Yair Antman , David Elooz , Avi Zadok

High Resolution Distributed Fiber-optic Sensors of Strain

and Temperature for Aerospace and Civil

Engineering ApplicationsOptical Engineering, Feb. 26, 2014

Yair Antman, David Elooz, Avi ZadokFaculty of Engineering, Bar-Ilan University, Ramat-Gan 52900, Israel

[email protected]

mailto:[email protected]

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Outline

What is distributed Brillouin sensing?

Commercial equipment: specification, deployment examples

New applications: improve resolution towards cm-scale

Solution principle

Experimental results

Ongoing work: integration of high-resolution distributed

measurements within composite materials.

Dr. Avi Zadok, Bar-Ilan Univ., COST Technical Meeting, Oct. 2013

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Stimulated Brillouin scattering

Dr. Avi Zadok, Bar-Ilan Univ., Optical Engineering, Feb. 2014

Image curtsy of Luc Thevenaz, EPFL Switzerland

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Brillouin Fiber Sensing Localization and measurement of strain variation

Source: www.neubrex.com

Pump Probe

Repeat for various positions, frequency offsets


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Commercial Deployment Examples Omnisens, Switzerland. Focus on Energy Sector. Pipeline integrity monitoring:


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Commercial Deployment Examples Omnisens, Switzerland. Undersea cables monitoring:


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Commercial Deployment Examples Omnisens, Switzerland. Downhole monitoring:


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Commercial Deployment Examples Omnisens, Switzerland. Power plants and cables


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B-OTDA CW probe amplified by counter-propagating, pulsed pump Most widely known and employed Brillouin analysis scheme Long range: towards 100 km! For a given frequency offset, one scan maps out the entire fiber That scan can be very fast (towards hundreds of Hz at 1 km) Resolution (of ‘classic’ scheme): on the order of 1 meter Many elaborate configurations for resolution enhancement:

pre-excitation, multiple pulse widths, etc. Centimeter-scale resolution is challenging

Y. Peled, A. Motil, and M. Tur, Opt. Express 20, 8584-8591 (2012) S. M. Foaleng, M. Tur, J.-C. Beugnot, and L. Thevenaz, JLT 28, 2993 (2010)


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Acoustic Field in Space and Time Intensity of stimulated acoustic field: where and when do the

pump and probe waves interact effectively? The B-OTDA case (reference for following schemes):

z [m]

t [ns

]Acoustic Field Amplitude

-3 -2 -1 0 1 2 3

0

10

20

30

40

50

60

70

Probe (CW)Pump (pulsed)

Output probe is ‘imprinted’ with SBS gain information that is accumulated over long section Dr. Avi Zadok, Bar-Ilan Univ., Optical Engineering, Feb. 2014

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Motivation

Provide distributed Brillouin sensing with:Centimeter-scale resolutionHundreds of meters range (tens of thousands of

resolution points)Reduced acquisition times: simultaneous

interrogation of a large number of high-resolution points.


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High Resolution: Correlation Domain Driving force for the acoustic field buildup: inner product of

pump and probe

Analogous to the charging of a capacitor with a time constant . Acoustic field: integrate over driving force cross correlation. Localization of SBS interactions through manipulations of the cross-

correlation between pump and probe envelopes B-OCDA (Prof. Hotate, Univ. of Tokyo): Sync. FM of both waves Inner product stable at few correlation peaks, oscillates elsewhere Sub-cm resolution. Periodic peaks restrict the unambiguous

measurement range to hundreds of resolution points

1

, 1 , , , .2 p s

Q t zj Q t z jg A t z A t z

t

K. Hotate and M. Tanaka, IEEE Photonics Tech. Lett. 14, 179-181 (2002).


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PRBS Phase Coding Pump and probe co-modulated by a binary PRBS phase code Code length: . Bit duration: Narrow correlation peaks: (100 ps bits 1 cm resolution) Arbitrarily long separation between neighboring peaks (and

range of unambiguous measurements): Resolution and range effectively decoupled Correlation peak can be scanned along the fiber under test,

through timing of modulated waves.

Y. Antman, N. Primerov, J. Sancho, L. Thevenaz, and A. Zadok, Optics Express 20, 7807 (2013)


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PRBS Phase Coding: Acoustic Field

SBS interaction is stationary and localized Output probe affected by SBS at a single location only

Pump (fast PRBS phasecode)

Probe (fastPRBS phasecode)


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Difficulties with Phase Coding High resolution and (in principle) long range, but… Off-peak acoustic fields are non-zero, and contribute parasitic

SBS amplification (‘coding noise’). Each noise contribution is weak,

but there are 10,000 of them… Large number of averages SBS is built and interrogated

one-point-at-a-time (for each frequency offset):

Number of scans equals the number of resolution points: long acquisition times

z [m]

t [ns

]

PRBS

-0.5 0 0.5

0

10

20

30

40

50

60

z [m]

t [ns

]

Golomb

-0.5 0 0.5

0

10

20

30

40

50

60


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Coding Noise Reduction Weaker coding noise with judicious choice of code. Simulations:

z [m]

t [ns

]

PRBS

-0.5 0 0.5

0

10

20

30

40

50

60

z [m]

t [ns

]

Golomb

-0.5 0 0.5

0

10

20

30

40

50

60

Y. Antman, N. Levanon, and A. Zadok, Optics Letters 37, 5269-5271 (2012)


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Coding Noise Reduction (Continued)

Experimental results:

Y. Antman, L. Yaron, T. Langer, N. Levanon, M. Tur, and A. Zadok, Opt. Letters 38, 4701 (2013)


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z [m]

t [ns


-3 -2 -1 0 1 2 3

0

20

40

60

80

100

120

140

160

180

Solution Paths: Interrogation of Multiple Points

Use short phase codes (~ 130 bits): multiple correlation peaks are introduced.

