NTUA. Physics Department, National Technical University of Athens. June IPRD10,, Siena, Italy S. Maltezos SIENA ITALY

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1 NTUA SIENA ITALY ATMOSPHERIC MONITORING FOR VERY HIGH ENERGY GAMMA RAY CHERENKOV TELESCOPES BASED ON HSRL: DEVELOPMENT OF HIGH ACCURACY NON-INVASIVE ETALON CHARACTERIZATION TECHNIQUES S. Maltezos,, E. Fokitis,, V. Gika,, A. Aravantinos,, N. Maragos, E. Koubli and G. Koutsourakis Physics Department, National Technical University of Athens 1

2 Outline The Atmospheric Showers in UHE and VHE Cosmic Ray Observatories Atmospheric monitoring with a HSRL Performance tests of the F-P F P etalon receivers A non-invasive F-P F P characterization method Conclusions and Prospects 2

3 The Atmospheric Showers in UHE and VHE Cosmic Ray Observatories The scientific objectives UHECR: Investigating the sources, the energy range and the composition of primary particles in the highest energy scale (i.e. P. Auger, TA, JEM-EUSO). VHE Gamma Ray: VHE Gamma Ray: Exploring and studying particular candidate sources of VHE gamma ray events (i.e. H.E.S.S., MAGIC, CANGAROOII, VERITAS,CTA). The detector s s principle of operation In both cases, the atmosphere is essentially the detector medium where the Extensive Air Showers are developed. In the UHECR detectors the fluorescence technique is often used, while the HE Gamma Ray detectors are designed to collect the Cherenkov radiation. 3

4 The induced light signals The Fluorescence Technique is based on the detection of the emitted UV radiation during an Extensive Atmospheric Shower (EAS). A very important parameter is the Air Fluorescence Yield specifying the total intensity and thus the possibility p to determine the energy of the primary particle. In the range of 50 nm ( nm) which is the sensitivity of the Fluorescence Telescopes are emitted by excitation of the nitrogen molecules: The Atmospheric Cherenkov Technique allows the imaging of an atmospheric shower, by means of collecting the Cherenkov photons spread on the ground level within a cone of ~1 o. In the same range ( nm) are emitted: N Ch _ ph = 2πα Z sin θc = 10.8 photons/m per e (e ) λ1 λ2 where N fl _ ph 4 photons/m per e 1 1 = = βn β n 2 cosθc sin θc 1 In both techniques the light signal has to be corrected due to the influence of the atmosphere (attenuation due to water vapour and scattering due to aerosol). - (e + ) 2 2 4

5 Atmospheric monitoring with HSRL The High Spectral Resolution Lidar The HSRL allows measurements of the scattering ratio aerosol/molecular with larger sensitivity in comparison with Raman LIDAR because of the large cross section. The technique has been advanced up to the level of laboratory testing t with a 100 mw Diode Pumbed Single Longitudinal mode (DPSS-SLM) SLM) CW laser at 532 nm with coherence length about 50 m. Fabry-Perot Perot etalon pair is used to disentangle the contributions of aerosol and molecular scattering. 5

6 Atmospheric monitoring with HSRL The signal analysis method The Lidar equation describing the backscattered signal by molecules (m)( ) and aerosols (p)) in the atmosphere: Where, α is the extinction coefficient and β the scattering coefficient. 6

7 The basic component The receiver telescope prototype is based on a massive parabolic mirror Type Diameter Focal Length Width Weight General Parabolic 370 mm 1600 mm 57 mm ~14 kg Material BK7 Quality λ/8 rms Coating Al protected SiO2. Mounting of the mirror on a stable support. The attachment to the rest of the mounting allows the rotation to any direction. 7

8 The configuration of the prototype In the High Spectral Resolution Lidar, the backscattered light (or the scattered light in bistatic mode) by the atmosphere is separated by the two dedicated F-P etalons of molecular and aerosol channel. The aim is to determine the aerosol to molecular ratio. A prototype HSRL in backscattering mode in performance testing phase Possibility to operate in backscattering and as well in bistatic mode 8

9 Performance tests of the F-P F P etalon receivers Resolving the laser modes using optical fiber Optical layout λ=632.8 nm The experimental setup The output of the optic fiber is placed at the focal plane of the collimating lens (1). The CCD camera is placed at the focal plane of focusing lens (4). We used two types of optical fibers: OF-1, a bundle of 50 μm m core diameter fibers with 1 mm outer diameter. OF-2, single with 1 mm core diameter. 9

10 What we were expected Let us calculate the resolving capability of the laser modes v δ v c= λv v = δv = c c δ v δ v = = cm c 3 10 cm/s 6 mod es Hz modes 10 (wavenumber or spatial frequency) -1 We tested 3 available F-P F P etalons with spacer thicknesses: d = 5 mm, 20 mm and 50 mm. The corresponding free spectral range (FSR) of each etalon is: FSR 1.00 cm 1 = = 0.25 cm 2d 0.10 cm Assuming a minimum Finesse equal to 20, we obtain a resolution: FSR δ vf P= F δv~ F P cm = cm cm modes: not resolved modes: partially overalapped modes: resolved 10

11 Evaluating and analyzing our results Fringe pattern obtained with of 5 mm etalon combined with the OF-2 2 (bundle). Fringe pattern obtained with of 20 mm etalon combined OF-1 1 (single). The modes are not resolved Improved image using a pinhole of 0.5 mm The modes are partially overlapped 11

