High Sensitivity Optical Coherence Detector Optimization

1
High Sensitivity Optical Coherence Detector
Optimization

• R.C. Coutinho, D.R. Selviah, H.A. French and H.D.
  Griffiths
• Department of Electronic and Electrical
  Engineering, University College London, United
  Kingdom

2
Outline

• Technique Description
• Derivation of Theoretical Responsivity
• Description of the Experiment
• Theoretical Vs. Experimental Results
• Conclusion

3
Basics

• Technique combining optical and digital signal
  processing to detect coherent or partially
  coherent sources in an incoherent environment
• Employs an optical narrowband filter to generate
  a specific feature in the self coherence function
  measured with an interferometer

• Unlike Fourier transform spectroscopy (FTS), the
  path difference is scanned in a tiny region
  surrounding the first minimum of the self
  coherence function (interferogram), thus
  achieving faster frame rates
• The recorded interferogram is processed using a
  computer algorithm to extract a phase step in the
  fringe signal its position is used to declare
  detection.

4
Theory
Detector Reading (mV)
F.T.
Path Difference (microns)

• If a spectrally narrow emission source enters the
  field of view, the net degree of coherence of the
  scene changes, shifting the position of the first
  minimum in the self coherence function (see next
  slide). This shift is measured and used for
  detection
• The approach senses the change in the spectrum
  through measurements of the change in a region of
  the interferogram, which makes it a lot faster
  than other spectral approaches.

5
The signal
6
Phase Step Detection Algorithm
7
Derivation of Theoretical Responsivity
8
Experimental Arrangement
9
Target/Filter Combinations
10
Results - Responsivity
11
Results - Wavelength Offset
12
Conclusions

• Responsivity already high can be increased
  twofold by careful choice of filter to target
  optical bandwidth ratio
• Design of filter transmission curve is another
  degree of freedom to be exploited