1. Liang Mei, Zheng Kong, and Peng Guan
The Scheimpflug Lidar Technique and Its Recent Progress in Atmospheric Remote Sensing
Liang Mei, Zheng Kong, and Peng Guan
School of Optoelectronic Engineering and Instrumentation Science
Dalian University of Technology (DLUT), Dalian , China *
*Tel:
Good afternoon, it is a great pleasure to give a presentation in CLRC. Thank Prof. Ishi for giving me this opportunity.
Today I will talk about ……

2. Background and Motivations
Contents
Background and Motivations
Principle of the Scheimpflug Lidar (SLidar)
Atmospheric Aerosol Monitoring
Differential Absorption Lidar (DIAL)
Conclusions and Outlook
First I will briefly talk the background and motivations, and the principle of SLidar. Then I will discuss the applications in atmospheric aerosol monitoring and DIAL, Finally I will give the conclusions and outlook.

3. Background and Motivations
Lidar techniques have been widely employed for atmospheric remote sensing, e.g., aerosol, trace gas, pollution, wind, temperature
Micro pulse lidar
Scanning lidar
Mobile lidar
The Scheimpflug lidar – a new possibility
Aerosol Profiling
Pollution tracking
Trace gas monitoring Hg
As we all know,…Various lidar technique or systems have been developed, e.g., MPL, mobile lidar, and scanning lidar system.
The prevailing lidar techniques are mainly based on the time-of-flight principle, which are high cost, complicated to some extent and critical maintenance
Conventional pulsed lidar techniques: high cost, complicated, critical maintenance

4. Principle of the Scheimpflug Lidar (SLidar)
The Scheimpflug lidar technique
Image
objects
CCD
Lens
Conventional imaging system
To achieve sharp focus both for the near & far range?
Reduce the aperture – deteriorate image quality
Scheimpflug Principle – the object plane, lens plane, and imaging plane intersect with each other to achieve infinite depth of focus without closing the aperture.
Before talking the Slidar technique, I will discuss the conventional imaging system first, where the object planes, image plane and lens plane are parallel to each other. As you can see here, when we get a clear image of the flower, the background is blurred.
One way to achieve sharp focus both for the near and far range is to reduce the aperture, which certainly deteriorate the image quality.
Another solution is to employ the Scheimpflug principle, which was discovered a century ago. According to the Scheimpflug principle, if XXX infinite depth of focus can be achieve.

5. Principle of the Scheimpflug Lidar (SLidar)
Features of the Scheimpflug Lidar
Laser beam image
1 km
Continuous-wave (CW) laser source for range-resolved measurements
Image sensor: pixel → distance
Large aperture → infinite depth of focus
Lidar signal/SNR does not decrease with the square of the measurement distance
0.4 km
0.1 km
nonlinear
Lens
Laser beam
Intersect
Backscattering
coefficient
Extinction
SLidar equation: no z2 factor
Overlap function=1
The application of the Scheimpflug principle in atmospheric remote sensing is shown in the right figure. Briefly, a cw laser beam is transmitted into atmosphere, we can then use a large aperture receiving lens or telescope to image the laser beam, if the laser beam plane, the lens plane and the CCD plane intersect into a single line, we can get a clear image both for the near and far range as you can see from the up figure. If we binning the pixel intensity along the vertical direction, we can retrieve the range-resolved lidar curve.
Clearly, a key feature of the Slidar technique is that CW laser source can be employed for range resolved measurements. Besides, the pixel of the image sensor corresponds to the distance. A very important aspect of this optical configuration is that large aperture can be employed to achieve infinite depth of focus, which is essential for detecting the weak atmospheric backscattering signal.
According to the geometrical optics, the lidar signal as well as the SNR does not decrease with the square of the measurement distance. In other words, there is not z2 factor in the lidar equation. By employing an area detect, the overlap function is equal to 1 in the field of view.
CCD
Laser CW
Lidar curve
Liang Mei, etc., LPR, (CW-DIAL)
Liang Mei, etc., Optics Express, (Aerosol)

6. Principle of the Scheimpflug Lidar (SLidar)
Advantages:
High-power continuous-wave laser diodes can be employed, low-cost, stable, compact...
Compact and highly integrated CCD/CMOS sensor: light detection, signal acquisition, data transfer.
Multiple-wavelength lidar system can be readily implemented.
Able to measure near distance with CCD/CMOS sensor, shorter blind- range
Tunability and narrowband very easy for laser diodes, good for gas sensing
There are a few advantages of the Slidar technique.

7. Principle of the Scheimpflug Lidar (SLidar)
Light sources: Multi-mode high-power laser diodes, 397 nm nm 1-10 Watt
Detectors: Si or InGaAs image sensor, 2D CMOS/CCD cameras
Receiving telescope: 200 mm diameter, 800 mm focal length
Filters: Interference filters + absorption filters, e.g., 3 nm FWHM
Here are the system schematic and picture. The laser diode in installed in a customized mount to stabile the temperature, as shown in the corner. Backscattering light is collected by a 200-mm Newtonian telescope, a 2D cmos camera which is 45 degree tilted to the optical axis is mounted to detect the backscattering image. The separation between the transmitting telescope and the receiving telescope is about 806 mm.

