design of a high spectral resolution lidar for atmospheric monitoring in eas detection experiments
Post on 29-Jun-2016
Embed Size (px)
Design of a High Spectral Resolution Lidar for Atmospheric Monitoringin EAS Detection Experiments
E. Fokitisa, P. Fetfatzisa, A. Georgakopouloua, S. Maltezosa, and A. Aravantinosb
aPhysics Department, National Technical University of Athens,9, Heroon Polytechniou, ZIP 15780, Athens, Greece
bPhysics Department, Technological Educational Institution of Athens,Agiou Spiridonos, ZIP 12210, Athens, Greece
A High Spectral Resolution Lidar (HSRL) is designed in order to achieve high accuracy measuring the aerosol tomolecular scattering coecients ratio. This type of Lidar consists of a Continuous Wave Single Longitudinal Modelaser beam at 532 nm, a receiver with a parabolic mirror and analyzes spectrally the scattered light in a Fabry-Perot Cavity. The great wavelength sensitivity of this system allows the separation of the aerosol and molecularcomponent due to their dierent levels of Doppler eects on the scattered laser light. We present a study of theeects of the various levels of CCD cooling on the sensitivity of this. Firstly we discuss preliminary experimentalresults based on a Fabry-Perot etalon with free spectral range (FSR) 0.1 cm1, the expected performance of aetalon with FSR of 0.05 cm1, under construction, and with nally an aerosol parameter analysis in SimulationCodes for EAS.
In this paper we present the work of our teamin the atmospheric monitoring issue for EAS De-tection experiments. In the measurements of air-uorescence yield for the detection of Ultra HighEnergy Cosmic Rays, there is a contribution dueto the air-Cherenkov signal scattered mainly bythe aerosol particles. Therefore, there is a need tomonitor the degree of this type of scattering in or-der to correct the total signal and obtain the air-uorescence signal. The most common methodfor measuring the aerosol induced scattering isusing the Elastic or Raman Lidar Method. Inthis work, we present the progress in an alter-native method using the High Spectral Resolu-tion Lidar (HSRL). This is continuation of recentwork , where now we emphasize on the studyof the performance of a candidate laser source forthe HSRL. This apparatus is designed in order toachieve high accuracy measuring the aerosol tomolecular scattering ratio. It consists of a Con-tinuous Wave Single Longitudinal Mode (SLM)Laser beam at 532 nm, a receiver with a parabolic
mirror, and analyzes spectrally the scattered lightpassing through Fabry-Perot cavities.
We mention that , rst reported a highspectral resolution lidar (HSRL) using a narrow-band laser and a high resolution Fabry-Perotetalon to separate the aerosol (Mie) and molecu-lar (Rayleigh) scattering. More recently, (see ),demonstrated aerosol and temperature measure-ments using a HSRL based on an iodine vapor l-ter, and the temperature in the stratosphere wasobtained from the Mie-ltered signal.
The HSRL can give simultaneously, for eachatmospheric height layer, both the aerosol andthe molecular scattered intensity from a groundbased laser emitter. This capability is due tothe interferometric measurement method, that is,the Fabry-Perot interferometer in the LIDAR re-ceiver. Additionally, in the aerosol scatteringHSRL, the narrow linewidth of the laser trans-mitter makes it possible both to separate the con-tributions from aerosol particles and atmosphericmolecules, and to calculate the aerosol extinctionwithout assuming the lidar ratio (dened as theratio between the extinction and backscattering
Nuclear Physics B (Proc. Suppl.) 190 (2009) 261265
0920-5632/$ see front matter 2009 Elsevier B.V. All rights reserved.
coecients) . We take advantage of the re-cent technological developments in the solid statediode lasers which have lead to cost aordableHSRL emitters.
In section 2 we describe the laser source usedin this HSRL. The section 3 deals with the char-acterization of the molecular channel to be usedfor determining the scattering coecient due tothe atmospheric molecules (3.1), while the aerosolchannel characterization is presented in 3.2. Thelaboratory emulation scattering studies of theSLM laser beam is presented in Section 4. Thesection 5 discusses the limitations in the sensitiv-ity of the HSRL prototype. The Section 6 consid-ers the prospects of the present research eorts.
