Schematic diagram of a HSRL in bistatic mode. ... camera (Nikon D40) with 6
Mpixel image anal- ... A representative diagram of the setup is given in. Fig. 2.
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High Spectral Resolution LIDAR Receivers to measure Aerosol to Molecular Scattering Ratio in Bistatic mode for use in Atmospheric Monitoring for EAS Detectors E. Fokitisa , S. Maltezosa , A. Papayannisa , P. Fetfatzisa , A. Georgakopouloua , and A. Aravantinosb 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 We present the design of a bistatic High Spectral Resolution Lidar (HSRL) aiming to measure the aerosol phase function for applications in Ultra High Energy Cosmic Ray experiments. The expectation is to give accurate data for the aerosol phase function, needed to correct the EAS signal of air -fluorescence detectors for the air Cherenkov contamination, caused mainly by the aerosols. In this work we mainly focus on the design principles of the HSRL receiver for recording the aerosol to molecular scattering ratio as a function of height. We present results from testing an SLM CW diode laser of 120 mW at 532 nm, to be used as a LIDAR emitter in the transmitting telescope, and verify the design coherence length according to the manufacturer. As receiver we consider a system two different Fabry-Perot etalons having free spectral ranges 0.1 cm−1 and 1 cm−1 respectively, corresponding to the molecular and aerosol channels. The fringe patterns are analyzed over 2π polar angle range using appropriate algorithms.
1. INTRODUCTON In Ultra High Energy Cosmic Ray experiments using fluorescence detectors, it is of great importance to correct the EAS signal of air fluorescence for the air Cherenkov contamination, caused mainly by the aerosols. Following our recent work [1], we study in the present work a High Spectral Resolution Lidar (HSRL) in bistatic mode for recording the aerosol to molecular scattering ratio as a function of height. Applying this method we concentrate basically on the receivers which could be two appropriate FabryPerot etalons in order to separate the aerosol and molecular scattering signals due to their different free spectral ranges (FSR). The molecular signal is expected to appear as Doppler broadened scattered laser beam line due to the small masses of the molecules. On the contrary, the aerosol signal is expected to be spectrally unchanged due to the large average mass of these particles. Thus the proposed receiver system will contain two chan-
nels, namely the aerosol channel and the molecular channel. They are based on Fabry-Perot etalons with their associated optical and optoelectronic components, and are supplying as observables sharp interference patterns. The expected fringe pattern in each channel has to be analyzed collecting all the signal information contained in the image. The performance tests of the receivers were useful to understand their response to a CW diode laser of 100 mW at 532 nm in SLM operation (called SLM laser in the remaining of the paper). In section 2 we present the technique of the HSRL in bistatic mode and a detail description of the two Fabry-Perot etalons is also given. In section 3 the performance tests and the data analysis are described. Finally, in section 4, we discuss the results and give some conclusions and prospects.
2 2. DESIGN OF HSRL IN BISTATIC MODE 2.1. Radiation scattering in the atmosphere Aerosols are non-gaseous substances that are divided into solid particles or liquid droplets and held in suspension in the atmosphere. The scattering processes of the light depends on the effective diameter D of the particle. The aerosols cause exclusively Mie scattering (D ∼ λ) and their presence in the atmosphere plays a significant role in the scattering of the Cherenkov radiation and it has to be monitored since it may be mixed, after scattering, with the air-fluorescence radiation. The expression of the signal S of the scattered light per unit length per sterad falling to any detector, is given in [2], quantifying the contributions of molecular and aerosol scattering. 2.2. Configuration of bistatic mode
Figure 1. Schematic diagram of a HSRL in bistatic mode.
The High Spectral Resolution Lidar (HSRL) is a device based on a narrow-band laser and a pair of high resolution Fabry-Perot etalons to separate the aerosol (Mie) and molecular (Rayleigh) scattering. The HSRL can give simultaneously, for each atmospheric height layer, both the aerosol and the molecular scattered intensity which can be recorded by a ground based detector. This
configuration will be used for obtaining measurements of the scattering intensity in particularpreselected heights h in order to measure the aerosol phase function, P (θ). It can be shown easily, that the height h is related to zenith angles, θlaser and ϕview as follows: h=
L tanϕview + tanθlaser
(1)
and thus, for each selected height h corresponds to a required φview to obtain a measurement at the scattering angle ωscat . Another task was to estimate of the overall efficiency of the system. Using Lidar equation and according the method described in [3], we tried to estimate the expected efficiency in this configuration. The efficiency was found to be 1.6 × 10−13 corresponding to a photon rate of 4 × 104 ph/s at the telescope of the receiver. This photon rate, in principle, could be recorded using a CCD camera. To achieve a satisfactory signal-to-noise ratio we have to use exposure times greater than 1 s and also we plan to use an optical filter to reduce the night sky background radiation. 2.3. Fabry-Perot etalons receivers Appropriate FSRs from our calculations based on the expected line width of the aerosol and molecular backscattering are 0.1 cm−1 and 1 cm−1 respectively. They have been realized using Fabry-Perot etalons with spacer thicknesses are 50 mm and 5 mm (from SLS Optics and Queensgate Instruments Ltd, respectively). Their role is to discriminate the aerosol scattered radiation from the molecular one. The expected performance and response of the two etalons used for the aerosol and molecular channel, has been simulated in a computer program according to the Fabry-Perot theoretical model including the defects given by the manufacturer. 3. PERFORMANCE MEASUREMENTS 3.1. Laboratory measurements In the Laboratory we have studied the resolution and the sensitivity of the etalons under various conditions, such as, the fringe pattern obtained after scattering of the laser beam by di-
3 luted microparticles in the water or by using diffusely scattering surfaces. Additionally, the stability of the laser beam, by means of wavelength or Doppler shift, has been investigated by off line analysis. We used the SLM laser at well stabilized wavelength at 532 nm and with coherence length exceeding 50 meters corresponding to wavenumber uncertainty δk ≈ 0.02 cm−1 . A colored CCD camera (Nikon D40) with 6 Mpixel image analysis (3040x2014) and pixel size 7.8 µm was used for recording the fringe patterns. A Newtonian telescope of diameter 250 mm and f-number=5.5 has been used for collecting the scattered light. A representative diagram of the setup is given in Fig. 2.
