A simpler approach to distributed sensing with ultra-low reflectivity fibre points
- Sutherland, Harriet Elizabeth
- Authors: Sutherland, Harriet Elizabeth
- Date: 2016
- Subjects: Optical fiber detectors , Optical fibers , Interferometers
- Language: English
- Type: Masters (Thesis)
- Identifier: http://hdl.handle.net/10210/212850 , uj:21035
- Description: Abstract: Distributed fibre sensors based on the frequency-domain analysis of Rayleigh backscattered light are well established. They exhibit excellent performance in both sensitivity and spatial resolution, but their application can be limited due to their cost and the complexity of the analysis. This work presents a system based on coherent optical frequency-domain reflectometry used in Rayleigh distributed sensors, but with some modifications to the fibre and the implementation of signal-conditioning algorithms that enable the use of more readily available components and simplified analysis. A sensing fibre is prepared by printing uniformly spaced (in this instance), ultra-low reflectivity points. When swept-wavelength light is introduced into the fibre, the reflections from the ultra-low reflectivity points interfere with the reflection from the tip of the fibre. These reflections can be processed with the techniques used in coherent optical frequency-domain reflectometry, providing information about the state of the fibre with regards to a parameter (such as temperature) between the reflective points. The function of the ultra-low reflectivity points is to provide stronger reflections than those produced by Rayleigh backscatter. The ultra-low reflectivity points are fibre Bragg gratings that act as reflectors and not as sensors per se. They are manufactured to reflect the same wavelength and, because of their low reflectivity, they also have a wider reflective spectral range than commonly used high-reflectivity fibre Bragg gratings. This removes the need for specialised detection equipment. Use of a reference interferometer and signal-processing algorithms (linearisation and phase fixing) makes it possible to replace a high-precision linearly tuneable laser with a standard tuneable laser or even a distributed feedback laser diode as the optical source. A novel technique is also presented for identifying the initial phase in the signal, post-acquisition, by means of a fibre Bragg grating in the reference interferometer. In combination with the linearisation and phase fixing algorithms, this locks the phase of the signal prior to analysis, dispensing with the need for precise synchronisation between the optical source and signal acquisition. The best spatial resolution that can be achieved by the system is 0.36 mm, and the best temperature resolution achieved (with a spatial resolution of 60 mm) had a standard deviation of... , M.Ing. (Electrical and Electronic Engineering)
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- Authors: Sutherland, Harriet Elizabeth
- Date: 2016
- Subjects: Optical fiber detectors , Optical fibers , Interferometers
- Language: English
- Type: Masters (Thesis)
- Identifier: http://hdl.handle.net/10210/212850 , uj:21035
- Description: Abstract: Distributed fibre sensors based on the frequency-domain analysis of Rayleigh backscattered light are well established. They exhibit excellent performance in both sensitivity and spatial resolution, but their application can be limited due to their cost and the complexity of the analysis. This work presents a system based on coherent optical frequency-domain reflectometry used in Rayleigh distributed sensors, but with some modifications to the fibre and the implementation of signal-conditioning algorithms that enable the use of more readily available components and simplified analysis. A sensing fibre is prepared by printing uniformly spaced (in this instance), ultra-low reflectivity points. When swept-wavelength light is introduced into the fibre, the reflections from the ultra-low reflectivity points interfere with the reflection from the tip of the fibre. These reflections can be processed with the techniques used in coherent optical frequency-domain reflectometry, providing information about the state of the fibre with regards to a parameter (such as temperature) between the reflective points. The function of the ultra-low reflectivity points is to provide stronger reflections than those produced by Rayleigh backscatter. The ultra-low reflectivity points are fibre Bragg gratings that act as reflectors and not as sensors per se. They are manufactured to reflect the same wavelength and, because of their low reflectivity, they also have a wider reflective spectral range than commonly used high-reflectivity fibre Bragg gratings. This removes the need for specialised detection equipment. Use of a reference interferometer and signal-processing algorithms (linearisation and phase fixing) makes it possible to replace a high-precision linearly tuneable laser with a standard tuneable laser or even a distributed feedback laser diode as the optical source. A novel technique is also presented for identifying the initial phase in the signal, post-acquisition, by means of a fibre Bragg grating in the reference interferometer. In combination with the linearisation and phase fixing algorithms, this locks the phase of the signal prior to analysis, dispensing with the need for precise synchronisation between the optical source and signal acquisition. The best spatial resolution that can be achieved by the system is 0.36 mm, and the best temperature resolution achieved (with a spatial resolution of 60 mm) had a standard deviation of... , M.Ing. (Electrical and Electronic Engineering)
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Development of a multi-point temperature fiber sensor based on a serial array of optical fiber interferometers
- Authors: Della Tamin, Michelin
- Date: 2015-06-29
- Subjects: Interferometers , Optical fibers , Bragg gratings , Optical fiber detectors
- Type: Thesis
- Identifier: uj:13641 , http://hdl.handle.net/10210/13823
- Description: M.Ing. (Electrical and Electronic Engineering) , An experimental study of a multi-point optic fibre sensor for monitoring temperature changes is presented. The multi-point optic fibre sensor is made of a serial array of weak-reflectivity identical gratings. The weak-reflectivity identical gratings form the interferometric cavities UV printed on the single mode fibre. The ability to measure temperatures changes at different cavities along the serial array is particularly interesting for the monitoring of power transformers, high temperature furnaces and jet engines. Changes in temperature in each respective cavity is measured based on the spectral shift in the phase of the light from each respective cavity. The performance of the multi-point fibre sensor system is evaluated. Further, a theoretical and experimental investigation of a serial array composed of two cavities of different lengths is conducted. This investigation is aimed at measuring the impact of the overlap of the two distinct cavities in their respective frequency domain and determining the accuracy of the measurement. The result found shows that the sensor phase response is no more linear to temperature changes. It is also found that the nonlinear response of the sensor to temperature changes increases with the magnitude of the overlap.
