Research on coherent differential absorption LiDAR based on Golay coding technology

Hu Yihua; Dong Xiao; Zhao Nanxiang

doi:10.12086/oee.2019.190081

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Hu Yihua, Dong Xiao, Zhao Nanxiang. Research on coherent differential absorption LiDAR based on Golay coding technology[J]. Opto-Electronic Engineering, 2019, 46(7): 19081-. doi: 10.12086/oee.2019.190081

Citation:

Hu Yihua, Dong Xiao, Zhao Nanxiang. Research on coherent differential absorption LiDAR based on Golay coding technology[J]. Opto-Electronic Engineering, 2019, 46(7): 19081-. doi: 10.12086/oee.2019.190081

Research on coherent differential absorption LiDAR based on Golay coding technology

1.
State Key Laboratory of Pulsed Power Laser Technology, College of Electronic Engineering, National University of Defense Technology, Hefei, Anhui 230037, China
2.
Anhui Province Key Laboratory of Electronic Restriction Technology, College of Electronic Engineering, National University of Defense Technology, Hefei, Anhui 230037, China

Fund Project: Supported by National Natural Science Foundation of China (61871389) and Major Fund Program of National University of Defense Technology (ZK18-01-02)

More Information

Corresponding author: Dong Xiao, E-mail:skl_dongxiao@163.com

Received Date 01 March 2019

Revised Date 10 May 2019

Published Date 01 July 2019

Abstract

Abstract

The detection of CO₂ based on coherent different absorption LiDAR (CDIAL) requires high signal-to-noise ratio (SNR). To improve the SNR and reduce the inversion error of CO₂, a coherent differential absorption LiDAR based on Golay coding is proposed and the corresponding decoding method is also studied. The coding gain of SNR in traditional atmospheric backscattering signal detection is also analyzed when the pulse code technology is used. The variations of coding gain with the power of local oscillator (LO), the code length and the splitting ratio of 3 dB coupler are discussed. The higher the local oscillator power is and the more the beam splitting ratio deviates from 50%, the lower the coding gains. In addition, there are optimal code lengths in practical systems. The influence of thermal noise on the detection system decreases when the LO power grows, and there is optimal LO power which is only related to the system noise characteristics. The optimal LO power decreases with respect to single pulse detection after pulse coding, but the SNR is still higher than the traditional single pulse detection. When the splitting ratio of the 3 dB coupler is 0.495, the optimal LO power in coded system is 0.93 mW. The effective detection ranges of CO₂ increase when the pulses are coded, and in the pulse accumulations of 10⁴~10¹⁰, the improvement ratios of effective detection range are higher than 15%.
- coherent LiDAR /
- differential LiDAR /
- Golay coding /
- coding gain

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References

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Overview

Overview

Overview: The differential absorption LiDAR (DIAL) can obtain the spatio-temporal distribution information of atmospheric CO₂, which needs high signal-to-noise ratio (SNR). To improve the detection SNR, the coherent detection and heterodyne detection are widely used and have been combined with DIAL due to the excellent noise-reducing ability. In this paper, we propose a coherent differential absorption LiDAR (CDIAL) based on Golay coding to further reduce the detection errors, and the decoding method is also analyzed. The coding gain formula of SNR due to Golay coding is deduced, which is related to the local oscillator (LO) power, the code length, the splitting ratio of the 3 dB coupler. When the LO power is lower, the thermal noise should not be neglected, and the coding gain is higher, which is mainly due to the suppression of thermal noise. The higher the local oscillator power is and the more the beam splitting ratio deviates from 50%, the lower the coding gains are, because these two factors can improve the shot noise and the relative intensity noise, and thus the influence of thermal noise decreases. In addition, there are optimal code lengths in actual heterodyne detection systems, when the code length is higher than the optimal code length, the increase of coding gain is not obvious. The influence of thermal noise on the detection system decreases when the LO power grows, and there are optimal LO power which is only related to the system noise characteristics. The optimal LO power decreases with respect to single pulse detection after pulse coding, but the SNR is still higher than the traditional single pulse detection. When the splitting ratio of the 3 dB coupler is 0.495, the optimal LO power in coded system is 0.93 mW, and the maximum SNR in traditional pulse LiDARs is 73.27% of that in coded pulse LiDARs. When the splitting ratio is 0.49, the optimal LO power can be further lower. To analyze the improvement of CDIAL performance when the Golay coding is used, we calculate the detection error of CDIAL under different LO power. Two operation mode of CDIAL system are considered, including the collimated mode and the focused mode. The focused mode has better performance in short range detection duo to its relatively higher system efficiency. The detection accuracy of CO2 should be better than 4 ppm, and we define the range corresponding the error of 4 ppm as the effective range. And the effective range is longer with the coded pulses. In the pulse accumulations of 104~1010, the improvement ratios of effective detection range are higher than 15%. In addition, the Golay coding technology can both improve the SNR and the spatial resolution of LiDARs, which will be discussed in the future research.