Open Access
Add to BibSonomyAbstract
We propose a novel wavelength-locking-free differential absorption lidar system for CO2 sensing. The ON-line wavelength laser was wavelength modulated around a specific CO2 absorption line to ensure that the emission from the ON-line laser hit the atmospheric CO2 absorption line peak twice a cycle. In the meantime, the intensity of the ON-line and OFF-line wavelength lasers were sinusoidally intensity modulated to enhance the SNR of the back-scattered signal. As a consequence, the system configuration was simplified and the measurement error caused by the deviation of CO2 absorption coefficient from the long-time ON-line wavelength drifting was completely eliminated. Furthermore, a more precise calibration method was developed which could simultaneously calibrate the offset and precision of the lidar detector. This method could be applied to other differential-absorption-based lidar systems. The result showed that a measurement precision of 0.525% for the column concentration was achieved in 1 s time interval through a path of 780m. We recorded the CO2 concentration variation for 12 hours starting from mid-night, the result showed that the course of the concentration derived from the DIAL was in good agreement with that of the in situ CO2 sensor only when the status of atmosphere was stable.
© 2014 Optical Society of America
1. Introduction
Carbon dioxide (CO2), one of the greenhouse gas, has gained increasing concern since the late nineteenth century for its rapid growth on the global scale. Identification of sink, source and flux of atmospheric CO2 can give important information about the climate change and related impacts on human beings. As an effective method for CO2 monitoring, differential absorption lidar (DIAL) has been studied in both pulsed mode [1–4] and continuous-wave (CW) mode [5, 6] so far. Most of the pulsed DIAL systems employed optical parametric oscillators pumped by solid state lasers as the transmitters making the whole DIAL systems very bulky. Since fiber lasers have moved into the mainstream of laser research owing to their compactness, robustness, ultra-wide gain spectral range and excellent beam quality [7], Erbium doped fiber amplifiers (EDFA) have been adopted as alternative sources of DIAL systems for CO2 sensing. In these DIAL systems, both scanning technique and wavelength locking technique have been adopted. In scanning-technique-based DIAL systems, there is only one laser which is wavelength scanned across one absorption line [3, 4]. However, the system offset could not be calibrated during measurement. And there are two lasers in their wavelength-locking-technique-based counterpart, in which at least one laser (the ON-line laser) should be precisely wavelength-locked onto one of the absorption peak by referencing the CO2 gas cell to get the accurate value of CO2 absorption in the atmosphere [5]. Nevertheless, the wavelength locking unit is complicated and the atmospheric CO2 absorption peak is likely different from that of the reference CO2 gas for their different pressure and temperatures.
The precision and the instrumental offset dominated the performance of DIAL systems. In a previous report, a diffuse hard target located in front of the optical antenna unit at a distance of 0 m was utilized to back-scatter the transmitting laser light for precision analysis [5]. Thus, the inference from the atmospheric turbulence was omitted in their study. Although the testing results of the DIAL system was in good agreement with that of an in situ sensor, the conclusion was valid only when the distribution of CO2 concentration was uniform. Because the in situ sensor records the mixing ratio of CO2 at a certain point while the DIAL system measures the whole integrated path. Later experiment demonstrated an indoor measurement to confirm the correlation of the instrumental response to a known CO2 density in the cell in which the atmospheric effect was still not considered [8].
In this paper, we demonstrate a novel continuous-wave (CW) DIAL system for CO2 sensing in which the ON-line wavelength laser was wavelength modulated by a triangle wave with low frequency instead of being wavelength locked onto one specific CO2 absorption peak. This method ensures the center of the atmospheric CO2 absorption line could be covered twice during a single cycle and avoids the error that caused by the wavelength drift of the ON-line wavelength laser. The OFF-line laser was wavelength stabilized (not wavelength locked) by the electronic control unit. Wavelength stability of 0.5 pm (peak-to-peak) was achieved, which was enough for precise CO2 measurement by DIAL systems. In addition, a novel calibration method was presented, by which the precision and instrumental offset could be measured during each scanning period. Thus, both self-calibration process and measurement process could be accomplished in each cycle. This method is applicable to different kinds of DIAL systems.
2. Experimental setups
The system configuration is shown in Fig. 1. The ON-line and OFF-line wavelength seed lasers (both from EM4 Inc. with maximum output power of 63 mW and line-width of ~1MHz) were driven independently by two control units. These units could provide both tunable driving current and temperature stabilization. Since the wavelengths of the laser diodes were driving current and temperature related, the stability of the OFF-line wavelength and the center of the ON-line wavelength were guaranteed by the electronics. The measured long time wavelength instability was about 0.5 pm (peak-to-peak). A 1 Hz triangle current signal was superimposed on the driving current of the ON-line wavelength laser leading to the corresponding wavelength sweeping range of about 20 pm. The center of the atmospheric CO2 absorption peak could be covered as long as the frequency shift of the ON-line wavelength laser was less than the half of the wavelength scanning range. The CO2 absorption line (on-line:1572.335 nm, off-line: 1572.184 nm) was selected according to the HITRAN 2004 database [9],ensuring the requirements of negligible absorption in H2O and low temperature sensitivity [5].
