Application Notes to Lidar Instrumentation

LIDAR (an acronym for Light Detection and Ranging) is a remote sensing technique similar to radar that is being used extensively in fields ranging from the surveillance of gaseous effluents from smoke stacks, airplanes engines etc. to atmospheric research.

The Firepond Lidar at Millstone Hill / MIT Haystack Observatory by Th. J. Duck
Differential Absorption LIDAR using a DA500A Transientrecorder
Fast ComTec offers a comprehensive range of LIDAR instrumentation ranging from Transientrecorders to ultra fast "time resolved" Single Photon Counting Systems.
The vertical laser beam from the atmospheric lidar being developed by the Australian Antarctic Division and the University of Adelaide against a background of trailing stars.


Short Range Lidar

A short range lidar system measures the characteristics of pulses of laser light scattered by gases and aerosols to determine the structure and composition of an effluent.
The main components of the lidar system are 
• laser, which produces brief pulses of high intensity light
• An optical device, which directs the laser emission to the object to be analyzed
• collects the light back-scattered by the object
• A light sensitive detector (PMT etc.)
• A Transientrecorder with a high sampling rate
• A computer system and appropriate software
At short ranges the light sensitive detector produces a current that is sampled by a fast Transientrecorder. The distance to the object determines the sampling rate which can be in the MHz to two GHz range. The back-scattered light from many laser shots are either stored in a fast memory or processed in a Digital Signal Processor or computer. Responses from repeated shots from the Laser must be summed to improve the statistical precision.
Most of our Transient recorders have a unique function which allows to segment the data memory. Successive shots can be sampled in successive memory segments which drastically increases the shot-rate capability of the lidar systemThe diagram shows a typical Lidar system using a Transientrecorder to collect the data To select the Transientrecorder best suited for your requirements click here.


Lidar in Atmospheric Research*

The following are excerpts from the web-page of the Australian Antarctic Division
Lidar is well suited for the remote sensing of the atmosphere, particularly in the upper stratosphere (between altitudes of 35km and 50km) and mesosphere (between altitudes of 50km and 90km). In these regions, in situ measurements are difficult, while other remote sensing methods (e.g. radiospectral measurement of molecular emission lines) do not achieve the level of spatial and temporal resolution that is possible with lidar.

Concern and uncertainty relating to global climate change has focussed attention on the upper atmosphere. Theoretical studies have suggested that anthropogenic climate change in the troposphere may lead to more clearly detectable changes at higher altitudes. In this context, there is considerable interest in Antarctica owing to its remoteness from major industrial regions, its considerable influence on the global climate, and the extreme climatic conditions that prevail above the region.

There are relatively few lidar systems capable of probing the mesosphere, particularly in the Southern Hemisphere. In order to address this situation and to contribute to the debate on global climate change, the Australian Antarctic Division and the University of Adelaide (South Australia) are collaborating in the development of novel atmospheric lidar. This instrument will be operated at Davis, Antarctica (68.6° S, 78.0° E) from 1999. Since early 1997, the lidar has been undergoing assembly and testing at Kingston, Tasmania.

Two main measurement techniques are employed in the lidar. Firstly, the Rayleigh lidar technique is used to profile atmospheric density, temperature and aerosol loading from the lower stratosphere (above 10 km altitude) up to the mesopause region (~95 km altitude). Secondly, the incoherent Doppler lidar technique is used to profile temperature and wind velocity up to the lower mesosphere (~65km altitude).

Technique
In Rayleigh mode, the lidar measures the intensity of laser light back-scattered by the atmosphere as a function of range. This information is combined with in-situ radiosonde measurements or an assumed density value at a particular altitude (usually where the contribution to scattering due to aerosols is minimal) to recover the density-altitude profile. Account is made of the background scattered light and instrument noise by ‘off-pulse’ measurements or by fitting a background term to the signal versus range profile. By assuming that the atmosphere obeys the perfect gas law and is in hydrostatic equilibrium, the density measurements are used to recover a temperature profile by an iterative method which works down in altitude from a level of known or assumed atmospheric pressure. In addition, the variation of back-scattered signal as a function of altitude, when combined with a model of the molecular atmosphere yields information on the importance of aerosols in the scattering process.

