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Abstract
We present a fiber-optic photoacoustic (PA) sensor for remote monitoring of gas micro-leakage. The gas sensing head is a miniature ferrule-top PA cavity with a cantilever beam. Gas diffuses into the cavity from the gap around the cantilever beam, and a small hole opens on the side wall. The volume of the optimized PA cavity is only 70 μL. An erbium-doped fiber amplified laser is used as a light source of acoustic excitation. The PA pressure signal is obtained by measuring the deflection of the cantilever beam with a fiber-optic white-light interferometric readout. The experimental result of leaking acetylene (C2H2) gas measurement shows a real-time response of 11.2 s. A detection limit is achieved to be 20 ppb with a 1 s lock-in integration time and a 1 km conductive fiber. Since both the excitation light and probe light are transmitted by the optical fiber, the designed sensing system has the advantages of remote detection and intrinsic safety.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Gas leakage monitoring is important in the places of gas station, gas pipeline and chemical plant [1–3]. Escaped flammable, explosive or toxic gases may cause major safety accidents. Micro-leakage monitoring can provide early warning of the accidents.
Various kinds of gas sensors based on electrochemistry [4], multi-walled carbon nanotube [5], infrared absorption spectrum [6] and laser spectroscopy [7–9] have been developed for gas leakage monitoring. Among them, the laser spectroscopy sensor is one of the most promising sensors owing to its advantages of high selectivity, high sensitivity and remote monitoring [10–13]. Because the infrared spectral region is the characteristic absorption spectral band of some gas molecules, the concentrations of these gases can be measured by using corresponding light sources. Tunable diode laser absorption spectroscopy (TDLAS) and laser photoacoustic (PA) spectroscopy (PAS) are the two most common mechanisms for trace gas detection using this principle. Due to the narrow line width of the laser, the cross-interference caused by the overlap between the absorption spectrums of gas molecules can be greatly eliminated. TDLAS based stand-off sensors have been developed for gas leakage in the open-path environments [14–16]. However, the detection distance is usually less than 50 m. Due to the advantages of low transmission loss and intrinsic safety, TDLAS based fiber-optic sensors have been designed for remote monitoring of gas leakage [17–19]. Since the gas detection sensitivity is usually proportional to the length of the absorption path, the minimum detection limit (MDL) of the sensor with a miniature probe is limited to about several tens of parts-per-million.
Laser PAS has advantages of high sensitivity, fast response and small sampling volume [7,20–24]. In recent years, a variety of all-optical PA spectrometers have been proposed. With both the excitation light and the probe light being transmitted by optical fibers, the designed gas sensing systems have the advantages of remote detection and intrinsic safety. In our previous work, highly sensitive fiber-optic Fabry-Perot (F-P) acoustic sensors were designed to detect the PA pressure [25–29]. Combined with the resonant PA cell, the fiber-optic F-P acoustic sensor based PAS can achieve part-per-billion-level acetylene (C2H2) gas detection. Moreover, it shows the potential for remote monitoring. However, due to the large volume of the PA cell, the response time is very long without a gas sampling pump. Cao et al. presented a fiber-tip PA sensor [30]. The designed sensor head has a miniature fiber-tip air cavity with a deflectable polymer diaphragm. The air cavity acts as both a PA cell and an F-P cavity. A single fiber is used to transmit excitation and signal light. A MDL of 4.3 ppm has been achieved for trace C2H2 detection. However, a tunable filter is used to separate signal light, which makes the PA signal demodulation system complicated. Gruca et al. subsequently demonstrated a miniature PA sensing head obtained by carving a micromachined flexural pressure transducer directly on top of a glass ferrule [31]. The ferrule is connected to two fibers: one for the laser excitation of the gas and the other for the interferometric readout of the transducer. The MDL for C2H2 gas detection is achieved to 300 ppb with a 1 s integration time. To further improve the gas detection sensitivity, Zhou et al. reported a new all-optical detecting sensor based on a miniaturized PA spectrometer [32]. The sensor has a cavity volume of less than 6 µL. The PA signal is detected by a cantilever transducer, which is equipped with a fiber-optic interferometric readout. The cantilever transducer works by the movement of the micromirror, which is connected to the free hanging end of the cantilever beam with a width of 14 µm. Viscous drag loss can be greatly reduced by this cantilever design. The designed sensor reaches a MDL of 15 ppb with a 0.3 s integration time for C2H2 detection. However, this fragile cantilever structure is difficult to be used for field leak monitoring. Moreover, none of the above all-optical PA sensors have been tested for remote monitoring.
