Distributed sensing using polarization sensitive optical low-coherence reflectometry and fiber gratings

Optical low coherence reflectometry (OLCR) is non-destructive interferometric technique that allows measuring amplitude and phase of the light reflected from the device under test. OLCR is a powerful tool for the characterization of the various optical devices such as fiber gratings and optical waveguides. This thesis work had explored possibilities to combine the OLCR technique with fiber gratings to perform distributed strain and temperature measurements, and to develop novel sensing techniques as an alternative to classical spectral methods. Special attention is devoted to polarization sensitive measurements and techniques, and OLCR techniques that use only amplitude information, as an alternative to more complicated phase sensitive measurements. Using a polarization sensitive system two methods for the measurement of local birefringence of fiber Bragg gratings (FBG) using OLCR were developed. The first technique uses oscillations in the OLCR amplitude signal to directly obtain the beat length and the birefringence. The second method is based on the measurement of the OLCR phase and inverse scattering algorithm. Both methods were compared with birefringence obtained from spectral measurements, and very good agreement was obtained. The indirect method, based on local Bragg grating determination requires two independent measurements and mathematical reconstruction, but provides a very high spatial resolution of ≈ 25 µm. The grating length limits the minimum measurable birefringence in the direct method, but a good sensitivity of 4×10-6 was obtained, which corresponds to a Bragg wavelength shift of 4 pm. The spatial resolution is in the millimeter range in this case. The novel methods were successfully applied for the distributed measurement of birefringence of FBG under diametric load. New types of tunable devices and sensors – fiber Bragg gratings written in high attenuation fibers (HAF) were developed. Active tuning by heating was achieved by optical pumping of a pure single mode fiber without any mechanical or electrical parts, or deposited light absorption coatings. Tuning can be controlled by the applied pump power, the position of the grating, the HAF attenuation level, and the pumping configuration. The proposed design assures a high repeatability and long lifetime. Using OLCR these devices were successfully applied to measure the liquid level with a spatial resolution of 100 micrometers. Liquid level measurements that use both phase and amplitude, or only amplitude were demonstrated. The same principles could be easily applied to design other fiber grating devices (long-period, tilted etc), and sensors based on hot-wire anemometry, like flow or vacuum sensors.

La diffusion Brillouin dans les fibres optiques

Among all non-linear optical effects observed in single-mode optical fibres, stimulated Brillouin scattering (SBS) is of particular importance since it has numerous practical implications. SBS occurs when laser waves are scattered through the interaction of light with hypersonic acoustic waves in the medium. It manifests through the generation of a backscattered Stokes wave that carries most of the input optical power once a definite intensity level is reached within the fibre core. It limits the maximum optical power that can be transmitted through an optical fibre and therefore can cause a severe penality for fibre optics telecommunications. The backscattered Stokes wave is down shifted in frequency with respect to the incident lightwave frequency. This frequency shift – often called the Brillouin frequency shift – depends on the fibre parameters and on the wavelength of the incident light. It is directly proportional to the acoustic velocity and ranges from 12 to 13 GHz for silica fibres at a 1.3 µm wavelength. The Brillouin frequency shift is also very sensitive to environmental quantities changing the acoustic velocity such as temperature and strain. This feature makes SBS very interesting for temperature and strain monitoring in optical fibres and has been used in the design of fibre optics sensors. In the present work, a novel method to measure SBS characteristics in single-mode optical fibres has been developed. It is based on the so-called pump and probe technique using two counterpropagating optical waves within the test fibre. It basically presents the originality to require only one laser source and relies on an electro-optic modulator to generate the probe signal by microwave modulation of the pump light. The inherent high stability of the experimental set-up has made possible the characterisation of different fibres in terms of SBS parameters with a higher accuracy than the traditional two lasers set-up. The influence of dopant concentration in silica fibres on the SBS properties has been fully characterised. Furthermore it has been determined that the presence of core dopants decreases the acoustic velocity resulting in a smaller Brillouin frequency shift. The temperature and strain effects on the SBS characteristics have been extensively investigated. The problem of the evolution of the polarisation of the interacting lightwaves has been studied and a model clarified the so far unexplained behaviour in low birefringence fibres. A new measurement procedure has been defined to achieve polarisation-independent Brillouin gain determination. The second facet of the present thesis deals with the concept of distributed fibre optics sensors based on SBS. The determination of the Brillouin frequency shift gives access to the temperature or strain experienced by the fibre, while a modified OTDR technique provides the information on the position. Here again the use of a single laser source together with an electro-optic modulator brings several advantages. Besides the convenient flexibility in the generation of the probe signal, it makes possible to pulse the optical signals to localise the interaction within the test fibre. The overall temperature or strain distribution of the test fibre can be carried out by measuring the Brillouin frequency shift at any locations throughout the test fibre. A sensor has been experimentally achieved and its performances can be summarised as follow: spatial resolution less than 10 meters over more than 10 kilometres, with a resolution on the Brillouin frequency determination of 1 MHz, that corresponds to a ±1°C temperature resolution or to a 2x10-5 strain resolution. The dynamic range can be increased up to 30 kilometres to the detriment of the spatial resolution. The ultimate performances of the SBS distributed fibre optics sensors have been investigated in terms of spatial resolution and dynamic range. It turns out that these sensors are eventually dedicated to distributed measurements over several tens of kilometres with a spatial resolution limited to the meter range. Finally two practical applications of such sensors are described: the measurement of the strain distribution in installed fibre optics cables for telecommunications and the temperature monitoring of electrical energy distribution cables. On site measurements using a Brillouin sensor have been performed for the first time thanks to the high stability and reliability of the sensor.