The Rayleigh technique is used in two additional types of measurement in the lower stratosphere. Firstly, sequential back-scatter measurements at particular azimuth angles with a fixed zenith angle are used in a ‘moving pattern’ analysis to infer wind velocity. Secondly, measurement of rotation-vibration Raman back-scatter is used to profile the density of molecular oxygen or molecular nitrogen.

During the scattering process, the laser spectrum is broadened by thermal motion of the molecules and aerosols. Bulk motion of the scattering medium relative to the lidar results in the Doppler shift of the laser spectrum. By measuring the spectrum of the back-scattered light and accounting for the spectral response of the laser and detection system, it is possible to infer both temperature and wind speed as a function of altitude. Doppler wind speed measurement for a particular altitude made along several lines of sight can be used to infer the prevailing wind direction.

Lidar measurements in the mesosphere are difficult to make, primarily because the atmosphere at these altitudes is extremely tenuous. Air density decreases by a factor of 10 for each altitude increment of 15 km. Normally, the back-scattered light collected by a lidar requires integration over many laser pulses to achieve a desired accuracy. The integration time required by the Davis lidar for mesospheric measurements will be comparatively lower than most other systems owing to the high power of the laser , the large collecting area of the telescope, and the high level of discrimination to background light afforded by the detection system.Model profiles of density and temperature for the atmosphere above Davis, Antarctica (*)


Lidar Instrumentation

The main components of the lidar are

• An injection-seeded Nd:YAG laser, which produces brief pulses of high intensity green light (at a wavelength of 532nm). The laser has an average power of 30W, a pulse length of 7ns and a repetition rate of 50Hz.
• 1 metre diameter Cassegrain telescope, which directs the laser emission into the sky, and collects the light back-scattered by the atmosphere. The primary mirror of the telescope is made from aluminium and has a nickel coating. The telescope has an alt-azimuth mounting, and can be pointed to a maximum zenith angle of 45° . In order to achieve maximum range performance for Rayleigh measurements, the laser beam is directed vertically via a beam expander rather than through the telescope.
• A high speed rotating shutter (the 'Mirror Shutter') which switches the system between transmit and receive modes.
• A dual-etalon Fabry-Perot spectrometer which analyses the back-scattered light. The central wavelength of the spectrometer is scanned for Doppler measurements and is fixed for Rayeligh measurements. The minimum bandpass of the spectrometer is approximately 1pm.
• A photomultiplier operating in photon-counting mode, which detects the light filtered by the spectrometer. Side elevation of the lidar showing the main structural arrangement. (*)

The Photon Counting System
The lidar employs a single fast EMI photomultiplier in a cooled housing as the detector. The photomultiplier is followed by a FAST ComTec Model 7011 amplifier/discriminator which outputs NIM pulses to a FAST ComTec Model MCD-2 multichannel scaler.

The diagram shows the MCD-2 Multichannel Scaler in a two input channel configuration
The MCD-2 is operated in two different modes depending on the type of observations being conducted. In the case of Rayleigh observations, the NIM pulses are simply binned according to range. The data are typically accumulated for 1500 laser firings (30 seconds) and then written to disk, after which the process is repeated. A typical profile is shown below, with the intensity of the received signal on the vertical axis, and range on the horizontal axis. In this example, the bin width is 625ns (equivalent to ~93m), and the maximum altitude is approximately 96km. The profile up to an altitude of approximately 20km is influenced by a mechanical shutter which progressively allows more light to reach the photomultiplier with range in order to counteract the high levels of back-scatter from the lower atmosphere. The flexibility of the MCD-2 allows a variety of different bin widths and number of bins to be selected.

For Doppler observations, the MCD-2 is operated in memory segmentation mode. While the Fabry-Perot spectrometer is scanning, range-gated counts are accumulated in a different segment of memory each time the spectrometer moves to a new wavelength channel. Typically, counts are range-gated into 512 bins and accumulated over 8 laser firings before moving to a new memory segment. In this way, up to 256 spectral channels are available. At the end of a spectral scan, the process is repeated. After an elapsed time of approximately 5 minutes the data are written to disk, and then a new accumulation commences.

To select the Single Photon Counting System best suited for your requirements click here.

For further information datasheets of the instruments can be downloaded.

If you have any questions please contact us at: info@fastcomtec.com
(*) reprinted with kind permission of the Atmospheric and Space Physics Section of the Australian Antarctic Division, Tasmania.