Based on cavity-enhanced absorption spectroscopy (CEAS) and wavelength modulated spectroscopy (WMS) technology, a cavity-enhanced spectrum measurement system was built to measure the volume fraction of CO gas. In an experiment, a distributed feedback laser (DFB) laser with a central wavelength of 2.3 μm was used as the light source, and an optical cavity with a base length of 30 cm was constructed with two highly reflective mirrors with a reflectivity of 99.8%. An effective absorption path of 147.15 m was achieved. On this basis, CO was detected using the CO absorption spectral line at 4297.705 cm-1 as the sensing target. In the experiment, the measurement accuracy of the system was verified using CO and N2 gas mixtures with different volume fractions of CO. The measured value was consistent with the reference value, and the measurement error was approximately 0.2%, which confirmed the measurement accuracy of the system. The detection limit of the system was analyzed using the second harmonic signal of CO gas with a volume fraction of 3×10-6, and the lowest detectable CO volume fraction with the system was 138×10-9.
The optical cavity in the sealed box was composed of two highly reflective mirrors. The length of the cavity was 30 cm, diameter of the highly reflective mirrors specified by the manufacturer was 25.4 mm, and radius of curvature was 1 m. Their reflectivity reached 99.8% in a wavelength range of 1.9-2.3 μm. In the experiment, the laser output from the 2.3 μm DFB laser was divided into two parts using a fiber beam splitter. One part was passed through an oscilloscope (Bristol 671) to obtain the real-time scanning output wavelength of the laser, and the other part passed through the sealed box made of tempered glass. The laser beam entered the optical cavity through the sealed box. Then, after multiple reflections in the cavity, it was transmitted from the other end and converged in the InGaAs detector through a lens with focal length of 5 mm for detection. After the detector signal was sent to the input port of a lock-in amplifier, it demodulated the signal according to the specific setting parameters. The demodulated signal was collected by a data acquisition card (National Instruments USB-6361) and stored in the computer for subsequent data processing. A triangle wave with a frequency of 2 Hz generated by a function generator and a sine wave with a frequency of 880 Hz generated by a phase-locked amplifier were superimposed together by an adder, and the superimposed signal was sent to the laser controller to achieve scanning and modulation of the laser wavelength.
The experiment was carried out under the fixed conditions of 101325 Pa and 300 K, and the second harmonic (2f) signals of different volume fractions of CO were measured. The 2f signals of CO with volume fraction of 3×10-6, 10×10-6, 30×10-6, and 50×10-6 are shown in Fig. 5. Figure 6 shows the linear relationship between the peak height of the 2f signal of each volume fraction of CO and the volume fraction. The fitting results show that the linearity, R2, between the data points was 0.998, while the relative standard deviation was 0.34%. It can be seen that the two maintained a good linear relationship within the volume fraction range measured in the experiment.
In this study, the measured CO volume fraction was compared with the known reference value during the gas distribution. This comparison is shown in Fig. 7. The linearity, R2, between the data points in the figure was 0.998, which shows that the measured CO volume fraction was consistent with the reference value, and there was a good linear relationship between them. The red line in the figure was obtained by the linear fitting of the scatter plots. The slope of the fitting line was 1.00282 ± 0.02261. It can be seen that the measurement accuracy of the experimental system was maintained at approximately 0.2%, which confirmed that the measurement of the experimental system had high accuracy.
The system was used to conduct a 500 s time series measurement of CO gas with a volume fraction of 3×10-6. Each data sampling time was 1 s. Thus, a total of 500 volume fraction points of CO gas were obtained, as shown in Fig. 8 (a). Figure 8 (b) shows the histogram distribution of the 500 data points. The histogram shows a good Gaussian distribution, and the half-height width (HWHM) of the curve could be used to evaluate the measurement accuracy of the system. The results show that the CO volume fraction measurement accuracy of the system was 286×10-9. The Allan variance was used to analyze the detection sensitivity of the system under the environmental conditions of a temperature of 300 K and pressure of 101325 Pa, as shown in Fig. 9. It can be seen from the figure that when the system time was approximately 1850 s, the Allan variance was the lowest, which meant that this time was the best detection time for the system, and the detection limit at this time could reach 138×10-9. Before the optimal detection time of 1850 s, the Allan variance of the system was higher than the minimum detection limit, which was due to the influence of the white noise of the system. The noise after this was mainly caused by the instability of the system. Therefore, when measuring a stable flow field, the noise could be reduced and the measurement accuracy could be improved by averaging the collected data for many measurements within a period of 1850 s.
The CO absorption spectrum line at 4297.705 cm-1 was selected in the experiment. Based on cavity-enhanced absorption spectroscopy (CEAS) technology, combined with wavelength modulated spectroscopy (WMS) technology, a cavity-enhanced spectrum measurement system was built with a self-made sealed box. The reflectivity of the cavity mirror at 2.3 μm was calibrated to be 99.8%, which was consistent with the data given by the manufacturer. At the same time, the effective absorption optical path of the system was calibrated to be 147.15 m, and the optimal modulation amplitude was determined to be 360 mV. During the experiment, a DFB laser with a central wavelength of 2.3 μm was used as the light source. Under the conditions of 101325 Pa and 300 K, the linear relationship between the peak height of the 2f signal of each each volume fraction of CO and the volume fraction was obtained by measuring the 2f signals of the CO with different volume fractions prepared using the CO and N2 mixture gas, and a good linear relationship between the two was determined within the measured volume fraction range. Finally, the CO volume fraction measured by the system was compared with the known value during the gas distribution, and the measurement error was approximately 0.2%, which verified the reliability and accuracy of the system.