EPFL1997

Mid-infrared surface plasmon resonance sensing applied to refractive index measurements of liquids

Sylvain Herminjard

There has been a very strong development of the sensors based on surface plasmon resonance during the last thirty years, mostly for biological and biomedical applications. If the first experiments in this field were carried out at the beginning of the 20t century, it would have been necessary to wait for the end of the sixties to see the first devices performing accurate measurements of the refractive indices of gas and liquid samples. Indeed, the surface plasmon resonance phenomenon allows to measure refractive index variations with a high sensitivity that is hardly obtained with other optical measurement techniques. The basic principle consists of exciting the free electrons of a metallic layer with light, which induces the formation of a surface wave called surface plasmon. This resonant phenomenon depends on the wavelength of the excitation light and the refractive indices of the metallic layer and the material located close to the metallic layer. A variation of the refractive index of the latter induces a variation of the resonance condition, which allows to realize optical sensors designed to measure determined samples. An appropriate calibration allows to set a relationship between the variation of the concentration of the sample and the measured refractive index value. Among the various parameters that describe the performances of the sensor, one of the most important is the sensitivity. It is possible to demonstrate that the excitation wavelength has an impact on the sensitivity of the system : the latter increases if the measured sample has an absorption peak at a wavelength close to the excitation wavelength. The main goal of this thesis is to explore this sensitivity enhancement technique based on the absorption properties of the probed medium. A proof-of-principle is experimentally demonstrated with an optical setup constituted of laser sources working in the visible range of the electromagnetic spectrum. However, many materials have absorption peaks located in the mid-infrared range due to their fundamental molecular vibrations. Therefore, the realization of a plasmonic sensor working in the mid-infrared could allow to measure these materials with an increased sensitivity. To the best of our knowledge, surface plasmon resonance based sensors have never been developed above the near-infrared. This thesis presents for the first time the design and realization of a plasmonic sensor working in the mid-infrared range able to measure refractive index variations of gas and liquids samples. Even if the optical sources, components and detectors designed for the mid-infrared are clearly different from the ones designed for the visible range, it has been possible to successfully realize two systems working in the mid-infrared range during this project. The first is able to perform gas measurements. As the excitation wavelength depends on the measured sample, it was necessary to select a laser source that had a wavelength located close to the absorption peak of the gas under measurement. For this purpose, we designed a plasmonic sensor working at a 4.4 µm wavelength able to measure carbon dioxide (CO2) absorbing at 4.3 µm. It has been demonstrated that the experimental sensitivity of this system is five times higher than the theoretical sensitivity obtained with a system working in the visible range at a 633 nm wavelength. The second has been conceived to measure liquid samples in order to measure variations of concentration of CO2 dissolved in water. If measurements of refractive index variations (without using the sensitivity enhancement technique) could be performed successfully with variations of concentration of alcohol in water, further work is necessary to get decisive results for CO2 dissolved in water. This project was carried out in collaboration with an industrial partner in order to take into account some industrial constraints during the development of the sensor. Therefore, particular care has been taken in order to design a system that is compact, mechanically robust, without any mechanical moving parts, cheap and easily reproducible. The technological development presented in this thesis finds many applications mainly in the chemical, biological and biomedical fields. Indeed, many proteins show absorption peaks in the mid-infrared. It could be possible to measure their concentration or the modification of their properties – like their composition or structure – with an increased sensitivity. The interest of surface plasmons is that the measurement is not a transmission-based technique, but rather a "reflection-based technique", as the light does not go through the gas or liquid sample. This measurement technique is also adapted for high volume samples that are present in industrial production processes like water or soft drink quality control.