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- Authors: Della Tamin, Michelin
- Date: 2015-06-29
- Subjects: Interferometers , Optical fibers , Bragg gratings , Optical fiber detectors
- Type: Thesis
- Identifier: uj:13641 , http://hdl.handle.net/10210/13823
- Description: M.Ing. (Electrical and Electronic Engineering) , An experimental study of a multi-point optic fibre sensor for monitoring temperature changes is presented. The multi-point optic fibre sensor is made of a serial array of weak-reflectivity identical gratings. The weak-reflectivity identical gratings form the interferometric cavities UV printed on the single mode fibre. The ability to measure temperatures changes at different cavities along the serial array is particularly interesting for the monitoring of power transformers, high temperature furnaces and jet engines. Changes in temperature in each respective cavity is measured based on the spectral shift in the phase of the light from each respective cavity. The performance of the multi-point fibre sensor system is evaluated. Further, a theoretical and experimental investigation of a serial array composed of two cavities of different lengths is conducted. This investigation is aimed at measuring the impact of the overlap of the two distinct cavities in their respective frequency domain and determining the accuracy of the measurement. The result found shows that the sensor phase response is no more linear to temperature changes. It is also found that the nonlinear response of the sensor to temperature changes increases with the magnitude of the overlap.
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Versatile interferometer system for inscription of fiber Bragg gratings
- Authors: Du Toit, Ruan W.
- Date: 2012-06-06
- Subjects: Bragg gratings , Optical fibers , Interferometry , Interferometers
- Type: Thesis
- Identifier: uj:2503 , http://hdl.handle.net/10210/4956
- Description: M.Ing. , Bragg gratings are important components for sensing and for wavelength-division multiplexed optical communication systems. These gratings are manufactured by either side-writing of the fiber with a high intensity UV light through a phase mask, or by exposing the fiber to interference fringes through an interferometer arrangement. With one phase mask, only a small range of grating wavelengths is possible. This is achieved by pre-straining the fiber during the writing process. The limitation arises from the break strength of the fi ber, allowing a maximum range of Bragg wavelengths of only approximately 10 nm. The interferometric technique uses a beam splitter to divide a single input UV beam into two and intersecting them at the fiber. The angle at which the beams intersect will determine the period of the interference fringes and thus the Bragg grating written in the optical fiber. The Argon-ion laser is used with a 1060 nm phase mask (used to split beam) to write Bragg gratings with reflections from 1012 to 1600 nm. Three accurate- translation and rotation stages are used to keep the fiber at the beam intersection. Alignment, mechanical stability and coherence of laser are critical.
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- Authors: Du Toit, Ruan W.
- Date: 2012-06-06
- Subjects: Bragg gratings , Optical fibers , Interferometry , Interferometers
- Type: Thesis
- Identifier: uj:2503 , http://hdl.handle.net/10210/4956
- Description: M.Ing. , Bragg gratings are important components for sensing and for wavelength-division multiplexed optical communication systems. These gratings are manufactured by either side-writing of the fiber with a high intensity UV light through a phase mask, or by exposing the fiber to interference fringes through an interferometer arrangement. With one phase mask, only a small range of grating wavelengths is possible. This is achieved by pre-straining the fiber during the writing process. The limitation arises from the break strength of the fi ber, allowing a maximum range of Bragg wavelengths of only approximately 10 nm. The interferometric technique uses a beam splitter to divide a single input UV beam into two and intersecting them at the fiber. The angle at which the beams intersect will determine the period of the interference fringes and thus the Bragg grating written in the optical fiber. The Argon-ion laser is used with a 1060 nm phase mask (used to split beam) to write Bragg gratings with reflections from 1012 to 1600 nm. Three accurate- translation and rotation stages are used to keep the fiber at the beam intersection. Alignment, mechanical stability and coherence of laser are critical.
