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Abstract
A high-sensitivity photoacoustic (PA) spectroscopy (PAS) system is proposed for dual enhancement from both PA signal excitation and detection by employing a miniaturized Herriott cell and a fiber-optic microphone (FOM). The length of the optical absorption path of the PA cell is optimized to ∼374 mm with 17 reflections. The volume of the PA cell is only 622 µL. The FOM is a low-finesse fiber-optic Fabry-Pérot (FP) interferometer. The two reflectors of the FP cavity are formed by a fiber endface and a circular titanium diaphragm with a radius of 4.5 mm and a thickness of 3 µm. A fast demodulated white-light interferometer (WLI) is utilized to measure the absolute FP cavity length. The acoustic responsivity of the FOM reaches 126.6 nm/Pa. Several representative PA signals of trace acetylene (C2H2) are detected to evaluate the performance of the trace gas detector in the near-infrared region. Experimental results show that the minimum detectable pressure (MDP) of the FOM is 3.8 µPa/Hz1/2 at 110 Hz. The noise equivalent minimum detection concentration is measured to be 8.4 ppb with an integration time of 100 s. The normalized noise equivalent absorption (NNEA) coefficient is calculated as 1.4×10−9 cm−1·W·Hz−1/2.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Among all the gas detection methods, photoacoustic (PA) spectroscopy (PAS) is famous for its high sensitivity, high selectivity and in situ measurements [1–3]. The gas molecule absorbs photons of the incident light, resulting in the excitation of the energy level. Typically, radiation processes and collisional relaxation are two main ways to release the absorbed energy. However, the long radiative lifetimes of vibrational levels suppress the effects of radiative emissions. Consequently, the excited state loses its energy mainly by collisional relaxation. Thus, the absorbed energy is mainly converted to the kinetic energy of the gas molecules, causing gas vibration and PA pressure wave. The amplitude of the generated PA pressure is proportional to the light intensity and the gas concentration [4,5].
The minimum detection limit (MDL) is one of the most important parameters to evaluate the performance of a PA gas detection system. Over the past few decades, researchers have generally improved the MDL in two main aspects: enhancing the PA pressure signal and optimizing the performance of sensitive microphones. Quantum cascade laser (QCL), interband cascade laser (ICL), and optical parametric oscillator (OPO) have been used in the PAS system [6–8]. This is because the mid-infrared QCL, ICL and OPO operate at the fingerprint region of the gas molecular, where the fundamental vibrations of many gas molecules are located. However, the prices of mid-infrared laser sources are too high in comparison with near-infrared lasers. Placing the gas chamber in the laser intracavity can make full use of the intracavity laser power and thereby the PA signal can be enhanced [9–12]. The gas detection sensitivity can also be improved by employing multi-pass cells with the advantage of low cost. Some typical types of multi-pass cells, such as White cell and Herriott cell, have been developed as alternatives to single-pass cell in a PAS system [13–16]. Concave mirrors and planar mirrors are employed to change the direction of the light propagation to increase the length of the optical path. In order to improve the sensitivity of PAS system, Han, et al. placed two cylindrical concave mirrors in opposite positions to form a PA cavity [17]. The distance between the two mirrors was set to be 160 mm. The beam passed the optimized cell 18 times, which increased the total light path to 2.88 m. The conventional Herriott cell and White cell with long length and large radius have long absorption path, but greatly increase the volume of the gas chamber. For many applications, especially in dissolved gas analysis of transformer and natural fire monitoring in coal mine goaf, the amount of the sample gas is very small (usually less than 10 mL). Besides, a finer cross-section size of a PAS cell makes the PA signal stronger [4]. To optimize the performance of PA cell, the sensitive microphone has been a hot topic in recent years. Conventionally, electric microphones such as MEMS microphones and condenser microphones are widely applied for PA gas detection systems because of their wide bandwidth and excellent stability. However, electromagnetic interference (EMI) restricts the performance of these electrical microphones in a strong EMI environment. In pursuit of an alternative to electric microphones, researchers have been focused on the study of interferometric fiber-optic microphones (FOMs), including Fabry-Pérot (FP) interferometer (FPI) [18], Michelson interferometer (MI) [19], and Mach-Zehnder interferometer (MZI) [20]. Compared with MI and MZI based FOMs, fiber-optic FPI based FOM has a more compact size and therefore more compatible with the PA cell. Fiber-optic FP cantilever microphones have an excellent signal-noise-ratio (SNR) at the resonant frequency and are mostly utilized in a resonant PA cell [21–23]. However, the length and the volume of a conventional resonant PA cell are much larger than that of a non-resonant PA cell. In a quartz-enhanced PAS (QEPAS) system, a short resonant tube is often used to match the high operating frequency of the quartz tuning fork. However, it is very difficult for a QEPAS system to detect gases (such as CO and CO2) with a low V-T relaxation rate. In addition, in the fiber-optic cantilever-enhanced PAS (CEPAS) system, a gap is inevitable between the cantilever beam and the bracket, suppressing the performance of the microphone at low operating frequencies. To solve this problem, researchers have intensively studied the low-frequency FOM for PAS gas detection systems in recent years. Gong, et al. developed a PAS system using a low-frequency FPI-based FOM [24]. The FOM with a nanothick-level Parylene-C diaphragm has a high SNR at the frequency of 30 Hz and has been used in a PAS multi-gases detection system. However, an intensity demodulation method is utilized to detect the PA pressure signal, and therefore the long-term stability of the system is difficult to be assured [25,26]. Moreover, a mid-infrared thermal radiation light source is employed for multi-gas detection, and the effects of cross interference increase consequently.
