DIAL

DIAL (Differential Absorption Lidar) is a diagnostic technique for the measurement of pollution molecules.
Two wavelengths are delivered. The first wavelength is absorbed by the molecule under check, while the second one is used to normalise the measurement of the intensity of the back-scattered light. 

A Ti:Sapphire tunable laser, pumped by a YAG laser is used for DIAL measurements.
All the regions of interest are well covered by the spectrum of the direct emission of the Ti:Sa laser and by its harmonics.
This solution has reached, nowadays, a high level of reliability and efficiency.

The Ti:Sa laser is pumped by the Nd:YAG laser. This solution is generally adopted because of many advantages, such as short pulse emission and longer lifetime.
Through the emitted wavelength it is, for example, possible to detect NO2 (50 MDC - ppb) , SO2 (20 MDC - ppb), O3 (17 MDC - ppb) and NO (10 MDC - ppb) molecules, benzene and toluene.

The DIAL set up can be also used as an improved technique in the determination of trace pollutants and physical parameters of the atmosphere.

Typical composition of a DIAL system

Transmitter

Receiver

Laser control
The laser control, i.e. the Nd:YAG pockels' cell driver, the wavelength tuning, the doubling and mixing crystal angular tuning, is performed by means of a PC.
A set of subroutines, written under LabVIEW, allows to determine, for a given pollutant gas, the most suitable wavelength pair and the control parameters for each laser subsystem.

Calibration cells
To ensure DIAL self calibration using the same gas to be detected, a set of calibration cells is mounted in the laser box. Cells consist of a fused silica tube with Brewster fused silica windows, filled with a mixture of a given concentration of pollutant gas with air or nitrogen. A couple of photodiode detectors allow to monitor the differential transmission of the gas inside the cell, thus allowing the evaluation of the differential absorption cross section at the two wavelengths emitted by the laser. This procedure overcomes the problems connected with an absolute wavelength calibration of the laser.
 
Control electronics
The control electronics essentially consist of an acquisition system and of a Nd:YAG laser-linked timing unit, a telescope zenithal and azimuthal movement control unit, and a computer interface. The purpose of this subsystem is to automate the DIAL measurement procedures by entrusting its control to a minicomputer.
The laser supplies alternating λ1 and λ2 pulses. A portion of the laser beam is sent to the data acquisition system for an energy and wavelength check by means of a sampling cell containing the gas being tested. The remaining laser beam is sent to the atmosphere.
Since the picked-up LIDAR signal has a very wide dynamic range, the receiving photomultiplier gain is time-modulated in accordance with the T2 law.
The pre-amplified photomultiplier signal is subsequently sent to the transient recorder where it is sampled, recorded and transferred to the computer memory.
The Nd: YAG laser operating at clock rate of 10Hz clocks the whole control electronics
through a flash light trigger (generated within the laser power supply unit) that is being sent to a broad-band operational amplifier and discriminated by a fast comparator.
The control electronics minimise the phase shift between the transient recorder start-up, laser pulse emission, T2 modulation pulse and other functions with the transient recorder clock. The control electronics also supplies the control signals to the step motors for the telescope zenithal and azimuthal motion. The computer enters the values of the zenith and azimuth angles in the registers provided for this purpose, and a servomechanism links the telescope position to the register code.
It is also possible to tune the titanium sapphire laser beam to one or two alternating wavelengths.

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