We present 100-μm crack detection and 100-km distributed strain and temperature sensing by fiber-optic distributed
strain and temperature sensor (DSTS) based on coherent interaction of probe-pump. The DSTS products have been
employed to detect cracks on ceramic by measuring the strain distributions along the surface of the ceramic and a
continuing long-term field monitoring of local temperature and stress changes in a 70-km buried fiber-optic cable.
Distributed Brillouin scattering sensor system was employed to measure the hoop strain in an internally pressurized steel
pipe with wall thinning due to erosion. The difference of Brillouin frequency shift from regions that have different
degree of wall thickness lost can be observed on strain distribution. The strain from thin wall region is higher than that
from the thick wall region. Therefore, the inner wall thinning can be discriminated from the corresponding strain
measurements, which shows that fiber optic sensor technology based on distributed Brillouin scattering offers the great
potential as a "nervous system" for infrastructure elements that allow high performance, cost effective health and
damage assessment systems to be achieved.
A Brillouin-scattering-based distributed strain and temperature sensor (DSTS) has been employed to detect cracks on
ceramic by measuring the strain distributions along the surface of the ceramic for the first time. The existence of cracks
and their locations are identified by measuring the strain distribution on a sensing fiber bonded on the ceramic surface.
Due to innovated design and signal processing, the distributed Brillouin sensor developed for this study achieves a
uniquely high resolution and accuracy. Experimental study on ceramic tile specimens demonstrated the efficacy of the
distributed Brillouin fiber optic sensor in detecting and locating fine cracks.
We monitored the distributed strain during the pipeline buckling process using distributed Brillouin sensor, which allows us to predict the buckling or crack location according to the sequence and location of the deformation for the first time using the broadening factor of Brillouin spectrum width. Two pipelines were designed and instrumented with polymer and carbon/polyimide coated fibers, and then the pipelines were subjected to internal pressure, axial tensile force and bending moment. We show that 1) the localized buckling occurred at the top, median and bottom of the pipeline, where the maximum broaden factors were obtained; 2) the deformation sequence can be measured using the nonlinearity of the broadening factor, 3) a high strength carbon/polyimide-coated fiber can detect higher stress accurately than standard telecom fibers. Our results strengthen the distributed Brillouin fiber sensor position as a nervous system to identify the potential problem in early stage for structural health monitoring.
In this paper, we demonstrated a distributed Brillouin sensor based on the offset locking of two 1550nm DFB lasers. We
use PID controller and optical delay line to lock and tune the frequency difference between two DFB lasers and we also
use lock in amplifier to stabilize the DC level of the EOM. The power depletion in the Brillouin scattering has been
studied to show optimized strain and temperature accuracy. With this sensor system, we can get the spatial resolution
which is 1m with temperature resolution of < 1°C.
We introduce a phenomenological model, based on steady state analytical solution adapted to transient regime through modification of the Brillouin spectrum with the pulse spectrum. This model can accurately de-convolve the strain profiles from measured spectra. The model includes experimental parameters such as the electro-optic modulator Extinction Ratio, the pulse width, pulse and pump powers, position and sensing fibre length. The pulse base is treated as pure steady state contribution. A systematic numerical analysis has been carried out and the results are qualitatively matched with our experimental results. The experimental results have been used to validate the model and evaluate its limitations. Within this context, the approach has been applied to experimental data obtained under well-controlled laboratory conditions. The agreement is good and reflects the Brillouin frequency and then the strain distribution along the fibre. The approach is also successful when used to deconvolve the main strain contributions of a pipe subjected to a compression stress. The strength of the model lies in its simplicity of implementation because it is quasi-analytical and is not restricted to short fibre lengths.
A conventional SMF-28 was used to conduct localized pipe-wall buckling monitoring in a section of energy pipe with 2,667 mm (105 in) in length and 762 mm (30 in) in diameter by a coherent probe-pump based distributed Brillouin fiber sensor with 15 cm spatial resolution. The locations of pipe-wall buckling have been found by measuring the strain distributions along the outer surface of the pipe. However the sensing fiber (SMF-28) was broken when the bending load increased above 1335 kN (300 kips), which caused the sensing fiber experienced more than the compressive strains of -8,084 με. In order to get strain data after pipeline buckling happens, a high strength sensing fiber with carbon coating instead of conventional acrylate coating should be used. The Brillouin measurement on the carbon coating single-mode fiber has Brillouin frequency shift of vB 12.479 GHz at wavelength of 1320 nm and room temperature. The measured Brillouin bandwidth ΔvB is equal to 66 MHz. The central Brillouin frequency shows a strong dependence on strain with 1.510 GHz shift at 2.5% elongation. The excellent linearity of the central frequency vB on strain is confirmed and the strain coefficient was measured as 16.21 με/MHz. Its strain-stress relation keeps linearity up to 2.5% elongation, which is much bigger than that of SMF-28.