In this paper, a PAS system is proposed for dual enhancement from both PA signal excitation and detection by employing a miniaturized Herriott cell and a FOM. The length of the optical path of the multi-pass PA cell has been optimized. The volume of the Herriott cell is only 622 µL. A titanium diaphragm and a fiber endface form the two reflectors of the low-finesse FP cavity. Moreover, a fast demodulated white-light interferometer (WLI) is utilized to measure the absolute FP cavity length. To eliminate the effects caused by the fundamental frequency (1f) interference signal, the second harmonic wavelength modulation spectroscopy (2f-WMS) technique is utilized to measure the PA pressure signal. Several representative PA signals of trace acetylene (C2H2) have been detected to evaluate the performance of the trace gas detector in the near-infrared region.
2. PA gas detection system design
2.1 Design of the PA cell
Figure 1(a) depicts the schematic of the miniature multi-pass PA cell. The PA cavity is made of brass tube, one end of which is the spherical concave mirror and the other end is the planar mirror. The PA cavity has a diameter (D) of 6 mm and a length (L) of 22 mm. The volume of the Herriott cell is only 622 µL. The collimator is placed in the planar mirror and the laser beam is launched into the PA cell with a certain deflection angle longitudinally. With the help of optical design software TracePro, multiple reflections between the concave mirror and the planar mirror could be achieved by optimizing the focal length of the concave mirror and the deflection angle of the collimator. Figure 1(b) shows an optical path of multiple reflections between the two mirrors when the deflection angle and the radius of curvature are 1° and 50 mm, respectively. The intensity of the light gradually decreases as the optical path increases. This is mainly because the reflectivity of the mirror is set to 96%. Figures 1(c) and 1(d) show the simulation results of the illuminance map on the inner surface of the concave mirror and the planar mirror, respectively. It is obvious that the reflection point is fixed in a circle with a radius of 3 mm on both mirrors. This effect means multi-reflections only happen in the cavity. Consequently, the impacts of sidewall absorption can be effectively suppressed. After 17 reflections, the laser intensity decays to less than one-half. The total length of absorption path reaches about 374 mm. Meanwhile, the laser returns to the collimator, whose reflectivity is extremely low. After this, the laser intensity is too weak to provide considerable contributions to the excitation of the PA signal. This can also be illustrated by the several blue color lines in Fig. 1(b), which represent the light with extremely low intensity. As a result, a reflection point is vanishes in Fig. 1(d).
Fig. 1. (a) Schematic of the multi-pass PA cell. (b) The optical path of multiple reflections between these two mirrors. (c) Simulation results of the illuminance map on the inner surface of the concave mirror. (d) Simulation results of the illuminance map on the inner surface of the planar mirror.
