Design of a High Spectral Resolution Lidar for Atmospheric Monitoring in EAS Detection Experiments

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1 Nuclear Physics B (Proc. Suppl.) 190 (2009) Design of a High Spectral Resolution Lidar for Atmospheric Monitoring in EAS Detection Experiments E. Fokitis a, P. Fetfatzis a, A. Georgakopoulou a, S. Maltezos a, and A. Aravantinos b a Physics Department, National Technical University of Athens, 9, Heroon Polytechniou, ZIP 15780, Athens, Greece b Physics 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 to molecular scattering coefficients ratio. This type of Lidar consists of a Continuous Wave Single Longitudinal Mode laser 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 molecular component due to their different levels of Doppler effects on the scattered laser light. We present a study of the effects of the various levels of CCD cooling on the sensitivity of this. Firstly we discuss preliminary experimental results based on a Fabry-Perot etalon with free spectral range (FSR) 0.1 cm 1, the expected performance of a etalon with FSR of 0.05 cm 1, under construction, and with finally an aerosol parameter analysis in Simulation Codes for EAS. 1. INTRODUCTION In this paper we present the work of our team in the atmospheric monitoring issue for EAS Detection experiments. In the measurements of airfluorescence yield for the detection of Ultra High Energy Cosmic Rays, there is a contribution due to the air-cherenkov signal scattered mainly by the aerosol particles. Therefore, there is a need to monitor the degree of this type of scattering in order to correct the total signal and obtain the airfluorescence signal. The most common method for measuring the aerosol induced scattering is using the Elastic or Raman Lidar Method. In this work, we present the progress in an alternative method using the High Spectral Resolution Lidar (HSRL). This is continuation of recent work [3], where now we emphasize on the study of the performance of a candidate laser source for the HSRL. This apparatus is designed in order to achieve high accuracy measuring the aerosol to molecular scattering ratio. It consists of a Continuous Wave Single Longitudinal Mode (SLM) Laser beam at 532 nm, a receiver with a parabolic mirror, and analyzes spectrally the scattered light passing through Fabry-Perot cavities. We mention that [1], first reported a high spectral resolution lidar (HSRL) using a narrowband laser and a high resolution Fabry-Perot etalon to separate the aerosol (Mie) and molecular (Rayleigh) scattering. More recently, (see [2]), demonstrated aerosol and temperature measurements using a HSRL based on an iodine vapor filter, and the temperature in the stratosphere was obtained from the Mie-filtered signal. The HSRL can give simultaneously, for each atmospheric height layer, both the aerosol and the molecular scattered intensity from a ground based laser emitter. This capability is due to the interferometric measurement method, that is, the Fabry-Perot interferometer in the LIDAR receiver. Additionally, in the aerosol scattering HSRL, the narrow linewidth of the laser transmitter makes it possible both to separate the contributions from aerosol particles and atmospheric molecules, and to calculate the aerosol extinction without assuming the lidar ratio (defined as the ratio between the extinction and backscattering /$ see front matter 2009 Elsevier B.V. All rights reserved. doi: /j.nuclphysbps