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Numerical modelling of a Raman-Rayleigh distributed temperature fiber sensor implementing correlation techniques
- Authors: Shimaponda, Mulundumina
- Date: 2015-06-29
- Subjects: Optical fiber detectors , Optical fibers , Interferometers
- Type: Thesis
- Identifier: uj:13647 , http://hdl.handle.net/10210/13831
- Description: M.Ing. (Electrical and Electronic Engineering) , A distributed temperature fiber sensor based on the ratio of the Raman anti-Stokes to Rayleigh backscattered light components is studied. The aim of the study is to propose a method of quantifying the noise exhibited in the Rayleigh backscattered signal and further propose correlation coding techniques to reduce the noise in the Rayleigh and Raman backscattered signals. The noise in the Rayleigh backscattered signal is referred to as “interferometric noise”. When Rayleigh scattering along the length of an optical fiber occurs, some of the scattered light travels in a direction opposite to the direction of propagation, and is called backscattered light. When the coherence length of the optical source permits interactions between the Rayleigh backscattered light, there is a possibility for the interacting backscattered light, within a distance that is half the coherence length, to interfere with each other. Furthermore, when the sensing optical fiber is greater than the coherence length of the optical source, there will be several interference sections along the length of the sensing fiber causing the intensity of the Rayleigh backscattered light at the photo-detectors to vary randomly. The intensity variation gives the Rayleigh backscattered signal a jagged appearance indicating the presence of interferometric noise. The longer the coherence length of the optical sources, the larger the intensity variations in the backscattered light, that is, the more the interferometric noise exhibited. The more the interferometric noise in the Rayleigh backscattered signal, the poorer the temperature accuracy of the distributed temperature sensor based on the ratio of the Raman anti Stokes to Rayleigh backscattered components. To quantify the interferometric noise affecting the Rayleigh backscattered signal, a mathematical model based on well-known scattering and interferometry theories is developed. Using the developed mathematical noise model, noise powers of approximately -52dBm and -40dBm for coherence lengths of 4m and 24m are respectively obtained...
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- Authors: Shimaponda, Mulundumina
- Date: 2015-06-29
- Subjects: Optical fiber detectors , Optical fibers , Interferometers
- Type: Thesis
- Identifier: uj:13647 , http://hdl.handle.net/10210/13831
- Description: M.Ing. (Electrical and Electronic Engineering) , A distributed temperature fiber sensor based on the ratio of the Raman anti-Stokes to Rayleigh backscattered light components is studied. The aim of the study is to propose a method of quantifying the noise exhibited in the Rayleigh backscattered signal and further propose correlation coding techniques to reduce the noise in the Rayleigh and Raman backscattered signals. The noise in the Rayleigh backscattered signal is referred to as “interferometric noise”. When Rayleigh scattering along the length of an optical fiber occurs, some of the scattered light travels in a direction opposite to the direction of propagation, and is called backscattered light. When the coherence length of the optical source permits interactions between the Rayleigh backscattered light, there is a possibility for the interacting backscattered light, within a distance that is half the coherence length, to interfere with each other. Furthermore, when the sensing optical fiber is greater than the coherence length of the optical source, there will be several interference sections along the length of the sensing fiber causing the intensity of the Rayleigh backscattered light at the photo-detectors to vary randomly. The intensity variation gives the Rayleigh backscattered signal a jagged appearance indicating the presence of interferometric noise. The longer the coherence length of the optical sources, the larger the intensity variations in the backscattered light, that is, the more the interferometric noise exhibited. The more the interferometric noise in the Rayleigh backscattered signal, the poorer the temperature accuracy of the distributed temperature sensor based on the ratio of the Raman anti Stokes to Rayleigh backscattered components. To quantify the interferometric noise affecting the Rayleigh backscattered signal, a mathematical model based on well-known scattering and interferometry theories is developed. Using the developed mathematical noise model, noise powers of approximately -52dBm and -40dBm for coherence lengths of 4m and 24m are respectively obtained...
