N.Maragos 1, S.Maltezos 1, V.Gika 1, E. Fokitis 1, T.Omatsu 2 1 National Technical University of Athens 2 Chiba University, Japan

 Framework Atmospheric monitoring in VHE Gamma Ray and UHE Cosmic Ray observatories Limitations of standard elastic lidar and Raman lidar. High Spectral Resolution Lidar  Recent progress Focus in HSRL laser transmitter. Design of an optical amplification configuration for the energy scaling of a pulsed SLM Nd:YVO 4 laser. Simulations Experimental setup and preliminary results 2

 When UHECR and VHE Gamma rays strike the Earth's atmosphere, they initiate cascades of secondary particles called Air Showers.  Detailed observation of the AS can give information about the energy and the nature of the primary cosmic ray.  A variety of Air Shower detection techniques have been studied and implemented in cosmic ray observatories. (fluorescence telescopes UHECR, Cherenkov telescopes for VHE-Gamma Rays)  Atmosphere can alter the received signal significantly and thus give misleading results about the energy spectrum of the Cosmic Rays.  Lidar systems (simple elastic, Raman) are installed to monitor the transparency of the atmosphere. Lidar data can be applied in the reconstruction process 3

 Simple elastic lidar β = β mol + β aer : atmospheric backscatter coefficient α = α mol + α aer : atmospheric extinction coefficient The molecular parts can be accurately predicted if atmospheric density is known. Two remaining unknowns: β aer, α aer Theoretical models ( eg Klett Applied Optics, Vol 20, pp 211 ) to predict β aer /α aer are implemented to solve the lidar equation but lead to significant systematic errors.  RAMAN lidar Raman lidar technique solves this problem with the help of an extra lidar profile where β aer =0. This is accomplished with the detection of the inelastic (Raman-shifted) backscattered signal from the atmospheric molecules. Limitations derives in this case because the Raman signal is almost 1000 times weaker than the elastically backscattered one. 4

This method utilizes the Doppler frequency shifts produced when photons are scattered from molecules in random thermal motion. The elastically backscatter signal consists of : a Doppler broadened (~1 GHz FWHM) signal from the lightweight, fast moving atmospheric molecules a much less “Doppler affected “ (~30 MHz FWHM) signal from the heavier aerosols or cloud particles. These signals are spectrally isolated with the use of narrow iodine filters or Fabry-Perot interferometers, giving an extra lidar profile with β aer =0, as in the Raman lidar method but with much stronger signal. First demonstrated in: "High spectral resolution lidar to measure optical scattering properties of atmospheric aerosols. 1: Theory and instrumentation", S. T. Shipley, D. H. Tracy, E. W. Eloranta, J. T. Trauger, J. T. Sroga, F. L. Roesler, and J. A. Weinman, APPLIED OPTICS / Vol. 22, No. 23 / 1 December 1983, 5

We are working on the development of a prototype HSRL to be used for the atmospheric monitoring in Cosmic Ray observatories. In our resent work we are focusing in designing the laser transmitter. Laser specification to be used in the HSRL transmitter: Narrow spectral output (<30 MHz linewidth). SLM operation required. Frequency stabilized or frequency locked with iodine absorption line or Fabry – Perot transmission peak in the receiver. Pulse width of <10 ns for sufficient range resolution. Pulse energy sufficient for increased signal/noise ratio (low energy limit depends on the receiver instrumentation) 6

In the first stage of the design we developed a simulation program for studying the performance of the experimental setup in order to fulfill our requirements in amplifying the pulsed SLM Nd:YVO 4 Master laser. 7

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 Simulation program is written in Matlab language and uses a Graphical User Interface  Simulates the propagation of laser pulses in four-level Solid State crystals using the relevant rate equations  No depopulation or thermal effects are considered in the method used  Some features:  Individual pulses or pulse-trains  Multiple passes through the crystal  End-pump or side-pump configuration with internal reflection  Flexibility in definition of the pump power distribution in pump surface  Flexibility in definition of the beam profile  Automated multiple runs with different set of parameters  “Real time” monitoring 9

1.We divide the crystal in orthogonal parallelepiped elements creating a grid. At every element we suppose that the density of population inversion is homogeneous. 2.We calculate the population inversion density at all elements (further description ►) 3.We split the input pulse in a sequence of square pulses with finite time intervals where we assume to have a constant optical power. 4.We calculate the amplification of the pulse as it propagates through the elements of the crystal (further description ►) 10

Initially, the intensity distribution (I 0 ) over the pumping face of the crystal is defined The rate equation for the pumping process is given below and it is well described at “Solid state laser engineering” of W. Koechner : I: pumping irradiance Δn: population inversion density σ: absorption cross section n t : total density of atoms τ: fluorescence lifetime of the upper state For every elementary part of the crystal, Δn k is calculated with the help of (1) and the knowledge of the intensity of the pump light (I k ) illuminating this element. On the other hand I k is derived from I k-1 : Where Δz is the thickness and a k-1 is the absorption coefficient of the element, derived from: which takes absorption saturation into account 11

Snapshot of the simulation program where the three - dimensional matrix of the population inversion density has been derived and illustrated 12

To simulate the pulse propagation through the crystal we use the following rate equations ( described at “Solid state laser engineering” of W. Koechner ): Equations (2), (3) have been analytically solved ( L. M. Frantz and J. S. Nodvik, J. Appl. Phys. 34, 2346 (1963). ) for square pulses and homogeneous population inversion density (φ, Δn=constant) The laser pulse is treated as a sequence of square pulses. Using the solutions, we calculate the amplified output of an input square pulse, as it passes through an elementary part of the crystal with homogeneous population inversion density. We convert the distorted (non-square) output pulse to an effective square pulse of equal integral and we continue to the next element. φ: photon density of the pulse Δn: population inversion density σ s : stimulated emission cross section Square input pulse Output pulse Transformed output pulse Crystal Element 1 Crystal Element 2 13

Snapshot of the simulation program while calculating the amplification of a Gaussian pulse which suffers internal reflection at the pump face in side-pump configuration. 14

The program has been tested by simulating a side-pump double pass experimental setup described in detail in publication: Experimental resultsOur Simulation results The predictions of the simulation are consistent to the experimental results within a ± 20% with respect to the 0.8 W. The observed slope in the simulation may be related to the non implemented depopulation effects in the simulation. K. Nawata, Y. Ojima, M. Okida, T. Ogawa, and T. Omatsu, “Power scaling of a pico-second Nd:YVO4 masteroscillator power amplifier with a phaseconjugate mirror”, 30 October 2006 / Vol. 14, No. 22 / OPTICS EXPRESS 15

Nd:YVO 4 Crystal 20 x 5 x 1.5 mm 1 % doping level Master Laser Nd:YVO 4, 1064 nm 3 ns, 10 kHz, 20 μJ SLM Temperature controlled intracavity etalon Diode single bar Max 40 W 10 mm length 100 μm thickness 35 o fast axis 8 o slow axis Closed loop water cooling system for heat abduction at: Nd:YVO4 Crystal Diode bar Master Laser 16

Total gain of 90 at 33 W pump power 17

 Optimization of the amplification setup and detailed characterization of the output. (Absolute pulse energy measurements, beam quality measurements)  Double pass configuration to achieve higher energy extraction from the crystal  Frequency mixing with a KTP and an LBO non linear crystal to get an output at 355 nm (Cherenkov light region in VHE Gamma Rays observations).  Enhancement of stability and compactness. 18

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