Pump (fast and short PRBS phasecode)

Probe (fastand shortPRBS phasecode)

Output probe is again amplified at numerous locations. Wasn’t that what we were trying to avoid?


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z [m]

t [ns


-3 -2 -1 0 1 2 3

0

10

20

30

40

50

60

Short Phase Codes with Overlaying Pulsed Pump

Use amplitude pulsed modulation of the pump wave on top of the phase codes

Pump (fast and short PRBS phasecode, withamplitude pulses)

Probe (fastand shortPRBS phasecode)

Correlation peaks are introduced at different times. Probe amplification can be monitored in time domain, unambiguously.


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0 200 400 600 800 1000 1200 1400 1600 1800 2000-1

0

1

2

3

4

5

6x 10

-3

t [ns]

Pow

er [A

.U]

Output Probe Power

Experimental Results 200 m-long fiber. 127 bits-long phase code. 2 cm resolution. Overlaying pump pulses: 26ns duration Example: output probe power as a function of time

Periodic, isolated peaks repeat every 26 ns (code period), corresponding to SBS gain of individual correlation locations.

Phase codes re-timed to move on to next 2 cm-long sections, and so on…


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Experimental Results: Continued A hot spot: one of the peaks is missing, and reappears at a

different frequency offset.

0 500 1000 1500 2000-1

0

1

2

3

4

5

6

7 x 10-3

t [ns]

Pow

er [A

.U]

Output Probe Power

1600 1650 1700 1750 1800 1850 1900 1950

3

3.5

4

4.5

5

5.5

6

6.5

x 10-3

t [ns]

Pow

er [A

.U]

Output Probe Power

0 200 400 600 800 1000 1200 1400 1600 1800 2000-1

0

1

2

3

4

5

6

7x 10

-3

t [ns]

Pow

er [A

.U]

Output Probe Power

1650 1700 1750 1800 1850 1900 1950

3

3.5

4

4.5

5

5.5

x 10-3

t [ns]

Pow

er [A

.U]

Output Probe Power


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Brillouin Gain Map This experiment: two spliced fibers, 400 m, 2 cm resolution.

All 20,000 resolution points covered with only 130 scans. Acquisition time: 20 minutes, mostly saving of scope traces

and equipment switching. Net acquisition time << 1 minute.


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Brillouin Gain Map 5 cm-long hot-spot towards the end of a 400 m-long fiber 128 averages used.

Uncertainty in Brillouin shift : ±3 MHz (±3 C): rather large…


D. Elooz, Y. Antman, N. Levanon, and A. Zadok, accepted for publication, Opt. Express

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Latest Results: 1600m Range 5 cm-long hot-spot towards the end of a 1600 m-long fiber (“More than a mile, less than an inch”). 512 averages


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Summary Co-modulation of pump and probe by short, high-rate phase

codes to obtain numerous, movable correlation peaks Amplitude modulation of pump pulses to generate correlation

peaks one after another. Time-domain analysis of probe to separate between peaks Demonstrated complete scan of 1,600 m fiber with 2 cm

resolution, 80,000 points, with only 127 scans per frequency offset.

Number of scans pre frequency offset for M resolution points:B-OTDA: 1 (low resolution)Phase-coding: M ~ 10,000 – 100,000Proposed hybrid method: code length N ~ 130 << M


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Next Phase Implement distributed Brillouin measurements over fibers

embedded in composite materials. Much work has been done, by Tel-Aviv Univ. (Tur group) and

IAI, and elsewhere world-wide:Discrete, point sensors (fiber Bragg gratings), including

measurements during flights! Distributed monitoring based on Rayleigh scatteringDistributed Brillouin monitoring, including dynamic

measurements, with lower resolution. MAGNETON project: Xenom Ltd. and Bar-Ilan University.

Demonstrate high-resolution Brillouin measurements within the products of the company.


27Dr. Avi Zadok, Bar-Ilan Univ., COST Technical Meeting, Oct. 2013

Xenom-BIU MAGNETON Project Early days… more to follow

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Acknowledgements Chief Scientist Office, Israeli Ministry of Industry, Trade and

Labor (MoITaL): KAMIN program European Union Cooperation on Science and Technology

(COST) Action TD-1001, OFSESA. EPFL, Switzerland: Prof. Luc Thevenaz, Nikolay Primerov,

Andrey Denisov Tel-Aviv University: Prof. Moshe Tur, Tomy Langer, Lior Yaron


Documents

Yair Antman , David Elooz , Avi Zadok