WDM Fiber Vibrometry Krzysztof M. Abramski

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

Introduction

Principles of operation

Principles of operation

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

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

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

Idea of laser-fiber vibrometer

Multipoint vibrometer

Experimental set-up

Fibre – free space out coupling

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

WDM characteristics

Optical spectra

Detection of heterodyne signals Balanced photodetector

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:

Experiment driving signal vibration at first point vibration at second point

4 - channel vibrometer

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.

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”

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