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A sub-ppb-level nitrogen dioxide (NO2) QEPAS sensor is developed by use of a cost-effective wide stripe laser diode (LD) emitting at 450 nm and a novel background noise suppression method called scattered light modulation cancellation method (SL-MOCAM). The SL-MOCAM is a variant of modulation spectroscopy using two light sources: excitation and balance light sources. The background noise caused by the stray light of the excitation light sources can be eliminated by exposing the QEPAS spectrophone to the modulated balance light. The noise in the LD-excited QEPAS system is investigated in detail and the results shows that > ~90% background noise can be effectively eliminated by the SL-MOCAM. For NO2 detection, a 1σ detection limit of ~60 ppb is achieved for 1 s integration time and the detection limit can be improved to 0.6 ppb with an integration time of 360 s. Moreover, the SLMOCAM shows a remote working ability in the preliminary investigation.
© 2016 Optical Society of America
1. Introduction
Photoacoustic spectroscopy (PAS) is one of the most widely used spectroscopic techniques for trace gas detection in the past decades because of its advantages of high sensitivity, high selectivity and compact detection module [1,2 ]. The principle of PAS is to detect the sound waves which are generated in the media upon absorption of the modulated optical radiation [3,4 ]. Quartz-enhanced photoacoustic spectroscopy (QEPAS), an alternative approach to PAS that utilizing a quartz tuning fork (QTF) as a sharply resonant acoustic transducer instead of microphone [5–8 ], has been applied to environmental monitoring, industrial process control, and medical diagnosis [9–22 ]. Two kinds of QEPAS spectrophone configurations, on-beam [23] and off-beam [24,25 ], are usually used to improve the sensor performance. On-beam spectrophone configuration, with two acoustic micro resonators (AmRs) positioned closely on the each side of the QTF, offers a strong acoustic coupling efficiency between the QTF and AmRs, but good beam quality was required in this configuration. Off-beam spectrophone configuration and its variant T-shape spectrophone configuration [26], eliminate the stringent requirement for the beam quality by placing the QTF alongside the AmR close to a sonic output. The off-beam spectrophone configuration provides an approach to combine the QEPAS with the light sources of poor beam quality such as broadband LEDs [27–29 ] or the boomingly developed mid-infrared laser sources [30]. X. Gao et al. combined a 450 nm blue laser diode (LD) with the off-beam spectrophone configuration to detect NO2 by use of amplitude modulation (AM) technique [27]. S. Böttger et al. developed an ozone (O3) sensor based on the off-beam QEPAS spectrophone configuration and the 285 nm UV-LED, achieving a detection limit of ppm level [28]. Most recently, they developed an absorption-QEPAS sensor for biogas detection by use of mid-infrared LEDs [29]. B. Lendl et al. utilized a 7.24 µm continuous wave distributed feedback quantum cascade laser (CW DFB-QCL) for sulfur dioxide detection based on the off-beam QEPAS technique [30].
However, undesirable background noise cannot be avoided by only using the off-beam spectrophone configuration. The background noises come from the absorption of incident optical power by photoacoustic (PA) cell windows and illumination on QTF and AmRs caused by the stray light. Among them, the illumination caused by stray light is the dominant limiting factor [29]. According to Ref [30], when a nonoptimum DFB-QCL was used in the off-beam QEPAS, the measured background noise was 8.2 times higher with respect to the typical thermal noise value of the QTF, thereby limiting the detection sensitivity significantly. A practical approach for background elimination in the photoacoustic system is to measure the zero background as a baseline reference [31]. Furthermore, a custom QTF with large prong spacing was designed to avoid the background noise caused by the poor beam quality in the hot THz QEPAS sensor [9,32,33 ] and power-boost QEPAS sensor based on fiber amplifiers [10]. Another effective approach which could suppress the QEPAS background noise is the modulation cancellation method (MOCAM), which utilizes two modulated laser sources with the opposite modulation phase to balance out the background noise. The MOCAM has been used for analysis of isotopic composition [34], measurements of small temperature difference in a gas mixture [35] and detection of broadband molecular absorbers [36].
