8.6 Faseroptische Sensorik
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This thesis deals with the development of a novel optical fiber sensing scheme based on geometric phase for sensing strain and its application to seismology. Interference of two coherent frequency offset electromagnetic waves gives rise to a geometric phase in the resulting beat signal. The existence of this phase was recently reported along with requisite conditions for its existence. This thesis proposes to detect and use this geometric phase in the context of distributed and dynamic fiber optic strain sensing, also known as distributed acoustic sensing (DAS). In the first part, I devise a novel DAS hardware setup capable of detecting the geometric phase considering that its measurement methods require the measurement of beam intensities and the beat signal’s envelope. The geometric phase is a function of relative intensity and polarisation state of two interfering beams. Therefore, its measurement is verified by determining its relation on these quantities using a polarisation scrambler and a piezoelectric transducer, inline an optical fiber. It is a fundamental study that has implications in coherent optical communication and novel sensing mechanisms.
The second part involves using the geometric phase in DAS for measurement of strain. I attempt to replace the traditionally measured dynamic phase in a DAS setup with the geometric phase. This is made possible by the fact that the geometric and dynamic phases are reportedly coupled over every beat period such that their sum remains constant. However, the spatial resolution for geometric phase is lower as it is measured per beat period. I determine an equivalence for the two phases empirically as well as optimum test parameters such as the required frequency offset between the interfering beams. The advantages offered by the use of geometric phase are demonstrated; geometric phase can be measured even when the two interfering beams have non-identical polarisation states, unlike the traditionally measured dynamic phase. Moreover, it does not require phase unwrapping and is therefore free from unwrapping errors.
In the third and final part, the setup, after optimisation, is tested in the field to detect seismic waves travelling on the surface of the Earth in response to a set of blasts carried out at a test-site. The surface waves are used for the characterisation of the structure and material properties of the first tens of meters of the Earth with applications in earthquake monitoring, resource exploration and infrastructure planning.
In short, this study is the first of its kind to measure geometric phase in beat signal of light using optical fiber medium and to measure strain with it, for which a novel hardware setup and a novel sensing mechanism is designed and tested in addition to its application in real-world seismology measurements.
The geometric phase in the beat signal from coherent interference of two frequency-offset light beams is measured using a novel distributed optical fiber sensing setup. In a fiber optic medium, with changing beam intensities, to the best of our knowledge, ours is the first measurement of the mentioned geometric phase. Experimental results of applying a 100-Hz sinusoidal stimulus to a polarization scrambler and a piezoelectric transducer inline to an optical fiber are presented. The results may enable novel distributed fiber sensing techniques.
We present a technique for distributed temperature gradient sensing in real-time along an optical fiber utilizing simple amplitude-based direct-detection coherent optical time domain reflectometry (C-OTDR) and a special sensing fiber. Our technique enables us to determine phase changes or low-frequency variations of the C-OTDR signal stemming from temperature variations. The distinct feature of the used sensing fiber is its structuring with equidistant strongly scattering dots. Consecutive pairs of these scatterers form the dominant local interferometers, effectively overwriting the otherwise highly nonlinear transfer function of common optical fiber. This enables a quasi-phase-resolved evaluation of perturbation responses originating from temperature changes at sensor positions between the scatterers. Using our method, we show the measurement of a nonlinear temperature transient from a heating process with a maximum temperature gradient of 0.8 °C over 20 s and a total temperature increase of 28.4 °C. This method requires almost no post-processing and can be used for simultaneous distributed vibration sensing (DVS) and quantification of local temperature gradients in a single fiber, e.g., for the use in condition monitoring of infrastructure or industrial installations.
We present a technique for distributed temperature gradient sensing in real-time along an optical fiber utilizing simple amplitude-based direct-detection coherent optical time domain reflectometry (C-OTDR) and a special sensing fiber. Our technique enables us to determine phase changes or low-frequency variations of the C-OTDR signal stemming from temperature variations. The distinct feature of the used sensing fiber is its structuring with equidistant strongly scattering dots. Consecutive pairs of these scatterers form the dominant local interferometers, effectively overwriting the otherwise highly nonlinear transfer function of common optical fiber. This enables a quasi-phase-resolved evaluation of perturbation responses originating from temperature changes at sensor positions between the scatterers. Using our method, we show the measurement of a nonlinear temperature transient from a heating process with a maximum temperature gradient of 0.8 °C over 20 s and a total temperature increase of 28.4 °C. This method requires almost no post-processing and can be used for simultaneous distributed vibration sensing (DVS) and quantification of local temperature gradients in a single fiber, e.g., for the use in condition monitoring of infrastructure or industrial installations.
We present results demonstrating several beneficial effects on distributed fiber optic vibration sensing (DVS) functionality and performance resulting from utilizing standard single mode optical fiber (SMF) with femtosecond laser-inscribed equally-spaced simple scattering dots. This modification is particularly useful when using traditional single-wavelength amplitude-based coherent optical time domain reflectometry (C-OTDR) as sensing method. Local sensitivity is increased in quasi-distributed interferometric sensing zones which are formed by the fiber segments between subsequent pairs of the scattering dots. The otherwise nonlinear transfer function is overwritten with that of an ordinary two-beam interferometer. This linearizes the phase response to monotonous temperature variations. Furthermore, sensitivity fading is mitigated and the demodulation of low-frequency signals is enabled. The modification also allows for the quantitative determination of local temperature gradients directly from the C-OTDR intensity traces. The dots’ reflectivities and thus the induced attenuation can be tuned via the inscription process parameters. Our approach is a simple, robust and cost-effective way to gain these sensing improvements without the need for more sophisticated interrogator technology or more complex fiber structuring, e.g., based on ultra-weak FBG arrays. Our claims are substantiated by experimental evidence.