The article information
 Bo Fang, Nana Yang, Chunhui Wang, Weixiong Zhao, Xuezhe Xu, Yang Zhang, Weijun Zhang
 方波, 杨娜娜, 王春晖, 赵卫雄, 徐学哲, 张杨, 张为俊
 Detection of Nitric Oxide with Faraday Rotation Spectroscopy at 5.33 μm
 5.33 μm处磁旋转吸收光谱NO分子探测研究
 Chinese Journal of Chemical Physics, 2020, 33(1): 3742
 化学物理学报, 2020, 33(1): 3742
 http://dx.doi.org/10.1063/16740068/cjcp1910182

Article history
 Received on: October 21, 2019
 Accepted on: November 13, 2019
b. University of Science and Technology of China, Hefei 230026, China;
c. School of Environmental Science and Optoelectronic Technology, University of Science and Technology of China, Hefei 230026, China
Atmospheric nitric oxide (NO) is an important compound of atmospheric reactive nitrogen. It is mainly formed in the combustion process carried out by human activities [1], and plays important roles in controlling the photochemical production of ozone (O
Laser spectroscopy technique provides an attractive and powerful chemicalfree tool for NO measurement with high time resolution and good precision. A wide range of laser spectroscopy methods, such as quartzenhanced photoacoustic spectroscopy [4], differential optical absorption spectroscopy [5], tunable diode laser absorption spectroscopy [6], cavity based techniques (integrated cavity output spectroscopy [7], cavityenhanced absorption spectroscopy [8], cavity ring down spectroscopy [9]), and Faraday rotation spectroscopy (FRS) [10], have been used for NO measurement. The reported detection precision ranged from several tens of pptv (parts per trillion by volume) to several ppbv (parts per billion by volume) levels.
FRS is a spectroscopic technique that relies on the magnetooptic effect (Zeeman split) for paramagnetic species [1113]. The background signals from the absorption of diamagnetic compunds are largely suppressed [14], thus providing a useful method for high sensitive measurement of NO [10, 1434]. There are two ways to modulate the Zeeman splitting of the absorption line: (ⅰ) an alternating magnetic field (ACfield) produces varying magnetic circular birefringence; (ⅱ) a static magnetic field (DCfield) combined with wavelength modulation spectroscopy (WMS) of the laser frequency to effectively vary the magnetic circular birefringence [35]. The FRS signal is then generated from the demodulation of the modulated magnetic circular birefringence with a phasesensitive lockin amplifier. Compared with ACFRS method, DCFRS method provides an alternative FRS scheme with excellent performance. The use of permanent magnet instead of AC magnetic coil has potential to reduce the power consumption, and the low frequency noise can be reduced by using high frequency modulation (in ACFRS method, the demodulation frequency is usually limited by the resonant frequency of RLC circule) [36].
In this work, DCFRS technique was studied for NO detection at 5.33 µm (1875.81 cm
For weak absorption and small rotation angle (
$ \begin{eqnarray} P(\varphi)=\frac{P_0}{2}\left(1\cos2\varphi+R_\Delta L\sin2\varphi\right) \end{eqnarray} $  (1) 
where
$ \begin{eqnarray} F=\frac{P_0}{2}R_\Delta L\sin2\varphi \end{eqnarray} $  (2) 
$ \begin{eqnarray} R_\Delta=\frac{NS\sqrt{\ln 2}}{\pi\gamma_\textrm{D}}\sum\limits_{M'_JM''_J}(1)^{M'_JM''_J}\textrm{Re}[Z(z)] \end{eqnarray} $  (3) 
where
$ \begin{eqnarray} Z(z)&=&\frac{1}{\sqrt{\pi}}\int_{\infty}^{+\infty}\frac{\exp(t^2)}{tz}\textrm{d}t \end{eqnarray} $  (4) 
$ \begin{eqnarray} x&=&\sqrt{\ln 2}\frac{\nu\nu_{M'M''}}{\gamma_\textrm{D}} \end{eqnarray} $  (5) 
$ \begin{eqnarray} y&=&\sqrt{\ln 2}\frac{\gamma_\textrm{C}}{\gamma_\textrm{D}} \end{eqnarray} $  (6) 
where
The total noise can be expressed as the following by adding the noise term with the extinction ratio (
$ \begin{eqnarray} N_{\textrm{tot}}=\frac{P_0}{2}\left(1\cos2\varphi+P_0\xi\right) \end{eqnarray} $  (7) 
which is a function of the offset angle
$ \begin{eqnarray} N_{\textrm{tot}}(\varphi)=\sqrt{{N_0}^2+{N_1}^2(\sin^2\varphi+\xi)+{N_2}^2(\sin^2\varphi+\xi)^2}\\ \end{eqnarray} $  (8) 
Among these noise sources, the detector noise
