Liang Mei 19th Coherent Laser Radar Conference

CLRC 2018, June 18 – 21 1

The Scheimpflug Lidar Technique and Its Recent Progress in Atmospheric Remote Sensing

Liang Mei(a), Zheng Kong(a) and Peng Guan(a) (a)School of Optoelectronic Engineering and Instrumentation Science, Dalian University of Technology,

Dalian 116024, China

Abstract: The Scheimpflug lidar technique has been recently developed for atmospheric remote sensing. The entire laser beam that is transmitted into atmosphere can be clearly focused over km-range on a tilted image sensor, if the optical configuration satisfies the Scheimpflug principle. The range-resolved lidar signal is in this case angular-resolved and obtained from pixel intensities of the image sensor. The special imaging principle enables the usage of low-cost, compact high-power CW laser diodes and highly integrated CMOS/CCD image sensors. This paper reports our recent progress of the Scheimpflug lidar technique in the applications of atmospheric aerosol and NO2 monitoring.

Keywords: Scheimpflug lidar, aerosol, NO2, extinction coefficient, depolarization.

1. Introduction

Lidar techniques have been of great interest for atmospheric aerosol [1], trace gases [2], temperature [3] and wind speed [4] monitoring for decades. Recently, the Scheimpflug lidar technique has been proposed for atmospheric remote sensing [5-7]. The laser beam that is transmitted into atmosphere can be clearly focused on a tilted image sensor, if the optical layout of the lidar system satisfies the Scheimpflug principle, as shown in Figure 1. Infinite depth-of-focus is achieved while employing large aperture optics. Thus, the range-resolved atmospheric backscattering signal can be retrieved from recorded pixel intensities. The pixel information can be transformed to measurement distance according to geometrical optics. The Scheimpflug lidar technique significantly reduces the complexity of laser sources and detection units, the requirement on system maintenance, and the overall cost of an atmospheric lidar system by utilizing high-power continuous-wave (CW) laser diodes as light sources and highly integrated CMOS/CCD sensors as detectors [8]. Moreover, the wide availability of high power laser diodes from 375 to 1550 nm provides a great possibility for developing multi-wavelength Scheimpflug lidar system, which is of great interest for precise particle sizing. This paper reports recent progress of the Scheimpflug lidar technique in the applications of atmospheric aerosol and trace gas monitoring in Dalian University of Technology (DLUT).

Figure 1. Principles of the Scheimpflug lidar technique. Infinite depth of focus can be achieved when the image plane, lens plane and the object plane intersect into a single line according to the

Scheimpflug principle.

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CLRC 2018, June 18 – 21 2

2. Atmospheric aerosol sensing

The schematic of the typical Scheimpflug lidar system as well as the photograph are shown in Figure 2. High power CW laser diodes are often employed as the light sources, which are housed by homemade aluminum mounts with large-capacity thermoelectric cooler to stabilize the case temperature and thus the center emission wavelength. The laser beam is collimated by a refractor telescope (F6). Additional cylindrical lens pairs may be used to reshape the laser beam for the Scheimpflug lidar system operating at e.g., 405 nm and 520 nm [9]. The sunlight background is suppressed by interference filters with 1.7-10 nm full width at half maximum. The backscattering light of the laser beam is collected by a Newtonian telescope with approximately 806 mm separation to the refractor telescope. Both the refractor telescope and the Newtonian telescope are mounted on an aluminum bar, carried by an equatorial mount. A 2D CMOS camera is mounted with 45˚ titled to the optical axis of the receiving telescope. A trigger signal generated by the camera is fed to a Johnson ring counter, and the output is utilized to switch the laser diode on and off through the driving current. The CMOS camera records the "laser on" and "laser off" images alternatively. The background signal can then be subtracted from the recorded raw images. Each image pairs are vertically binned to extract the "on" and "off" curves. After background subtraction, signal averaging, and digital filtering, etc, a single lidar curve can be obtained in 45 s (1000 times signal averaging). The pixel-distance relationship is calibrated by measuring backscattering echo from a hard target with known distance.

Recently, single-band Mie scattering Scheimpflug lidar systems operating at various wavelengths, e.g., 405 nm, 450 nm, 520 nm and 808 nm, have been implemented and employed for real-time, large area atmospheric aerosol remote sensing [9,8]. Atmospheric measurements were performed in urban area to investigate the performance of the lidar systems. All-time operation is feasible for the 405-nm and 808-nm Scheimpflug lidar system due to relatively lower sunlight background and the narrow bandwidth of interference filters (less than 3 nm). However, the 450-nm and 520-nm Scheimpflug lidar systems, where 10-nm FWHM interference filters are often used, could be saturated when the lidar system is facing to the direction of the sun. The maximum measurement distance of the Scheimpflug lidar system can reach up to 7-10 km during a sunny and clean weather conditions, while the measurement distance is limited during severe haze particularly for the 405 nm lidar system due to severe atmospheric attenuation. The noise level of the raw lidar signal is dominated by the sunlight background noise and the photon-response non-uniformity (PRNU) noise of the image sensor [10]. During nighttime measurements when the sunlight background noise is negligible, the signal-to-noise ratio (SNR) of the raw lidar signal is inverse proportional to the PRNU noise ratio of the image sensor, which is about 0.5% for the CMOS camera. The maximum SNR is thus limited to about 200:1. Thus, the Savitzky–Golay filter is utilized for signal de-noising, and the maximum SNR can then reach up to 1000:1 during nighttime. The Klett-Fernald inversion algorithm was also employed for atmospheric extinction coefficient retrieval. Figure 3 shows the distribution of aerosol extinction measured by the 405 nm Scheimpflug lidar system. In several case studies, the trends of the extinction coefficients retrieved from the Scheimpflug lidar system were in good agreement with the PM10/PM2.5 concentrations measured by a conventional air pollution monitoring station, which successfully demonstrates the feasibility of the Scheimpflug lidar technique for atmospheric remote sensing. Practical applications of the Scheimpflug lidar technique in atmospheric pollution monitoring or tracking can be expected in the near future.