Fig. 1 Schematic diagram of the DIAL system. LD: laser diode; DAC: digital-to-analog convertor; ADC: analog-to-digital convertor; DC: direct current; PC: personal computer.
The intensities of the ON-line and OFF-line wavelength lasers were then sinusoidally modulated with different frequencies for discrimination of their signals in the frequency domain. The modulation frequency is higher than 10 kHz because the input electric noise spectral density of our preamplifier stays at high level in frequency below 10 kHz.
For practical CO2 concentration monitoring, the DIAL system usually needs to work for hours. However, the bias of intensity modulation will change as the ambient temperature shifts. This may lead to the “clipped - off” distortion of the sinusoidal wave, such as “flat tops” or “flat bottoms”. This distortion will affect the result of the succeeding signal processing. Taking this problem into account, the bias control servo system containing Detector1, Detector 2, analog-to-digital converter (ADC) and digital-to-analog converter (DAC) was introduced adjusting the DC offset for the intensity modulator to keep the waveform of the output sinusoidal wave without distortion.
The two modulated lasers were amplified simultaneously by the EDFA (IPG EAR-10K-1571-LP-SF) after passing through a 3 dB coupler. The amplified laser emission was then expanded and reflected by a tilted plane mirror fixed on the secondary mirror of the Cassegrain telescope. Detector 3 monitored transmitted radiation, by reflecting a part of its edge, which was different from the optical splitter [5]. This eliminates the intensity error of the monitored laser emission caused by the laser polarization instability.
The back-scattered light from the hard target was collected by the telescope, coupled to a multimode optical fiber, and detected by the Detector 4. The signals from all the photo-detectors were converted by an analog-to-digital card with four channels and were processed in a personal computer.
3. Offset calibration and precision analysis
Figure 2 shows the information about the system calibration process. Firstly, a zero calibration process was performed as is shown in Fig. 2(a). The wavelength of the ON-line laser was tuned to OFF-line wavelength (1572.184nm). As a consequence, the inference of the environment was the same between the two laser emissions and the retrieved results should be zero in theory. However, the actual recorded curve was slightly below zero as is shown in Fig. 2(d) (section a), which was defined as the instrument offset. Because the distribution of the absorption coefficient near the OFF-line wavelength is “flat”, the estimated error of the system offset was less than 0.12% for a wavelength deviation of 2.5 pm between the two lasers. Actually, the difference between the two lasers was less than 0.5pm which was secured by the control electronics, resulting in an even better precision in the DIAL system. Secondly, the measurement process was carried out, changing the ON-line laser to working wavelength (1572.335nm) as is shown in Fig. 2(b). The measured curve, shown in Fig. 2(d) (section b), was noisier than that in the zero calibration process. We attribute this to the fluctuation of the concentration of the atmospheric CO2. Finally, zero calibration process was repeated for verifying the system reliability, as is shown in Fig. 2(d) (section c).
Fig. 2 Result of the system offset calibration and precision analysis. (a) zero calibration process. (b) measurement process. (c) repeated zero calibration process. (d) DIAL signal during the whole calibration process.
The curve of the whole calibration process is depicted in Fig. 2(d). The average measured value in the first zero calibration process was −0.0174 which should be zero theoretically. And the calculated root mean square (RMS) of this section was 0.00084. The average value of the succeeding measurement process was 0.143. The difference between the zero calibration process and the measurement process was computed to be 0.16. Therefore, the calculated precision in percentage was 0.525%.
Fig. 3 shows the long-time record of the offset measurement. Figure 3(a) compares the CO2 concentration measured by the in situ sensor and the zero line drift of the DIAL system in nine hours starting from mid-night. The black line and gray line represent the atmospheric CO2 mixing ratio and the system zero offset, respectively. Figure 3(b) plots the environment temperature variation during the measurement. The stability of the offset was calculated to be 0.0006957 (RMS value). Clearly, the system offset was temperature insensitive and was independent of the CO2 concentration, as is illustrated in Fig. 3.
Fig. 3 Measurement of the system offset. (a) system zero drift and atmospheric CO2 fluctuation (b) environment temperature change during the measuring period.