We applied axial and lateral stresses on a composite FRP/concrete column to produce earthquake like deformation. We monitored strain distribution with the Distributed Brillouin Sensor while the column was tested. We can correlate strain profile with column deformation and crack condition.
A distributed strain sensor based on Brillouin scattering has been employed to inspect localized pipe-wall buckling on a length of 2.58 m and diameter of 0.75 m steel pipe by measuring the axial strain distributions along the outer surface of the pipe. An optical fiber with 10 sections was laid along the longitudinal direction of the pipe. Both compressive load and bending load were applied on both end of tested pipe up to 2,013 and 300 kIPS respectively, to induce the localized pipe-wall buckling. The buckling locations comprising tension and compression of the pipe wall are found and distinguished using their corresponding strain distribution data. The compressive strains of - 4330 and - 6856 με measured by our distributed strain sensor on the bending load of 220 and 300 kIPS, respectively, at one location near buckling point, match the readings from strain gauge at the same location where the strain gauge attached. The tensile strain happened at different sides of the pipe are found too.
KEYWORDS: Spatial resolution, Continuous wave operation, Signal to noise ratio, Single mode fibers, Temperature sensors, Sensors, Signal attenuation, Pulsed laser operation, Numerical simulations, Wave propagation
We present a sensing principle of the distributed fiber Brillouin strain and temperature sensor by coherent probe-pump technique that offers a new method to achieve centimeter spatial resolution with high frequency resolution. A combination of continuous wave (cw) and pulse source as the probe (Stokes) beam and cw laser as the pump beam have resulted in stronger Brillouin interaction of Stokes and pump inside the pulse-length in the form of cw-pump and pulse-pump interactions. We find that the coherent portion inside the pulse-length of these two interactions due to the same phase has a very high Brillouin amplification. The Brillouin profile originating from the coherent interaction of pulse-pump with cw-pump results in high temperature and strain accuracy with centimeter resolution, which has been verified by successfully detecting 1.5 cm out-layer crack on an optical ground wire (OPGW) cable.
Because of the phase modulation (PM) and amplitude modulation (AM) of the probe signal from the electric optical modulator (EOM), the Brillouin gain/loss spectrum becomes asymmetric. The central Brillouin frequency is shifted from that of AM pulse. The maximum extinction ratio of the EOM is limited to ~29 cB for power splitting ratio of 51% to 49%. The PM also induces sub-peaks in the Brillouin spectrum due to the interaction of phonon field and AM/PM based probe beam. The sub-peaks is enhanced when the beat frequency of pump and probe beam is off from the Brillouin frequency.
Optical fiber sensor technology has progressed at a rapid pace over the last decade. Many different sensing techniques have been developed to monitor specific parameters. In particular, distributed Brillouin scattering-based sensor systems provide an excellent opportunity for structural health monitoring of civil structures by allowing measurements to be taken along the entire length of the fiber, rather than at discrete points, by using fiber itself as the sensing medium. One class of Brillouin-based sensors is based on the Brillouin loss technique, whereby two counter-propagating laser beams, a pulse and a CW, exchange energy through an induced acoustic field.
This type of sensing has tremendous potential for structural health monitoring since the spatial resolution can be adjusted for different applications simply by altering the pulse duration, even after the fiber is installed. Although the spatial resolution can be improved using short pulse, the loss spectrum broadens as the pulse width decreases below the phonon lifetime t. Hence, it was generally believe that sub-meter resolutions were unachievable due to rapid linewidth increases when pulse width W < t = 10 ns provided a 1 m spatial resolution limitation.
In this paper, we will report the development of distributed optical fiber sensor with centimeter spatial resolution. The sensing principle will be presented. We will also report the test results of pipeline buckling and corrosion fatigue monitoring and small damages/cracks of 1.5 cm in an optical ground wire (OPGW) cable with centimeter spatial resolution.
We solved the three coupled partial differential equations in transient regime for the probe-pump Brillouin sensor to explain the sub-peaks in Brillouin loss spectra, which have been experimentally observed. We discovered that the Fourier spectrum of the pulsed signal and the off-resonance oscillation attributed to sub-peaks. The off-resonance oscillation at frequency [v - vB] is the oscillation in the Brillouin time domain when the beat frequency v of the two counter-propagating laser beams does not match the local Brillouin frequency vB. This is important in differentiating the sub-peaks from strain/temperature peaks.
Pipeline failures induce costly repair and cleaning spending that could be avoided by the implementation of proactive approaches. Distributed sensors based on Brillouin scattering are attractive candidates to monitor structural health of pipelines. They can measure local strain and allow real-time control over lengths ranging from a few meters to tenths of kilometres. One of the possible degradations that must be detected is buckling. We describe in this paper what is to our knowledge the first time report of buckling detection with a Brillouin sensor.
