1. 1 A High Sensitivity Interferometer- Based Spectrometer Without a Fourier Transform Ricardo C. Coutinho a, David R. Selviah b, Hugh D. Griffiths c a Brazilian Navy Weapon Systems Directorate, Rua Primeiro de Março, 118, 20 o andar, Rio de Janeiro, Brazil, riccout@globo.com b Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, United Kingdom c Defence College of Management and Technology, Cranfield University, Shrivenham SN6 8LA, United Kingdom

2. 2 Contents 1 - Motivation 2 - Description of the technique 3 - Comparison with FTS 4 - Experiments and results 5 – Algorithm 6 - Conclusions

3. 3 Contents 1 - Motivation 2 - Description of the technique 3 - Comparison with FTS 4 - Experiments and results 5 – Algorithm 6 - Conclusions

4. 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).

5. 5 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. (thanks to Nick Davies - NPL) ( m)

6. 6 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

7. 7 Example 2: Deoxyhaemoglobin absorption spectra visible infrared (UCL Biomedical Optics Research Group web-site)

8. 8 Example 3: Forensic science (spectrum of Diazepam – Valium) (Brazilian Army Research Institute)

9. 9 Example 4: Detection of plastic explosives (with THz radiation) (spectrum of pentaerythritol tetranitrate – PETN) (Cook, Decker, Dadusk & Allen, Physical Sciences Inc., 2003)

10. 10 Contents 1 - Motivation 2 - Description of the technique 3 - Comparison with FTS 4 - Experiments and results 5 - Conclusions

11. 11 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.

12. 12 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). opticalelectronicdigital

13. 13 IPSS - Interferogram shaping Path Difference ( m) Amplitude Phase FT. Inst. freq. narrowband filter

14. 14 The detected signal Path Difference ( m) ADC Input (V)

15. 15 Phase step shift

16. 16 Contents 1 - Motivation 2 - Description of the technique 3 - Comparison with FTS 4 - Experiments and results 5 - Conclusions

17. 17 IPSS vs. FTS COMPARISON FEATUREIPSSFTS Length of scanSmall (a few m) Large (up to a few cm) Need for FTNoYes Data volumeSmallCan be very large SensitivityVery high (-46 dB) High

18. 18 FURTHER PERFORMANCE DATA Spectral resolution – Not measuring spectrum, but from the laser linewidth of 0.002 nm, corresponds to 0.05 cm -1 ; ROC characteristics – Laser target, PD 90%, PFA 2.10-4, SCR -26 dB; Sensitivity (SCR) advantage of 10 dB (wrt Eismann et al. SPIE 1997.

19. 19 IPSS vs. FTS COMPARISON FEATUREIPSSFTS Calibration laser Not requiredRequired Pre-knowledgeRequiredNot required BandwidthNarrowbandBroadband

20. 20 Contents 1 - Motivation 2 - Description of the technique 3 - Comparison with FTS 4 - Experiments and results 5 - Conclusions

21. 21 Basic experimental system Detection system iris

22. 22 Results: He-Ne laser filtered signal-to-clutter ratio (dB) Phase step shift ( m)

23. 23 Targets detected

24. 24 MDSCR vs. target coherence length log coherence length MDSCR modulus (dB)

25. 25 Contents 1 - Motivation 2 - Description of the technique 3 - Comparison with FTS 4 - Experiments and results 5 – Algorithm 6 - Conclusions

26. 26 Algorithm waveforms Input Filtered Data Phase Demodulation Frequency Demodulation Path Difference ( m)

27. 27 Contents 1 - Motivation 2 - Description of the studied technique 3 - Comparison with FTS 4 - Experiments and results 5 – Algorithm 6 - Conclusions

28. 28 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.

29. 29 ACKNOWLEDGEMENTS Prof. Herbert French - UCL Duleep Wickramasinghe – DSTL Portsdown West - UK