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Sensing characteristics of an optical fibre long-period grating Michelson refractometer
- Authors: Van Brakel, Adriaan
- Date: 2009-02-26T12:22:37Z
- Subjects: Optical fiber detectors , Interferometers , Refractometers
- Type: Thesis
- Identifier: uj:8179 , http://hdl.handle.net/10210/2183
- Description: D.Ing. , Most optical fibre-based ambient refractive index sensors (including individual long-period gratings) rely on spectral attributes obtained in transmission. However, a probe refractometer has been proposed that is based on self-interference of a long-period grating (LPG), thus providing reflectance spectra containing the relevant data. This sensor operates as a Michelson interferometer by virtue of the fact that its constituent LPG acts as both a mode converter and coupler. Its construction is such that optical power coupled into the cladding (when light impinges on the LPG) is reflected at a fibre mirror and returns towards the grating, where it is re-coupled into the fundamental guided mode. Since light waves propagating along the core and cladding material of the fibre cavity beyond the LPG experience different optical path lengths (due to differing mode indices), a phase difference exists between these modes upon recombining at the grating location. This causes interference, which is manifested as a characteristic fringe pattern in the sensor’s reflectance spectrum (analogous to that obtained in the transmission of a twin LPG cascade operating as a Mach-Zehnder interferometer). Research was conducted towards implementing a unique method of temperature compensation in this LPG-based Michelson interferometer. Sensing attributes of individual LPGs were investigated first, with specific emphasis on the temperature characteristics of two different types of host fibre. It was found that LPGs manufactured in conventional ATC SMF-28 fibre (previously hydrogen-loaded to inscribe the grating and annealed after fabrication) and B/Ge co-doped PS1500 fibre from Fibercore exhibited temperature characteristics of opposite polarity. This led to the implementation of a compound-cavity Michelson interferometer whose constituent LPG is written in one type of fibre, while a specific length of the other type of fibre is fusion spliced onto the host fibre section. Experiments verified the success of this temperature-compensation technique, which caused a measured reduction in temperature sensitivity of up to in interferometer phase shift. Measurements of the refractive index of the test substance surrounding the cladding material of the Michelson interferometer’s fibre cavity (and not the LPG itself) could therefore be done without being adversely affected by environmental temperature fluctuations. This was demonstrated experimentally by comparing the interferometer’s phase shift – devoid of temperature-induced effects – due to increasing refractive index of the analyte (as a result of escalating temperature) with index of refraction readings from a temperature-controlled Abbe refractometer. Numerical gradients of linear curves fitted to these results differed by two orders of magnitude less than the resolution of readings obtained from an Abbe refractometer – proof of the success of the temperature compensation technique applied in this LPG-based Michelson refractometer.
- Full Text:
- Authors: Van Brakel, Adriaan
- Date: 2009-02-26T12:22:37Z
- Subjects: Optical fiber detectors , Interferometers , Refractometers
- Type: Thesis
- Identifier: uj:8179 , http://hdl.handle.net/10210/2183
- Description: D.Ing. , Most optical fibre-based ambient refractive index sensors (including individual long-period gratings) rely on spectral attributes obtained in transmission. However, a probe refractometer has been proposed that is based on self-interference of a long-period grating (LPG), thus providing reflectance spectra containing the relevant data. This sensor operates as a Michelson interferometer by virtue of the fact that its constituent LPG acts as both a mode converter and coupler. Its construction is such that optical power coupled into the cladding (when light impinges on the LPG) is reflected at a fibre mirror and returns towards the grating, where it is re-coupled into the fundamental guided mode. Since light waves propagating along the core and cladding material of the fibre cavity beyond the LPG experience different optical path lengths (due to differing mode indices), a phase difference exists between these modes upon recombining at the grating location. This causes interference, which is manifested as a characteristic fringe pattern in the sensor’s reflectance spectrum (analogous to that obtained in the transmission of a twin LPG cascade operating as a Mach-Zehnder interferometer). Research was conducted towards implementing a unique method of temperature compensation in this LPG-based Michelson interferometer. Sensing attributes of individual LPGs were investigated first, with specific emphasis on the temperature characteristics of two different types of host fibre. It was found that LPGs manufactured in conventional ATC SMF-28 fibre (previously hydrogen-loaded to inscribe the grating and annealed after fabrication) and B/Ge co-doped PS1500 fibre from Fibercore exhibited temperature characteristics of opposite polarity. This led to the implementation of a compound-cavity Michelson interferometer whose constituent LPG is written in one type of fibre, while a specific length of the other type of fibre is fusion spliced onto the host fibre section. Experiments verified the success of this temperature-compensation technique, which caused a measured reduction in temperature sensitivity of up to in interferometer phase shift. Measurements of the refractive index of the test substance surrounding the cladding material of the Michelson interferometer’s fibre cavity (and not the LPG itself) could therefore be done without being adversely affected by environmental temperature fluctuations. This was demonstrated experimentally by comparing the interferometer’s phase shift – devoid of temperature-induced effects – due to increasing refractive index of the analyte (as a result of escalating temperature) with index of refraction readings from a temperature-controlled Abbe refractometer. Numerical gradients of linear curves fitted to these results differed by two orders of magnitude less than the resolution of readings obtained from an Abbe refractometer – proof of the success of the temperature compensation technique applied in this LPG-based Michelson refractometer.
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