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Can laser radar systems be designed to be more powerful?

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Xie Chenbo,
Key Laboratory of Atmospheric Composition and Optical Radiation (LACOR),
CAS of the Anhui Institute of Optics and Fine Mechanics

As an advanced means of atmospheric sensing, laser radar systems need profile information with high spatial resolution, high time resolution and high measurement accuracy. There is no doubt that outstanding data acquisition units are a key link for such systems.

With the development of current electromagnetic wave radar detection technology from the centimeter or millimeter wave level to the light wave level, the combination of conventional laser technologies and modern laser technologies leads to laser radar having shorter light wave lengths. This is convenient for realizing active remote transmission of optical and physical characteristics and meteorological parameters of the atmosphere, making use of the mutual effects of the laser and the air molecules and aerosol particles in the atmosphere. Meanwhile, with the beneficial characteristics of lasers in terms of monochromaticity, directionality, coherence and high luminance, lasers are unmatched by conventional atmospheric sensing means in terms of measurement levels and accuracy, time resolution, spatial resolution and continuous automatic monitoring. After dozens of years of development, laser radar is now extensively applied in such study fields as laser propagation in the atmosphere, aerosols and cloud radiation, atmospheric environment monitoring, global climate models, and more.

Laser radar is an advanced means of atmospheric sensing, and its characteristics result in its high speed and high accuracy requirements for data information. Profile information with high spatial resolution, high time resolution and high measurement accuracy is necessary for laser radar. This is also the biggest challenge for laser radar data acquisition units. Specifically, according to the aerosol and cloud time and spatial distribution in the atmosphere, it is necessary in the design of a laser radar systems to make sure data acquisition has a spatial resolution within 30m, a time resolution within 15min, a measuring signal dynamic range above 104 and a minimum measurement accuracy below 1mV. Considering the challenges and requirements, we finally chose the ADLINK PCI-9846 data acquisition digitizers as the data acquisition units for the laser radar systems to realize the purpose of sensing the aerosols and clouds in the atmosphere at heights of 0km to 15km.

The key to success

A laser radar system consists essentially of a laser emission unit, an optical receiving unit, a signal detection unit, a data acquisition unit, and a control unit. Mainly used for A/D conversion and sampling for the electrical signals output by the signal detection unit, the data acquisition unit determines the success of the laser radar system directly.

Due to the time and spatial distribution characteristics of the atmosphere and the measuring principles of laser radar systems, laser radar systems also have the following special requirements for their data acquisition units.

  • Measuring sensitivity: the actual atmospheric echo signals measured by laser radar systems are typically weak signals; therefore, the data acquisition units of laser radar systems must have high measuring sensitivity.
  • Data acquisition range: air molecules and aerosol particles, which are the detection targets of laser radar systems, exist widely in the troposphere, with a height of 0km to 15km, and their concentrations have an exponential decrease with an increase in height; therefore, the data acquisition units of laser radar systems must have wide data acquisition ranges to meet the sampling needs of signals with large dynamic ranges.
  • Time resolution and spatial resolution: actual atmospheric conditions are non-uniform in both time and space; therefore, the data acquisition units of laser radar systems must have high time resolution and spatial resolution, so that the detailed atmospheric distribution information can be determined effectively.
  • Response capability: laser radar systems must provide accurate spatial distance information; therefore, their data acquisition units must have super-fast response capabilities.
  • Anti-electromagnetic interference capability: laser radar systems are precise optical, mechanical and electrical measuring systems containing both strong currents and weak currents; therefore, their data acquisition units must have excellent anti-electromagnetic interference capabilities.

Data acquisition units are important parts of laser radar systems, and their performance parameters determine the detection performance and accuracy of the laser radar systems directly.

Making laser radar systems more powerful


Fig. 1 Outside View (Left) and Structural Diagram (Right) of a PRML Laser Radar System

Polarization-Raman-Mie-scattering LIDAR systems (PRML laser radar systems) are active atmospheric sensing units specially developed for the China Meteorological Administration. They are mainly used for the continuous and automatic measurement of the time and spatial distribution characteristics of the optical parameters of the aerosols (inclusive of polluted aerosols, dust, haze, and so on) and clouds (inclusive of water clouds, ice crystal clouds, and so on) in the atmosphere in the troposphere, from a height of 0km to 15km (Fig. 1).

The PRML laser radar system uses a 532nm frequency-doubled, linearly polarized laser of a Nd:YAG laser. Its subsequent optical detection unit is composed of three detection channels, including a Raman detection channel for measuring the 607nm Raman scattering signal generated following the reaction between the 532nm outgoing laser and the particles in the atmosphere, a vertical detection channel for measuring the vertical component of the Mie scattering signals generated following the same reaction, and a parallel detection channel for measuring the parallel component of the Mie scattering signals generated following the same reaction. The vertical distribution characteristics of the optical parameters of the aerosols and clouds in the troposphere can be obtained based on the measured data of the three detection channels and the abovementioned inversion method. The 532nm vertical detection channel and the parallel detection channel both use ADLINK PCI-9846 high-resolution digital acquisition digitizers for data acquisition.

