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Abstract
An FDM-assisted opposite two-way OA-CEAS system is reported in this paper. Compared with the traditional OA-CEAS system with one-way transmission configuration, the new system has two main advantages. One of the advantages is that four lasers can be employed for simultaneous measurements of multiple species in this system. Another advantage is the combination of the silver-coated concave spherical mirror and the narrow bandpass filter employed to realize the opposite two-way transmission in the optical cavity which can also serve as a re-injection mirror and optical enhancement gotten for free in the system. The performance of the system is demonstrated by simultaneous measurements of CO, CO2, C2H4, and CH4. This work highlights a new strategy for simultaneous detection by using four lasers in a single optical integrated cavity, which can improve the utilization rate of the optical cavity and reduce the cost for multiple gas species sensing.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Cavity enhanced absorption spectroscopy (CEAS), which is also referred to as ICOS (integrated cavity output spectroscopy), has been developed into a remarkable spectroscopy technique in the past 30 years [1–3]. Benefiting from the properties of high-finesse optical cavity which can trap photons for finite periods of time, CEAS instruments yield high sensitivity because the effective optical path length can be increased by 105 times or more compared with the base path length. Off-axis cavity enhanced absorption spectroscopy (OA-CEAS) is a development of CEAS, which was introduced by Paul et al. in 2001 [4]. In OA-CEAS system, the laser beam is introduced into the optical cavity with an off-axis geometry and passes in the manner of a Herriott type multipass absorption cell [5–7]. In this configuration, more high-order transverse modes can be excited to achieve a higher mode density with a narrower free spectral range (FSR) in the high-finesse optical cavity by choosing a suitable incident direction and position of the laser beam. Then the laser frequency can effectively always resonate with the denser mode structure of such an optical cavity [8]. So the mode matching is not needed and the intensity fluctuations owing to mode instabilities can be minimized. The main advantages of OA-CEAS including compact system configuration, simple optical alignment, and high sensitivity comparable to CEAS. So OA-CEAS is particularly suitable for development of small and robust trace gas sensors for field measurements.
Within OA-CEAS, the high-finesse optical cavity, which acts as a gas-detection cell, is the core component of the system. The high-finesse optical cavity consists of highly reflective end mirrors aligned parallel to each other and works as a Fabry–Perot etalon [9–11]. In order to achieve an extremely long optical path length, dielectric-coated mirrors with high reflectivity must be used to construct the optical cavity. The low-loss dielectric-coated mirrors with a high reflectivity (>99.99%) in near- and mid-infrared region are very expensive and not easy to get. In traditional OA-CEAS technique, the coupling and detection of the laser beam in and out the optical cavity is in a one-way transmission configuration [12–15]. Due to the limitation of the cavity mirror size, only a single laser can be employed for gas sensing in most of the existing OA-CEAS sensors. Obviously, utilization rate of the optical cavity is limited in this configuration and then constraint the utilization of the OA-CEAS technique in the field of simultaneous multi-gas detection which need employing multiple lasers.
