Research on coherent differential absorption LiDAR based on Golay coding technology
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摘要:
针对差分吸收相干激光雷达在CO2浓度反演时对信号的高信噪比需求,研究了一种基于Golay脉冲编码的差分吸收相干激光雷达及其解码方法,以改善系统信噪比,降低浓度反演误差。分析了采用脉冲编码技术对传统大气后向散射信号相干探测信噪比的编码增益,研究了编码增益随本振光功率、编码长度和3 dB耦合器分束比的变化规律,本振光功率越高、分束比偏离50%越多,则编码增益越低,且在实际系统中,存在最优的编码长度。当本振光逐渐增强时,热噪声对系统的影响逐渐降低,相干探测系统存在最优的本振光功率,该功率与回波无关仅与系统的噪声水平有关。脉冲编码后最优本振光功率相对于单脉冲探测时下降,但其探测信噪比仍优于单脉冲探测,当3 dB耦合器分束比为0.495时,最优本振光功率为0.93 mW。脉冲编码后,系统对CO2的有效探测距离增加,且在104~1010范围内进行脉冲积累时,相较于原系统距离增长率大于15%。
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%.
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Key words:
- coherent LiDAR /
- differential LiDAR /
- Golay coding /
- coding gain
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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.
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表 1 外差激光雷达参数表
Table 1. Heterodyne LiDAR system parameters
Detector items Value ρ1, ρ2 1.0 ε 0.495/0.505 Bandwidth/MHz 20 1/Ts/(Msps) 800 PLO/mW 0.5~1 E0/mJ 1 tT/ns 400 NRIN/(dBc/Hz) -120 A/m2 0.152π/4 Beam truncation[18] $1 / \sqrt{2}$ Transmittance 1.0 Beam type Gaussian -
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