2 262 E. Fokitis et al. / Nuclear Physics B (Proc. Suppl.) 190 (2009) coefficients) [5]. We take advantage of the recent technological developments in the solid state diode lasers which have lead to cost affordable HSRL emitters. In section 2 we describe the laser source used in this HSRL. The section 3 deals with the characterization of the molecular channel to be used for determining the scattering coefficient due to the atmospheric molecules (3.1), while the aerosol channel characterization is presented in 3.2. The laboratory emulation scattering studies of the SLM laser beam is presented in Section 4. The section 5 discusses the limitations in the sensitivity of the HSRL prototype. The Section 6 considers the prospects of the present research efforts. 2. Laser sources 2.1. SLM laser We need a very narrow line-width laser source so that the line source uncertainty is much smaller than the Doppler broadening of the scattered radiation from the air molecules. Only in such case we may separate the Mie from molecular scattering using a high spectral resolution spectrometer as the one we will be describing. We are using a Diode Pumped Solid State Laser continuous wave (CW) SLM laser at 532 nm (called SLM at the rest of the paper) with a nominal power of 100 mw and coherence length greater than 50 m or correspondingly, wavenumber uncertainty k =0.02 cm 1. The tests conducted were aiming at studying the stability of the interference fringe patterns of the 5 cm spacer etalon as the current driving the SLM laser gradually is increased to the nominal value. The results will be presented in the Section about the aerosol channel. However, the question of the absolute laser wavelength cannot be answered by such measurements. The measurements so far conducted have indicated that at the nominal operation conditions, there is a dominant operation mode and a secondary mode with intensity of the order of 2-4 % of the one of the dominant mode as will be shown in 2.3. For the absolute wavelength (or frequency) determination, one would need a facility which would include a laser frequency standard. However, we believe that the knowledge of the absolute frequency is not necessary for the aims of the HSRL Procedure for laser stabilization The specific solid state laser we use includes some proprietary design, but basically, it consists of an optical cavity and some lasing medium, while the emitted line structure depends on the temperature of the lasing medium as the temperature is a thermodynamic quantity which is important in affecting the longitudinal and transverse modes. For this reason, the SLM laser has a controllable excitation current and we had to study the laser line structure as a function of the current as well as of its rate of change. The use of the available Fabry-Perot spectrometers can greatly help in revealing the laser line structure and its changes as a function of excitation current. In comparison with He-Ne mode competition, we have the dominance of a single mode as discussed. The minimum level of the intensity in this plot is consistent with the expected curve of Airy function. No noise substraction has been performed since the latter is expected to be negligible as the exposure time was around 1/4000 s. 3. Molecular and Aerosol channels 3.1. Molecular channel The radiation approaching the receiver, after being scattered by the air molecules, is typically reflected by a beam splitter and is directed to the molecular channel, while the rest is transmitted to the aerosol channel. In order to characterize the molecular channel we perform the following measurements: We have obtained interferograms of a platinum-neon (Pt-Ne) Hollow Cathode spectra lamp operating at ambient as well as at higher temperatures. We have also used an interference filter at 562 nm, with a passband 30 ± 5nmin order to select a specific spectral line of Pt-Ne spectrum. In this range there are two line of Ne, namely at 585 and 588 nm, possibly some platinum lines. The interferogram presented is taken with the 2 cm etalon spacer and the above interference filter. The two intensity histograms indicate that the first one, corresponding to cold lamp, is less influenced by the Doppler effect than

3 E. Fokitis et al. / Nuclear Physics B (Proc. Suppl.) 190 (2009) Figure 1. The two intensity histograms indicate that the first one, corresponding to cold lamp, shows smaller Doppler Width than the one at the second histogram. the one at the second histogram, where the temperature is higher. The result is presented in Fig. 1. This plot is a display of the superposition of the interference patterns corresponding to the atomic transitions involved. As the FSR is given by FSR =1/2d, where d is the etalon spacer length, selecting an etalon of d=5 mm we are able to study effectively the molecular scattering. Thus, we plan to make corresponding measurements with available etalons having spacer lengths 2 mm and 5 mm, respectively, which are capable to contain the whole line-breath of the laser radiation scattered and therefore broadened by the air molecules Aerosol channel We have studied in a laboratory the performance of the aerosol channel using two different lasers, one at nm, based on an available He- Ne laser, and the other was the SLM laser, the latter having appropriate power and linewidth to be used in a HSRL. Characterization of the longer spacer etalon, such as the one with d=5 cm, using He-Ne lasers can be nicely studied by observing Figure 2. Modes of He-Ne Laser recorded with a 5 cm spacer etalon and 110 cm focal length lens projecting the interferometer output. On top of figure, we show the result of a preliminary simulation of the intensity distribution. the separation of the He-Ne longitudinal modes, as seen in Fig. 2. This plot is obtained with focal length 110 cm after the etalon. The laser resonator s length is 38 cm, so the mode spacing c/2l = 400 MHz. This corresponds to Δk around 0.08 cm 1. We observe that some fringes have angular distance as small as 1/5 of the free spectral range. As we see in this Figure, such several modes are seen separated but there may also be some overlapping within the narrow free spectral range of the etalon used. In the same figure we show result of a simulation of the total intensity distribution of the modes involved. This verifies the High Spectral Resolution of the etalon. This result should not have been taken for granted since the effective finesse of our interferometer depends on several factors which must combine to

