1. WDM Fiber Vibrometry
Krzysztof M. Abramski

2. Outline Introduction The idea of multipoint fibre vibrometer
Heterodyne signals
Demodulators
Experiment and signal processing
Conclusions

3. Introduction

4. Principles of operation

5. Principles of operation

6. Homodyne vs. heterodyne detection
The low frequency noise spectrum
Advantages of using heterodyne technique

7. Features
Advantages:
Visible radiation, an analyzed point can be easy localized
High resolution, it depends on a source wavelength
Disadvantages:
1. Low output power, less than 1mW
2. Multipoint measurement in the same time is impossible

8. Motivation for 1550 nm
Relatively low prices of fibre elements, such like: optical isolators, couplers, photodetectors, fibre Bragg gratings etc.
2. Narrow linewidth laser diodes (DFB, DBR) are available
3. ”Eye safe” 1550 nm radiation
4. Unique possibility of weak optical signals amplification by very useful device: Erbium Doped Fibre Amplifier (EDFA)
5. WDM technique allows to make a multichannel vibrometer

9. Idea of laser-fiber vibrometer

10. Multipoint vibrometer

11. Experimental set-up

12. Fibre – free space out coupling

13. EDFA - spectral gain characteristic
The best wavelength for single point vibrometer
The best area for multipoint vibrometer

14. WDM characteristics

15. Optical spectra

16. Detection of heterodyne signals Balanced photodetector

17. Demodulation of heterodyne signals
l1 – observed wavelength, l0 – source wavelength, c – light velocity, V – light source velocity, l – unit vector in the direction of observation, f0 – frequency of light source
For 1550 nm we obtain carrier signal with frequency deviation:
For 632,8 nm frequency deviation is:

18. Experiment driving signal vibration at first point
vibration at second point

19. 4 - channel vibrometer

20. Conclusions
WDM systems can be particularly useful for multichannel fibre vibrometry.
We obtain heterodyne signals with SNR good enough for FM demodulation.
Our experiment showed practical advantage of the system which can be adapted to many measurement application, particularly can be very flexible.
The signal processing for speed – displacement – acceleration measurement based on LabView environment is under development.

21. Acknowledgments
This work has been carried out in the frame of „The EEA Financial Mechanism & Norwegian Financial Mechanism” (PL0471) entitled:
„Developing novel laser-fiber monitoring technologies to prevent environmental hazards from vibrating objects”

22. Thank You