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Analysis of a Dual Etalon Fabry Perot
Cavity for application in a
Geostationary Coastal Water Imager
Viviana Vladutescu, Fred Moshary,
Barry Gross, Samir Ahmed
Optical Remote Sensing Lab, City College
and Graduate Center of CUNY
New York, New York 10031, USA
City College
of New York
Motivation
-Development of a compact Geostationary
Hyperspectral Imager that can be easily tuned to
different bands of interest for coastal water and
urban observations
-The main operating principle is the use of
synchronized multiple cavity Fabry Perot etalons
to provide single resonant spectral scanning
capabilities with very low out of band light over the
entire range from 400nm to 800nm coupled to
appropriate optics to image onto a compact
Charge Coupled Device (CCD) receiver.
-This design avoids the need for various
interference filters rotating into the beam.
Multicavity Fabry Perot Interferometer
Design
A high resolution single wavelength Fabry Perot Interferometer with
no filter to join the system was only proofed by now to be
achievable by use of three etalons in series. The restrictions due to
the free spectral range of one etalon with no additional filter can be
overcome by the three parallel cavities in series with an absorbing
medium between them.
t
t
Ein
T1T2T3Eint2
T1T2T3EinR1R22R3t6
T1T2T3EinR12R24R32t10
R1
R2
R3
Multicavity Fabry Perot Interferometer
Design
If reflections between etalons
T1T2T3t 2
T
(1  R1 R2t 2 )(1  R2 R3t 2 )  R1 R3T22t 4
t  transmissi on of seperating region
R j  1  T j 
With appropriate Optical Isolation
Tsys 
The lengths are connected by the so called res 
“vernier” ratio
2L p
p 1

2 Lq
q 1
j
j
2L p
p
If p=n, q=n+1, it is easy to
prove that all neighboring
resonances are eliminated
until
Therefore, maximum tuning range is limited. So
T

2 Lq
q
2 L2 p
2p


Lp
Lq

2 L2 q
2q
p
q

max  2min
res
2
Optical train
A
L2
60mm
S
H=100km
R
M
C
θ/2=2.6775°
α=0.02916
26.6
mm
Q
y=300mm
N
B
11
”
α=0.02916
O
49.3mm
O’
L’=
L=32000km
B’
2800mm
D
spotsize
α=0.02916
456.1mm
P
h’=8.75mm
A’
L1
60mm
150mm
•Schmidt Cassegrain telescope (11’ diameter)
•Double etalon Fabry Perot (60cm diameter ~
2.5um spacing) t=.85
• 2048x2048 CCD with a pixel size of 13umx13um.
•Pixel resolution ~ 100 meters
Time Exposure of the CCD Detector
SNR 100
-3
4
x 10
3.8
Required exposure time across the
spectrum for an SNR=100
assuming an optically thick aerosol
layer covering a typical deep ocean
signal
3.6
Time(s)
3.4
3.2
3
2.8
2.6
2.4
2.2
4
4.5
5
5.5
6
6.5
Wavelength(m)
7
7.5
8
-7
x 10
Etalon Transmissions
Transmission
of each
etalon
tuned to
T
600nm
a)
b)
Wavelength(m)
Transmission of the a) three uncoupled cavities and
b) three and two detuned vernier cavities
Detected and retrieved water leaving radiance
0.3
0.05
Retreived water leaving
radiance including out
of band compensation
0.045
0.25
0.04
Ignoring out of bands effects
Reflectance
0.2
0.035
0.03
0.15
0.025
0.1
0.02
TOA reflectance
Water leaving radiance
Ignoring out of band effects
0.05
0.015
0.01
0
0.005
True water leaving radiance
-0.05
4
5
6
7
8
0
4
-7
x 10
5
6
7
8
-7
Wavelength
x 10
Conclusions
The system presented here describes the transmission of a dual cavity planemirror Fabry–Perot interferometer. We illustrate in principle the design for a
VIS/NIR Fabry Perot Imaging Spectrometer and develop a proceedure to
calculate the characteristics of the needed components. We have also shown
that if we use synchronously scanned multiple etalon cavities, we may
significantly increase the FSR without decreasing the bandwidth of the
resonance allowing single mode operation over the 400nm-800nm band with
only a single fixed interference filter. The S/N between the main resonance and
the sideband resonance is somewhat impacted by the multiple reflections of the
cavities but we show that a moderate decoupling of the etalons is sufficient to
reduce the sidebands. Furthermore, we have performed a SNR analysis and
show that sufficient SNR may be achieved for limited integrations times on the
order of 4 ms. With the inclusion of the scanning times and CCD dump times,
an overall scan of 200 hyperspectral channels can be performed over 10
seconds In addition, we show that the appearance of off resonant side bands in
the spectrum need to be considered. If these bands are ignored, the overall
water leaving radiance is extremely overestimated but by a suitable inversion
taking into account the spectral details of the out of band signals, accurate
water leaving radiances may be obtained even in the presence of noise.
Acknowledgements: This work is partially supported by a
NASA-COSI grant #NCC-1-03009
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