In this paper, a hollow-core anti-resonant fiber (HC-ARF) based light-induced thermoelastic spectroscopy (LITES) sensor is reported. A custom-made silica-based HC-ARF with length of 75 cm was used as light medium and gas cell. Compared to a traditional multi-pass cell (MPC), the using of HC-ARF is advantageous for reducing the sensor size and easing the optical alignment. A quartz tuning fork (QTF) with a resonant frequency of 32766.20 Hz and quality factor of 12364.20 was adopted as the thermoelastic detector. Acetylene (C2H2) and carbon monoxide (CO) with absorption lines located at 6534.37 cm−1 (1530.37 nm) and 6380.30 cm−1 (1567.32 nm) were chosen as the target gas to verify such HC-ARF based LITES sensor performance. It was found that this HC-ARF based LITES sensor exhibits excellent linearity response to the analyte concentrations. The minimum detection limit (MDL) for C2H2 and CO detections were measured as 4.75 ppm and 1704 ppm, respectively. The MDL for such HC-ARF based LITES sensor can be further improved by using a HC-ARF with long length or choosing an absorption line with strong strength.
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Trace gas sensing is widely used in various fields such as environmental monitoring, manned spaceflight, life sciences and combustion diagnosis [1–6]. Compared with non-optical techniques of electrochemical and semiconductor sensors, optical sensing methods have the advantages of fast response, online measurement, high selectivity and sensitivity, and so on [7–11].
Photoacoustic spectroscopy (PAS) is an extensively adopted technique in optical gas sensing fields due to its merits of zero background and wide dynamic range [12,13]. In 2002, quartz-enhanced photoacoustic spectroscopy (QEPAS), an improvement to the traditional microphone based PAS, was firstly reported . In QEPAS, a quartz tuning fork (QTF) is used as an acoustic wave transducer. Due to the high quality factor, narrow response frequency band and tiny size of QTF [15–18], QEPAS is recognized as an advanced optical gas sensing technique with the advantages of excellent detection performance, strong noise suppression capability and compact volume [19–22]. However, QTF should be immersed in the target gas, which means that QEPAS is a contact measurement method. This feature limit its application in corrosive or acidic gases measurement because the performance of resonance characteristic and quality factor of QTF will be degraded when the QTF exposures to the such environment [23–25].
QEPAS technique not only has the limitation on its range of application in gas species, but also does not allow for remote and standoff trace gas detection. In 2018, light-induced thermoelastic spectroscopy (LITES) was firstly reported to effectively solve the above issues . In LITES, after passing through the target gas, a laser beam is irradiated on the surface of QTF and the laser energy is absorbed by the quartz crystal. The QTF produce a thermoelastic deformation after being heated by the laser absorption [27,28]. Due to the laser modulation the QTF form a periodic mechanical vibration, and this vibration is enhanced because of the resonant characteristic of QTF. On the basis of piezoelectric effect, an electrical signal is generated from the mechanical vibration of QTF, which is used for retrieving concentration of the target gas. In LITES, a QTF can be placed far from the analyte [29,30]. Therefore, LITES can be a non-contact measurement method and used for remote trace gas detection [31,32]. Due to these merits, LITES technique has been widely adopted in various trace gases detection [33–35].
In LITES technique, in order to improve the laser absorption, a multi-pass cell (MPC) is usually used [36,37], where the optical path length is extended to tens meters. However, in that situation, the LITES sensor is bulky due to the large size of a MPC and a large number of optical elements needed for laser beam alignment. Furthermore, plenty of used optical components reduce the system stability of the sensor. Hollow-core anti-resonant fiber (HC-ARF) has become popular in the field of sensing recently. On the one hand, HC-ARF has significant advantages in nearly single-mode transmission, low optical loss and wide spectral range. In addition, its design that the ring of silica capillaries around a hollow core can effectively suppress the mode interference between the core mode and other cladding modes . On the other hand, it typically contains a tiny hollow core in the central region, where propagates light modes and can confine the gas sample within the hollow core simultaneously . In the hollow core, the gas samples could interact with the light perfectly. Compared to a MPC, a HC-ARF with a low optical transmission loss provides merits of reducing the size and easing the optical alignment for an optical sensor.
In this paper, a compact LITES sensor based on HC-ARF was reported. A custom-made silica-based HC-ARF was used as light medium and gas cell. A QTF with a resonant frequency f0 of 32.768 kHz was adopted. Acetylene (C2H2) and carbon monoxide (CO) were chosen as the target gas to verify such HC-ARF based LITES sensor performance. Two diode lasers with emission wavelength of 1530 nm and 1567 nm were used as the excitation source for C2H2 and CO detection, respectively.