Of course, if division multiplexing techniques are combined with OA-CEAS technique, more than one laser also can be employed in an OA-CEAS sensor. Recently, a few works have reported the development of this kind of sensors by the combination of division multiplexing and OA-CEAS techniques [16–17]. For example, Zheng et al. developed an OA-CEAS sensor for simultaneous measurements of C2H2 and CH4 employing two near-infrared diode lasers by frequency division multiplexing (FDM) [16]. The beams of the two lasers operating at the wavelengths of 1532 nm and 1653 nm were combined by a fiber combiner and then coupled into the optical cavity off-axis. The transmitted light from the optical cavity which contains two wavelength bands were acquired by a detector, and the detector signal was sent to two lock-in amplifiers to isolate the second harmonic signals of the target C2H2 and CH4 absorption lines according to responding modulation frequency. Profiting from the combination of the OA-CEAS and FDM-based wavelength modulation spectroscopy (WMS), both C2H2 and CH4 can be detected simultaneously at ppb level. Although FDM can isolate different signals with different modulation frequencies, it requires that the modulation frequencies of the employed lasers must be separated enough to suppress the cross-talk between the different harmonic signals [18–19]. Therefore, it is usually hard to employ more than two lasers in FDM due to the limitation of the available lock-in amplifiers. Besides FDM, time division multiplexing (TDM) has also been used for development of the OA-CEAS sensor. Wang et al. developed a dual-laser OA-CEAS sensor based on TDM-assisted direct absorption spectroscopy technology [17]. The beam of two diode lasers were simultaneous coupled into the optical cavity by a home-made dual-channel microcollimator. The two lasers at 1603 nm and 1651 nm were alternately scanned to pitch across the sample gas and each transmitted laser beam was caught and sent to one detector to record the direct absorption signals of CO2 and CH4 in responding period. The main virtue of the TDM-assisted OA-CEAS sensor is that it allows for a relatively simple optical construction [20–21]. But the TDM scheme would limit the bandwidth of the OA-CEAS sensor since each laser can only work in its own periods. Besides that, the transmitted laser beams in the TDM scheme are not acquired at exactly the same time. This drawback would add extra measurement uncertainties when the TDM-assisted OA-CEAS sensor is applied in rapidly fluctuating flow fields. Through a literatures investigation we find that no more than two lasers can be employed in the one-way transmission configuration of the traditional OA-CEAS.
In order to improve the utilization rate of the optical cavity, we have proposed an opposite two-way transmission scheme which can be used to couple the laser beams into the optical cavity from both ends [22]. An opposite two-way OA-CEAS system was developed based on the transmission scheme and two DFB diode lasers near 2.0 µm and 2.3 µm were employed to detect CO2 and CH4 simultaneously. In the present work, we will develop a FDM-assisted WMS based opposite two-way OA-CEAS system. In order to demonstrate the improvement of the utilization rate of the optical cavity, four diode lasers in the near-infrared region will be employed in the system. CO, CO2, C2H4, and CH4 will be detected simultaneously to validate the performance of the system for multiple gas species detection.
2. Development of the sensor
2.1 Target lines selection
Proper target lines can not only improve the system sensitivity but also reduce the development and maintenance costs of the absorption spectroscopic instruments, especially for the OA-CEAS system which the cavity mirror only can have high reflectivity in a relatively narrow wavelength band. Because the relative weak absorption can be enhanced by the long effective path length, the lines in the near-infrared region can be employed in the OA-CEAS system. Here a pair of cavity mirrors which effective reflectivity is bigger than 99.99% in the wavelength of 1550-1660 nm was used to form the integrated cavity, so as to reduce the cost of the system. In this work, the optimum target lines were selected by using a series of selection criteria including the position of the target lines, the adequate line strength, and the interference from the nearby lines. According to the high resolution spectroscopic parameters given in HITRAN 2020 [23] and the C2H4 parameters measured by ourselves, the absorption spectra of CO, CO2, C2H4, and CH4 were investigated to find the optimum target lines for this work. Finally, the four lines listed in Table 1 were selected as the target probe lines.
Table 1. Spectral parameters of the four selected lines at 300 K
The spectral simulations of the four selected target probe lines along with the nearby lines of the main interference gas molecules in the atmosphere are depicted in Fig. 1. The simulations were performed at a temperature of 300 K, a gas pressure of 0.4 atm, and an effective optical path length of 1 km. It can be seen from the simulations that all selected lines locate in the expected wavelength region and have sufficient absorption strength to guarantee the sensitivity of the measurement. The simulations also show that there are not serious overlap or interference from other lines at the simulated temperature and pressure conditions which will also be used in the measurement. The tuning characterization of each DFB diode laser measured by wavelength tuning experiment is also illustrated in the responding graph. As shown in the figure, complete absorption spectra for all of the selected four target probe lines can be acquired when the employed DFB diode lasers are operated at the normal temperature and current range.
Fig. 1. Simulated spectra of CO, CO2, C2H4, and CH4 near the selected regions in comparison with H2O and CO2. The circle dots represent the tuning characterization of each DFB diode laser.