2.2 Design of the FOM
Figures 2(a) and 2(b) show the structure diagram and the image of the extrinsic FPI based FOM, respectively. The sensor head contains an extrinsic FP cavity, which is formed by a ceramic ferrule, a titanium diaphragm, and an air micro-chamber. A small hole exists in the front end of the stainless steel bracket to balance the air pressure inside and outside of the FOM. The diaphragm is fixed on the bracket by the epoxy glue. For a diaphragm-based FOM, the responsivity of acoustic pressure and the resonant frequency can be respectively expressed as [27,28]:
2.3 Experimental setup
The schematic diagram of the high-sensitivity PA gas detector is illustrated in Fig. 3. A distributed feedback (DFB) laser was employed as the PA excitation source. An isolator integrated in the laser source protected it from being hurt by the reflection light. The collimator was placed in the planar mirror and the laser beam was launched into the PA cell with a deflection angle of 1° longitudinally. The 2f-WMS signal was generated by the DFB laser, which was modulated by a combined signal. The combined signal consisted of a sawtooth wave and a sine wave, which were provided by a signal generator (DG4102, RIGOL) and the PA demodulator, respectively. The FOM was placed in the center of the multi-pass gas cell to detect weak PA pressure and convert it to the variations of the FP cavity length. Two valves were used to control the inlet and outlet of the sample gas. In the gas detection experiments, two mass flow controllers (MFCs) (CS200, Sevenstar) were used to control the proportions of C2H2/N2 gas mixture and pure N2. The nominal flow control error of the MFC was ± 1%. In order to avoid the PA pressure signal from being attenuated by balancing the pressure inside the PA cell with the external air pressure, the two valves were turned off to seal the PA cell during the measurement. During the gas detection experiments, the FOM was sealed well in the PA cell with a wall thickness over 10 mm. Besides, the PA cell was placed in an acoustic isolation box to further isolate the environmental sound noise. The PA pressure signals were converted to be the variations of the FP cavity length and then were detected with both high sensitivity and fast speed by the designed PA demodulator, which mainly contained a superluminescent light emitting diode (SLED) (DL-CS5077, Denselight) and a near-infrared spectrometer (FBGA analyzer, BaySpec) with a maximum sampling rate up to 5 kHz. The emitting probe light, which was supplied by the SLED, propagated into the FOM. The low-fineness FP interference spectrum was detected and achieved by the FBGA. Then, the FP interference spectrum and digital signal was acquired by a computer. Finally, the demodulated digital acoustic signal was further processed by a LabVIEW based virtual lock-in amplifier [30].
A 1000 ppm C2H2/N2 gas mixture (all test gases were produced by Dalian Special Gases CO., LTD) was used as the calibration gas of the sensor. The absorption coefficients of the gas mixture in the wavelength range from 1525 nm to 1540 nm were shown in Fig. 4(a). The central wavelength of the DFB laser was selected to be 1531.588 nm, corresponding to the highest absorption coefficient of C2H2 gas molecule. As a consequence, the 2f-WMS signals could be stronger. The absorption coefficient C2H2 gas molecule at 1531.588 nm is 0.58 cm−1 [31]. The modulation of the DFB laser was optimized by adjusting the modulation current [32], as shown in Fig. 4(b). A polynomial fit curve was added to shown the trend more intuitively. The amplitude of the modulation current was selected to be 19 mA, corresponding to the highest amplitude of the 2f-WMS signal. According to the current tuning coefficient of ∼0.019 cm−1/mA, the modulation depth was calculated to be 0.36 cm−1. The modulation index was obtained to be 2.4 by dividing the modulation depth by the half width at half maximum (HWHM) of 0.15 cm−1.
Fig. 4. (a) Absorption coefficients of 1000 ppm C2H2/N2 gas mixture in the wavelength range from 1525 nm to 1540 nm. (b) Amplitude of 2f-WMS signal as a function of modulation current.
3. Experimental results and discussion
3.1 Test of the FOM
The amplitude-frequency response characteristics of the designed FOM were investigated with the help of an acoustic test setup [33]. The FOM and a condenser microphone (Type 4187, B&K) were placed side by side in an acoustic isolation box. A loudspeaker driven by a lock-in amplifier provided acoustic field to the two microphones. The root-mean-square (RMS) value of the FP cavity length variation was measured and recorded in the frequency range from 40 Hz to 510 Hz. The condenser microphone, with an acoustic responsivity of 45.7 mV/Pa, was utilized to monitor the real-time acoustic pressure. The real time sensitivity of the FOM was then calculated by the ratio of RMS values to the real-time acoustic pressure, as shown in Fig. 5. The sensitivity of the FOM decays as the frequency decreases when the frequency is lower than 100 Hz. This is mainly caused by the small hole. On one hand, the small hole on the side wall of the FOM helps gas exchanges inside and outside of the air cavity. On the other hand, the small hole is equivalent to a high-pass filter on frequency response [34,35]. The resonant frequency of the FOM was measured to be 410 Hz, which was close to the calculated value of 381 Hz in Section 2.2. The deviation between them was mainly caused by the change of the effective radius of diaphragm, which was caused by many factors, such as size error of the stainless steel bracket, and the extension of the epoxy glue. Because the resonant frequency would vary with temperature and humidity. Therefore, the operating frequency should be in the flat region of the frequency response of the FOM. As a result, the FOM can work stably at the frequency around 100 Hz.