In this paper, a fiber-optic PA sensor for remote monitoring of gas micro-leakage is demonstrated. The gas sensing head is a miniature ferrule-top PA cavity with a cantilever beam. Gas diffuses into the air cavity from the gap around the cantilever beam and a small hole opened on the side wall. The PA pressure signal is obtained by measuring the deflection of the cantilever beam with a fiber-optic white-light interferometric readout. The optimization of the PA sensing head is analyzed. Moreover, the designed PA sensor has been tested for remote detection of C2H2 by using a 1 km double-core conductive fiber.
2. Sensing system design
2.1 Sensing head design
The fiber-optic PA sensing head is a miniature PA cavity with a cantilever beam. Figure 1(a) shows the schematic structure of the gas sensing head. The gas sensing head consists of two optical fibers, two ceramic ferrules, a stainless steel shell and a cantilever diaphragm. The optical fiber is fixed by inserting the uncoated bare fiber into the slender hole of the ceramic ferrule. The outer diameter of the ceramic ferrule is 2.5 mm. The PA excitation light is launched into the PA cavity via the upper ceramic ferrule. In order to accelerate gas exchange, a small hole with a diameter of ~0.5 mm is opened on the side wall. Gas can diffuse into the air cavity from the gap around the cantilever beam and the small hole. Figure 1(b) shows the schematic diagram of the cantilever diaphragm, which is fabricated by a laser marker (YLP-F10, Han's Laser). The thickness, length and width of the cantilever beam are 10 μm, 3 mm and 1 mm, respectively. The gap between the cantilever beam and the frame is ~100 μm. The air cavity between the endface of the lower fiber and the inner surface of the cantilever beam constitutes an extrinsic F-P cavity [34]. The generated PA pressure wave will deform the cantilever beam. As a result, the F-P cavity length varies with the PA pressure.
Fig. 1 (a) Schematic structure of the fiber-optic PA sensing head. (b) Schematic diagram of the cantilever diaphragm.
The amplitude of the generated PA pressure in an unsealed non-resonant PA cell can be expressed as [33]:
where is the absorption coefficient of the target gas, γ is the specific heat ratio, P0 is the power of the PA excitation light source, l is the length of the PA cavity, V is the volume of the cavity, f is the frequency, is the thermal damping time, and is the time constant of the damping effect caused by gas and heat flows. and can be calculated by: where r is the radius of the PA cavity, DT is the thermal diffusivity of the sample gas, Ag is the area of the gap and hole for gas diffusion, and is the sound speed of the sample gas.According to Eqs. (1)-(3), in the case of l = 10 mm and Ag = 1 mm3, the amplitude of the PA pressure as a function of frequency is calculated with the inner radius of a PA cavity being 0.5 mm, 1 mm, 2 mm, 3 mm and 4 mm, as shown in Fig. 2(a). It indicates that the amplitude decreases as the frequency decreased in the low-frequency range. This is due to gas and heat leakages resulting in a decrease of PA pressure. In addition, the amplitude of the PA pressure as a function of the inner radius of a PA cavity is calculated with the frequencies of 100 Hz, 200 Hz, 300 Hz, 500 Hz, 1000 Hz and 1500 Hz, as shown in Fig. 2(b). It indicates that the inner radius should be around 0.4 mm at a frequency near 1000 Hz to achieve a large amplitude. However, a cavity with such a small inner radius greatly increases the difficulty of machining. In addition, the Gaussian spatial distribution of the emission light from the end face of the lead-in fiber should be considered. The mode-field radius of the emission light can be expressed as:
where z is the distance from the end face of the fiber, r0 is the mode-field radius in the fiber, and is the wavelength of the laser light. For a 1532 nm wavelength, the mode-field radius is calculated to be 1.06 mm with a distance of 10 mm. Therefore, the radius of the PA cavity is determined to be 1 mm.Fig. 2 (a) Calculated amplitude of the PA pressure as a function of frequency. (b) Calculated amplitude of the PA pressure as a function of the inner radius of a non-resonant PA cavity.
The amplitude-frequency response of the fiber-optic cantilever microphone to a sinusoidal external force can be expressed as [33]:
where m is the effective mass of the cantilever beam, D is the damping constant, f0 is the resonant frequency of the cantilever. and f0 can be calculated by:where k is the effective spring constant of the cantilever, E is the Young’s modulus, p is the gas pressure, mc is the mass of the cantilever beam, is the density of the sample gas, Ac is the area of the cantilever beam, lc, wc and tc are the length, width and thickness of the cantilever beam, respectively.The amplitude-frequency response of the PA sensing system can be expressed as:
According to Eqs. (1)-(3) and Eqs. (5)-(7), the calculated amplitude-frequency response of the PA system is plotted in Fig. 3(a). It shows that different cavity volumes have a large effect on the resonant frequency and peak response of the cantilever. The peak amplitude of the PA response as a function of the PA cavity length is shown in Fig. 3(b). It indicates that a large PA cavity volume helps to improve gas detection sensitivity.