Figure 2. The configuration of the Laboratory setup. The green light of the laser beam is scattered by the microparticles in the box.
3.2. Fringe pattern analysis The analysis procedure deals with two basic features: a) The calculation of the total intensity (CCD counts) along the x-y plane and b) Estimation of the effective finesse of the fringes. For the first task, we scan the image plane using an elementary circular sector (a cluster of pixels) δθ of around 1.5 deg which should be rotated by
of equal angular steps covering the whole polar range. This is realized by rotation transformation of each pixel cluster belonging to the corresponding sector by angle θ = −kδθ, where k is a positive integer. By this technique the integration is always done in the initial angular position θ = 0, as shown in Fig. 3. For the second task, we apply a transformation of the fringe intensity plot along the x-axis (in units pixel) as follows: x0 −→ 1/cos(cx), where c an arbitrary scale factor (see Fig 4). The argument cx expresses the incidence angle to the etalon. By this transformation the intensity has a linear dependence with the new variable x0 and thus the effective finesse can be calculated by the well known formula, F = (F SR)/(F W HM ), where F W HM is the full width at half maximum of the peak. The finesse is estimated to be equal to 15.0±0.5 for the specific focal length used for projection of the interference pattern, which was 30 cm. The fringe pattern (system of concentric circles) allows also the determination of the excess fraction ε by a two-dimensional direct fit method as it is described in [4]. This method has been applied in two independent interferograms and has resulted in giving the excess fraction ε which was 0.535 and 0.462 respectively. According to these authors, the error in ε is of the order ±5 × 10−4 . This error can be reduced in Fabry-Perot spectrographs with better resolution than the one used in this reference (i.e. using etalons with higher reflectivity, better flatness, longer focal lengths etc). However, in order to verify the correctness of our results we used a simpler chi-square line fitting method but potentially less accurate based on the linear relationship between fringe diameter squared versus fringe order number. We observed general consistency with the by the above two-dimensional direct fit method. In conclusion, this method has the limitation that the ellipse orientation is a priori unknown. We try to overpass this difficulty in order to improve the accuracy of the fitting parameters of our interference data based on the method proposed in [5].
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Figure 4. The intensity plot along a radial direction ± 1 pixel wide obtained with the etalon of 50 mm. The variable x0 has arbitrary units. The FSR is typically calculated as the average of 10 successive intensity periods. Figure 3. The fringe pattern image obtained using diffusely scattering surface with etalon of 50 mm. The circular sector (white) is also shown.
4. CONCLUSIONS AND PROSPECTS The feasibility of a High Spectral Resolution Lidar in bistatic mode has been studied. We have concentrated mainly on the optimization of the receivers performance, that is, of the two Fabry-Perot etalons. From the performance tests, we have estimated the resolving power of the two etalons corresponding to the two channels of HSRL. From the fringe pattern analysis the excess fraction parameter has been determined by using a direct least chi-square fitting methodology. One of our next plans is the optimization of the design of a 100 mm etalon, by means of its absorption coefficient and its reflectance using thin film deposition method. This will increase the signal-to-noise ratio as well as the finesse. 5. ACKNOWLEDGMENTS This work has been funded by the project PENED 2003. 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 sec-
tor, under measure 8.3 of OPERATIONAL PROGRAMME ”COMPETITIVENESS” in the 3rd Community Support Programme. REFERENCES 1. E. Fokitis, P. Fetfatzis and S. Maltezos, Proc. of 30t h ICRC2007, ID 1114, track classif. HE.1.5, Merida, Yucatan, Mexico (2007). 2. S. Westerhoff, ”Measurements of Aerosol Phase Function”, presented at ATMON’08 (2008) Phase Separation in Glass, NorthHolland, Amsterdam, 1984. 3. S. Fukagawa, I. Kouga, H. Kuze, N. Takeuchi, M. Sasaki, Y. Asaoka and S. Ogawa, ”Simulation Study for Aerosol Distribution Retrieval from Bistatic, Imaging Lidar Data”, Lasers and Electro-Optics, CLEO/Pacific Rim Conference (2005). 4. M. O’Hora, B. Bow and V. Toal, ”Least squares algorithm for rabid analysis of fabryPerot fringe patterns”, J. Opt. A: Appl. Opt. 7 (2005)S364-S367. 5. R. Halir, J. Flusser, ”Numarically Stable Direct Least Squares Fitting of Ellipses”, Proc., 6th International Conference in Central Europe on Computer Graphics and Visualization. WSCG ’98. CZ, Plzen (1998), pp. 125132.
