Dissertations / Theses on the topic 'Optical time domain reflectometry (OTDR)'

The statistics of the phase dependent optical time-domain reflectometer have been analyzed. The optical fiber is modeled by the use of a discrete set of reflectors positioned randomly along the fiber. The statistics of the reflected light from a traveling pulse are derived. The statistics of the signal are used to calculate the characteristics of shot noise in the photodetector, and the probability that noise of certain intensity will occur. An estimation of the backscattered power is made by calculating the fraction of the backscattered power that is captured in a guiding mode. Upper power limits are calculated by considering nonlinear optical effects. An estimation of noise from thermally excited sound waves, amplified by Brillouin scattering, is derived. This noise considers the parameters of a photodetector, giving a model for the noise in the measurable photocurrent. Two models are used to describe the fading probability of the signal. The first model, based on the Fabry-Perot interferometer with a random phase perturbation in the middle, is used to calculate the probability that the whole signal vanishes for any value of phase perturbation. The second model, by calculating the correlation between two signals, one perturbed and one unperturbed, predicts the fading of the signal of interest. The present work gives the theoretical basis for the phase dependant Optical Time Domain Reflectometry, allowing its optimization and setting the fundamental limitations to the performance of the system.

This thesis explores and evaluates the temperature measuring capabilities of a phase-sensitive optical time-domain reflectometer (φ-OTDR), which exploits Rayleigh backscattering in normal single mode optical fibers. The device is constructed and its setup explained, and a protocol for making temperature measurements with it is developed. Performance tests are made and the device is shown to achieve fully distributed temperature measurements on fibers hundreds of meters in length with a spatial resolution of 1 m and a temperature resolution of 0.1 K. In addition, the capabilities of the device to measure normal strain in the measurement fiber are tested using the same approach, albeit with less success. The device is capable of very precise measurements, making it very sensitive to the environmental conditions around the measuring fiber but also susceptible to disturbances. Some discussion is had on how to avoid or deal with these disturbances. Furthermore, the technique is shown to be able to run in conjunction with other φ-OTDR measurement techniques from the same device simultaneously.<br>Det här examensarbetet utforskar och utvärderar förmågorna att mäta temperatur hos en fas-känslig optisk tidsdomän-reflektometer (φ-OTDR), som utnyttjar bakåtriktad Rayleigh-spridning i vanliga optiska singelmodfibrer. Anordningen konstrueras och dess komponentstruktur förklaras, och ett protokoll tas fram för att utföra mätningar med den. Prestandatester utförs och anordningen visas kapabel att göra fullt distribuerade temperaturmätningar längs hundratals meter långa fibrer, med en rymdsupplösning på 1 m och en temperaturupplösning på 0.1 K. Dessutom testas förmågan att mäta normaltöjning hos testfibern med samma metod, dock med mindre framgång. Anordningen är väldigt känslig för förhållandena i omgivningen runt mätningsfibern, vilket gör den kapabel till mätningar med mycket hög precision, men också mottaglig för störningar. Lite diskussion hålls kring hur dessa störningar kan undvikas eller hanteras. Vidare visas att mätningstekniken kan köras samtidigt som andra φ-OTDR-baserade tekniker från samma anordning.

The wavelength-division-multiplexed passive optical network (WDM-PON) offers many advantages such as large bandwidth per user, easy management, high network security and low insertion loss enabling long-reach and/or high split-ratio. However, with the rapid development and growing interest for deployment of WDM-PON, the PON systems are becoming increasingly vulnerable to various failures. Therefore, network supervision becomes extremely important to guarantee an appropriate level of network reliability performance. The aim of PON supervision is to provide automated test and diagnostic capability without compromising the available bandwidth for services. An essential function is to detect any kind of deterioration in the network that can cause suspended services and to remotely localize these faults in order to avoid the high cost arising from troubleshooting. In this thesis, we evaluate several existing WDM PON supervision methods, based on which two simple and cost-effective new solutions are proposed to localize fiber failures in WDM PON systems. In the first solution, a conventional optical time domain reflectometry (OTDR) and a switch unit are employed, while in the second method, a novel architecture based on N×N arrayed waveguide grating (AWG) is proposed. Both methods are able to localize failures in feeder fiber and drop fibers without e.g. using expensive tunable OTDRs. A patent has been filed based on the second solution. The first proposed method is validated by simulations using VPI Transmission Maker. According to the results, this supervision method is able to give accurate measurement of many different types of failures that may happen in a fiber link. Furthermore, VPI is examined in terms of its future applicability in modeling complete PON supervision methods based on OTDR.