By employing a fast demodulated WLI, the absolute FP cavity length was detected with merits of high sensitivity, fast speed, and high stability [30,33]. Figure 6(a) shows the low-fineness interference spectrum of the FOM detected by the spectrometer. The static length of the F-P cavity was calculated to be 266.44 µm by the spectrum demodulation algorithm. Figure 6(b) shows acoustic signals retrieved in time domain by fast spectrum demodulation algorithm under 10 mPa acoustic pressure when the frequency is 40 Hz, 80 Hz, 120 Hz, and 160 Hz, respectively. The FOM exhibited sensitive responses and high SNR at different acoustic frequencies.
Fig. 6. (a) Low-fineness interference spectrum of the FOM. (b) Time domain responses at different frequencies when the external acoustic pressure is 10 mPa.
3.2 Test of the PA cell
To analyze the performance of the designed PA multi-pass cell, the frequency response of the PA gas detector was measured by introducing the 1000 ppm C2H2 gas sample into the PA cell. Figure 7(a) shows the deformation value of the diaphragm as a function of operation frequency. The PA signal induced deformation values were recorded by adjusting the sinusoidal modulation frequency from 1 Hz to 250 Hz, which corresponds to an operation frequency of 2 Hz to 500 Hz. The maximum response occurs at the frequency of 8 Hz. When the operating frequency is higher than 100 Hz, the frequency response becomes relatively flat. The noise induced deformation as a function of frequency when the lock-in integration time is 1 s was depicted in Fig. 7(b). A 1/f curve was added to show the trend more intuitively. The measured noise data has a similar decrease trend with the 1/f curve. This means that the noise of the designed PA gas detection system noise is dominated by flicker noise (1/f noise) when the operating frequency is lower than 500 Hz.
Fig. 7. (a) PA signal induced deformation of the diaphragm as a function of frequency. (b) Noise induced deformation of the diaphragm as a function of frequency.
The ratio of the PA signal induced deformation value in Fig. 7(a) to the noise induced deformation value in Fig. 7(b) was calculated to further investigate the SNR of the PA gas detector, which was plotted in Fig. 8. A fifth-order polynomial fit curve was added to show the trend of SNR more intuitively. As shown in Fig. 8, the SNR is the highest in the neighborhood with a frequency of 110 Hz. Therefore, in the following experiments, the operating frequency of the system was fixed at 110 Hz, corresponding to the sinusoidal modulation frequency of 55 Hz. This optimized frequency also coincides with the optimal operating frequency of the FOM.
3.3 Trace gas detection
To investigate the linear response characteristics of the PA gas detector, several representative concentrations of C2H2/N2 gas mixture were introduced into the PA cell and the corresponding PA signal induced deformation values of the diaphragm were measured. By controlling MFCs, the 1000 ppm C2H2/N2 gas mixture and a pure N2 were introduced into the photoacoustic cell at several specific proportions to obtain 100 ppm, 500 ppm, 1000 ppm C2H2/N2 gas mixture, respectively. In the same way, 5 ppm, 10 ppm and 50 ppm C2H2/N2 gas mixture were introduced into the PA cell by diluting a 50 ppm C2H2/N2 gas mixture with pure N2. The scanning current of the DFB laser was set in the region of 135 mA∼183 mA. The representative 2f-WMS scans were recorded at the obtained C2H2 concentrations for a 1-s data acquisition time, as shown in Fig. 9(a). The relationship between the maximum peak value of 2f-WMS signals and the tested concentrations was shown in Fig. 9(b). A linear fit was utilized to better show the trend of responsivity. Furthermore, the linear fit provides a slope, which means that the PA gas detector has a responsivity for trace C2H2 detection of 3.0 pm/ppm within 1000 ppm. The intercept represents the background noise level of the system, which is corresponding to the value in Fig. 7(b). In addition, the R-square value was estimated to be 0.9998, which verifies the linearity of the PA signal versus C2H2 concentration of less than 1000 ppm.