Figure 4 illustrates the diurnal variation of horizontal column averaged CO2 concentration obtained by this system in Shanghai on 11 July 2014. In the experiment, the solid target was a building located at the range of 780m from our experimental building. We also measured CO2 concentration using the in situ CO2 sensor located on the top of our experiment building for comparison. The result indicated that the courses of measured CO2 concentration were in good agreement between the DIAL system and the in situ CO2 sensor before 6:00 am while the later courses deviated. Probably, the status of the atmosphere was relatively stable before 6:00 am and the measurement was not affected by the human activities. In addition, there are two roads and a community between the integrated path. Not only the automobile exhaust but also the photosynthesis of the green foliage would affect the distribution of the CO2 concentration after 6:00 am. Furthermore, there was more human influence on the DIAL than on the in situ sensor which was located on the top of our experimental building (10 floors). After 8:30, the wind became active and the cloud became thick. It started to rain at 11:00 and it rained heavily after 12:00. Neither the DIAL nor the in situ sensor could work under such condition, which affected the measuring results. It is worth mentioning that, by comparing the measured data in our previous report [10], evident decrease in measured CO2 concentration was also found after sunrise in sunny weather. But we could not find any other common tendency during other measuring period in urban areas.
4. Conclusion
In summary, we report a novel DIAL system using wavelength modulation for ON-line wavelength laser instead of locking its wavelength onto a specific absorption line. A novel calibration method was also developed in this report, which could be applied to different kinds of DIAL systems. Thanks to the proposed method, both the self-calibration process and the measurement process could be accomplished during each cycle. Therefore, the system offset and precision, which are the crucial parameters of the DIAL system, as well as the measured CO2 concentration could be obtained during each scanning period. A measurement precision of 0.525% for the column concentration was achieved in 1 s time interval through a path of 780m. The variation of CO2 concentration, affected by complicated weather condition and human activities, was recorded starting from mid-night. The result showed that the course of the concentration derived from the DIAL was different from that of the in situ CO2 sensor unless the status of atmosphere was stable.
References and links
1. A. Amediek, A. Fix, M. Wirth, and G. Ehret, “Development of an OPO system at 1.57 μm for integrated path DIAL measurement of atmospheric carbon dioxide,” Appl. Phys. B 92(2), 295–302 (2008). [CrossRef]
2. D. Sakaizawa, C. Nagasawa, T. Nagai, M. Abo, Y. Shibata, M. Nakazato, and T. Sakai, “Development of a 1.6 microm differential absorption lidar with a quasi-phase-matching optical parametric oscillator and photon-counting detector for the vertical CO2 profile,” Appl. Opt. 48(4), 748–757 (2009). [CrossRef] [PubMed]
3. J. B. Abshire, H. Riris, C. J. Weaver, J. Mao, G. R. Allan, W. E. Hasselbrack, and E. V. Browell, “Airborne measurements of CO2 column absorption and range using a pulsed direct-detection integrated path differential absorption lidar,” Appl. Opt. 52(19), 4446–4461 (2013). [CrossRef] [PubMed]
4. J. B. Abshire, H. Riris, G. R. Allan, C. J. Weaver, J. Mao, X. Sun, W. E. Hasselbrack, S. R. Kawa, and S. Biraud, “Pulsed airborne lidar measurements of atmospheric CO2 column absorption,” Tellus B Chem. Phys. Meterol. 62(5), 770–783 (2010). [CrossRef]
5. S. Kameyama, M. Imaki, Y. Hirano, S. Ueno, S. Kawakami, D. Sakaizawa, and M. Nakajima, “Development of 1.6 microm continuous-wave modulation hard-target differential absorption lidar system for CO2 sensing,” Opt. Lett. 34(10), 1513–1515 (2009). [CrossRef] [PubMed]
6. M. Imaki, S. Kameyama, Y. Hirano, S. Ueno, D. Sakaizawa, S. Kawakami, and M. Nakajima, “Laser absorption spectrometer using frequency chirped intensity modulation at 1.57 μm wavelength for CO2 measurement,” Opt. Lett. 37(13), 2688–2690 (2012). [CrossRef] [PubMed]
7. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives [Invited],” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). [CrossRef]
8. S. Kameyama, M. Imaki, Y. Hirano, S. Ueno, S. Kawakami, D. Sakaizawa, and M. Nakajima, “Performance improvement and analysis of a 1.6 μm continuous-wave modulation laser absorption spectrometer system for CO2 sensing,” Appl. Opt. 50(11), 1560–1569 (2011). [CrossRef] [PubMed]
9. L. S. Rothman, D. Jacquemart, A. Barbe, D. Chris Benner, M. Birk, L. R. Brown, M. R. Carleer, C. Chackerian Jr, K. Chance, L. H. Coudert, V. Dana, V. M. Devi, J. M. Flaud, R. R. Gamache, A. Goldman, J. M. Hartmann, K. W. Jucks, A. G. Maki, J. Y. Mandin, S. T. Massie, J. Orphal, A. Perrin, C. P. Rinsland, M. A. H. Smith, J. Tennyson, R. N. Tolchenov, R. A. Toth, J. Vander Auwera, P. Varanasi, and G. Wagner, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96(2), 139–204 (2005). [CrossRef]
10. H. Liu, R. Shu, G. L. Hong, L. Zheng, Y. Ge, and Y. H. Hu, “Continuous-wave modulation differential absorption lidar system for CO2 measurement,” Acta Phys. Sin 63, 104214 (2014).