This thesis considers cost effective optical time domain rcflectometry for distributed fibre sensors, using new developments in light sources, detectors, fibres and computational power, suitable to interrogate distributed fibre sensors from 20m to 10km, within a £5,000 hardware budget. The characteristics of 200μm core diameter polymer clad silica (PCS) fibre and 980μm core diameter PMMA POF (polymethyl methacrylate plastic optical fibre) were theoretically evaluated including damage thresholds, optimum sensitisation (evanescent field attenuation and micro-bending) and launch conditions for optimum performance as a distributed sensor. Rayleigh backscattered signals, forward-propagating power and Fresnel reflections for different fibre types at different distances along the fibre were considered. PCS fibre allows solutes and gases ready access to the core-cladding interface and is preferred for distributed optical fibre sensors (DOFS) of 1000m. 50-125 graded index multimode fibre is preferred as a sensor of mechanical measurands for DOFS 10km long. The higher backscattering coefficient of PMMA POF returns the highest reflected signals for DOFS of up to 30m. Test beds of simulated distributed optical fibre sensors built from single- and multimode silica, hybrid and PCS fibres were assembled for interrogation by visible and NIR wavelengths using mechanical measurands and misaligned splices as point losses, to determine the relationships between launch pulse power, pulse width and backscattered power and the factors determining spatial resolution and dynamic range. Commercial analogue OTDRs (optical time domain reflectometers) and custom-built photon counting OTDRs (ν-OTDRs) were used to probe the fibre sensors. This enabled the design, custom build and evaluation of an OTDR-based DOFS system where the light source may easily be changed for one of a different power or wavelength. The performance of intrinsic or modified fibres in applications of chemical and pH sensing was evaluated: A simulated distributed PMMA POF was demonstrated to sense pH to a resolution of ±1 pH when placed in aqueous solutions of 4.2 x 10⁻⁴ M methyl red between pH 2.89 and 9.70 and probed with 648nm light using a ν-OTDR. An undyed PCS fibre was used to sense aqueous methyl red when probed with 657nm light from a ν-OTDR. An undyed PCS fibre was used to detect 6.5 x 10⁻⁴ M methyl red in ethanol using 657nm light from a ν-OTDR by modifying the cladding refractive index from 1.401 to 1.370. A PCS fibre dyed with 4.4 x 10⁻⁶ M chlorophyll a in ethanol solution then dried was probed by ν-OTDR at 657nm, returning a 4.0dB peak on the trace indicating detection by Fresnel reflection by light in the cladding. A slope of -2.0dB/m on the trace indicated evanescent field absorption due to the 662nm absorption peak. An avalanche photodiode (APD) detection system with inherent stability suitable for long term monitoring of Rayleigh back-scattered signals was designed and built. The modules included an active quench and recharge circuit capable of 20MHz count rate with a novel quenching circuit bias arrangement to provide immunity from spurious triggering, and a Peltier cooler circuit to regulate the APD to ± 0.1°C. A dynamic bias control system based around a PCI-6602 (NI) counter-timer card was designed, built, evaluated and shown by calculation to limit the tolerance on 2.0V excess bias to ± 0.048V. The tighter control of the excess bias stabilises quantum efficiency, resolution and dark count.