2. Experimental setup
2.1 HC-ARF characteristics
The custom-made HC-ARF used in this research is a 75 cm long piece formed by a single layer of seven nontouching silica capillaries. The scanning electron microscope (SEM) image of its cross section is shown in Fig. 1(a). The HC-ARF has an outer diameter of 220 µm and a hollow core with inscribed inner diameter of 57 µm. The thickness of glass wall of 0.63 µm ensures its operation in the first anti-resonance band covering from 1.45 µm to at least 2.4 µm, as shown in Fig. 1(b), which is limited by the optical spectrum analyzer. This covers all the wavelength used in this experimental investigation.
2.2 HC-ARF based LITES sensor configuration
The experimental schematic of HC-ARF based LITES sensor is shown in Fig. 2. Two fiber-coupled, distributed feedback (DFB) diode lasers with emission wavelength of 1530 nm and 1567 nm were selected as an excitation source, respectively. Firstly, the output light of the diode laser was collimated by a convex lens (L1), and then the beam was transmitted to a focusing lens (L2) with a focal length of 60 mm. The laser beam was reshaped and could couple into the HC-ARF easily. The fiber was fixed in a designed gas cell, which was installed with a calcium fluoride (CaF2) window for optical access and placed on a six-dimensional translation mount for precise adjustment. Since the LITES signal is directly proportional to the absorption length, a HC-ARF with a length of 75 cm was employed to realize long absorption path. The coupling efficiency of the HC-ARF was about 81% at 1.5 µm. Two absorption lines located at 6534.37 cm−1 with line strength of 1.21×10−20 cm−1/(cm−2×mol.) and 6380.30 cm−1 with line strength of 2.18×10−23 cm−1/(cm−2×mol.) were chosen for C2H2 and CO detection, respectively. A QTF with a resonance frequency f0 of 32.768 kHz (in vacuum) was utilized as the thermoelastic detector. The length, thickness and width of the prong of QTF were 3.9 mm, 0.36 mm and 0.62 mm, respectively. Wavelength modulation spectroscopy (WMS) with second harmonic detection (2f) was adopted to suppress the background noise and simplify the data processing. A sinusoidal wave with high frequency f1 (f1 = f0/2) and a direct current ramp with low frequency f2 (f2 = 20 mHz) generated from a lock-in amplifier were added together and sent to the laser driver. After modulating, the laser beam passed through the coupling system and transmitted through the HC-ARF, where the gas interacted with the laser. Eventually, the exiting laser beam from the HC-ARF hit on the base of QTF to produce the strongest signal level  due to the light-induced thermoelastic effect. The electrical signal coming from the QTF was further sent into the lock-in amplifier for demodulation and analysis. The integration time of this lock-in amplifier was set to 200 ms. Two flow controllers were adopted to mix the target gas with pure nitrogen (N2) to generate analyte with different concentrations. A pyrocamera was used to capture the beam profiles at different position. Before entering the HC-ARF, the laser beam presents a symmetric Gaussian profile. The far-field beam profile at the output of this 75 cm HC-ARF demonstrates the LP01 fundamental mode of the transmitted laser.
3. Results and discussions
Firstly, the resonant characteristic of QTF was measured. The QTF was stimulated using an electrically excitation method. The experimental data were fitted by Lorentzian function to get the resonant frequency (f). The obtained results are depicted in Fig. 3. It could be found that the used QTF had a resonant frequency of 32766.20 Hz and the response bandwidth (△f) was 2.65 Hz. The quality factor (Q) was calculated to be 12364.20 according to the definition of Q = f/△f. The narrow △f and high Q imply that this QTF is beneficial to noise immune and energy conversion.
3.1 HC-ARF based C2H2-LITES sensing
In this research, wavelength modulation spectroscopy with second harmonic detection (2f) was adopted. Therefore, the laser wavelength modulation depth should be optimized to obtain the maximum 2f signal amplitude for this HC-ARF based LITES sensor. In the experiment, the injection current of 1530 nm DFB diode laser was adjusted from 62 mA to 102 mA to obtain the 2f signal. The output power was 20.8 mW when the laser emission wavelength reached the C2H2 absorption line at the conditions of TEC temperature of 30 °C and the injection current of 82 mA. The variation between the 2f signal amplitude and modulation depth was investigated. The result is displayed in Fig. 4. It can be seen that when the injection current for the used 1530 nm diode laser was 23.0 mA C2H2-LITES sensor had the strongest signal response. The experimentally determined current tuning coefficient for this laser was −0.0078 cm−1/mA.