The schematic diagram and the corresponding photograph of the experimental setup are shown in FIG. 2. A 5.33 µm room temperature continuouswave (CW) quantum cascade laser (QCL, Thorlabs), controlled by a laser diode controller (ITC4002QCL, Thorlabs), was used for probing the Faraday rotation effect via measurement of the Q
Laser wavelength scan was realized by feeding an external voltage ramp from a function generator (Agilent 33622A) to the injection laser diode current at a rate of 100 Hz. An internal reference sinusoidal signal (
The light path diagram and actual light spot pattern of the Chernin type optical multipass cell [3841] are shown in FIG. 3 (a) and (b) respectively. The cell consisted of two rectangular filed mirrors (FIG. 3(c), with dimensions of 75 mm
The static magnetic field was provided by a superconducting wires (NbTi) wrapped coil [38]. In order to maintain superconducting sate, the magnet coil was sealed in a Dewar vessel (with 1280 mm long, and 500 mm inner diameter), and was cooled to temperature below 5 K with a Hecycle cryocooler (SHI F50, Sumitomo Industries). The intensity of the magnetic field intensity was adjustable (17.9 Gauss/A) with a resolution of 2 Gauss. The maximum field intensity tested was about 1800 Gauss, which was limited by the current source. The intensity could be further increased as the excitation current increased.
Ⅳ. RESULTS AND DISCUSSION A. Optimization of magnetic field and rotation angleTo maximize the FRS signal in the experimental pressure of 100 mbar, a series of experiments were performed to determine the optimum magnetic field strength (
The linear relationship between FRS
Performance comparison between FRS and WMS was taken to depict the improvement of DCFRS, as shown in FIG. 7. Continuous time series measurement of NO with the two methods are shown in FIG. 7(a, b). The time resolution of the data was 1 s (wavelength scanning with a 100 Hz ramp, and 100 spectral averaging). Measurement fluctuations (
A histogram plot of time series depicting an approximate normal distribution around the mean value is plotted in FIG. 7(c, d), which was used to assess the measurement repeatability. A Gaussian profile was fitted to the distribution histogram, resulting in a halfwidth at halfmaximum (HWHM) of 1.71 ppbv and 8.32 ppbv, and a
The stability and precision were investigated using Allan deviation analysis, which is shown in FIG. 7(e, f). For FRS, the measurement precision was 1.15 ppbv with a 1 s data acquisition time, and was improved further to 0.43 ppbv with averaging time of 15 s. For WMS, the precision over 1 s was 3.12 ppbv, and may be improved to 1.28 ppbv in 150 s. The precision for FRS was several times better than WMS.
A comparison of the detection precision with some literature report results is shown in Table Ⅰ. Though further improvement can be made, the achieved precision in this work with DCFRS was comparable to ACFRS methods combined with laser frequency locking [10, 22], heterodyneenhanced [23], dualmodulation [34], and cavity enhanced methods [32].
Ⅴ. CONCLUSIONAn experimental study was carried out on NO detection with DCFRS method. By using a Chernin type multipass cell, a precision of 1.15 ppbv in 1 s dataacquisition time was achieved. This precision was reduced to 0.43 ppbv by increasing the sampling time to 15 s. The experimental system in this work can be further miniaturized by using a compact multipass cell and a small solenoid magnet or a permanent magnet to make it suitable for field application.
Ⅵ. ACKNOWLEDGMENTSThis work was supported by the National Key Research and Development Program of China (No.2016YFC0202205), the National Natural Science Foundation of China (No.41805104, No.41875151, and No.41627810), the Natural Science Foundation of Anhui Province (No.1508085J03), the Youth Innovation Promotion Association CAS (No.2016383), and the CASHIPS Director's Fund (YZJJ2018QN7, BJPY2019B02).
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b. 中国科学技术大学，科学岛分院，合肥 230026;
c. 中国科学技术大学环境科学与光电技术学院，合肥 230026