Distributed fibre optic sensing (DFOS) is essential for structural health monitoring (SHM) of strain changes induced during the lifetime of a structure. Among different DFOS systems, the Brillouin Optical Time Domain Reflectometry (BOTDR) takes the advantages of obtaining full frequency spectrum to provide strain and temperature information along the optic fibre. The key parameters of distributed fibre optic sensors, spatial and frequency resolution, are strongly linked with the time-frequency (T-F) localisation in the system in three parts: pulse, hardware design and optical fibre. T-F localization is fundamentally important for the communication system, whereas in this study the importance of the T-F localisation to the spatial and frequency resolution, repeatability and the measurement speed are introduced in BOTDR. In this dissertation, the development of DFOS is first introduced, including both traditional methods and new developed designs. The literature review shows the signal to noise ratio (SNR) of BOTDR can be improved by investigating its T-F localisation. In the hardware design, in order to improve the T-F localisation in hardware architecture, a Short-Time Fourier Transform-Brillouin Optical Time-Domain Reflectometry (STFT-BOTDR), which implements STFT over the full frequency spectrum to measure the distributed temperature and strain along the optic fibre, is applied so that the conventional frequency sweeping method can be replaced for high resolution and fast speed measurement, providing new research advances in dynamic distributed sensing. The STFT based BOTDR has better T-F localisation, which in turn provides an opportunity for off-line post signal processing that is more adaptable for fast speed measurements. The spatial and frequency resolution of dynamic BOTDR sensing is limited by the Signal to Noise Ratio (SNR) and the T-F localization of the input pulse shape. The T-F localized input pulse shape can enhance the SNR and the spatial and frequency resolution in STFT-BOTDR. In this study, simulation and experiments of T-F localized different pulses shapes are conducted to examine the limitation of the system resolution. The result indicates that a rectangular pulse should be selected to optimize the spatial resolution and a Lorentzian pulse could be chosen to optimize the frequency resolution, while a Gaussian shape pulse can be used in general applications for its balanced performance in both spatial and frequency resolution. Meanwhile, T-F localization is used for pulse T-F localisation optimisation. A set of Kaiser-Bessel functions is used to simulate different pulse shapes and to compare their parameters in terms of T-F localisation and their Brillouin scattering spectrum. A method using an iterative filtering algorithm to achieve the optimised pulse in terms of T-F localisation is introduced to converge the Effective-pulse Width (TEW) in the time-domain and Effective-pulse Linewidth (FEL) in the frequency domain to identify the fundamental limitations. The optimised pulse can be fitted with a 7th order Gaussian (super-Gaussian) shape and it offers the best experimental performance compared to a Rectangular pulse. The sensitivity of a sensor to strain or temperature variations due to distributed Brillouin scattering is closely related to the power distribution on the Brillouin scattering spectrum which is related to the property of the optic fibre. The performance of a highly nonlinear fibre that can generate a higher Brillouin scattering signal is compared to that of a standard single mode fibre. The results show that much higher SNR of the Brillouin scattering spectrum and smaller frequency uncertainties in the sensing measurement can be achieved by using a highly nonlinear fibre for comparable launched powers. With a measurement speed of 4 Hz, the frequency uncertainty can be 0.43 MHz, corresponding to 10 με in strain or 0.43°C in temperature uncertainty for the tested highly nonlinear fibre. In contrast, for a standard single mode fibre, the value would increase to about 1.02 MHz (25 με or 1.02°C), demonstrating the advantage of the tested highly nonlinear fibre for distributed strain/temperature sensing. Results show that, by using a small effective area highly nonlinear fibre, the strain or temperature resolution can be improved because it generates stronger Brillouin scattering signal with high SNR and high Q factor spectrum, both of which determine the optimal averaging time in a single measurement. In general, the STFT-BOTDR can achieve 1 m spatial resolution, 10 με frequency resolution on a 10 km fibre with measurement speed at about 2.5 kHz.

<p>A method of using optical fiber to measure strain and correct for the effects of temperature is proposed. A means of measuring apparent strain is given, pure temperature is measured using Fresnel-backscatter based Optical Time Domain Reflectametry, and a method for combining the two measurements to obtain a measurement of mechanically-induced strain alone is developed. The background, theory and experimental results that demonstrate the feasibility of such a system are presented and the results are compared with the performance of existing fiber-based means of measuring temperature. Experiments on several OTDR-addressed, intensity-based optical temperature sensors are performed and a method for manufacturing small air gap splices for use in measuring strain at several places along an optical fiber are presented.<br>Master of Science

High resolution Optical Time Domain Reflectometry (OTDR) measurements have recently allowed spatial resolutions of less than one millimeter. These capabilities indicate that OTDR techniques may be suitable for non-conventional applications such as the determination of fiber strain. This thesis presents an investigation of how high resolution OTDR techniques can be used in such applications. The concept of fiber segmentation via partially reflective optical splices for local strain measurements is discussed both from a theoretical and practical standpoint. Experimental results demonstrating the feasibility of such local strain measurements are also given. Another part of this investigation considers the practical details of the proposed strain measurement technique, addressing such topics as launching conditions, and environmental factors. Possible applications of the local strain measurement techniques, such as two- and three-dimensional stress analysis, and strain determination of fiber optic cables, are also presented. These applications also include the development of small, easy to manufacture elastomeric optical splices, which were shown to yield acceptable performance < 0.2 dB losses) for multimode fibers.<br>Master of Science

