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, riccout@globo.com

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

0

0.1

0.2

0.3

0.4

0.5

0.6

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”)

0

0,1

0,2

0,3

0,4

0,5

0,6

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

0

0.2

0.4

0.6

0.8

1

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

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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