Based on the QEPAS and MOCAM, V. Spagnolo et al. seeks to develop a compact hydrazine (N2H4) sensor for National Aeronautics and Space Administration (NASA) to measure N2H4 at part per billion (ppb) concentration levels after astronauts carry out EVA tasks [36]. The developed N2H4 sensor spectrophone consists of two commercial wide stripe Fabry-Perot diode lasers emitting at ~6370 cm−1 and ~6570 cm−1 respectively, a QTF, a half-wavelength plate and a Glan polarizer. The radiation emitted by the two diode lasers was combined by the Glan polarizer and the waist of the combined laser beam was located between the QTF prongs. It should be noticed that no acoustic micro-resonator (AmR) was used in this QEPAS spectrophone, thus reducing the probed optical path to the QTF thickness of ~0.3 mm. In fact, it is still a big challenge to use a Glan polarizer to combine two quasi laser beams with big divergence angles and then focus the combined beam through the AmR.
In this work, a scattered light modulation cancellation method (SL-MOCAM) was developed for background noise suppression in QEPAS based on an off-beam spectrophone configuration. Compared with the conventional MOCAM, the combination of two laser beams is no longer required in the SL-MOCAM. Instead, the balance light was operated in scattering mode, directly illuminating the QTF surface. For its practical implementation, a nitrogen dioxide (NO2) QEPAS sensor was developed by use of two wide stripe laser diodes (LDs), emitting at 450 nm and 635 nm, respectively. Nitrogen dioxide is one of the most prevalent air pollutants causing photochemical smog and acid rain. For the human beings, continuous exposure to even low NO2 levels can cause respiratory health hazards. Therefore, our motivation is to develop a robust, cost-effective and ppb-level NO2 QEPAS sensor by use of the proposed SL-MOCAM.
2. SL-MOCAM and experimental setup
The modulation cancellation method (MOCAM), based on the differential principle [37,38 ], is a variant of modulation spectroscopy using two light sources: the excitation and balance light sources. The basic concept of MOCAM is that the intensity and phase of the two light sources are adjusted to balance out the background noise which would otherwise limit the accuracy or interfere with the measurements. Of the two light sources, one is centered on the target absorption feature, while the other source is located at the absorption band wing. The two light sources are modulated in opposite phase so that no signal is detected in the absence of absorbing gas. Figure 1 shows the emitting wavelengths of the used LDs and the NO2 absorption cross sections in the visible region. According to the HITRAN database [39], NO2 exhibits a broadband absorption from 300 nm to 650 nm with a maximum cross section of 7.4 × 10−19 cm2/molecular at 414nm. However the photochemical dissociation occurs below 415 nm [40], lowering the photoacoustic signal intensity. Therefore a 450 nm blue LD with a typical spectra linewidth (full width at half maximum) of ~4 nm (ORSAM PLTB450) was selected as the excitation light source for NO2 photoacoustic detection. The cross section at 450 nm is ~4.7 × 10−19 cm2/molecular. A 635 nm red LD (Mitsubishi ML520G71) was selected as the balance light source. The cross section at 635 nm is ~9.8 × 10−21 cm2/molecular which is ~50 times lower than that at 450 nm. The used LDs are multimode sources, emitting a stripe shape with a beam divergence θ┴ ~25 degree, θǁ ~15 degree.
The schematic of the QEPAS spectrophone based on SL-MOCAM is depicted in Fig. 2 . The QEPAS spectrophone was assembled in the off-beam configuration. The used AmR was made of a stainless steel tube with the length of 5.24 mm and inner diameter of 0.9 mm. The intensity of the 450 nm blue LD was sinusoidally modulated at a frequency f 0, where f 0 is the resonance frequency of the QTF. The light beam from the blue LD was focused to pass through the AmR by use of a lens (Daheng Optics, focal length = 30 mm, not shown in Fig. 2) with a beam waist of ~300 µm in diameter. However, due to the large divergence of a multi-mode LD, it is quite difficult to focus the excitation light beam to pass through the AmR without any touching, especially with the AmR which is absent in conventional MOCAM. However there is a significant improvement in the proposed SL-MOCAM, the QEPAS spectrophone is just exposed in the scattered balance light without any optical alignment. In the SL-MOCAM, the 635 nm red LD acted as the balance light source was used to illumine the QTF surface intentionally. The red LD was operated in scattering mode to cover the QTF and AmR with an azimuth , as shown in Fig. 2. It is known that the QTF can be used as a high sensitive radiation detector and the force F generated on the QTF surface can be expressed as [41]:
where c and P represents the light velocity and the incident optical power, respectively; is a factor that when the incident radiations are reflected by the QTF surface the factor Φ becomes 2 and when they are absorbed by the QTF surface it is equal to 1. Due to the piezoelectric property of the QTF, the force F generated by the radiation could be transferred into output current, thus contributing to the background noise. The distance d between the red LD and the QTF was ~10 cm and the azimuth was equal to 120 degree in the SL-MOCAM setup. The modulation amplitudes of the two LDs were carefully adjusted to balance out the background noise when the acoustic detection module (ADM) was filled with pure N2.