The concentration response of this HC-ARF based C2H2-LITES sensor was investigated. The C2H2:N2 with concentration of 1% was diluted with pure N2 to produce different concentrations of the gas mixture. As depicted in Fig. 5(a), the 2f signal of HC-ARF based C2H2-LITES sensor was recorded when these gas mixtures were used. After linear fitted, as shown in Fig. 5(b), it can be found that the R-square for the linear fitting was 0.99, which indicated that such HC-ARF based C2H2-LITES sensor has excellent concentration response.
The noise level for the HC-ARF based C2H2-LITES sensor system was determined when the HC-ARF was filled with pure N2. The signal was recorded for 60 seconds and displayed in Fig. 6. The standard deviation (1σ) was 0.34 µV. Based on the obtained results shown in Figs. 5 and 6, a minimum detection limit (MDL) of 4.75 ppm was acquired for this HC-ARF based C2H2-LITES sensor.
3.2 HC-ARF based CO-LITES sensing
In the following research, the analyte was changed from C2H2 to CO to further investigate this HC-ARF based LITES sensor performance. For CO detection, there are three available absorption bands of the fundamental band (v) located at 4.6 µm, the first overtone band (2v) located at 2.33 µm and the second overtone band (3v) located at 1.57 µm. Although the 3v band with line strength in the magnitude of 10−23 cm−1/cm−2·mol. is very weak, the excitation source of fiber-couple diode lasers are mature. Accordingly, the 1530 nm diode laser was replaced by a 1567 nm diode laser. Firstly, the optimization of laser modulation depth was performed. The relationship between the 2f signal amplitude and modulation depth for the CO-LITES sensor is shown in Fig. 7. Different from the optimum value of 23.0 mA for C2H2-LITES sensor shown in Fig. 4, due to the different linewidth of C2H2 and CO, the optimal injection current for the selected CO absorption line was 24.5 mA. The experimentally determined current tuning coefficient for this laser was −0.0053 cm−1/mA.
The concentration response for this HC-ARF based CO-LITES sensor was performed. Similarly, the laser injection current was tuned from 60 mA to 100 mA to obtain the 2f signal. The output power was 9.5 mW when the laser emission wavelength reached the CO absorption line at the conditions of TEC temperature of 25 °C and the injection current of 80 mA. Due to the very weak absorption strength in 1.57 µm band, the concentration of CO:N2 mixture was diluted from 50%. As depicted in Fig. 8(a), the 2f signal of HC-ARF based CO-LITES sensor was measured when the CO:N2 mixture with concentration ranged from 50% to 10% was used. The measured 2f signal amplitude of CO-LITES sensor as a function of CO concentrations is depicted in Fig. 8(b). After a linear fitting procedure, the R-square was found to be 0.99, which indicates that this HC-ARF based CO-LITES sensor has an outstanding linearity response to the CO concentration.
After the used HC-ARF was filled with N2, the noise level for the HC-ARF based CO-LITES sensor system was measured. The obtained results are shown in Fig. 9. The 1σ noise of the sensor system was 0.22 µV. It is obvious that the noise measured in CO-LITES sensor system was less than that measured in C2H2 measurement due to its laser power was only half of that used in C2H2 detection, which reduced the thermal noise of the QTF. Based on the acquired results of signal and noise amplitudes shown in Figs. 8 and 9, respectively, a MDL of 1704ppm was realized for this HC-ARF based CO-LITES sensor. If a laser with emission wavelength of 2.3 µm or 4.6 µm is used to target the first overtone band or fundamental vibrational band of CO, the MDL can be improved by two to four orders of magnitude to ppb-ppt level in theory.
A compact LITES sensor based on HC-ARF was reported in this paper. A custom-made silica-based HC-ARF with length of 75 cm was used as light medium and gas cell. The gas samples interacted with the light perfectly in the central region of the HC-ARF. Compared to a traditional MPC, the HC-ARF with a low optical transmission loss is beneficial for reducing the sensor size and easing the optical alignment. A QTF with a resonant frequency of 32766.20 Hz and quality factor of 12364.20 was adopted as the thermoelastic detector. C2H2 and CO were chosen as the target gas to verify such HC-ARF based LITES sensor performance. Two diode lasers with emission wavelength of 1530 nm and 1567 nm were used as the excitation source, respectively. With experimental verifications, such HC-ARF based LITES sensor exhibits excellent linearity response to the analyte concentrations. The MDL for C2H2-LITES and CO-LITES sensors were measured as 4.75 ppm and 1704ppm, respectively. The MDL for such HC-ARF based LITES sensor can be further improved by using a HC-ARF with long length or choosing an absorption line with strong strength.
National Natural Science Foundation of China (61505041, 61875047, 62022032); Natural Science Foundation of Heilongjiang Province (YQ2019F006); Fundamental Research Funds for the Central Universities; Heilongjiang Provincial Postdoctoral Science Foundation (LBH-Q18052).
The authors declare no conflicts of interest.
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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