1 a high sensitivity interferometer- based spectrometer without a fourier transform ricardo c....
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1
A High Sensitivity Interferometer-Based Spectrometer Without a Fourier
Transform
Ricardo C. Coutinhoa, David R. Selviahb, Hugh D. Griffithsc
aBrazilian Navy Weapon Systems Directorate, Rua Primeiro de Março, 118, 20o andar, Rio de Janeiro, Brazil, [email protected]
bDepartment of Electronic and Electrical Engineering, University College London, London WC1E 7JE, United Kingdom
cDefence College of Management and Technology, Cranfield University, Shrivenham SN6 8LA, United Kingdom
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Contents
1 - Motivation
2 - Description of the technique
3 - Comparison with FTS
4 - Experiments and results
5 – Algorithm
6 - Conclusions
3
Contents
1 - Motivation
2 - Description of the technique
3 - Comparison with FTS
4 - Experiments and results
5 – Algorithm
6 - Conclusions
4
Motivation• Detection of optical dim emissions in brighter
backgrounds;• Increased sensitivity requires new dimensionality in the
detection process;• FTS uses the spectral signature or some features in it as
discriminants;• FTS instruments generate large amounts of data;• We propose a technique that measures a spectral feature
without measuring or estimating the whole spectrum;• This measurement is performed directly in the coherence
domain (interferogram).
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Applications of coherence-based approaches
• anything where a coherent target is in an incoherent background;
• In terms of spectrum, a coherent target means one that displays narrow spectral features.
4.1 4.15 4.2 4.25 4.3 4.35 4.4 4.45 4.5 4.55 4.6 4.65 4.7-0.1
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0.1
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Wavelength (microns)
Inte
nsity
(a.
u.)
(thanks to Nick Davies - NPL)
(m)
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Example 1: A helicopter infra red emission spectrum
(m)
(G.J. Zissis, ed., Sources of Radiation, vol. 1 of The Infrared and Electro-Optical Handbook. ERIM/SPIE Press, Bellingham, 1993)
unsuppressed
suppressed
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Example 2: Deoxyhaemoglobin absorption spectra
visible infrared
(UCL Biomedical Optics Research Group web-site)
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Example 3: Forensic science(spectrum of Diazepam – “Valium”)
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0,1
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0,7
0,8
0,9
1
2 3 4 5 6 7 8 9 10
wavelength (microns)
no
rm. i
nte
nsi
ty
(Brazilian Army Research Institute)
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Example 4: Detection of plastic explosives (with THz radiation)(spectrum of pentaerythritol tetranitrate – PETN)
(Cook, Decker, Dadusk & Allen, Physical Sciences Inc., 2003)
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Contents
1 - Motivation
2 - Description of the technique
3 - Comparison with FTS
4 - Experiments and results
5 - Conclusions
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FTS• Interferometer (Michelson)
measures self-coherence function (interferogram);
• The interferogram is Fourier transformed to obtain the power spectrum;
• Resolution proportional to scan length;
• Reference laser required to calibrate the path difference.
-10 -8 -6 -4 -2 0 2 4 6 8 10-1
-0.8
-0.6
-0.4
-0.2
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0.2
0.4
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0.8
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Path Difference (microns)
Det
ecto
r R
eadi
ng (
volts
)
400 430 460 490 520 550 580 610 640 670 700 730 760 790 8200
0.2
0.4
0.6
0.8
1
1.2
Wavelength (nm)
Pow
er S
pect
ral D
ensi
ty (
a.u.
)
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Interferogram Phase Step Shift (IPSS)
• Filter rejects clutter and creates interferogram minima;
• Interferometer measures self-coherence in the vicinity of the first minimum (much shorter scan);
• Signal processing algorithm extracts position of the phase step in the fringe carrier, proportional to self-coherence (no FT).
optical electronic digital
Signal Conditioning
Extraction Algorithm
detector output optics
interferometer interference filter
input optics
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IPSS - Interferogram shaping
0
Wavenumber
TotalSpectral PowerDensity
PB/
PB/+PE/
Path Difference (m)
Amplitude
Phase
FT.
Inst.
freq.narrowband filter
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The detected signal
Path Difference (m) Path Difference (m)
AD
C I
nput
(V
)
AD
C I
nput
(V
)
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Phase step shift
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Contents
1 - Motivation
2 - Description of the technique
3 - Comparison with FTS
4 - Experiments and results
5 - Conclusions
17
IPSS vs. FTS COMPARISON
FEATURE IPSS FTS
Length of scan Small (a few m)
Large (up to a few cm)
Need for FT No Yes
Data volume Small Can be very large
Sensitivity Very high (-46 dB)
High
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IPSS vs. FTS COMPARISON
FEATURE IPSS FTS
Calibration laser
Not required Required
Pre-knowledge Required Not required
Bandwidth Narrowband Broadband
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Contents
1 - Motivation
2 - Description of the technique
3 - Comparison with FTS
4 - Experiments and results
5 - Conclusions
21
Basic experimental system
driver
controller
audio amplifier
oscilloscope
light source (target)
detector
beamsplitter
ND filters
interference filter and rotation stage
interferometer
translation stage
light source (background)
Detection system
iris
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Results: He-Ne laser
-50 -45 -40 -35 -30 -25 -20 -15 -10 -5-5
0
5
10
15
20
filtered signal-to-clutter ratio (dB)
phas
e st
ep s
hift
(mic
rons
)
filtered signal-to-clutter ratio (dB)
Pha
se s
tep
shif
t (
m)
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Targets detectedTarget Target
central
wavelength
and FWHM
(nm)
Target
coherence
length
Filter
central
wavelength
and FWHM
(nm)
Dispersion
coefficient
(standard
deviation /
average)
Minimum
detectable
SCR at
input (dB)
Maximum
phase step
displacement
(m)
He-Ne laser 632.8/0.002 20 cm 632.8/11 1.05 -46.42 16.7 +/- 1.6
RCLED 650/10 22.77 m 651.9/36.2 0.21 Less than
-30
6.8 +/- 1.1
Tungsten
halogen
bulb with
filter
648.7/12.2 11.48 m 651.9/36.2 1.7 -31.96 2.9 +/- 2.0
LED 644/18 5.44 m 651.9/36.2 2.5 -13.07 1.06 +/- 0.37
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MDSCR vs. target coherence length
100
101
102
103
104
105
106
10
15
20
25
30
35
40
45
50
log of coherence length in microns
min
imum
det
ecta
ble
SC
R (
dB)
log coherence length
MD
SC
R m
odul
us
(dB
)
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Contents
1 - Motivation
2 - Description of the technique
3 - Comparison with FTS
4 - Experiments and results
5 – Algorithm
6 - Conclusions
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Algorithm waveforms
Input
Filtered Data
Phase
Demodulation
Frequency
Demodulation
Path Difference (m)
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Contents
1 - Motivation
2 - Description of the studied technique
3 - Comparison with FTS
4 - Experiments and results
5 – Algorithm
6 - Conclusions
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Conclusions • IPSS is an alternative spectroscopic method not requiring
a Fourier Transform;• The method compares favorably with FTS in speed,
update rates and sensitivity;• IPSS produces a reduced data volume;• Pre-knowledge of spectral characteristics of the target is
required;• The method is narrowband;• IPSS is advantageous for detection of known targets with
very high sensitivity.
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ACKNOWLEDGEMENTS
• Prof. Herbert French - UCL
• Duleep Wickramasinghe – DSTL Portsdown West - UK