PL0471

KKrzysztofrzysztof M. Abramski M. Abramski

WDM WDM Fiber VibrometryFiber Vibrometry

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• IntroductionIntroduction

• The idea of multipoint fibre vibrometerThe idea of multipoint fibre vibrometer

• Heterodyne signalsHeterodyne signals

• DemodulatorsDemodulators

• Experiment and signal processingExperiment and signal processing

• ConclusionsConclusions

OutlineOutline

PL0471IntroductionIntroduction

PL0471Principles of operationPrinciples of operation

PL0471Principles of operationPrinciples of operation

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Homodyne Homodyne vvs. heterodyne detections. heterodyne detection

The low frequency noise spectrumThe low frequency noise spectrum Advantages of using heterodyne techniqueAdvantages of using heterodyne technique

PL0471FeaturesFeatures

Advantages:Advantages:

1.1. Visible radiation, aVisible radiation, an analyzedn analyzed point can be easy localized point can be easy localized

2.2. High resolution, it depends on a source wavelengthHigh resolution, it depends on a source wavelength

Disadvantages:Disadvantages:

1. Low output power, less than 1mW1. Low output power, less than 1mW

2. Multipoint measurement in the same time is impossible2. Multipoint measurement in the same time is impossible

PL0471Motivation for 1550 nmMotivation for 1550 nm

1.1. Relatively low prices of fibre elements, such like: optical Relatively low prices of fibre elements, such like: optical isolators, couplers, photodetectors, fibre Bragg gratings isolators, couplers, photodetectors, fibre Bragg gratings etc.etc.

2. Narrow linewidth laser diodes (DFB, DBR) are available2. Narrow linewidth laser diodes (DFB, DBR) are available

3. ”Eye safe” 1550 nm radiation3. ”Eye safe” 1550 nm radiation

4. Unique possibility of weak optical signals amplification by 4. Unique possibility of weak optical signals amplification by very useful device: Erbium Doped Fibre Amplifier (EDFA)very useful device: Erbium Doped Fibre Amplifier (EDFA)

5. WDM technique allows to make a multichannel vibrometer5. WDM technique allows to make a multichannel vibrometer

PL0471Idea of laser-fiber vibrometerIdea of laser-fiber vibrometer

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Multipoint vibrometerMultipoint vibrometer

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Experimental set-upExperimental set-up

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Fibre – free space out couplingFibre – free space out coupling

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EDFA - spectral gain characteristicEDFA - spectral gain characteristic

The best The best area for area for

multipoint multipoint vibrometervibrometer

The best The best wavelength wavelength for single for single

point point vibrometervibrometer

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WDM characteristicsWDM characteristics

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Optical spectraOptical spectra

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Detection of heterodyne signalsDetection of heterodyne signals

)(

)(

)cos()sin(

)sin()cos(

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)(

2

1

2

1

fE

fE

lli

lile

fE

fE

i

ili

o

o

Balanced photodetectorBalanced photodetector

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Demodulation of heterodyne signalsDemodulation of heterodyne signals

c

lV

f

c

f

lVc0

0001

1 1 – observed wavelength, – observed wavelength, 00 – source wavelength, c – light velocity, V – light source velocity, l – unit – source wavelength, c – light velocity, V – light source velocity, l – unit

vector in the direction of observation, fvector in the direction of observation, f00 – frequency of light source – frequency of light source

0101

ccfff

c

lVfcf 02

0

0lV

f

For 1550 nm we obtain carrier signal with frequency deviation:For 1550 nm we obtain carrier signal with frequency deviation:

sm

MHzlVD /

29,120

For For 632,8632,8 nm frequency deviation nm frequency deviation is is::

sm

MHzlVD /

16,320

PL0471ExperimentExperiment

drivingdrivingsignalsignal

vibration vibration at first at first pointpoint

vibration vibration at second at second

pointpoint

PL04714 - channel vibrometer4 - channel vibrometer

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ConclusionsConclusions

• WDM systems can be particularly useful for multichannel fibre vibrometry.WDM systems can be particularly useful for multichannel fibre vibrometry.

• We obtain heterodyne signals with SNR good enough for FM demodulation.We obtain heterodyne signals with SNR good enough for FM demodulation.

• Our experiment showed practical advantage of the system which can be Our experiment showed practical advantage of the system which can be adapted to many measurement application, particularly can be very flexible. adapted to many measurement application, particularly can be very flexible.

• The signal processing for speed – displacement – acceleration The signal processing for speed – displacement – acceleration measurement based on LabView environment is under development.measurement based on LabView environment is under development.

PL0471AcknowledgmentsAcknowledgments

This work has been carried out in the frame of This work has been carried out in the frame of „The EEA Financial Mechanism & Norwegian Financial „The EEA Financial Mechanism & Norwegian Financial

Mechanism” (PL0471) entitled: Mechanism” (PL0471) entitled:

„„Developing novel laser-fiber monitoring technologies to prevent Developing novel laser-fiber monitoring technologies to prevent environmental hazards from vibrating objectsenvironmental hazards from vibrating objects””

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Thank YouThank You