Fig. 3 (a) Calculated amplitude-frequency response of the PA system with different PA cavity lengths. (b) The peak amplitude of the PA response as a function of the PA cavity length.
Although the longer PA cavity length can improve the PA response, it will increase the response time. In order to study the relationship between the response time of fiber-optic PA sensor and the volume of air cavity, the gas diffusion process was simulated by the fluid simulation software of ANSYS Fluent. Figure 4(a) shows the simulated gas diffusion concentration cloud map. Figure 4(b) shows the simulated average gas concentration curve in the PA cavity with different PA cavity lengths. In order to achieve a 10% to 90% step response time in 10 s, the PA cavity length is chosen to be 16 mm. Considering that the inner radius of the PA cavity is 1 mm, the calculated volume of the air cavity is only 70 μL.
Fig. 4 (a) Simulated gas diffusion concentration cloud map. (b) Simulated average gas concentration curve in the PA cavity with different PA cavity length.
The excitation light beam is reflected multiple times in the PA cavity. The reflectivity of the stainless steel and ceramic pin surfaces are ~70% and ~90%, respectively. Regardless of the Gaussian divergence of light, the effective absorption length, which is defined as the length of the optical path when the light is attenuated to half, is 48 mm. A very small fraction of the laser energy is absorbed by the target gas in the PA cavity. Other laser energy is mainly absorbed by stainless steel.
To detect the PA pressure with high sensitively and high stability, a fast demodulated white-light interferometry (WLI) is used [35,36]. The cavity length of the low-finesse F-P interferometer (FPI) is absolutely measured by a high-speed demodulation method utilizing full spectrum. Figure 5 shows the interference spectrum of the fiber-optic cantilever acoustic transducer measured by a high-speed near-infrared spectrometer (FBGA Analyzer, BaySpec Inc.) with an integration time of 65 μs. The static cavity length is demodulated to be 354.02 μm at room temperature. The sound pressure signal is recovered from the alternating current (AC) component of the fast demodulated cavity length.
2.2 Experimental setup
The test experiment of remote gas micro-leakage monitoring has been performed by using a 1 km double-core single-mode fiber. Figure 6 shows the schematic structure of the test setup. To generate second-harmonic wavelength modulation spectroscopy (2f-WMS) signal, a distributed feedback (DFB) laser is modulated by a combined signal which is summed by a sawtooth wave and a sine wave. The sawtooth wave and the sine wave are supplied by a signal generator (DG4102, RIGOL) and a direct digital synthesizer (DDS), respectively. To avoid interference from moisture absorption, the central wavelength of the DFB laser is chosen to be 1532.831 nm [24,28]. The power of the wavelength-modulated laser is amplified to 100 mW by a miniature erbium-doped fiber amplifier (EDFA) module (EDFA-BA-20-M, Max-ray photonics Ltd.). The high-power PA excitation light is then launched into the designed PA sensing head, which is placed in a gas chamber with a volume of ~150 mL. To demodulate the cavity length in time, a WLI based demodulator is designed. A superluminescent light diode (SLD) (SLD-76-LP, Superlum) is used as the probe light source, with a central wavelength near 1550 nm and a spectral width of ~60 nm. The emitting probe light is launched into a broadband fiber optical circulator, and then propagates to the cantilever beam. The reflected light from the FPI based cantilever acoustic transducer is detected by the high-speed spectrometer. The sampling of the spectrum is synchronously triggered by an external square wave signal which is phase locked to the output signal of a DDS by a phase locked loop (PLL). The acquired interference spectrum is first processed by the high-speed WLI demodulation algorithm through a computer program. Finally, the demodulated PA signal is further processed by a LabVIEW based virtual lock-in amplifier [29]. C2H2/N2 gas mixtures of different concentrations are flowed into the PA cavity continuously with a constant flow rate of 50 sccm. The gas mixing system mainly consists of a bottle of pure N2 gas, a bottle of 100 ppm C2H2/N2 gas mixture and two mass flow controllers (MFCs) (D07-19, SevenStar Electronics). A variety of gas concentrations are obtained by controlling the flow rate ratio of the two MFCs.