In this thesis, the work focuses on developing distributed optical fiber vibration sensors based on phase-sensitive optical time domain reflectometry (Φ-OTDR). Three works have been accomplished to improve the performances of Φ-OTDR for distributed vibration sensing. Firstly, Φ-OTDR based on a polarization diversity scheme is demonstrated to mitigate the polarization mismatch effect occurring in traditional systems. A theoretical analysis is performed in different polarization cases corresponding to coherent and polarization diversity detection. Φ-OTDR based polarization diversity shows a great potential in the multi-events sensing application. Two vibration events are simultaneously detected and their signal to noise ratios are improved by 10.9 dB and 8.65 dB, respectively, compared to the results obtained by a conventional coherent scheme. Intensity fluctuation in a phase-sensitive optical-time domain reflectometry (Φ-OTDR) system caused by stochastic characteristics of Rayleigh backscattering has limited relative vibration strength measurement, which is proportional to dynamic strain. A trace-to-trace correlation coefficient is thus proposed to quantify the Φ-OTDR system stability and a novel approach of measuring the dynamic strain induced by various driving voltages of lead zicronate titanate (PZT) is demonstrated. Piezoelectric vibration signals are evaluated through analyzing peak values of the fast Fourier transform spectra at fundamental frequency and high-order harmonics based on Bessel functions. Experimental results show high correlation coefficients and good stability of our Φ-OTDR system, as well as the small measurement uncertainty of measured peak values. To reduce the intra-band noise caused by the finite extinction ratio of optical pulses, Φ-OTDR based on high extinction ratio generation is studied. Two methods are developed for achieving high extinction ratio of optical pulse generation. One of the approaches is to synchronize two cascaded electro-optic modulators to achieve high extinction ratio operation. The other one is to use the nonlinear optical fiber loop mirror as an optical switch to suppress the continuous wave portion of optical pulse. The sensing range of 1.8 km and 8.4 km with corresponding spatial resolution of 0.5 m and 2 m have been demonstrated based on cascaded two electro-optic modulators and nonlinear optical fiber loop mirror setup, respectively.

Distributed fibre sensors have been successfully demonstrated and used in various industries for performing continuous measurements of the physical parameters such as temperature and strain. However, the measurement range and the performance of such sensors are limited by the intrinsic attenuation of the signal in the sensing fibre. This thesis investigates using optical amplification within the sensing fibre to enhance the range and the performance of the distributed Brillouin based sensors. To extend the range beyond this, remote amplification was also investigated using CW Raman amplification in which a Raman pump is injected at the midpoint of a 100km sensing fibre and a remotely pumped Erbium doped fibre amplifier is also situated at the midpoint. The performance achieved using each technique was measured and the relative advantages of each configuration were identified and discussed

