N.Maragos1, S.Maltezos1, V.Gika1, E. Fokitis1, T.Omatsu2
1National
Technical University of Athens
2Chiba University, Japan
SLM LASER OPTICAL AMPLIFICATION DESIGN
FOR HSRL ATMOSPHERIC MONITORING IN UHE
COSMIC RAY OBSERVATORIES
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
2
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


 A 
 ct 










P r  P0  2  O r    r exp   2  a r d r 
r 
 2 
0


β = βmol+ βaer : atmospheric backscatter coefficient
α = αmol+ αaer : atmospheric extinction coefficient
The molecular parts can be accurately predicted if atmospheric
density is known.
Two remaining unknowns: βaer , αaer
Theoretical 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 lidar
Raman 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.
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HIGH SPECTRAL RESOLUTION LIDAR (HSRL)
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,
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.
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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)
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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 – OVERVIEW

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
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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
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 ►)
(further
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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 :
 n
t
I
Δn
σ
nt
τ
  I n t   n  
n

(1)
: pumping irradiance
: population inversion density
: absorption cross section
: total density of atoms
: 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 :
I k  I k 1 e
a
k 1
z
Where Δz is the thickness and ak-1 is the absorption coefficient
of the element, derived from:
a k 1   I k 1  n t   n k 1 
which takes absorption saturation into account
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SIMULATION – POPULATION INVERSION DENSITY CALCULATION
Snapshot of the simulation program where the three - dimensional matrix of the population
inversion density has been derived and illustrated
12
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):

t
 c s  n 

x
c
 n
(2)
t
  c s  n
(3)
φ : photon density of the pulse
Δn : population inversion density
σs : stimulated emission cross section
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
.
Square input
pulse
Crystal Element
1
Output
pulse
Transformed
output pulse
Crystal Element
2
13
SIMULATION – POPULATION INVERSION DENSITY CALCULATION
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.
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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:
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
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.
15
EXPERIMENTAL SETUP
Nd:YVO4 Crystal
20 x 5 x 1.5 mm
1 % doping level
Master Laser
Nd:YVO4,
1064 nm
3 ns, 10 kHz, 20 μJ
SLM
Temperature
controlled
intracavity etalon
Closed loop water cooling system for
heat abduction at:
•Nd:YVO4 Crystal
•Diode bar
•Master Laser
Diode single bar
Max 40 W
10 mm length
100 μm thickness
35o fast axis
8o slow axis
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EXPERIMENTAL RESULTS
Total gain of 90 at 33 W pump power
17
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.
18
DESIGN
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