8. Atmospheric Aerosol Sensing
Urban area monitoring, Dalian
All-time measurements in March
Extinction coefficient Map – Klett-Fernald Retrieval
Extinction coefficient and particle concentration – highly correlated
Measurement scheme
Wavelength: 808 nm, 3.2 W
Exposure time: 20 ms
Signal averaging: 1000 times
Response time: 45 s for single measurement
SNR: beyond 100:1 under sunshine
In 2017, we have carried out measurements on a near horizontal path in the urban area of Dalian, Northern China by using a Mie scattering 808 nm Scheimpflug lidar system, The output power of the laser diode is about 3.2 W, the exposure time is 20 ms, the signal has been averaged for 1000 times. The SNR for the near range lidar signal is beyond 100:1 under full sunshine. It should be emphasized that the SNR does not decrease with the distance square. The extinction coefficient is retrieved according to the Klett-Fernald method. You can clear see from this figure. During this period, it is a severe haze, and the pollution is blew away by wind, and we get a small extinction coefficient, the atmosphere could also be homogeneous. If we calculate the statistic median extinction coefficient along the laser beam path, we can compare the result with the particle concentration measured by a local monitoring station.

9. Atmospheric Aerosol Sensing
Portable Scanning Scheimpflug Lidar System
Laser: 4 W 808 nm laser
Receiver: F5 refractor, 150 mm diameter
Collimator: F6, 100 mm diameter
Receiver-transmitter separation: ≈756 mm
Pollution source tracking
90° scanning, 1° step
10 s for each,15 mins in total
haze condition
Dalian city
Motivated by the promising results.
Liang Mei, et al., low-cost portable SLidar system, in manuscript, 2018.

10. Atmospheric Aerosol Sensing
405 nm SLidar system
All-time operation, mist/cloud emitted
1.7 nm filter
1.2 W laser
Extinction coefficient Map
520 nm SLidar system
Saturation under strong sunlight condition
Stronger sunlight background
Broader FWHM of filter, 10 nm
Solutions:
Narrowband filter, or larger full-well capacity
10 nm filter
1 W laser

11. Atmospheric Aerosol Sensing
Dual-band Scheimpflug Lidar System
Angstrom map
808 nm backscattering
405 nm backscattering
4 W 808 nm laser diode, 3 nm filter
1.2 W 405 nm laser diode, 1.7 nm filter
24-hour operation in May
45 s for single recording
808 nm extinction
405 nm extinction
Zheng Kong, et al., dual-band SLidar in manuscript, 2018.

12. Atmospheric Aerosol Sensing
Polarization Scheimpflug lidar
Time-multiplexing principle: dual light sources, single CMOS detector
Parallel and perpendicular polarized lidar signals:
Transmitting
Receiving
Depolarization ratio
Polarization system constant:
Liang Mei, et al., Optics Letters, 2017

13. Atmospheric Aerosol Sensing
Polarization Scheimpflug lidar – vertical looking
Typical lidar signals
Backscattering map
Depolarization map
Performance limited by SNR of the perpendicular channel
Liang Mei, et al., Optics Letters, 2017

14. Differential absorption lidar – NO2
System specifications
Schematic and Principle
Multimode high-power laser diodes：1.6 W, 450 nm, Osram, wavelength stabilized
High sensitivity CCD sensor for spectrometer（Hamamatsu S11071）， 28*0.896 mm,14 μm pixel pitch
On-wavelength: nm, Off-wavelength nm, controlled by driving current
Monitored by side-placed spectrometer
Liang Mei, et al., Optics Express, 2017

15. Differential absorption lidar – NO2
NO2 DIAL Measurements
Nighttime measurements on a near horizontal path
Detection sensitivity: 0.9 km, 1.5 km, 2.4 km, 15 mins.
SNR limited by the photon-response non-uniformity (PRNU) noise of the image sensor
Signal de-noising method should be applied

16. Conclusions and Perspectives
The Scheimpflug Lidar technique has a great potential for atmospheric aerosol and gas monitoring.
The system cost and complexity can be greatly reduced. The cost is only about 10%-20% of the conventional pulsed lidar system.
Low-cost, compact and portable SLidar system has been developed, and improvements are planned in the near future.
Scheimpflug lidar systems operated in various wavelengths have been demonstrated. Multiple-wavelength aerosol lidar for particle sizing studies, etc., to be demonstrated soon.
Range resolved NO2 monitoring is feasible, and the performance is continuously improving.

17. Thanks for attention! Q&A?
Research group
Liang Mei
Peng Guan
Zheng Kong
Zhi Liu
Lishan Zhang
Limei Li
Yiran Xu ……
Funding and Support:
National key R & D program 2016
Natural Science Foundation, China
Natural Science Foundation of Liaoning Province
Fundamental Research Funds for the Central Universities

18. Differential absorption lidar
DIAL principle
（1）
（2）
（3）
（4）
Concentration distribution
Absorption curve
Lidar signal
Measurements