2.2 Sensor configuration
The optical architecture and the ray tracing simulation by TracePro of the opposite two-way OA-CEAS system is presented in Fig. 2. In the simulation, only a small amount of the rays is exhibited for the sake of clarity. The high-finesse optical cavity, which is the core of the CEAS system, is consisted of two highly reflective mirrors (R≈99.995%) with the diameter and radius of curvature is 25.4 mm and 1 m, respectively. An optical cage system using four rigid steel rods is employed to mount the cavity mirrors for stabilization, and the region with a length of 20 cm between the two mirrors constitutes the sample volume. Two 2-inch silver-coated concave spherical mirrors (Thorlabs, R > 97%), with a 4 mm diameter hole located at 2.7 mm off the center, are located at ∼15 cm outside the cavity and are tilted at ∼3° with respect to the cavity axis. All of these mirrors are mounted in a sealed box as a sample chamber for gas measurement. The incident light beam is firstly collimated by an adjustable aspheric collimator which mounted on the end wall of the chamber, then the collimated beam passes through the hole on the concave spherical mirror and inject into the optical cavity. The adjustable aspheric collimator can provide adequate adjustment for injection angle needed by the off-axis light. The output from the cavity is reflected by the concave spherical mirror at the opposite side and focused onto an InGaAs detector. A narrow bandpass filter is added in front of the detector, so as to remove the light of the lasers from the other side. In order to ensure the beam injection to meet the requirement of the off-axis alignment approach, all the positions and sizes of each element in the architecture are determined by theoretical optimization and careful optical design.
Fig. 2. The optical architecture and the ray tracing simulation by TracePro of the opposite two-way OA-CEAS system.
Figure 3 is a schematic of the FDM-assisted opposite two-way OA-CEAS based sensor. In this system, four lasers at 1.567 µm, 1.571 µm, 1.620 µm, and 1.653 µm are employed to exploit the four selected target lines of CO, CO2, C2H4, and CH4, respectively. Each laser is placed in a custom made mount and driven by a commercial laser driver (ILX Lightwave LDC-3724C). A triangle ramp and a sine wave generated from two function generators are added by an electrical adder and sent to the laser driver to scan the frequency of each laser and provide wavelength modulation. As illustrated in Fig. 3, the outputs of every two lasers, which are modulated at different frequencies, are multiplexed together by using a standard single-mode fiber combiner. The combined beam goes through the adjustable aspheric collimator mounted on the chamber wall and passes through the hole in the concave spherical mirror, then injects into the high-finesse optical cavity in an off-axis way. The light leaking out from the cavity is reflected by the silver-coated concave spherical mirror at the other side and focused onto an InGaAs detector (Thorlabs, APD410C/M). The acquired signals are sent to digital lock-in amplifiers to demodulate the 2f harmonic (WMS-2f) signals at the specific modulation frequencies for the four selected target lines. An acquisition system consisting of a data acquisition (DAQ) card (National Instruments, USB-6356) and a laptop is used to acquire the WMS-2f signals.
Fig. 3. The schematic of the FDM-assisted opposite two-way OA-CEAS based sensor that can employ as much as four lasers for simultaneous multi-species detection.
3. Optimization of the sensor
3.1 Cross-talk elimination
It is known that the signal acquired by the single detector must be sent to different lock-in amplifiers to demodulate the harmonic components at the specific modulation frequencies in the FDM strategy, which similar to how an FM radio operates to separate individual channels. In order to prevent the crosstalk between channels, the modulation frequency for each laser must be chosen cautiously to eliminate aliasing and harmonic interference. Here the selection of the optimum modulation frequencies is mainly based on two aspects. Firstly, interference between different WMS-2f signals must be minimum. Secondly, the WMS-2f signal demodulated at the frequency must have a high signal to noise ratio (SNR). So a series of tests was done at different scanning frequencies, modulation frequencies, and modulation frequency separations for the four lasers employed in the system. Figure 4 shows the measured WMS-2f signals with different modulation frequency separations from 0 kHz to 9 kHz with a step of 3 kHz. It can be seen from the figure that a separation of 6 kHz is enough to allow sufficient suppression of the cross-talk harmonics. Then some measurements of the WMS-2f signals were performed to determine the scanning frequency and modulation frequency for each laser while the modulation frequency separation was fixed at 6 kHz. Finally, the optimum combination of the scanning frequency and modulation frequency listed in Table 2 was chosen for each laser.
