What is LIDAR?
LIDAR is the acronym for Light Detection and Ranging. LIDAR implies probing the atmosphere optically, and then gathering information based on the returned scattered signal. LIDAR has higher precision and better vertical resolution than satellites, and actually serves to calibrate them. As a remote sensing system, it is particularly well suited to the observation of aerosols and pollutants, wind and turbulence measurements, concentrations of various gases, as well as humidity and temperature profiling. The advantage of LIDAR lasers is that one can have an on-line monitoring system that is non-intrusive, giving real-time profiles over a range of altitudes.
The intended application of the LIDAR laser system determines the specifications of the laser used. In the every increasing field of environmental monitoring, the CO2 laser is found to be an ideal system for LIDAR monitoring of pollutants at low altitudes. A typical application might see a CO2 LIDAR laser system used to monitor the emissions from a primary stack in a refinery. At PaR Systems (Pty ) Ltd we have designed several custom TEA CO2 systems for LIDAR applications.
The LIDAR lasers have been designed according to the specifications required for the application. One improvement to the design that is now offered on all of our LIDAR systems is a dual grating control system for fast switching between laser lines.
It has recently been found that fast switching between two laser lines can realize better detection resolutions. The idea is based on the differential absorption/scattering of the two lines. For this to be realized in a LIDAR system, the number of pulses on each line must be known, and the time to switch between lines must be very fast. At PaR Systems (Pty) Ltd we have developed an agile grating control system that can be fully integrated into our laser control. A schematic is shown below.
In this system, the rotating mirror selects the wavelengths based on user-defined selections. The switch over time between the wavelengths selected allows single pulse wavelength selection at repetition rates up to 200Hz.
The system is controlled via a Windows type interface, and is seamlessly integrated into our laser control. Menu options allow the user to specify recipe type operations. This might include a sequence of operations to follow one another. The system stores this into memory, and executes the sequence. If the sequence is to be used again, it can be stored in file format, and recalled.
The wavelength selection range is determined by both the bandwidth of the CO2 lasing lines, and the positional accuracy of the grating tuning system. The laser used determines to a large extent the wavelength tuning range available. In high-pressure CO2 lasers, pressure broadening of the gain lines allows the laser to tune continuously from 9 to 11μm.
In atmospheric CO2 lasers, or TEA CO2 lasers as some are commonly named, the laser is line tunable in roughly the same range.
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