Transcript FABRY-PEROT ETALONS

CENTRO DI CULTURA SCIENTIFICA
A.VOLTA
National Technical
University of Athens
NTUA
VILLA OLMO COMO-ITALIA
Design and Data Analysis Method of Receivers
of HSRL for Atmospheric Monitoring in Ultra
High Energy Cosmic Ray Experiments
S. Maltezos, E. Fokitis, P. Fetfatzis, A. Georgakopoulou, V. Gika,
I. Manthos and G. Koutelieris
11th ICATPP Conference on
Astroparticle, Particle, Space Physics, Detectors and Medical Physics
Applications
Villa Olmo, 5-9 October, 2009
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.
Three main shower components
Muonic
The intensity increases with altitude
Hadronic
The Electromagnetic
detected
cosmic
and it changes with latitude.
We consider
that: ray
90% ofpeaks
the cosmic
rays are15
protons,
flux
at about
km
9% are alpha
particles and
in altitude.
1% are electrons.
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Cosmic rays
Atmospheric fluorescence experiments
•ASHRA [All-sky Survey High Resolution Air-shower detector]
•Auger Project Fluorescence Group
•EUSO (Extreme Universe Space Observatory)
•HiRes (The High Resolution Fly’s Eye)
•OWL (Orbiting Wide-angleLight collectors)
Expected SOURCES
Supernova Remnant SN1006
(AGN) Active galactic nuclei
(with black-holes at their
center)
Cygnus X-3
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The active galaxy NGC-4261
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Extensive Air Showers
EAS

Almost 90% of primary radiation is fluorescence radiation and that’ s why this
method is useful.

The remaining energy is then distributed over the secondary particles.

Beyond the "shower maximum", the shower particles are gradually absorbed with
an attenuation length of ~200 g/cm2.
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
The passage of charged particles in an extensive air shower through the
atmosphere results in the ionization and excitation of the gas molecules
(mostly of nitrogen). The emitted radiation by de-excitation of the
nitrogen molecules is extended mainly in the UV region.
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

The aerosols cause exclusively
Mie scattering 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.
To correct the Extensive Air
Shower
signal
of
air
fluorescence for the air
Cherenkov
contamination,
caused mainly by the aerosols,
accurate data for the aerosols
are needed. This can be made
by atmospheric monitoring.
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
The atmospheric monitoring forms what is known as remote sensing of
atmospheric properties with use in Air Fluorescence telescopes.

In the experiments for Ultra High Energy Cosmic Rays the signal
detected by the fluorescence telescopes have to be corrected by
means of mixing with the scattered Cherenkov radiation mostly by the
aerosols in the atmosphere.

The main target of the monitoring is to determine the concentration of
the aerosols, which is variable in time.
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LIDAR
LIght Detection And Ranging
One of the main methods
of atmospheric monitoring
is the LIDAR.
Types of Lidar




Doppler Lidar
Raman Lidar
DIAL Lidar (Differential
Absorption Lidar)
High Spectral Resolution
Lidar HSRL
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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.
Different geometries have been proposed for LIDAR atmospheric monitoring.
One is that the emitter and the receiver are in backscatter mode. This means that light is collected only at
the angle of 1800 as measured from the emitter. The light source is a pulsed Laser (expensive), so that the
time interval determines the distance from the system.
The other is in Bi-static mode with a cw (continuous wave) laser (low cost) on which we present the
development of a prototype. By this technique we combine low cost and high accuracy.
Spectral profile of backscattering from a mixture
of molecules and aerosols for a temperature of
300 K. The spectral width of the narrow aerosol
return is normally determined by the line width of
the transmitting laser.
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
Newtonian telescope of
D=250mm and f-number 5.5.

A
solid
state
CW
laser
(OEM
manufacturer) 120 mW at 532 nm, with
coherence length exceeding
50m
-1
corresponding to δk~0.02 cm .

Two different Fabry-Perot etalons
(spacer thickness: 50 mm for aerosol
channel and 5 mm for molecular
channel) with verified overall finesse of
17.5.