Fig. 4. The WMS-2f signals of the four selected lines acquired at different modulation frequency separations from 0 kHz to 9 kHz with a step of 3 kHz.
Table 2. The scanning and modulation frequency of the four lasers employed in the FDM-assisted opposite two-way OA-CEAS system
3.2 Modulation depth optimization
Due to the WMS-2f signal can be affected by the modulation depth (a), an optimum modulation depth (aopt) should be selected for each laser to maximize the amplitude of WMS-2f signal and yield the highest SNR. Here a gas mixture gas (150 ppm CO, 400 ppm CO2, 150 ppm C2H4, and 2 ppm CH4 in pure N2, P = 0.4 atm) prepared by a gas-mixing instrument (Environics, N-4000) was filled into the sample chamber and the WMS-2f signals of the four selected target lines were recorded at different modulation depth which can be varied by adjusting the modulation voltage applied to the lasers. The WMS-2f signal amplitudes of the four target species as a function of the modulation depth are shown in Fig. 5. The optimum modulation depth and the corresponding modulation voltage is shown in the figure for each laser. According to this figure, there is a linear relationship between the modulation voltage applied to each diode laser and the modulation depth. The half width of each target line can be calculated using the parameters listed in HITRAN 2020 database and it can be find that each selected optimum modulation depth occurs around the proper modulation index m (∼2.2) for wavelength modulation with 2nd harmonic demodulation [24], which confirms the validity of the modulation depth optimization. In the following experiment, the selected optimum modulation depth will be applied for all the measurements.
Fig. 5. Variation of the measured WMS-2f signal amplitudes of the four target species with modulation depth, adjusted by setting different modulation voltages for each laser.
4. Results and discussions
4.1 Influence of the FDM-assisted opposite two-way configuration on WMS-2f signal
In our FDM-assisted opposite two-way OA-CEAS system, four diode lasers can be employed for simultaneous multi-species detection. In order to evaluate the influence of the FDM-assisted opposite two-way configuration on WMS-2f signals for each species, a series of measurements were performed in the same gas mixture at 300 K and 0.4 atm. The measured signals of the four species with only one laser on and with four lasers on are compared in Fig. 6. For example, Fig. 6(d) shows the comparison of measured WMS-2f signals for CH4 at a concentration of ∼2 ppm. It can be seen from the figure that there is only a trivial distinction in the waveforms and peak heights of the WMS-2f signals before and after the other three lasers were activated. The deviation in the WMS-2f peak heights is only 1.09%. There is also a little difference in the shoulders and far-wings of the WMS-2f signals, but it does not have obvious influence on the calculation of the magnitudes of the WMS-2f signals. The comparison of the measured WMS-2f signals with one laser on and with four lasers on for CO, CO2, and C2H4 are shown in Fig. 6 (a), (b), and (c), respectively. Similarly, there is only a slight difference in the waveforms and peak heights between the two WMS-2f signals. All the deviation in the WMS-2f peak heights is no more than 2%. All information discussed above indicates that there is no obvious influence in the WMS-2f signal measurements and no immense sensor performance degradation associated with the employment of the FDM-assisted opposite two-way configuration.
Fig. 6. Comparison of the measured WMS-2f signals with one laser on and with four lasers on for the four target gas species.
