Calibration and Improved Speckle Statistics of IM-CW Lidar for Atmospheric CO2 Measurements

Xindong Liang,Wei Kong,Hao Liu,Guanglie Hong,Tao Chen

doi:10.3390/atmos11070737

Abstract

An intensity modulated, continuous-wave (IM-CW) integrated path differential absorption (IPDA) fiber-based lidar is developed herein for measuring atmospheric carbon dioxide (CO2). There are two main challenges in improving measurement accuracy, which have not been given enough attention in the previous research: one is that temperature sensitivity in optical components causes biases, due to the drift of component characteristic, and the other is that speckle noise deteriorates the signal-to-noise ratio. With the components thermally controlled, a target calibration accuracy of 0.003 dB is realized, corresponding to a CO2 concentration precision of better than 1 ppm for a 1 km path. A moving diffuser can reduce speckle noise by time averaging. In this paper, movement of the diffuser is substituted by the perturbation of the emitted laser beam by using a vibrating motor mounted on the optical antenna. Selecting on and off wavelengths with a small wavelength separation can improve the correlation between two laser speckle fields. These improvements result in the improved accuracy of the IPDA lidar system. Finally, the lidar performance was analyzed after the improvements described above were implemented. The diurnal variations of the atmospheric CO2 concentration using a topographic target were performed, and the results showed good agreement with the data measured by an in situ sensor. The root mean square (rms) of the deviation between the IPDA lidar and the in situ sensor was less than 1.4%.

Highlights

Atmospheric CO2 is presently regarded as the largest anthropogenic forcing factor for climate change, but the global CO2 budget remains considerably uncertain [1,2]
Amediek et al reported an integrated path differential absorption (IPDA) lidar operating at 1.57 μm using direct-detection, where an injection seeded KTP -OPO system pumped by Atmosphere 2020, 11, 737; doi:10.3390/atmos11070737
The modulated lights were amplified by an erbium-doped fiber amplifier (EDFA) after being combined with an optical coupler

Summary

Introduction

Atmospheric CO2 is presently regarded as the largest anthropogenic forcing factor for climate change, but the global CO2 budget remains considerably uncertain [1,2]. To be useful in reducing uncertainties about carbon sources and sinks, atmospheric CO2 measurements need to have a high resolution, with 1% precision [3]. To meet this need, differential absorption lidar (DIAL) or laser absorption spectrometer (LAS) systems, which use conventional coherent or incoherent lidar methods, have recently been developed. The detected power fluctuated obviously, due to the speckle effects in these systems. This is quite a serious problem, as it implies that a large number of pulses are necessary to obtain signals with a stable intensity [8]. Amediek et al reported an integrated path differential absorption (IPDA) lidar operating at 1.57 μm using direct-detection, where an injection seeded KTP -OPO system pumped by Atmosphere 2020, 11, 737; doi:10.3390/atmos11070737 www.mdpi.com/journal/atmosphere

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