4 264 E. Fokitis et al. / Nuclear Physics B (Proc. Suppl.) 190 (2009) Figure 3. The interferogram obtained with SLM laser and 5 cm spacer length etalon, used for determining the etalon finesse. achieve this result. Such factors are etalon plates flatness, their parallelism, the etalon reflectivity, the etalon s angular finesse, the laser linewidth, the effect of externally caused mechanical vibrations etc. The finesse of the Aerosol Channel parameter is quantified for the 5 cm spacer length etalon by using the SLM laser. The interferogram obtained with the SLM laser is presented in Fig. 3. We observe that the interferogram is dominated by one mode when operating the laser at the nominal power output of 100 mw. These data correspond to an overall finesse of Laboratory emulation the SLM laser beam scattering This was done by using the SLM laser beam expanded by a small telescope. The scattered radiation was collected by a Newtonian telescope of 250 mm diameter, and 1.6 meter focal length. The focused radiation was directed to the 5 cm spacer etalon combined with 110 cm focal length lens. The resulting interferogram shows that the finesse is again around This result can be considered as proof-of-principle that the experimental setup has the capability to measure at a resolution of FSR/ Finesse = 0.1 /17.5 cm 1 = By using an etalon, under development, with FSR=0.05 cm 1, we expect to improve somewhat on the above achieved in the laboratory spectral sensitivity, and therefore allow the separation of the aerosol and molecular scattering contributions via the two described channels. We have, however, to caution that the achievable sensitivity in field measurements of atmospheric aerosols may be worse than the one achieve under the described lab conditions.for instance, one expects that the shot-noise level is expected to be higher at atmospheric heights giving low aerosol scattering signal, an also the night sky background level may affect the sensitivity of the proposed setup. Finally, we are soon commissioning a CCD based system operating at Liquid nitrogen temperature so that the dark count contribution in the overall error budget is minimized. 5. Limitations on the sensitivity of HSRL The sensitivity of the HSRL is mainly affected by the Night Sky Background, shot noise, dark count, read noise of each CCD pixel. The operation of the HSRL is foreseen only during the nighttime, corresponding to the period when the EAS Fluorescence Detector Telescopes are operating. During this period, in the absence of artificial UV radiation, the main limitation in the HSRL is due to the night sky background in the near UV range, i.e. in the range nm, which is the window of sensitivity of the EAS Fluorescence Telescopes [4]. To get a feeling for the Night Sky Background, we may use a criterion that the NSB number of photons recorded in a time interval of 1-5 minutes, by the LIDAR receiver, is considerably smaller than the noise of the receiver CCD (combined dark count and electronic noise) at the same interval. For a typical thermoelectrically cooled CCD, the expected dark count is on the average 0.06 e-/pixel/electron, while it does not exceed 0.1 e-/pixel/s at 0 o C. The next requirement is that the total rate of night sky background plus dark count plus electronic noise is much smaller than the signal corresponding to the SLM radiation scattered at a specific height

5 E. Fokitis et al. / Nuclear Physics B (Proc. Suppl.) 190 (2009) corresponding to a scattering angle. 6. CONCLUSIONS AND PROSPECTS In this work we have explored the possibility to measure the aerosol scattering coefficient by comparing to molecular scattering coefficient recorded at the same time. We tried a High Spectral Resolution LIDAR method. The results done at this phase, obtained by microparticles dispersed in water, seem to retain the linewidth of the Laser beam. They show a Δk =0.02 cm 1 as measured by a system combining a Newtonian type telescope and a Fabry Perot Interferometer. We are at the stage to implement the optical design in permanent optical bench for both channels, aerosol and molecular, in order to have a stable receiver which will be able to move in various directions for operating in bistatic Lidar mode. Also we consider SLM Laser at the UV Range as well as Pulsed Laser operation mode field test of this LIDAR will be done by recording, in bistatic mode, the scattering of the SLM laser beam by aerosol and molecules near the PBL heights. We are developing an etalon with FSR 0.05 cm 1, compatible for operation in the near UV range which is appropriate for operation of Fluorescence Detector telescopes. The biggest challenge towards this goal is to acquire a UV laser source with the correspondingly narrow linewidth. 2. Z. Liu, I. Matsui and N. Sugimoto: Proc. SPIE 3504 (1998) E. Fokitis, P. Fetfatzis, S. Maltezos, N. Antonakakis Spyropoulos and A. Aravantinos, Proc. of the 10 th ICATPP 2007, Como, Italy, World Scientific Publ., (2007) pp N. Antonakakis Spyropoulos, P. Fetfatzis, E. Fokitis, V. Gika, S. Maltezos, Proc. of the 10 th ICATPP 2007, Como, Italy, World Scientific Publ., (2007) pp J. D. Klett: Appl. Opt. 20 (1981) M. Imaki, Japanese Journal of Applied Physics Vol. 44, No. 5A, 2005, pp ACKNOWLEDGMENTS This work has been funded by the project PENED The project is co-financed 80 % of public expenditure through C - European Social Fund, 20 % of public expenditure through Ministry of Development - General Secretariat of research and Technology and through private sector, under measure 8.3 of OPERATIONAL PRO- GRAMME COMPETITIVENESS in the 3 rd. REFERENCES 1. S. T. Shipley, D. H. Tracy, E. W. Eloranta, J. T. Trauger, J. T. Sroga, F. L. Roesler and J. A. Weinman: Appl. Opt. 22 (1983) 3716.