Two colored CCD cameras (Nikon D40)
with 6 Mpixel with analysis (3040x2014)
and pixel size 7.8 μm have been used
for recording the fringe images for the
two channels respectively.
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diameter
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2
1
3
3
4
5
4
5
1
2
1) Focus system
Input collimating lens
Narrow band filter
Diagonal mirror
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2) Fabry-Perot etalon
5) CCD cameras (Nikon D40)
3) Output Lens
4) Optical benches
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
Main idea: exploit both the light intensity and the spectral information.

The total intensity is calculated by integration along the fringe ring
system (rotated scanning).

Possible errors caused by the optical defects may partially destroy the
circular symmetry of the fringes.

In the next we describe an analysis method which can be used on one
fringe pattern system produced from a monochromatic beam.
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
First step : we select the data points close to the peak of the fringes rings. Finally
we identify each data point attributed to it the fringe order in which it belongs.
These can be achieved by appropriate algorithms.

Second step: we apply a direct two-dimensional non-linear chi-square fit considering
a model of a system of ellipses described in a arbitrary orthogonal coordinate
system.
ax2   xy   y 2   x   y    0
  p 1  
where α,β,γ,δ,ζ and Φ are free parameters to be determine, p is the order of
the fringe and ε is the excess fraction.
By introducing vectors :
A=[α,ζ,β,γ,δ,Φ]T , X=[x2,χy,y2,x,y,1]
the equation above can be rewritten to the vector form
FA(χ)= χ·Α=0
This equation corresponds to a linear system with six equations.
We expect to have an ellipses.
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
Third step: we determine free parameter values in order to calculate the
geometrical parameters of the ellipses and the excess fraction of the
fringe pattern. The excess fraction ε is a significant parameter because
its variation is proportional to the wavelength variation and thus it is
used to study the laser beam frequency stability obtaining a number of
successive interferograms.

2

y0  
2
x0  
 cc 
x0 and y0 are the coordinates of the center
εcc is the eccentricity
θ is the rotation angle of the ellipses
1  2
2
  
1
  arc sin 

2
   
2 2
  1  

4 4 
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STABILITY STUDY

The value of ε is related to the etalon spacing and source wavelength by
the equation
h  (m   )
0
2
 m 
2h
0
0
2h
h  (m   ) 
 0
2
m
for δε=1 we have:
 
and
2h
0
m
0 
2h

2
m
2h 
m 2 
2h  0 2
0 2
2h
 0 

 0 
2
2
2h 
2h
 2h 
 2h 
m
 
0 
 0 
0 
where h is the spacer thickness of etalon and m the integer number of λ0/2.
If h=50 mm and λ0= 532 nm then δλ0=0.0028 nm.
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First step
300
200
y (pixel)
100
0
-100
-200
-300
-400
-300
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-100
0
100
x (pixel)
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300
εcc= 0.9989577
θ = 31.77542
ε = 0.455362
Assuming an expected
wavelength 532 nm the
abοve value of ε lead to an
exact wavelength equal to
531.99849 nm.
200
y (pixel)
100
0
-100
-200
-300
-400
-300
-200
-100
0
100
x (pixel)
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Circular scanning
- red is the initial, - blue is the final normalized
to the initial for comparison reasons.
5
3
x 10
2.5
 = 358.5
Counts
2
1.5
1
0.5
0
1250
1300
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1400
1450
1500
Pixel No
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1650
1700
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This work is extension of our instrumentation development for atmospheric monitoring
using the High Spectral Resolution LIDAR as described above. We present spectra of
natural mercury lines selected by interference filters, and using Fabry-Perot etalon with
0.25cm-1 free spectral range (FSR). In Figure 1, we see the intensity pattern
corresponding to the central and the next fringe. We observe, although with low
resolution, splitting of spectral lines due to isotopic shift.The interferogram taken with
the 2 cm spacer etalon has smaller resolution than the 5 cm spacer etalon available.
Figure 1
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Figure 2
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
We are developing a prototype of HSRL in bi-static mode using
two channels to separate the molecular and the aerosol signal.
We also obtain a set of scattered signals in the laboratory in
order to characterize the Fabry-Perot receivers.

We further developed and applied an analysis method based
on two dimensional direct fit to fringe pattern.

Using this method we are able to evaluate the stability of the
laser.

An alternative thermoelectrically cooled CCD sensor from SBIG
and a liquid nitrogen cooled CCD sensor will be tested for the
bi-static LIDAR.
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