Ultra-High Spatial Resolution in Distributed Fibre Sensing

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1 Presentation of EPFL-Group for Fibre Optics: Ultra-High Spatial Resolution in Distributed Fibre Sensing Prof. Luc THEVENAZ & co-workers 2 Postdoc, 4 PhD students, 3 visiting students 1/5 Adm. Assistant - 1/10 Technician

GFO Activities Optical Fibre Sensors Electrical current sensors (closed) Distributed fibre sensing using stimulated Brillouin scattering (core research) Optical signal processing using Fibres

2 GFO Activities Optical Fibre Sensors Electrical current sensors (closed) Distributed fibre sensing using stimulated Brillouin scattering (core research) Optical signal processing using Fibres Slow & fast light, optical storage Dynamic fibre gratings, microwave photonics Sensing using laser spectroscopy Photoacoustic gas trace sensing (closed) Photonic crystal fibres and waveguides

3 Fibre sensing, a response to a societal concern Safer infrastructures More efficient energy use Environmental & human threats Anticipating catastrophic failures

seamlessly and densely integrated in a structure or the environment

4 «Optical fibre nerves» a realistic answer to this concern The optical fibre can play the role of a sensitive nerve that is seamlessly and densely integrated in a structure or the environment The optical fibre can inform about the amplitude and the position of the «sensation»

Examples of distributed fibre sensing A standard optical fibre is the sensing element and gives a value of measurand for each point along the fibre.

5 Examples of distributed fibre sensing A standard optical fibre is the sensing element and gives a value of measurand for each point along the fibre. Temperature, degc Optical fibre cable February 6 May 12 November 14 Position along the fibre, m The optical fibre may replace many thousands of point sensors. Range: 50 km, ext. to 150 km Temperature resolution: 0.1 deg Strain resolution: % Spatial resolution: m (range dependent)

6 Spontaneous scattering-based sensors Pulsed laser Directional coupler Light pulse Detection Fibre under test P(z) = 1 2 v gr P o t S(z) a d (z) e - 2az S(z) = 3 8 lo

6 6 Spontaneous scattering-based sensors Pulsed laser Directional coupler Light pulse Detection Fibre under test P(z) = 1 2 v gr P o t S(z) a d (z) e - 2az S(z) = 3 8 lo n pwo(z) 2 : Recapture factor of the backscattered light (wo: mode radius) ad(z) : Scattering coefficient a t : Attenuation : Pulse temporal width v gr : Group velocity

7 7 Rayleigh-based sensing principle Coherent optical time-domain reflectometry An optical pulse is use to interrogate the fiber Backscattered light originated from the different scattering points interfere, resulting in a zigzag-shaped trace. fiber optical frequency, refractive index and pitch

8 Characteristic of Rayleigh traces 1.2 frequency f f 0, 0, temperature T T 0 0 Signal amplitude [a.u.] frequency f MHz, temperature T Distance [m] Frequency dependence Signal amplitude [a.u.] Temperature dependence frequency f 0, f 0, temperature T 0 T 0 frequency f 0, temperature T 0-26 mk Distance [m] Distance [m] 3

Rayleigh-based sensing principle The shape change induced by temperature can be fully compensated by the effect of changing optical frequency. Signal amplitude [a.u.] 1.2 1 0.8 0.6 0.

9 Rayleigh-based sensing principle The shape change induced by temperature can be fully compensated by the effect of changing optical frequency. Signal amplitude [a.u.] frequency f 0 f, 0, temperature T 0 frequency f 0 f, 0, temperature T 0-26 mk frequency f MHz, temperature T 0-26 mk relative temperature change (, ) temp change Distance [m] freq change (, + ) ( +, + ) Trace comparison, The similarity is determined by cross-correlation, ( +, + ) 4

10 C-OTDR Spectral measurements Spectral cross-correlation 1.

2-200 -150-100 -50 0 50 100 150 200 Frequency shift (MHz) (b) Frequency accuracy: 3 MHz (2mK

10 10 C-OTDR Spectral measurements Spectral cross-correlation Experimental data Quadratic fitting Correlated value (a.u.) Frequency shift (MHz) (b) Frequency accuracy: 3 MHz 300K) 1k averages Scanning step: 10 MHz Time: 40s Frequency shift [MHz] Distance [m]

11 Km-range C-OTDR measurements Temperature change: -0.

11 11 Km-range C-OTDR measurements Temperature change: K (around 2-km distance) Frequency shift in standard fibre: MHz Frequency change [MHz] connector Distance [m]

12 12 Nonlinear coupling-based sensors P p P s g P P P z B s P s Aeff The nonlinear interaction creates a dynamic Bragg grating and 100% of the diffracted light is back-coupled: ideal recapture! 1.40 But the probe signal is needed to generate the dynamic grating and the response is on top of the CW probe signal. Response Distance (km)

13 13 Brillouin sensing principle P p f P s g P P P z B s P s Aeff t

14 Generation of local stationary gratings Signal phase-modulated by a PRBS or chaotic signal 0+ p- 0+ p- p- 0+ p p p- p- p- 0+ p p- p p The driving force for the acoustic wave ~ the product of the wave amplitudes X (time averaged) 0 1 0

15 z [m] Generation of local stationary gratings Signal phase-modulated by a PRBS 0+ p- 0+ p- p- 0+ p p p- p- p- 0+ p p- p p The driving force for the acoustic wave ~ the product of the wave amplitud

16 Generation of local stationary gratings

17 Results Fibre length: 40 m PRBS Clock rate: 8.14 GHz Spatial resolution: 1.2 cm PRBS length =32,767 symbols

18 Results Fibre length: 40 m PRBS Clock rate: 8.14 GHz Spatial resolution: 1.2 cm PRBS length =32,767 symbols A. Zadok, Y. Antman, N. Primerov, A. Denisov, J. Sancho, and L. Thevenaz, "Random-access distributed fiber sensing, Laser & Photonics Reviews, online preview, 2012.

Examples of fibre components inspection 10.89 10.9 Brillouin frequency (GHz) 10.88 10.

19 Examples of fibre components inspection Brillouin frequency (GHz) Distance (mm)

Examples of fibre components inspection 10.89 10.88 Brillouin frequency (GHz) 10.88 10.87 10.87 10.86 10.

20 Examples of fibre components inspection Brillouin frequency (GHz) Distance (mm)

21 Perspectives State-of-the-art: distributed temperature and strain sensing over 50km with 1m spatial resolution Research art: >100km with 1m spatial resolution, 10km with 1 cm spatial resolution ( resolved points). Future: Contact detection with instantaneous response Future: Distributed pressure sensing Future: Distributed sensing of magnetic field, illumination, radiation, etc Future: Shape sensing

Brillouin distributed time-domain sensing in optical fibers: state of the art and perspectives

Front. Optoelectron. China DOI 10.1007/s12200-009-0086-9 REVIEW ARTICLE Brillouin distributed time-domain sensing in optical fibers: state of the art and perspectives Luc THÉVENAZ ( ) Ecole Polytechnique