Fig. 2 (a) Schematic of scattered light modulation cancellation method. QTF: quartz tuning fork, AmR: acoustic micro resonator. (b) Photo of the SL-MOCAM QEPAS set up.
The schematic diagram of the experimental setup was depicted in the Fig. 3 . A dual-channel function generator (Tektronix AFG3102) was employed to generate modulation signals to the LD driver boards. The modulation amplitude, frequency, and phase of the two signals could be controlled independently. The used LD driver board (Wavelength Electronics LDTC2/2) could generate injection current from 0 to 1A for the two LDs. The QEPAS spectrophone was assembled in off-beam configuration, as shown in the Fig. 2. The output of the QEPAS spectrophone was firstly processed by a custom transimpedance pre-amplifier with a feedback resistance of 10 MΩ. And then the signal was sent to the lock-in amplifier (Stanford Research SR830 DSP) for 1f demodulation. A personal computer running with Labview program was used to record the data via a NI data acquisition card (National Instrument PCI-6251).
Fig. 3 The schematic diagram of the experimental setup. DAQ: data acquisition card, ADM: acoustic detection module.
3. Results and discussion
3.1 Noise analysis
The background noise in the QEPAS can be sorted into four types: the electromagnetic noise, gas flow noise, stray light noise and QTF thermal noise. The electromagnetic noise, from the electronic devices especially the modulation source, must be shielded by enclosing the QTF and the transimpedance pre-amplifier with metal housing. This is of significant importance because the QTF itself is a high sensitive detector for electromagnetic radiation [41]. With an excitation power of dozens of mW for traditional QEPAS sensors, no excessive flow noise was generated when the gas flow is below 200 sccm [42]. However, a background offset voltage resulting from the gas flow was observed in high-power QEPAS system when the gas flow rate > 35sccm [13,43 ]. Therefore the gas flow in the SL-MOCAM QEPAS system was set to 30 sccm. Among the three noises, the stray light noise is dominant, which can be orders of magnitude higher than the other two noises [13]. Therefore the SL-MOCAM is employed to mainly remove the stray light noise. Figure 4 shows the background noise measured in the SL-MOCAM QEPAS system. The blue LD with an output optical power of 4.8 mW measured after the ADM, was modulated with the amplitude of 195 mV and the offset of 206 mV. The obtained background noise level is 37.8 µV with a standard deviation (SD) of 2.2 µV. When the SL-MOCAM was activated, the red LD was switched on, and modulated with the amplitude of 71 mV and the offset of 81 mV. The modulation frequency of the blue and red LDs were equal to 32738 Hz, while the modulation phase shift between the two LDs was 178 degree. The SL-MOCAM reduced the background noise level by 91.3% from 37.8µV to 3.3 µV, with the SD of 2.2 µV. The background noise of SL-MOCAM is comparable with the QTF thermal noise with a noise level of 1.17µV and SD of 2.3µV. The QTF thermal noise was measured in the pure N2, with the two LDs turned off. The 91.3% reduction of background noise indicates that the SL-MOCAM is effective for the background offset suppression, but the SL-MOCAM cannot be used for thermal QTF noise suppression.
Fig. 4 The background noise before and after SL-MOCAM. Black squares: QTF thermal noise, red rounds: background noise, blue triangles: SL-MOCAM noise.
3.2 Remote working ability investigation of the SL-MOCAM
The advantage of SL-MOCAM is that the balance-out effect of the background noise is not dependent on the position of the balance light source. Therefore the distance d between the ADM and the balance light source was changed to evaluate the balance-out effect over a long distance. The distance d in Fig. 2 was changed from 10 to 50 cm. The background noise was measured in pure N2 with the blue LD modulated with the modulation amplitude of 40 mV and the offset of 50 mV. The measured background noise level is 13 µV. With the red LD activated with the modulation amplitude of 28.5 mV and the offset of 38.5 mV, the background noise level of the QEPAS sensor can be reduced from 13 µV to 2 µV in the case of d = 10 cm, as shown in the Fig. 5 . Subsequently the d was increased to 50 cm to evaluate the remote working ability of the SL-MOCAM. Due to the decrease of the optical power per unit area on the QTF surface, the output power of the red LD must be increased. In order to keep the noise level at ~2 µV, the modulated amplitude of the red LD was increased from 28.5 mV to 33.5 mV. The phase shift between the blue and red LDs was 176 degree. As shown in the Fig. 5, the noise measured at d = 10 cm and 50 cm are comparable with a QTF thermal noise. It indicates that the SL-MOCAM has a potential to be operated for remote working, suppressing the background noise, such as the QTF stand-off detection [44].