4.2 Optical enhancement
It is well known that a re-injection mirror can be added into the traditional one-way OA-CEAS system to achieve optical enhancement by injecting the laser power reflected from the first cavity mirror back into the cavity [25–26]. In the experiment, we found that the magnitude of measured WMS-2f signal by the opposite two-way OA-CEAS system were bigger than that measured in the traditional OA-CEAS system. In order to identify the enhancement of the signals, the WMS-2f measurements of the four target species were performed for a same gas mixture by using our opposite two-way OA-CEAS system and the traditional OA-CEAS system, respectively. The measured signals are compared in Fig. 7, where the solid lines represent the signals acquired by the opposite two-way OA-CEAS system and the dashed lines represent the signals acquired by the traditional OA-CEAS system. It can be seen from the figure that there is an approximately 1.3-1.7 times improvement in magnitude and SNR for the WMS-2f signals acquired by the FDM-assisted opposite two-way OA-CEAS system. According to our analysis, the enhancement comes from the combination of the silver-coated concave spherical mirrors and the narrow bandpass filter which can serve as re-injection mirror to enhance the transmission power of the cavity in our opposite two-way configuration. It can be seen from the ray trace simulation visualized in Fig. 2 that the spherical mirror and the filter can re-inject part of the light reflected from the input high-reflectivity mirror back into the cavity. In traditional OA-CEAS system with re-injection mirror mounted parallel to the cavity mirror, the optical enhancement factor is related to some parameters of the re-injection mirror, such as the mirror reflectivity, the radius of curvature, and the reinjection cavity length. It has been experimentally verified that the signal magnitude and SNR can be enhanced 4-22.5 times in the re-injection model by optimizing these parameters [26–28]. Because the main purpose of silver-coated concave spherical mirrors and the narrow bandpass filters is to realize the opposite two-way transmission in the optical cavity, the silver-coated concave spherical mirrors are tilted at ∼3° with respect to the cavity axis to detect the output from the cavity in our opposite two-way configuration. Owing to the angle between the spherical mirror and high-reflectivity mirror, the enhancement factor is smaller than that in the traditional OA-CEAS system with re-injection mirror mounted parallel to the cavity mirror, but the enhancement of the WMS-2f signals can also contribute to the improvement of the system performance.
Fig. 7. Comparison of the measured WMS-2f signal by the opposite two-way OA-CEAS system and the traditional OA-CEAS system.
4.3 Linearity and accuracy
Based on above optimization and evaluation, the FDM-assisted opposite two-way OA-CEAS system were first calibrated before applied for quantitative measurement of the target four species. A series of WMS-2f measurements was performed for different CO-CO2-C2H4-CH4-N2 mixtures prepared by the gas-mixing instrument. Due to the linear relationship between the peak height of the WMS-2f signal and the gas concentration, four fitting linear equations which can be used to infer the target gas concentrations from the measured WMS-2f signal for a random CO-CO2-C2H4-CH4-N2 mixture were obtained.
In order to evaluate the linearity and accuracy of the system, eight different CO-CO2-C2H4-CH4-N2 mixtures were generated by the gas-mixing instrument and were directed into the gas chamber in turn for measurement. The representative WMS-2f signals measured at each concentration are shown in the inserted graphs in Fig. 8. For each representative mixture, 10 groups of WMS-2f signals were measured for each species and each signal were averaged 100 times to reduce the noise. The measured concentration for each mixture were inferred from the average of the 10 signals.
Fig. 8. Linearity of the measured species concentrations and the WMS-2f signal amplitudes by the FDM-assisted opposite two-way OA-CEAS system versus the nominal concentrations. Inserted graphs: the representative WMS-2f signals measured at different concentrations for the four target species.
The relationship between the species concentration and the amplitude of the measured WMS-2f signal (blue circle) are displayed in Fig. 8 indicating the excellent linearity of the developed systems. For detailed analysis, the measured species concentration and the nominal one are also compared in this figure. As shown in the figure, the measured values agree well with the nominal concentrations for the four target species over the entire concentration range. The average deviation between the two values is less than 3% for each species. According to the linear fitted slopes of the scatter plot of the measured concentration (black block) which is marked in each graph, the accuracy of the concentration measurements are 0.82%, 0.47%, 1.21%, and 0.27%, for CO, CO2, C2H4, and CH4, respectively. The linear relationship with R2 values of 0.997, 0.998, 0.997, and 0.998 between the measured and nominal concentrations indicate the excellent linearity of the sensor for concentration measurements of the four target species.