Fig. 9. (a) Several representative 2f-WMS scans at the obtained C2H2 concentrations. (b) PA 2f-WMS maximum peak values as a function of trace C2H2 concentrations within 1000 ppm.
The noise level was tested when 300 ppb C2H2/N2 gas mixture flowed in the PA cell over a period of 1000 s to analyze the MDL of the PA gas detector. A 3 ppm C2H2/N2 gas mixture and pure N2 were mixed with a specific proportion to obtain the 300 ppb C2H2/N2 mixture. During the measurement, the central wavelength of the DFB laser was fixed at the wavelength of 1531.588 nm. The deformation values were recorded with a lock-in integral time of 1 s. The deviation of the measured values was 0.24 pm. According to the acoustic responsivity of 126.6 nm/Pa and the equivalent noise bandwidth of 0.25 Hz, the minimum detectable pressure (MDP) of the FOM was calculated as 3.8 µPa/Hz1/2. Besides, considering that the responsivity of the PA gas detector was 3.0 pm/ppm, the ratio of deformation values to the responsivity was calculated as the detectable gas concentration, as shown in the inset figure. The MDL (1σ) was calculated to be 79.6 ppb with a 1-s lock-in integral time. In addition, by utilizing an optical power meter (MT9810B, Anritsu), the output power of the DFB laser used in the PA gas detection system was measured to be 15.1 mW. With the absorption coefficient of C2H2 gas molecule being 0.58 cm−1 at 1531.588 nm, the noise-equivalent minimum detection absorption coefficient was calculated as 4.6×10−8 cm−1. Therefore, the normalized noise equivalent absorption (NNEA) was obtained as 1.4×10−9 W•cm−1•Hz−1/2. An Allan deviation analysis was performed to investigate the stability and the sensitivity of the PA gas detector [36–38], as shown in Fig. 10. A $1/\sqrt t$ baseline was added to show the trend of the Allan analysis results. The excellent trend fit between them indicates that Gaussian noise is the dominant noise during the 1000-s test time. The Gaussian noise mainly originates from the noise of the linear array detector used for spectral sampling, brown noise of the diaphragm, laser power fluctuation and laser wavelength jitter. Furthermore, when the average time is 100 s, the deviation can be recognized as 8.4 ppb in Fig. 10.
4. Conclusion
In conclusion, we have developed and proposed a PA gas detector by employing a miniaturized Herriott-type multi-pass cell and a fiber-optic FP microphone. With the help of TracePro, the optical path of the Herriott cell has been optimized. A titanium diaphragm based FOM is utilized as an ultra-high sensitive acoustic detector and this system realizes dual enhancement from both PA signal excitation and detection. The FOM has an acoustic responsivity of 126.6 nm/Pa at 110 Hz. Trace C2H2 gas is measured to test the performance of the PA system. Experimental results show that the PA detector has a responsivity of 3.0 pm/ppm for C2H2 detection and the FOM exhibits a MDP as low as 3.8 µPa/Hz1/2 at 110 Hz. When the integration is 1 s, the MDL is 79.6 ppb, which corresponds to a noise-equivalent minimum detection absorption coefficient of 4.6×10−8 cm−1. The NNEA coefficient was calculated as 1.4×10−9 W•cm−1•Hz−1/2. Allan deviation analysis results showed that the long-term stability of this system can be assured and the MDL can be reached 8.4 ppb for an average time of 100 s. In addition, the volume of the PA cell is only 622 µL (less than 1/300 of the volume of a conventional Herriott cell and White cell). Considering the high sensitivity and excellent stability of the PA detector, the PA detector has potential to be used in dissolved gas analysis of transformer and natural fire monitoring in coal mine goaf. Other gases can be detected with merits of high sensitivity and long-term stability by just changing the DFB laser.
Funding
National Natural Science Foundation of China (61905034, 61727816); Science and Technology Project of State Grid (521205190014); Natural Science Foundation of Liaoning Province (2019-MS-054); Fundamental Research Funds for the Central Universities (DUT18RC(4)040).
Disclosures
The authors declare no conflicts of interest.
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