EPFL2010

Brillouin Echoes for Advanced Distributed Sensing in Optical Fibres

Stella Foaleng Mafang

Brillouin scattering is particularly efficient and attractive for the implementation of strain and temperature distributed sensing in optical fibres. Recently a trend has been observed that modern advanced applications require a substantial step towards better spatial resolution, while preserving temperature/strain precision over a long range. For this purpose the state of the art does not satisfy all these requirements. In this thesis we present a radically new approach named Brillouin Echoes distributed sensing (BEDS) that allows covering these requirements. In the first part, we propose an updated configuration of the classical existing Brillouin sensor for time domain analysis allowing drastic noise reduction. Then we investigate the limitations (due to non-linear effects) of the classical Brillouin sensor in terms of long distance range measurements. The identified nonlinear effects are pump depletion due to SBS itself, self-phase modulation (SPM), modulation instability (MI), which occurs only in fibres presenting an anomalous dispersion at the pump wavelength and Raman scattering (RS). We propose the modeling of the pump depletion effect to obtain analytical expressions that are useful for the proper design of a BOTDA sensor and for the determination of a very small depletion. The model confirmed by experimental measurements is informative on the conditions maximizing the depletion effect; therefore a standard configuration can be defined to test the value of the depletion in the set-up. Furthermore, we demonstrate that SPM-induced spectral broadening can have a significant effect on the measured effective gain linewidth. Modeling and experiments have undoubtedly demonstrated that the effective gain linewidth can easily experience a two-fold increase in standard conditions when the pulse intensity profile is Gaussian. We showed that the problem can be practically circumvented by using a clean rectangular pulse with very sharp rising and falling edges. The theoretical and experimental analysis of the undesirable effects of MI and forward RS in distributed BOTDA sensors systems gives a simplified expression to predict the critical power for a given distance range. MI turns out to be the dominant nonlinear limitation since it shows the lowest critical power, but it is less critical since it can be avoided to a wide extent by using the fibre in the normal dispersion spectral region such as a DSF in the C-band. On the other hand Raman scattering can be avoided only by limiting the optical pump power and therefore is the ultimate nonlinear limitation in a distributed sensing system. Under similar conditions RS shows a critical power ∼5 times larger than MI. In the second part, we present the new approach Brillouin echo distributed sensing (BEDS) which has proved to be a powerful solution to realize sub-metric spatial resolutions in Brillouin distributed measurements. We have demonstrated both theoretically and experimentally that an optimized configuration is reached when the optical wave is π-phase shifted. The experimental tests have shown a spatial resolution down to 5 cm, with a clear margin for further improvement down to a real centimetric spatial resolution. This optimized configuration produce the best contrast independently of the pulse intensity, with a factor 2 of improvement compared to other techniques based on the same approach (dark pulse, bright pulse). This extends the dynamic range by 3 dB, which corresponds in standard loss conditions to a 5 km extension of the sensing range. An analytical developed model has proved to be an excellent tool not only for optimizing the pumping scheme but also in post-processing the measured data. Finally the potentialities of BEDS technology provide solutions in real contexts. Using the BEDS technology in landslide monitoring at laboratory scale, for the first time it became possible to observe the failure propagation in laboratory scale with an accurate precision. Furthermore, using BEDS we have proposed and demonstrated the possibility of mapping geometrical structure fluctuations along a photonic crystal fibre (PCF). Both long- and short-scale longitudinal fluctuations in the Brillouin frequency shift have been identify and quantify. Observation of Brillouin linewidth broadening in PCF fibre through distributed measurement of the Brillouin gain spectrum using BEDS has allowed fundamental understanding of SBS in PCF fibre and in their design in view of applications to optical-strain/temperature sensing.

EPFL2011