3. Experimental results and discussion
3.1 Performance of the fiber-optic cantilever acoustic transducer
The frequency response characteristics of the fiber-optic cantilever acoustic transducer were measured by an acoustic test setup [36]. By adjusting the frequency of the loudspeaker from 100 Hz to 1800 Hz, the root-mean-square (RMS) of the variation of the F-P cavity length was recorded. The sensitivity was calculated by the ratio of the RMS to the calibrated sound pressure, as shown in Fig. 7(a). The sensitivity is highest at the resonant frequency of 1132 Hz. When the frequency is lower than 400 Hz, the sensitivity is significantly reduced. This is mainly caused by the small hole opened on the side wall and the gap around the cantilever beam. This can result in gas leakage between the inside and outside of the air chamber. The effect on the frequency response of the cantilever beam is equivalent to a high-pass filter [34,37]. Figure 7(b) shows the sound pressure response of the designed cantilever acoustic transducer at 1132 Hz, when the sound pressure increases from 0 Pa to 0.5 Pa. The calculated R-square value of 0.9998 proves the linear response of the designed microphone to sound pressure. In addition, the sound pressure responsivity is estimated to be 3629.9 nm/Pa at 1132 Hz by linear fitting.
Fig. 7 (a) Frequency response of the fiber-optic cantilever acoustic transducer. (b) Sound pressure response of the fiber-optic cantilever acoustic transducer at the frequency of 1132 Hz.
3.2 Monitoring of gas micro-leakage
In order to optimize the operating frequency, the frequency response of the designed fiber-optic PA sensor was measured. In this experiment, the gas chamber was filled with 100 ppm C2H2/N2 gas mixture. Since the laser wavelength is dependent on both temperature and injection current, the DFB laser was thermostatically controlled by a PID controller (MAX1978, Maxim integrated) and the bias current was adjusted by the signal generator. The wavelength of the DFB laser was locked at 1532.831 nm, which is equal to the central wavelength of one of the C2H2 absorption lines. The sinusoidal modulation frequency of the laser was scanned from 100 Hz to 800 Hz, and the 2f-WMS signal was recorded to measure the frequency response from 200 Hz to 1600 Hz, as shown in Fig. 8(a). The maximum response value corresponds to a frequency of 1132 Hz, which is equal to the resonant frequency of the cantilever beam. To increase the amplitude of the PA pressure signal, the sinusoidal modulation frequency of the laser is set to 566 Hz in the following experiment. Differing from Fig. 7(a), the frequency response in Fig. 8(a) has no attenuation at frequencies below 400 Hz. This is due to the much higher PA response in the low frequency region when the PA cell is working in the non-resonant mode. The response to different laser excitation light power was also measured at the modulation frequency of 566 Hz. By adjusting the output power of the EDFA from 20 mW to 100 mW, the 2f output of the lock-in amplifier was recorded, as shown in Fig. 8(b). It indicates that the PA signal increases linearly with the excitation light power. In the following experiment, the output power of the EDFA was set to 100 mW.
Fig. 8 (a) Frequency response of the fiber-optic PA sensor. (b) Response to different laser excitation light power at the modulation frequency of 566 Hz.
To verify the linear concentration response of the designed fiber-optic PA sensor, different concentrations of C2H2/N2 gas mixture flowed into the gas chamber. The concentrations of the gas mixture were adjusted to 10 ppm, 20 ppm, 40 ppm, 60 ppm, 80 ppm and 100 ppm, successively. By increasing the bias drive current of the DFB laser from 45 mA to 70 mA, 2f-WMS spectrums were measured around the wavelength of 1532.831 nm, as shown in Fig. 9(a). The output powers of the DFB laser are 5.2 mW and 9.1 mW at the driving current of 45 mA and 70 mA, respectively. In addition, the corresponding output powers amplified by the EDFA are 99.4 mW and 100.3 mW, respectively. The maximum peak value of the measured spectrum as a function of concentration is plotted in Fig. 9(b). Each point represents the average value of the peaks of the second harmonic signal. In addition, error bars represent one standard deviation with 1 s lock-in integration time. The responsivity of the designed fiber-optic PA sensor can be estimated to be 146.6 pm/ppm for trace C2H2 detection by linear fitting. The calculated R-square value, which represents the approximate degree of regression line to the actual data point, is equal to 0.9997. It indicates that the designed PA sensor has an excellent linear response to C2H2 concentration of less than 100 ppm.
Fig. 9 (a) 2f-WMS spectrum with different concentrations of C2H2/N2 gas mixture. (b) Peak value as a function of concentration. Error bars represent one standard deviation with 1 s lock-in integration time.
