SLM LASER OPTICAL AMPLIFICATION DESIGN FOR HSRL ATMOSPHERIC MONITORING IN UHE COSMIC RAY OBSERVATORIES

N.Maragos1, S.Maltezos1, V.Gika1, E. Fokitis1, T.Omatsu21National Technical University of Athens2Chiba University, Japan

OUTLINE

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:YVO4 laser.• Simulations• Experimental setup and preliminary results

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CR OBSERVATORIES AND ATMOSPHERIC MONITORING 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

LIMITATIONS OF SIMPLE ELASTIC AND RAMAN LIDAR Simple elastic lidar

r

rdrarctrOrAPrP

020 2exp

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β = βmol+ βaer : atmospheric backscatter coefficientα = αmol+ αaer : atmospheric extinction coefficient The molecular parts can be accurately predicted if atmospheric density is known. Two remaining unknowns: βaer , αaerTheoretical models (eg Klett Applied Optics, Vol 20, pp 211) to predict βaer /αaer are implemented to solve the lidar equation but lead to significant systematic errors. RAMAN lidarRaman lidar technique solves this problem with the help of an extra lidar profile where βaer =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

HIGH SPECTRAL RESOLUTION LIDAR (HSRL)

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 βaer =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,

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HSRL DEVELOPMENT

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

OPTICAL AMPLIFICATION SIMULATION 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:YVO4 Master laser.

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FOUR LEVEL SYSTEM DIAGRAM

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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

SIMULATION – OVERVIEW

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SIMULATION – METHOD OVERVIEW

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 ►)

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SIMULATION – POPULATION INVERSION DENSITY CALCULATION

Initially, the intensity distribution (I0) 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 densityσ : absorption cross section

nt : total density of atoms

τ : fluorescence lifetime of the upper state

For every elementary part of the crystal, Δnk is calculated with the help of (1) and the knowledge of the intensity of the pump light (Ik ) illuminating this element. On the other hand Ik is derived from Ik-1 :

Where Δz is the thickness and ak-1 is the absorption coefficient of the element, derived from:

which takes absorption saturation into account

zakk

keII

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111 ktkk nnIa

(1)

nnnItn

t

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SIMULATION – POPULATION INVERSION DENSITY CALCULATIONSnapshot of the simulation program where the three - dimensional matrix of the population inversion density has been derived and illustrated

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SIMULATION – PULSE PROPAGATION

To simulate the pulse propagation through the crystal we use the following rate equations (described at “Solid state laser engineering” of W. Koechner):

(3) (2) c nctn

xnc

t ss

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 densityσs : stimulated emission cross section

Square input pulse

Output pulse

Transformed output pulse

Crystal Element 1

Crystal Element 2

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SIMULATION – POPULATION INVERSION DENSITY CALCULATIONSnapshot of the simulation program while calculating the amplification of a Gaussian pulse which suffers internal reflection at the pump face in side-pump configuration.

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SIMULATION – TEST

The program has been tested by simulating a side-pump double pass experimental setup described in detail in publication:

Experimental results

Our 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

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EXPERIMENTAL SETUPNd:YVO4 Crystal20 x 5 x 1.5 mm1 % doping level

Master LaserNd:YVO4, 1064 nm3 ns, 10 kHz, 20 μJSLMTemperature controlled intracavity etalonDiode single barMax 40 W10 mm length100 μm thickness35o fast axis8o slow axis

Closed loop water cooling system for heat abduction at:•Nd:YVO4 Crystal•Diode bar•Master Laser

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EXPERIMENTAL RESULTS

Total gain of 90 at 33 W pump power

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FUTURE PLANS

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.

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DESIGN

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