Made available in DSpace on 2016-03-15T19:37:31Z (GMT). No. of bitstreams: 1 Rodrigo Cesar Ribeiro Miraglia.pdf: 2147088 bytes, checksum: 67087392fe4b03bb24d9a164fd92ceed (MD5) Previous issue date: 2010-08-20<br>Fundo Mackenzie de Pesquisa<br>Sensors based on optical fibers are being increasingly used in hostile environments for measuring pressure, temperature, stress, chemical and biological parameters, etc. These sensors have the advantage of having reduced size and weight, immunity to electromagnetic interference, of being chemically inert, and also allowing the distributed measurement of the respective parameter along the fiber. Microstructured optical fibers have characteristics that are relevant to sensing applications, such as freedom of design of their internal structure obtained by varying the diameter of the holes and the distances between them, among other parameters, which is not applicable in conventional optical fibers. The present work aims to use a microstructured optical fiber as a distributed sensor for hydrostatic pressure, using the POTDR (Polarization Optical Time-Domain Reflectometry) and the OFDR (Optical Frequency-Domain Reflectometry) measurement techniques. The application of hydrostatic pressure on the fiber changes its birefringence and, consequently, the evolution of the states of polarization, which is the parameter monitored by the techniques. By monitoring the changes of these states, it is possible to infer the changes in applied pressure. The study is undertaken via simulations and analysis, both performed in the software MatLab.<br>Sensores baseados em fibras ópticas vêm sendo cada vez mais utilizados em ambientes hostis para medição de pressão, temperatura, stress, análise química e biológica, etc. Esses sensores têm a vantagem de possuírem tamanho e peso reduzidos, imunidade à interferência eletromagnética, de serem quimicamente inertes, e também permitirem a medição distribuída do parâmetro a ser analisado ao longo da fibra. As fibras ópticas microestruturadas possuem características relevantes na aplicação de sensoriamento, tal como a liberdade de construção de sua estrutura interna variando o diâmetro dos buracos e a distância entre eles, entre outros parâmetros, que não são aplicáveis em fibras ópticas convencionais. Este trabalho tem como objetivo estudar uma fibra óptica microestruturada como sensor distribuído de pressão hidrostática, utilizando as técnicas de medição POTDR (Polarization Optical Time-Domain Reflectometry) e OFDR (Optical Frequency-Domain Reflectometry). A aplicação de pressão hidrostática sobre a fibra altera sua birrefringência e, consequentemente, a evolução do seu estado de polarização, sendo este último o parâmetro monitorado pelas técnicas. Analisando-se a mudança deste estado, pode-se inferir a mudança de pressão aplicada. O estudo é realizado através de simulações e análises efetuadas no software MatLab.

Monitoring optical line for a long term monitoring and evaluating both transfer quality and running reliability of optical line links' physical layer (optical cable routes) fiber optical transfer systems. Monitoring indicates any kind of security erosion of transmission medium incited by either "natural" climatic and mechanical environmental effects, indeliberated cable violation (building operation) or by intended intervention into transfer path.

The purpose of this master’s thesis is foremost to provide a simple guide how to build elements of optical metropolitan area network. The basic model consists to sequence of construction, network topology, passive and active parts. The collection contains examples of alternative technology such as Wireless LAN with different frequency. The optical network construction based on optical cable, fibres, splices, trays, adapters, connectors and active parts for example a lot of media convertor models. After that there are demonstrating type of wavelength division multiplexer used in metropolitan area network – passive planar PCL splitter. One of the passive planar splitter are used to increase optical fibre channel. At the end of the collection a simplified examples of used measurements – optical time domain reflectometry and optical fibre transmission. Contains standard protocols or reflectogram. The conclusion of this thesis summarizes costs of FTTb (Fibre To The Building) model of optical metropolitan area network in Czech republic and future contribution for society.

碩士<br>國立清華大學<br>光電工程研究所<br>104<br>The objective of this study is to demonstrate a new technique for fiber ranging and fault locating. By using semiconductor laser and extracting the time delay signature (TDS) of dynamical states resulting from the optical feedback, we developed the selfmixing optical time domain reflectometry (SMOTDR). Experimental results show that 2 cm resolution and -32 dBm sensitivity had been achieved. And due to the improve of setup, SMOTDR has already existed in fiber system so it has the potential to become a built-in self-detector. On the other hand, we compare SMOTDR with chaotic correlation optical time domain reflectometry (CCOTDR), besides SMOTDR has the compacter setup and simplified processing program, moreover it has the higher ranging sensitivity, furthermore due to the difference on setup the ranging performance would be better than CCOTDR in long testing length.

碩士<br>國立成功大學<br>機械工程學系碩博士班<br>92<br>Optical time domain relfectometer (OTDR) is an commonly used optical fiber test instrument which is capable to detecting fiber length, fiber transmission loss, fiber connector, fiber splice loss, fiber break and fiber　bending, etc, and it only needs on end of the fiber link for measurement. Traditional OTDR uses Rayleigh scattering level as the measuring indicator, but measurable dynamic range and signal stability are limited due to the intrinsic weakness of Rayleigh signal. In this thesis, we improve the problem described above by applying the Fresnel reflection signals caused by fiber Gragg grating as the measuring indicator, besides, we adapt the dual wavelength reference scheme to resolve the degradation of spatial caused by the widening of laser pulse in OTDR measurement. We further apply these techniques in optical fiber salinity measurement, in the future, further multiplexing of the sensor link can be investigated.