We conducted an experiment reproducing buckling monitoring in a laboratory environment. Two specimens (steel pipeline and beam) were prepared by locally thinning the inner wall to provoke buckling. Fibre was laid along the external walls of the specimens. Strain gauges were glued in thinned wall area. An axial load was applied to the specimens and increased while strain measurements were carried out with the Brillouin sensor and the strain gauges. All the measurements showed a progressive compression increase in the neighbourhood of the thinned wall. Finally buckling aroused and was visually identified as well as localized with the Brillouin sensor. Strain gauges readings and strain measurements with the Brillouin sensor were in good agreement.
High sensitivity, real time distributed and cost effective sensor system is in great need for structure healthy monitoring in civil engineering. In our lab, we are developing a distributed, Stimulated Brillouin Scattering based, fiber optic sensing system at 1550nm wavelength. Our current SBS-based fiber optic sensor system works at 1310nm wavelength. Two expensive Nd: YAG Lasers (US$40,000 each) are being used, which leads to a soaring high cost to the entire system and eventually limits its application. Distributed Feedback (DFB) lasers have large tenability, compact size and low cost (less than US$1000 each). But they are not stable enough for the sensing system. In this project, we use the frequency offset locking technique with optical delay line and electrical feedback circuit to optimizing the stability of DFB lasers so that the lasers of 1310 nm in the sensor system can be substituted by the lasers of 1550 nm that is the most often used band in modern fiber optic telecommunication system. Less than 100 kHz stability of the beat frequency is required to achieve temperature accuracy of 0.1°C and strain accuracy of 2me. In our system we have realized 20 kHz stability of beat frequency of two DFB lasers. Greater than 800MHz turning range is necessary for the detection of temperature range of 600 °C and strain range of 10,000 me. In our system we have achieved 925 MHz in 18.75 seconds. In the sensing part, we can vary the pulse width from 120ns to 5ns that means we can realize the spatial resolution of 50cm at least. Because the total optical loss in the setup is comparably smaller, the measurable fiber length is mainly determined by the optical power launched to the fiber, normally it is in tens kilometers.
A distributed Brillouin scattering sensor with high special precision has been developed for the measurement of small damages/cracks of 1.5 cm. The out-layer damaged regions in an optical ground wire (OPGW) cable have been identified successfully by measuring the strain distributions every 5 cm using this technology. The stress increased to 127 kN which corresponds to more than 7500 micro-strain in the fibers. The locations of structural indentations comprising repaired and undamaged regions are found and distinguished using their corresponding strain data. The elongation of repaired region increases with time on 127 kN. These results are quantified in terms of the fiber orientation, stress, and behavior relative to undamaged sections.
The strain and temperature dependence of the Brillouin loss spectra using Brillouin frequencies in a photonic crystal fiber with a partially Ge-doped core are studied for the first time. The spectrum of the photonic crystal fiber displays a multi-peak structure due to waveguide and antiwaveguide types of sound propagation. Our experiments demonstrate that the Brillouin frequency shift vB depends on strain and temperature of the photonic crystal fiber. For two main peaks that are attributed to the scattering from longitudinal acoustic waves in the Ge-doped center region and solid pure silica region of the core of the photonic crystal fiber, the strain coefficients are 0.048 and 09.055 MHz/με and temperature coefficients are 0.96 and 1.25 MHz/°C at wavelength of 1320 nm. Combining the measurements of the strain dependence of Brillouin frequency shift with measurements of the temperature dependence of Brillouin frequency shift, we demonstrate a simultaneosly distributed strain and temperature sensor with a spatial resolution of 15 cm, a temperature resolution of 1.3°C, and a strain resolution of 15με along a single photonic crystal fiber.
Distrbuted Brillouin intensity vs. temperature measurements along an optical fiber are studied theoretically and experimentally using counter-propagating laser beams interacting at a fixed beat frequency. By monitoring the Brillouin temperature spectrum without scanning the beat frequency, one can acquire distributed temperature measurements within seconds rather than minutes, making this acquisition method suitable for dynamic processes such as hot spots and fire detection. This technique requires knowledge of a threshold temperature condition, which is mathematically derived by considering the temperature dependence on Brillouin peak power and linewidth in the frequency domain. Temperature varying fiber regions are monitored using 0.2 and 2 m spatial resolutions along 2 and 11 km fiber lengths respectively.
Brillouin scattering spectra in a photonic crystal fiber with partially Ge-doped core is demonstrated, for the first time to our knowledge, using pump-probe technique. We have observed and characterized Brillouin spectra in the photonic crystal fiber at 1320 nm. One main peak and several sub-peaks are observed. Brillouin loss spectrum of main peak has a Lorentzian shape. The bandwidth ΔνB is 66 MHz and the Brillouin frequency shift νB is 12.054 GHz at room temperature. The peak originating from a higher order guided longitudinal acoustic mode in the graded-Ge-doped core of the photonic crystal fiber is observed for the first time. The temperature related behavior of the photonic crystal fiber is investigated. It is shown a linear dependence for the photonic crystal fiber.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.