## 5 Conclusions  
A 1.57 \(\upmu\)m IM-CW IPDA fiber-based lidar capable of remotely sensing atmospheric CO\({}_{2}\) was demonstrated. The system utilizes advanced EDFA technology to achieve the simultaneous transmission of on and off lasers through the same single-mode fiber-based optical system, sharing common transmit-receive apertures. The on and off lasers are intensity modulated with CW signals at slightly different frequencies (e.g., 99.7 kHz and 101.8 kHz), in order to distinguish the received on and off radiation. In the first trial, the detected CO\({}_{2}\) mixing ratio strongly fluctuated around the expected 410 ppm ambient concentration, with minimum and maximum values of 300 and 600 ppm, respectively. Therefore, it was believed that unknown sources of systematic errors caused large fluctuations in the CO\({}_{2}\) concentration, indicating the necessity for system calibration. The temperature sensitivity of some optical components and the speckle effect caused by the random scattering of targets and atmospheric turbulence were determined to be the main reasons for the large fluctuations in the DAOD measurements through the accumulation of results from preliminary experiments. Following the results of the OFLT experiment, a thermal control function was added to the SM filter beam splitter and the monitoring photodetector, after which the fluctuation of the normalized intensity ratio was less than 0.003 dB, corresponding to a CO\({}_{2}\) concentration precision of 1 ppm for a 1 km path. In the preliminary experiments, the CNRs were very high, but the random error of the CO\({}_{2}\) concentration measurement value was quite large, which means that increasing the transmitting optical power does not contribute much to improve the performance. This is because the speckle-related SNR limits short-term fluctuations. We added a vibration motor at the end of the optical antenna to reduce the speckle correlation time, which has not yet been reported in the literature, to the best of our knowledge. The experimental results demonstrated that the fluctuation of the normalized intensity ratio was much smaller when the motor vibrated than when the motor was stationary. The faster the motor vibrated, the smaller the fluctuation of the normalized intensity ratio. We have demonstrated, using theoretical and modeling techniques, that highly correlated speckle has a great advantage in reducing the variance of the DAOD. The degree of correlation is determined by the wavelength separation between the on and off lasers. Therefore, a smaller wavelength separation has the potential for improving the accuracy of IPDA lidar systems. In a laboratory situation without atmospheric absorption and interference, the reflective target was approximately a Lambertian scatter, and the fluctuation of the normalized intensity ratio with the on and off frequencies separated by 36.4 GHz was twice as large as that when the wavelength separation was 16 GHz. The results of the laboratory experiments were consistent with the theoretical predictions. We also analyzed the  
Figure 22: The measurement results of the IPDA lidar over five consecutive days in autumn (18–22 October 2019). CO\({}_{2}\) concentration measurement performance of this system after implementing the improvements described above. The diurnal variations measured by the IPDA lidar were consistent with an in situ sensor. Unfortunately, we cannot discuss the speckle decorrelation caused by atmospheric turbulence any further, as we had no chance to measure the turbulence conditions. Conceptualization, G.H. ; methodology, H.L., W.K. and T.C. ; software, X.L. ; validation, X.L. ; formal analysis, X.L. ; investigation, X.L. ; resources, G.H. ; data curation, X.L. ; writing--original draft preparation, X.L. ; writing--review and editing, T.C. ; visualization, X.L. ; supervision, G.H. ; project administration, G.H. ; funding acquisition, G.H. All authors have read and agreed to the published version of the manuscript.