3.3 Sensor performance evaluation
The linearity and detection sensitivity of the SL-MOCAM-based QEPAS sensor (d = 10 cm) were evaluated by measuring its response to varying NO2 concentrations with a flow of 30 sccm. A gas dilution system (Beijing Sevenstar Electronics) was employed to generate different NO2 concentrations. The measurements were carried out at room temperature ~25°Cand atmospheric pressure. The results shown in Fig. 6(a) were processed with the lock-in amplifier with a time constant of 1 s and a filter slope of 12 dB, corresponding to a 0.25 Hz detection bandwidth. In the case of the 883 ppb NO2 measurement, the signal-to-noise ratio (SNR) is 14 and the calculated 1σ detection limit is ~60 ppb. With the absorption cross sections of ~4.67 × 10−19 cm2/molecular weighted by the LD emission spectrum [27], a 1σ normalized noise equivalent absorption coefficient (NNEA) of 7.1 × 10−9 W∙cm−1∙Hz -1/2 was achieved. Same data averaged as a function of the calibration of the standard gas generator are plotted in Fig. 6(b). A linear fitting is carried out and the obtained R square value of 0.9988 confirms the linearity of the sensor response to concentration.
Fig. 6 (a) QEPAS signal acquired repetitively while the NO2 concentration was varied by changing of the carrier gas flow using a gas dilution system. (b) Same data averaged and plotted as a function of the calibration of the gas dilution system.
To assess the long-term stability of the QEPAS sensor based on the SL-MOCAM, the QEPAS ADM was filled with pure N2 with a flow rate of 30 sccm. An Allan deviation analysis was carried out as depicted in Fig. 7 . The white noise remains the dominant noise source for 360 s. It reached the ultimate detection limit of 0.6 ppb. After that, the instrumental drift started dominating.
Table 1 shows the comparison of three NO2 QEPAS sensors by use of SL-MOCAM and other techniques. Although the NNEA of SL-MOCAM is slightly higher than other two techniques, the SL-MOCAM shows the better long-term stability. The SL-MOCAM obtains an ultimate sub-ppb-level detection limit and it can be further enhanced by improving the power of the excitation LD or constructed the spectrophone with on-beam configuration.
Table 1. Intercomparison of Three Kinds of NO2 QEPAS Sensor
4. Conclusions
A sub-ppb level NO2 QEPAS sensor is developed by use of a cost-effective wide stripe blue LD. A scattered light modulation cancellation method, i.e. SL-MOCAM, is proposed to suppress the strong background noise caused by the poor beam quality of the LD. Compared with the conventional MOCAM, the SL-MOCAM eliminates the requirement of light beam combination by simply exposing the QEPAS ADM to the scattered balance light. The background noise in the LD -excited QEPAS system can be reduced by ~90%. A 1σ detection limit of ~60 ppb is obtained at 1 s integration time, corresponding to a NNEA of 7.1 × 10−9 W∙cm−1∙Hz-1/2. With the integration time of 360 s, a detection limit of 0.6 ppb is achieved. Due to the advantage of the SLMOCAM, the balance-out effect is independent to the position of the balance light source. The remote working ability of the SL-MOCAM was preliminarily investigated in the experiment, showing the possibility that the SL-MOCAM can be used in the QTF stand-off spectroscopy for remote sensing where the undesirable background noise is present. Compared with the other NO2 detection techniques such as photochemistry and differential optical absorption spectroscopy (DOAS) [45], the SL-MOCAM NO2 QEPAS sensors have the advantages in in situ monitoring due to its compact sizes and the ability of localized measurement. However in the regional atmospheric air monitoring, multiple gaseous pollutants and aerosols in the atmosphere need to be monitored simultaneously. Hence, further improvement can be made by combining the compact QEPAS sensor with other spectroscopic techniques such as DOAS, Fourier Transform Infrared Spectroscopy (FTIR), and Raman Light Detection and Ranging (Lidar) etc. to form an integrated spatiotemporal monitoring system [46,47 ].
Acknowledgments
Lei Dong acknowledges support by the 973 program (Grant No. 2012CB921603), the National Natural Science Foundation of China (NSFC) (Grant #s. 61575113, 61275213, 61475093, 61378047 and 61205216).
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