The ability of continuous measurement of gas flow was validated. Different concentrations of C2H2/N2 gas mixture were sequentially injected to the gas chamber. Figure 10 shows the outputs of PA demodulator with a lock-in integration time of 0.5 s and a flow rate of 50 sccm. The signal fluctuations in the flat portion of Fig. 10 are primarily due to air vibrations caused by airflow near the sensor and ambient sound noise coupled into the gas chamber. The response time of 11.2 s was estimated by using the 10% to 90% step response corresponding to the rising edge of a concentration change. The measured response time is longer than the result of the simulation analysis, this is because the gas takes time to fill the chamber. In addition, actual sensor probe parameters deviate from design parameters.
In order to estimate the MDL of the designed PA sensor, the outputs of the demodulator were recorded by filling the gas chamber with a 300 ppb C2H2/N2 mixture and pure N2 gas. The 300 ppb C2H2/N2 mixture is obtained by controlling the flow rate of a 3 ppm C2H2/N2 standard gas and the high purity N2 gas. During this experiment, both the DFB laser and the EDFA module were turned on. In addition, the wavelength of the DFB laser was locked at 1532.831 nm. Figure 11(a) shows the output of the PA demodulator with the integration time of the virtual lock-in amplifier being set to 1 s. The deviation of the measured value of the 300 ppb C2H2/N2 mixture is calculated to be 3.2 pm. According to the responsivity of 146.6 pm/ppm, the MDL (1σ) is calculated as 20 ppb for a measurement time of 1 s. The corresponding minimum detectable absorption coefficient and the normalized noise equivalent absorption (NNEA) coefficient can be calculated to be 5.2 × 10−10 cm−1 and 2.3 × 10−9 W·cm−1·Hz-1/2, respectively. To evaluate long-term stability and the sensitivity of the PA sensor, an Allan-Werle deviation analysis [21,36,38] was implemented by continuously measuring a C2H2/N2 mixture with a concentration of 300 ppb over a period of 1500 s. Figure 11(b) shows the Allan-Werle analysis result for detection of the 300 ppb C2H2/N2 mixture. The deviation curve almost follows a dependence, which indicates that the PA system is dominated by white noise within the test time of 1500 s. From Fig. 11(b), the deviation is 0.36 pm with an averaging time of 100 s. Accordingly, the corresponding MDL (1σ) is calculated as 2.5 ppb.
Fig. 11 (a) Output of the PA demodulator with a lock-in integration time of 1 s when the chamber was filled with a 300 ppb C2H2/N2 mixture and pure N2 gas. (b) Allan-Werle deviation for detection of the 300 ppb C2H2/N2 mixture as a function of the data averaging time.
4. Conclusion
In conclusion, we have proposed and developed a fiber-optic PA sensor for remote monitoring of gas micro-leakage. The gas sensing head is a miniature ferrule-top PA cavity with a cantilever beam. Gas diffuses into the cavity from the gap around the cantilever beam and a small hole opened on the side wall. The inner radius of the PA cavity is 1 mm, and the optimized length is chosen to be 16 mm. The volume of the air cavity is only 70 μL. A fiber amplified laser with a central wavelength of 1532.831 nm is used as a light source of acoustic excitation. Meanwhile, the PA pressure signal is obtained by measuring the deflection of the cantilever beam with a fiber-optic white-light interferometric readout. Both the PA excitation laser light and the PA detection probe light are conducted by the optical fiber. Therefore, the fiber-optic PA sensor has the advantages of remote detection and intrinsic safety. The sound pressure responsivity of the cantilever acoustic transducer is measured to be 3629.9 nm/Pa at the resonant frequency of 1132 Hz. A simulated remote gas micro-leakage monitoring experiment has been carried out by using a 1 km double-core single-mode fiber. Experimental results demonstrated that the designed PA sensor has an excellent linearity response to C2H2 concentrations less than 100 ppm. The responsivity is estimated to be 146.6 pm/ppm. The ability of continuous measurement of gas flow has been validated. The response time is estimated to be 11.2 s by using the 10% to 90% step response. In addition, the MDL is achieved to be 20 ppb with a 1 s lock-in integration time. The minimum detectable absorption coefficient and the NNEA coefficient are calculated as 5.2 × 10−10 cm−1 and 2.3 × 10−9 W•cm−1•Hz-1/2, respectively. With this fiber-optic PA sensing method, remote monitoring of various gases such as NH3, H2S, C2H4 and CH4 can be achieved by replacing the laser source. For practical applications, the effects of ambient sound noise and vibration interference need to be considered. The active noise reduction technique could be used by adding an additional cantilever beam to detect ambient noise and vibration.
Funding
Fundamental Research Funds for the Central Universities (DUT18RC(4)040, DUT 15RC(3)112); Doctoral Starting Foundation of Liaoning Province (201601040).
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