12 Evaluating and analyzing our results Fringe pattern obtained with of 20 mm etalon combined with the OF-2 2 (bundle). Fringe pattern obtained with of 50 mm etalon combined OF-1 1 (single). The modes are well resolved The modes are well resolved Using a pinhole of ~0.5 mm 12

13 A non-invasive F-P F P characterization method How we could measure the spacer thickness of a mounted (commercial) Fabry-Perot etalon with a precision of few nm! THE ONLY REQUIREMENT! To obtain interferograms of two well-known wavelengths λ 1 and λ 2 with an accuracy of the order of More than 2 wavelengths can be, certainly, used for higher reliability. λ λ d λ = ελ = m + ε λ The knowledge of d of such accuracy it is very important in the cases of measuring the unknown wavelength of a SLM laser and its variation or studying the overall stability of the etalon. mλ 13

14 The stages of our methodology 1 st analysis step: Determination of the excess fractions ε i (i=1,2) of the fringe pattern for both wavelengths (presented at ICATPP 09). The e accuracy of ε i can be of the order of nd analysis step: Investigation of the (unique, eventually optimum) solution of the equations relating the unknown d with λ i and ε i. Concerning the 2 nd step, we are writing the equation system as λ λ 1m1+ λε 1 1 = 2 2m2 + λε 2 2 = 2 λ1m1 λ2m2 = λ2ε2 λε 1 1 d d leading to a 1 st order linear Diophantine Equation (DE) = c (the unknowns are the orders m 1 and m 2 while the 4 parameters λ 1, λ 2, ε 1 and ε 2 have to be converted to round numbers by a multiplier) We could expect: either infinite solutions (if c is a GCD of λ 1 and λ 2 ) or no-one solution (if c is not a GCD)! Because of the finite precision of the 4 parameters, it is not feasible f to find a integer solutions compatible to the physical problem. 14

15 The optimum against the exact solution Let us writing the equation system to be solved using the modulo operator ( 2 d mod ) ( 2 d mod ) ε = λ ε ε = λ ε (It has been shown, that an accuracy of ε i better than is adequate!) The key point is to consider the Cartesian two-dimensional space of ε 1 and ε 2 and to scan candidate values of unknown d within a sufficiently wide range with a step equal to the required precision. The solution will be the optimum one, by means of finding the point we the minimum geometrical distance in the Excess Fraction Space (EFS): r = 2 2 ε ( d) + ε ( d) 1 2 As we can see in the next, the modulo operation produces a family of parallel lines in the EFS corresponding to different orders m i,1 and m i,2. 15

16 How to obtain the family lines Assuming a value of spacer thickness d m in the proximity of an hypothetical solution point (that is d t =d m +d, where d is the variable spacer thickness), the corresponding line equations can be written as folows: 2d 2d ε = λ ε = λ + ε = ε + ε ( 2 dt mod ) ( 2 dm mod ) m λ1 λ1 2d 2d ε = λ ε = λ + ε = ε + ε ( 2 dt mod ) ( 2 dm mod ) m λ2 λ2 The line equation containing the solution point λ λ λ ε = ε + ε ε + ε ε = w ε + w ε ε + ε w ε ( ) ( ) m m m 12 m λ2 λ2 λ2 where, w 12 λ1 λ

17 Summarizing the possible branches Unfeasible to give a physical result Single Diophantine Equation Modular Equation system Exact and infinite solutions for m 1, m 2 Only the exact solution Optimum solution for ε 1 and ε 2 No solution Not feasible * Ideal means a solution point on the line equation and acceptable outside of the line. Optimum-ideal * solution for d Optimum-acceptable * solution for d 17

18 Application on real data We used the real interference data given by Karl W. Meissner I (Journal of Optical Society of America) to verify the agreement, the effectiveness of our method and to identify the kind of the particular solution. Variation of m i Variation of d Optimum-ideal ideal case (for λ 1, λ 2 ) Optimum-acceptable case (for λ 1, λ 3 ) 18

19 The obtained results The data we used are referred to 3 known lines of Krypton with well-known wavelengths and measured excess fractions. λ 1 = nm λ 2 = nm λ 2 = nm ε 1 = ε 2 = ε 2 = The corresponding modular equations for the lines 1,2 are: ( d 9 ) ( d 9 ) ε 1 = 2 mod ε 2 = 2 mod r min = 1 2 i min = d1 2 = (4) mm We also used simulated data in order to study the limits of the accuracy requirements. 19

20 Planning for operating in UV region Narrower FSR etalon Reflectivity curve of the two F-P mirrors for building an etalon of 100 mm spacer thickness appropriate for the aerosol channel. Operation with pulsed SLM Laser We are going to assemble a pulsed coherent laser system at 355 nm. n This may give, via appropriate RAMAN cells, additional UV and near UV wavelengths, so that we can achieve a dispersion curve in the factor β of LIDAR equation. 20

21 Conclusions and Prospects A HSRL prototype receiver based on a massive parabolic mirror having also rotation capability has been constructed and is ready to be tested. The performance tests of the F-P F P channels have shown the resolving capability of the modes of a He-Ne laser. A complete non-invasive method to measure the F-P F P etalon spacers with the highest possible precision was established. Etalon plates appropriate for the near UV range are going to be tested. The development and assembly of a pulsed coherent SLM laser system at 355 nm with discrete tunability in UV region is on the way. 21