Nhng tin b trong Quang hc, Quang ph và ng dng VI ISSN 1859 - 4271

Những tiến bộ <strong>trong</strong> <strong>Quang</strong> học, <strong>Quang</strong> <strong>ph</strong>ổ và Ứng dụng VI ISSN 1859 - 4271M2. A half-wave plate allows the reorientation of the polarization direction of the laser beam at532 nm. That allows us to measure the depolarization of the LIDAR signal at 532 nm. Thebackscattered light is collected by a Cassegrain telescope of diameter 20 cm and f/D=10. Thefield of view of the telescope is selected by using a pinhole of appropriate diameter located at thefocal plane of the telescope. In our measurements we usually use a field of view of 2 mrad,which corresponds to a pinhole size of 4 mm. A dichroic mirror then separates the backscatteredlight into 532 nm and 1064 nm channels. We also use a polarizing beamsplitter cube to split thebackscattered light into parallel and perpendicular polarization with respect to the polarizationdirection of the laser beam.Fig. 1. Block diagram of the lidar system developed at the Institute of PhysicsThe lidar signal is then detected by an avalanche <strong>ph</strong>otodiode (Hamamatsu S9251) in the 1064nm channel and by a <strong>ph</strong>otomultiplier tube (Hamamatsu R7400U) in the 532 nm channel. A blockdiagram showing the details of our LIDAR system is presented in Figure 1. The signals comingfrom the detectors are then processed in either analog or <strong>ph</strong>otoncoun<s<strong>trong</strong>>tin</s<strong>trong</strong>>g mode. In analog mode,the signals are pre-amplified and then sampled directly by a 12-bit digitizer connected tocomputer through the USB port. To detect the faint LIDAR signal from large distances or theRaman signal from Nitrogen molecules, our system also uses the <strong>ph</strong>oton coun<s<strong>trong</strong>>tin</s<strong>trong</strong>>g technique. Theshort single <strong>ph</strong>oton pulses (FWHM ~ 1.5 ns) coming out of the <strong>ph</strong>otomultiplier tube R7400U(from Hamamatsu) are amplified and stretched by a high speed amplifier. As the result, theamplified <strong>ph</strong>oton pulses are stretched to a FWHM of ~10 ns. These broader pulses are thensampled by a fast digitizer at a sampling rate of 100 Ms/s. True <strong>ph</strong>oton pulses are thendiscriminated against noise pulses by set<s<strong>trong</strong>>tin</s<strong>trong</strong>>g a pre-selected voltage level. The coun<s<strong>trong</strong>>tin</s<strong>trong</strong>>g of<strong>ph</strong>otons and forming of the LIDAR signal histogram are done in real time by a program writtenin Labview.III. AEROSOL MEASUREMENTS WITH THE LIDAR SYSTEMOur LIDAR system has been used to make measurements of the aerosols at two wavelengths532 nm and 1064 nm together with the Raman signal from Nitrogen at wavelength of 607 nm.An example of a series of the measured lidar signal is shown in Figure 2. The data from theelastic Mie/Rayleigh channel at 532 nm clearly indicates the a thick aerosol layer, which extendsup to 3000 m. The measurements clearly show the temporal evolution of the aerosol layer which145