4.4 Precision, stability, and sensitivity
To test the performance of the system about the measurement precision and the stability for simultaneous detection of the four target species, time series measurements were carried for a CH4- CO-C2H2-CO2-N2 mixture (∼108 ppm CO, ∼388 ppm CO2, ∼103 ppm C2H4, and ∼1.8 ppm CH4, P = 0.4 atm) in 1000 seconds. In the continuous measurements, each data point was averaged by 100 scans and 1000 data points were obtained. All the 1000 concentration points are shown in the upper panels of Fig. 9 to illustrate the stability of the measurement. Histogram plots of the measured data, which depict approximate Gaussian distributions, are shown in each bottom panel of Fig. 9 and can be used to evaluate the measurement precision. The measured mean concentrations for the four target species, the half width at half maximum (HWHM) and the R2 value of each Gaussian profile, are shown in responding graph, respectively. According to the HWHM of each histogram plot, the measurement precisions are 0.5 ppm, 0.41 ppm, 0.78 ppm, and 0.94 ppb, for CO, CO2, C2H4, and CH4, respectively.
Fig. 9. Continuous measurements of (a) CO, (b) CO2, (c) C2H4, and (d) CH4 in 1000 s along with the histogram plots obtained from the time-series data.
The stability of the FDM-assisted opposite two-way OA-CEAS system was further evaluated by Allan analysis. The Allan-Werle deviation technique was used to analyze the above time series data and the analysis results for the four target species are shown in Fig. 10. As can be seen from the figure, the optimum averaging time is better than 100 s for each target species, indicating the fine stability of the system. The minimum detectable concentration for each species with different typical signal averaging times are depicted in the corresponding graph. With the averaging time of 1 s, the detection limit is 0.48 ppm, 0.5 ppm, 0.8 ppm, and 6 ppb, for CO, CO2, C2H4, and CH4, respectively. When the signal averaging time is increased to 100 s, the detectable concentration can be reduced to 4.9 ppb, 1.4 ppb, 12 ppb, and 0.037 ppb for the four target species. The Allan analysis confirms that the excellent stability and detection sensitivity can be achieved in the FDM-assisted opposite two-way OA-CEAS system which can employ four lasers for simultaneous measurements of multi-species in a single optical integrated cavity.
Fig. 10. Allan-Werle deviation plots as a function of integration time for (a) CO, (b) CO2, (c) C2H4, and (d) CH4.
5. Conclusion
In this work, we proposed and developed a FDM-assisted opposite two-way OA-CEAS system which can employ as much as four lasers for simultaneous multi-species detection. Because the combination of the silver-coated concave spherical mirror and the narrow bandpass filter employed in the opposite two-way configuration also can act as re-injection mirror to re-inject part of the light reflected from the adjacent cavity mirror back into the cavity, optical enhancement is got for free in the system and the magnitude of the measured WMS-2f signal and signal-to-noise ratio have an approximately 1.5 times improvement. In order to validate the practicability of the FDM-assisted opposite two-way OA-CEAS system, CO, CO2, C2H4, and CH4 were selected as the target species to be detected simultaneously by using four diode lasers at 1.567 µm, 1.571 µm, 1.620 µm, and 1.653 µm, respectively. The system was optimized by eliminating the cross-talk and selecting the optimum modulation depth at first, and then a series of measurements was performed to evaluate the accuracy, linearity, precision, and stability of the system. All the measurement results validate the potential of the system for simultaneous multiple species measurements by four lasers in a single optical integrated cavity. The reported FDM-assisted opposite two-way OA-CEAS system is particularly useful in some applications that multiple species need to be detected, such as atmospheric and environmental monitoring, industrial process and chemical reaction control, combustion process diagnosis, and so on.
Funding
National Natural Science Foundation of China (42275136, 62275110, 61875079, 61805110, 61475068, 11104237); Science and Technology Support Program of Jiangsu Province (BE2022314, BE2021634).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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