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 Slow & fast light, optical storage Dynamic fibre gratings, microwave photonics Sensing using laser spectroscopy Photoacoustic gas trace sensing (closed) Photonic crystal fibres and waveguides
Fibre sensing, a response to a societal concern Safer infrastructures More efficient energy use Environmental & human threats Anticipating catastrophic failures
«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. 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: 0.001 % Spatial resolution: 0.01-3 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 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 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
Characteristic of Rayleigh traces 1.2 frequency f f 0, 0, temperature T T 0 0 Signal amplitude [a.u.] frequency f 0 + 20 MHz, temperature T 0 1 0.8 0.6 0.4 0.2 0 0 2 4 6 8 10 12 Distance [m] Frequency dependence Signal amplitude [a.u.] 1.2 1.2 1 0.8 0.6 0.4 0.2 Temperature dependence frequency f 0, f 0, temperature T 0 T 0 frequency f 0, temperature T 0-26 mk 0 10 12 0 2 4 6 8 10 12 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 0.4 frequency f 0 f, 0, temperature T 0 frequency f 0 f, 0, temperature T 0-26 mk frequency f 0 + 30 MHz, temperature T 0-26 mk relative temperature change (, ) temp change 0.2 0 0 2 4 6 8 Distance [m] 10 12 freq change (, + ) ( +, + ) Trace comparison, The similarity is determined by cross-correlation, ( +, + ) 4
10 C-OTDR Spectral measurements Spectral cross-correlation 1.2 1 Experimental data Quadratic fitting Correlated value (a.u.) 0.8 0.6 0.4 0.2 0-0.2-200 -150-100 -50 0 50 100 150 200 Frequency shift (MHz) (b) Frequency accuracy: 3 MHz (2mK @ 300K) 1k averages Scanning step: 10 MHz Time: 40s Frequency shift [MHz] -150-100 -50 0 50 100 150 200 0 4 8 12 16 20 24 Distance [m]
11 Km-range C-OTDR measurements Temperature change: -0.018 K (around 2-km distance) Frequency shift in standard fibre: 18.96 MHz Frequency change [MHz] 150 100 50 0-50 -100 connector -150 1995 2000 2005 2010 2015 Distance [m]
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 1.35 1.30 1.25 1.20 1.15 1.10 1.05 1.00 0 10 20 30 40 Distance (km)
13 Brillouin sensing principle P p f P s g P P P z B s P s Aeff t
Generation of local stationary gratings Signal phase-modulated by a PRBS or chaotic signal 0+ p- 0+ p- p- 0+ p- 0+ 0+ p- 0+ 0+ p- p- p- 0+ p- 0+ 0+ 0+ p- p- 0+ 0+ p- 0+ - + + - - + + + - + - - - + + - + + - + - - + - + The driving force for the acoustic wave ~ the product of the wave amplitudes X (time averaged) 0 1 0
-0.2-0.1 0 0.1 0.2 z [m] Generation of local stationary gratings Signal phase-modulated by a PRBS 0+ p- 0+ p- p- 0+ p- 0+ 0+ p- 0+ 0+ p- p- p- 0+ p- 0+ 0+ 0+ p- p- 0+ 0+ p- 0+ - + + - - + + + - + - - - + + - + + - + - - + - + The driving force for the acoustic wave ~ the product of the wave amplitud
Generation of local stationary gratings
Results Fibre length: 40 m PRBS Clock rate: 8.14 GHz Spatial resolution: 1.2 cm PRBS length 2 15-1=32,767 symbols
Results Fibre length: 40 m PRBS Clock rate: 8.14 GHz Spatial resolution: 1.2 cm PRBS length 2 15-1=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 10.88 10.87 10.86 10.86 10.84 10.85 10.84 10.82 10.83 10.8 10.82 0 50 100 150 Distance (mm)
Examples of fibre components inspection 10.89 10.88 Brillouin frequency (GHz) 10.88 10.87 10.87 10.86 10.86 10.85 10.84 10.84 10.83 10.82 10.83 10.81 10.82 10.8 10.81 0 20 40 60 80 80 100 Distance (mm)
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 (1 000 000 resolved points). Future: Contact detection with instantaneous response Future: Distributed pressure sensing Future: Distributed sensing of magnetic field, illumination, radiation, etc Future: Shape sensing