A polarization Scheimpflug lidar system based on the Scheimpflug principle has also been developed by employing two linearly polarized 808-nm laser diodes and a CMOS image sensor [11]. The polarization of one laser diode is rotated 90° by a half-wave plate. The two laser beams with orthogonal polarizations are combined by a polarization beam splitter, and then transmitted into atmosphere. The corresponding parallel and perpendicular polarized backscattering echoes are detected by the 45° tilted CMOS sensor using a time-division multiplexing scheme. 24-hour continuous atmospheric vertical profiling of depolarization ratio successfully demonstrated that the polarization Scheimpflug lidar system has a potential for the depolarization studies of atmospheric aerosols. However, the SNR of the perpendicular polarized backscattering signal should be improved for daytime measurements.

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CLRC 2018, June 18 – 21 3

Figure 2. (a) Principle and (b) photograph of the Scheimpflug lidar system developed in DLUT [8].

Table 1. Primary specifications of the Scheimpflug lidar techniques.

Model Specifications

Laser source Continuous-wave high power multi-mode laser diodes

Wavelength: 405 nm, 450 nm, 520 nm, 638 nm, 808 nm, 850 nm, 1064 nm; Output power: 1-5 W

Collimator F6 refractor Focal length: 600 mm, Diameter: 100 mm

Receiver Newtonian telescope Focal length: 800 mm; Diameter: 200 mm

Detector 2D CMOS Sensor, e.g., CMV2000

Tilt angle: 45˚; Area: 2048×1024 Pixels Frame rate:170 fps; Bit depth: 12/8 bit Exposure time: 20-500 ms

Filters Interference filters + color filters

1.7-10 nm FWHM

Figure 3. Distribution of extinction coefficient retrieved from all-time atmospheric measurements employing the 405-nm Scheimpflug lidar system. The PM2.5/PM10 concentrations were about 60

µg/m3, and the relative humidity is 80% during the severe haze on August 19th, 2017.

Liang Mei 19th Coherent Laser Radar Conference

CLRC 2018, June 18 – 21 4

3. Atmospheric NO2 monitoring

Differential absorption lidar (DIAL) technique provides unique capability of remotely monitoring urban/rural area localized NO2 emissions and profiling tropospheric vertical NO2 distribution. By measuring and evaluating atmospheric backscattering echoes of transmitted laser pulses at on-line wavelength and off-line wavelength of NO2, range-resolved NO2 concentration can be retrieved. Conventional pulsed lidar techniques often employ Nd:YAG pumped dye lasers [12] or Raman lasers [13], sum frequency generation based on solid state lasers [14], etc. Nevertheless, the laser sources mentioned above are extremely complicated and require intensive maintenance during the measurements, which prevents practical applications of the NO2 DIAL technique. A continuous-wave (CW) NO2 DIAL system based on the Scheimpflug principle has been recently developed by employing a compact high-power multimode 450-nm laser diode as the laser source [15]. Laser emissions at the on-line and off-line wavelengths of the NO2 absorption spectrum are readily implemented by tuning the injection current of the laser diode, i.e., 448.6 nm and 452.1 nm. Lidar signals are detected by an area CCD image sensor, which is tilted 45˚ to satisfy the Scheimpflug principle. Range-resolved NO2 concentrations on a near-horizontal path are obtained by the NO2 DIAL system in the range of 0.3-3 km during nighttime, and show good agreement with those measured by a national air pollution monitoring station. A detection sensitivity of ±0.9 ppbv in the region of 0.3-1 km is achieved with 15-minute averaging and 700 m range resolution, which allows accurate concentration measurement of ambient NO2. The low-cost and robust DIAL system opens up many possibilities for field NO2 remote sensing applications in the near future.

Figure 4. NO2 absorption spectrum and the on- and off-wavelengths of the NO2 DIAL technique.

4. Conclusions

The Scheimpflug lidar technique has been successfully demonstrated for atmospheric extinction coefficient retrieval, depolarization studies and NO2 monitoring. It has shown a great potential for practical atmospheric applications with the advantages of being low cost, less complexity, and low maintenance. Scheimpflug lidar systems operating at various wavelengths are also developed and validated for atmospheric remote sensing, which are of great interest for the development of multi-wavelength Scheimpflug lidar system in the near future.

This work was supported by the National key research and development program of China (2016YFC0200600), the National Natural Science Foundation of China (61705030), the Fundamental Research Funds for the Central Universities (DUT18JC22), and the Natural Science Foundation of Liaoning Province, China (201602163).

5. References

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CLRC 2018, June 18 – 21 5

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