2. Laser sources
2.1. SLM laserWe need a very narrow line-width laser source
so that the line source uncertainty is much smallerthan the Doppler broadening of the scattered ra-diation from the air molecules. Only in such casewe may separate the Mie from molecular scat-tering using a high spectral resolution spectrom-eter as the one we will be describing. We areusing a Diode Pumped Solid State Laser contin-uous wave (CW) SLM laser at 532 nm (calledSLM at the rest of the paper) with a nominalpower of 100 mW and coherence length greaterthan 50 m or correspondingly, wavenumber uncer-tainty k =0.02 cm1. The tests conducted wereaiming at studying the stability of the interfer-ence fringe patterns of the 5 cm spacer etalon asthe current driving the SLM laser gradually is in-creased to the nominal value. The results will bepresented in the Section about the aerosol chan-nel. However, the question of the absolute laserwavelength cannot be answered by such measure-ments. The measurements so far conducted haveindicated that at the nominal operation condi-tions, there is a dominant operation mode and asecondary mode with intensity of the order of 2-4 % of the one of the dominant mode as will beshown in 2.3. For the absolute wavelength (or fre-quency) determination, one would need a facilitywhich would include a laser frequency standard.However, we believe that the knowledge of the
absolute frequency is not necessary for the aimsof the HSRL.
2.2. Procedure for laser stabilizationThe specic solid state laser we use includes
some proprietary design, but basically, it con-sists of an optical cavity and some lasing medium,while the emitted line structure depends on thetemperature of the lasing medium as the tem-perature is a thermodynamic quantity which isimportant in aecting the longitudinal and trans-verse modes. For this reason, the SLM laser hasa controllable excitation current and we had tostudy the laser line structure as a function of thecurrent as well as of its rate of change. The useof the available Fabry-Perot spectrometers cangreatly help in revealing the laser line structureand its changes as a function of excitation current.In comparison with He-Ne mode competition, wehave the dominance of a single mode as discussed.The minimum level of the intensity in this plot isconsistent with the expected curve of Airy func-tion. No noise substraction has been performedsince the latter is expected to be negligible as theexposure time was around 1/4000 s.
3. Molecular and Aerosol channels
3.1. Molecular channelThe radiation approaching the receiver, after
being scattered by the air molecules, is typicallyreected by a beam splitter and is directed to themolecular channel, while the rest is transmittedto the aerosol channel. In order to characterizethe molecular channel we perform the followingmeasurements: We have obtained interferogramsof a platinum-neon (Pt-Ne) Hollow Cathode spec-tra lamp operating at ambient as well as at highertemperatures. We have also used an interferencelter at 562 nm, with a passband 30 5 nm inorder to select a specic spectral line of Pt-Nespectrum. In this range there are two line ofNe, namely at 585 and 588 nm, possibly someplatinum lines. The interferogram presented istaken with the 2 cm etalon spacer and the aboveinterference lter. The two intensity histogramsindicate that the rst one, corresponding to coldlamp, is less inuenced by the Doppler eect than
E. Fokitis et al. / Nuclear Physics B (Proc. Suppl.) 190 (2009) 261265262
Figure 1. The two intensity histograms indicatethat the rst one, corresponding to cold lamp,shows smaller Doppler Width than the one at thesecond histogram.
the one at the second histogram, where the tem-perature is higher. The result is presented inFig. 1. This plot is a display of the superpo-sition of the interference patterns correspondingto the atomic transitions involved. As the FSRis given by FSR = 1/2d, where d is the etalonspacer length, selecting an etalon of d=5 mm weare able to study eectively the molecular scatter-ing. Thus, we plan to make corresponding mea-surements with available etalons having spacerlengths 2 mm and 5 mm, respectively, which arecapable to contain the whole line-breath of thelaser radiation scattered and therefore broadenedby the air molecules.
3.2. Aerosol channelWe have studied in a laboratory the perfor-
mance of the aerosol channel using two dierentlasers, one at 632.8 nm, based on an available He-Ne laser, and the other was the SLM laser, thelatter having appropriate power and linewidth tobe used in a HSRL. Characterization of the longerspacer etalon, such as the one with d=5 cm, usingHe-Ne lasers can be nicely studied by observing
Figure 2. Modes of He-Ne Laser recorded with a5 cm spacer etalon and 110 cm focal length lensprojecting the interferometer output. On top ofgure, we show the result of a preliminary simu-lation of the intensity distribution.
the separation of the He-Ne longitudinal modes,as seen in Fig. 2. This plot is obtained withfocal length 110 cm after the etalon. The laserresonators length is 38 cm, so the mode spacingc/2L = 400 MHz. This corresponds to k around0.08 cm1. We observe that some fringes haveangular distance as small as 1/5 of the free spec-tral range. As we see in this Figure, such severalmodes are seen separated but there may also besome overlapping within the narrow free spectralrange of the etalon used. In the same gure weshow result of a simulat