In this thesis, an alternative approach to the well-known optical time domain reflectometry (OTDR) technique is presented. A thorough analysis regarding distributed backscatter measurement in optical fibres is provided and its prospects for optical fibre sensing applications are demonstrated and discussed. The measurement approach is referred to as incoherent optical frequency domain reflectometry (I-OFDR): the frequency response of the fibre under test is measured and transferred into its time domain equivalent using inverse Fourier transform. This general technique has been studied and used for the measurement of nonlinear scattering effects in optical fibres. The requirements, limitations and prospects for general backscatter measurement, however, are different and have not been studied in detail prior to this work. Distributed sensing using Rayleigh scattering and reflective events in the fibre is first demonstrated using I-OFDR with remarkable measurement resolution. The incoherent detection technique allows for measuring singlemode fibres as well as multimode fibres. The first part of this work deals with the theoretical analysis and optimized implementation of the frequency domain approach. Necessary signal processing and its impact on the time domain response are presented. Sources of deviation from the linearity of the I-OFDR system are identified and an optimized laboratory setup is introduced; the crucial impact of the source coherence is thoroughly discussed. Suitable system parameters for the I-OFDR approach are defined: the system dynamic range and sensitivity are determined. A technique to suppress the dynamic range-limiting signal originating from strong reflections in the fibre is suggested. It is demonstrated that the I-OFDR technique has advantages over OTDR in terms of implementation for high-resolution measurement, measurement accuracy and signal stability. These advantages and measurement possibilities specific to the frequency domain approach are utilized for spatially resolved sensing applications in the second part of this work: A low optical loss polymer optical fibre (POF) is for the first time studied and analyzed for distributed strain sensing. The backscatter level dependence on strain in the fibre can be used to detect and locate strained fibre sections. Also, a correlation algorithm is proposed and demonstrated to measure length changes along the fibre with mm-resolution by correlating the typical backscatter signature of this fibre type. The fibre type is analyzed in detail regarding cross-sensitivities to temperature, relative humidity as well as mode propagation influences. The proposed sensing principles in combination with the highresolution I-OFDR allow for promising distributed sensing applications. Special interest is expressed by the structural health monitoring (SHM) sector since the fibre can measure strain values exceeding 100 %. Another sensing technique, specific to I-OFDR, is proposed for quasi-distributed and dynamic measurement of length changes and optical power changes at reflective events along the fibre. Precise calculation of the positions and reflected powers of multiple reflections can be conducted in parallel from the measurement of a few sampling points of the complex-valued frequency response. That allows for measuring with an increased repetition rate up to 2 kHz or at μm-scale length changes resolution at lower measurement frequencies. The approach is demonstrated in the laboratory and in a field application by measuring the deformation of a masonry building on a seismic shaking table. The I-OFDR exhibits competitive performance for general high-resolution backscatter measurement and the proposed optical fibre sensor principles may have promising prospects in the structural health monitoring (SHM) sector.
We present the latest advances in distributed strain measurement in perfluorinated polymer optical fibres (POFs) using backscatter techniques. Compared to previously introduced poly(methyl methacrylate) POFs, the measurement length can be extended to more than 500 m at improved spatial resolution of a few centimetres. It is shown that strain in a perfluorinated POF can be measured up to 100%. In parallel to these investigations, the incoherent optical frequency domain reflectometry (OFDR) technique is introduced to detect strained fibre sections and to measure distributed length change along the fibre with sub-millimetre resolution by applying a cross-correlation algorithm to the backscatter signal. The overall superior performance of the OFDR technique compared to the optical time domain reflectometry in terms of accuracy, dynamic range, spatial resolution and measurement speed is presented. The proposed sensor system is a promising technique for use in structural health monitoring applications where the precise detection of high strain is required.
We present a novel technique for dynamic and simultaneous measurement of displacement and refractive indices at multiple reflection points in optical fibers. This quasi-distributed sensor is based on the incoherent optical frequency domain reflectometry (I-OFDR) technique and allows for m-resolution length change measurement and precise refractive index measurement. We show that the dynamic measurement ability and the simple sensor design allows for new applications in the field of structural health monitoring and chemical process control.
We present, to our knowledge for the first time, the possibility of measuring the backscatter signal of perfluorinated polymer optical fibers (POF) using an incoherent optical frequency domain reflectometry (OFDR) technique. The OFDR setup is described and it is shown that the dynamic range and measurement speed are superior to standard OTDR systems. It is shown for the first time that distributed detection of strain in POF is possible using the OFDR technique.
A novel quasi-distributed long-gauge fiber optic strain sensor system for dynamic measurement
(2011)
We present a novel technique based on incoherent optical frequency domain reflectometry (OFDR) to measure length changes quasi-distributed between reflection points in optical fibres. The technique enables length changes to be measured with a resolution better than 1 µm and allows for static and dynamic measurement capabilities up to 2 kHz. We demonstrate that dynamic measurements of multiple fibre sections can be conducted independently from each other with high precision. Due to the precise and dynamic measurement capabilities, the proposed sensor system is expected to open new fields of application, especially in the structural-health-monitoring sector. Possible applications are discussed in the paper.
We propose a digital implementation of the incoherent optical frequency domain reflectometry (I-OFDR) technique for precise backscatter measurement and optical fiber sensing applications. Specific performance parameters of the I-OFDR are discussed and compared to an analog vector network analyzer-based I-OFDR system. Improved sensitivity, dynamic range and signal stability of the digital I-OFDR is presented and demonstrated by means of quasi-distributed length change measurement.
A dynamic and quasi-distributed sensor principle for simultaneous measurement of length changes and optical power changes between reflection points in an optical fiber is presented. The technique is based on the incoherent optical frequency domain reflectometry (I-OFDR). Length change resolutions < 1 µm and measurement repetition rates up to 2 kHz can be achieved using standard single-mode and multi-mode optical fibers. Simultaneous length change and refractive index measurement as well as field test results showing the deformation of a masonry building under seismic load are presented. Promising fields of application for this technique are the structural health monitoring sector and chemical process control.
We propose to use focused femtosecond laser pulses to create scattering damage in standard singlemode optical fibres as reference points for quasi-distributed sensing applications. Such sensor fibres are interrogated with incoherent optical frequency domain reflectometry (I-OFDR) technique and optical time domain reflectometry (OTDR). A performance comparison of both techniques with the clear advantage of the I-OFDR is presented as well as a quasi-distributed length change measurement application. Also dynamic measurement based on the I-OFDR technique is demonstrated on a femtosecond laser-induced sensor chain.
We present a novel technique based on incoherent optical frequency domain reflectometry (OFDR) to measure length changes quasi-distributed between reflection points in optical fibres. The technique enables length changes to be measured with a resolution better than 1 µm and allows for static and dynamic measurement capabilities up to 2 kHz. We demonstrate that dynamic measurements of multiple fibre sections can be conducted independently from each other with high precision. Due to the precise and dynamic measurement capabilities, the proposed sensor system is expected to open new fields of application, especially in the structural-health-monitoring sector. Possible applications are discussed in the paper.