According to this feature, a scanning method based on the bisection principle was proposed (Fig.13). This can improve the positioning speed and accuracy of the scanning projection systems for target detection. The theoretical error of this scanning method was analyzed, and a comparison experiment between the grid-scanning method and the proposed method was conducted. The grid-scanning method can be used to obtain detailed point-cloud data for cooperative targets. The Canny operator and triangulation algorithm were implemented in MATLAB to extract the edges of the targets. These measurement results were adopted as benchmarks, and the same cooperative targets were measured using the proposed method under the same conditions. Finally, the number of scanned points and positioning deviations of the two methods were compared.
The sampling circuit designed for this study is capable of withstanding the power ripple influence of 60 mVp-p (Fig.4). Furthermore, although the signal-to-interference ratio (RSI) of -29.5 dB was calculated (Tables 1,2), the CDS detection module can stably output the signal. In this study, the scanning positioning error resulting from the perspective projection relation was determined as less than 1/10 of the galvanometer resolution. This suggests that the theoretical accuracy of laser scanning positioning is sufficiently high. Compared with the 10000 scanning points of the grid scanning method (Fig.18), the number of scanning points in the proposed approach is decreased by 97.4%. Moreover, the positioning deviation of the cooperative target is less than 0.06 mm (Tables 5,6). The new scanning method minimizes the irrelevant scanning regions and eliminates the image calculation process, thereby reducing the need for computational resources.
In this study, a novel method for detecting cooperative targets using CDS is proposed, which can achieve reliable detection even in the presence of ambient light interference (RSI=-29.5 dB). In addition, a discontinuous scanning method based on the bisection principle is introduced and verified. The results show that with the reduction of 97.4% in the number of scanning points, the deviation of the proposed method is better than 0.06 mm and simplifies the arithmetic process. Applying this technique to the developed laser scan projection system can improve the anti-interference performance during calibration. In addition, the proposed method can reduce time consumption and ensure position accuracy.
Optical vortex lasers, with good beam quality in the mid-infrared spectral region, has many interesting applications such as super-resolution molecular absorption microscopy and molecular spectroscopy. The optical parametric oscillator (OPO) has been established as the most direct method to change the wavelength and transition the orbital angular momentum (OAM) of an optical vortex pump beam. A single-idler resonant cavity can produce a high-quality mid-infrared vortex output. However, one of the main challenges has been to manage the transfer of OAM from the pump beam to the mid-infrared idler output, especially given the significant wavelength difference—over three times—between the 1 μm pump and 3.5 μm idler beam. This discrepancy complicates achieving high spatial overlap efficiency between the pump and idler vortex modes in the optical vortex pumped idler-resonant parametric oscillator. By choosing cavity mirrors with the correct radius of curvature, a half-symmetric OPO system can facilitate the transfer of the pump beam's OAM to the idler output, ultimately producing a high-quality mid-infrared vortex beam.
In the paper, the idler single resonant optical vortex parametric oscillator based on KTA was examined. A conventional flash-lamped Q-switched Nd∶YAG laser (with a Gaussian spatial form, pulse duration of 25 ns, wavelength of 1.064 μm, and pulse repetition frequency of 50 Hz) was employed as the pump source. The laser output was converted into a first-order optical vortex beam using a spiral phase plate. This beam was then focused into a non-critically phase-matched KTA crystal with dimension of 5 mm×5 mm×30 mm. A plane-parallel cavity was formed using M1, which had high transmission for the pump and high reflection for the idler output beam, and an OC that had high transmission for the pump and signal beams, and a partial reflectivity (80%) for the 3.5 μm (idler) beam. A plane-concave cavity was created using a plane-concave M2 (with a curvature radius of 500 mm) that was anti-reflection coated for the pump field and high-reflection coated for the idler beam. An OC, which was partially reflective (R=80%) for the idler field and high-transmitting for the pump and signal fields, was used. The pump beam was observed using a conventional CCD camera. The spatial forms and wavefronts of the signal and idler outputs were measured with a pyroelectric camera (Spiricon Pyrocam III; with a spatial resolution of 75 μm). A lateral shear interferometer with a Mach-Zehnder geometry was used, allowing the optical vortex output to interfere with its own copy, given a proper lateral displacement.
By using an input mirror with an appropriate radius of curvature and a flat output mirror, plane-parallel and plane-concave cavities are established, respectively. This setup enables the selective transfer of the pump beam's orbital angular momentum to either the signal or idler outputs. The plane-concave cavity produces a high-quality mid-infrared vortex beam with M2 factors of 2.1 and 2.2 in the two orthogonal directions, as shown in Fig. 4. We achieve 0.82 mJ of 3.468 μm mid-infrared vortex output and 3.04 mJ of 1.535 μm near-infrared vortex output, with a maximum pump energy of 20.6 mJ. This corresponds to slope efficiencies of 28.21% and 7.62%, as depicted in Fig. 5. The transfer principle of OAM is theoretically elucidated by considering the spatial overlap efficiency between pump and idler fields in the two cavities. The spectral bandwidths (FWHM) of the signal and idler outputs are measured as Δλs=0.85 nm and Δλi=1.08 nm (Fig. 3
To produce high beam quality and high energy vortex laser in the near/mid-infrared band, an idler-resonant mid-infrared optical vortex parametric oscillator, formed by a 1 μm optical vortex pumped KTA, is constructed. We obtain 0.82 mJ of 3.468 μm mid-infrared vortex output and 3.04 mJ of 1.535 μm near-infrared vortex output at the maximum pump energy of 20.6 mJ, corresponding to a slope efficiency of 28.21% and 7.62%, respectively. With appropriate radius curvature of the cavity mirrors, the plane-concave OPO system enables the OAM of the pump beam transfer to the idler output, and it delivers high beam quality mid-infrared vortex beam. Combined with the advantages of the idler single resonant optical vortex parametric oscillator, the beam quality factors of mid-infrared idler beam in the horizontal and vertical directions are
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Editor (s): Chao Lyu, Songnian Fu, Bo Liu, Xinyuan Fang, Shanting Hu