To meet the measurement requirements for aerosols and clouds in the atmosphere, PRML laser radar products should have a spatial resolution of 7.5m, a time resolution of 8min, a measurement range of 0km to 15km, and dual-channel signal input. Thus, ADLINK PCI-9846 high-resolution digital acquisition digitizers are used as the data acquisition units. In Table 1, the performance parameters of ADLINK PCI-9846 high-resolution digital acquisition digitizers and the performance parameters of the corresponding laser radar systems are compared.

Performance parameters of ADLINK PCI-9846 high-resolution digital acquisition digitizers Performance parameters of the corresponding laser radar systems
Input channel 4 Measuring channel 4 (2 channels are used, in fact)
Sampling rate 20MHz Spatial resolution 7.5m
Sampling accuracy 16bit Dynamic range of signals 32768
Input voltage ±1V Measurement accuracy (voltage) 0.03mV
Input impedance 50Ω Measurement accuracy (current) 0.61uA
Triggering response time < 20ns Range accuracy 3m

Table 1. Main Performance Parameters of ADLINK PCI-9846 High-resolution Digital Acquisition Digitizers and the Main Performance Parameters of the Corresponding Laser Radar Systems

It can be seen that each ADLINK PCI-9846 high-resolution digital acquisition digitizer has four channels for the concurrent analog data input of four channels, and each laser radar system can realize the concurrent analog data input of four channels. The PRML laser radar system mentioned in this paper uses only two input channels. ADLINK PCI-9846 high-resolution digital acquisition digitizers have a sampling rate of 20MHz, and the corresponding laser radar systems have a spatial resolution of 7.5m, which fully meets the high spatial resolution requirement of atmospheric sensing. ADLINK PCI-9846 high-resolution digital acquisition digitizers have a sampling accuracy of 16bit, and there are 65,536 corresponding sampling points. However, the laser radar detectors output negative currents, so there are actually 32,768 sampling points, and the dynamic range of the sampled signals is above 104, which covers the actual signals detected at a height of 0km to 15km. ADLINK PCI-9846 high-resolution digital acquisition digitizers have an input voltage of ±1V and an input impedance of 50Ω, and the corresponding actual measurement accuracies of laser radar systems are 0.03mv and 0.61uA, respectively, so the digitizers have excellent weak signal detection capabilities. The external triggers of ADLINK PCI-9846 high-resolution digital acquisition digitizers are TTL pulse electrical signals with a response time shorter than 20ns, and the corresponding laser radar systems have a range accuracy of 3m and a high range positioning accuracy. Meanwhile, ADLINK PCI-9846 high-resolution digital acquisition digitizers can be conveniently inserted into the standard PCI slots of industrial personal computers, have fast data transmission speeds, and great anti-electromagnetic interference capabilities. All of the parameters above show that ADLINK PCI-9846 high-resolution digital acquisition digitizers are suitable for the data acquisition of PRML laser radar systems.

Verifying the high quality of results

According to the results of the comparison of the data and graphs with the original signal, effective signal and signal-to-noise ratio of ADLINK PCI-9846 high-resolution digital acquisition digitizers, the quality of the data measured by the digitizers complies with the measurement requirements of laser radar systems for aerosols and clouds in the atmosphere.

The ultimate goal of atmospheric sensing by laser radar systems is not just to obtain high quality echo signals but to obtain the actual optical characteristics of the atmosphere through an inversion method. The quality of the inversion results of the data measured by laser radar systems is directly determined by the precision and accuracy of the sampled signals; therefore, the only standard for measuring the quality of the sampled signals of laser radar systems is the inversion result of the sampled signals. It is proved by the analysis and comparison results of the actual optical characteristics of the atmosphere that the performance parameters of ADLINK PCI-9846 high-resolution digital acquisition digitizers comply with the measurement requirements for the optical properties of the aerosols and clouds in the troposphere at a height of between 0km and 15km. Furthermore, according to the measurement results of the depolarization ratio, ADLINK PCI-9846 high-resolution digital acquisition digitizers have two channels for simultaneous high quality signal sampling.

In order to verify the capability of the long-time continuous operation of ADLINK PCI-9846 high-resolution digital acquisition digitizers, a continuous and automatic measurement lasting for six days, from August 10th to 15th, 2010, was conducted for PRML laser radar systems. The time interval and spatial resolution of the measurements were 30min and 7.5m, respectively. According to the results of the measurements, ADLINK PCI-9846 high-resolution digital acquisition digitizers have reliable capability for continuous operation and are suitable for deployment in the continuous and automatic operating mode of laser radar systems.

In summary, it can be observed from the high quality results obtained in real-world tests that ADLINK PCI-9846 high-resolution digital acquisition digitizers can meet the requirements of laser radar systems for data acquisition. This enables the laser radar detection systems to be designed to be more powerful.

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