Impact of the fiber coating on the temperature response of distributed optical fiber sensors at cryogenic ranges

The thermomechanical behavior of a standard single mode fiber with different coating materials is theoretically analyzed under different temperature conditions. Results show that the thermal expansion/shrinkage of the fiber coating introduces an extra strain on the optical fiber and can modify its thermal response. Distributed fiber sensors based on coherent Rayleigh and Brillouin scatterings are employed to characterize the impact of different coatings on the temperature sensitivity. The standard coating with dual-layer demonstrates little influence on the thermal response at room temperature due to the softness of primary coating, but it increases the temperature sensitivity by some 50 % at ~ 220 K as the primary coating becomes stiffer at low temperature. Optical fibers with aluminum (Al) and Ormocer® coatings are also experimentally tested. All the measured results agree well with the theoretical analysis.

Coherent Rayleigh time domain reflectometry

Xin Lu

Owing to the advantages of the optical fibre, such as lightweight, small size, low cost and immunity to electromagnetic interferences, the fibre sensors have been applied in diverse fields and the scientific research on fibre sensing keeps improving to meet new industrial demands. In this thesis, a novel distributed fibre sensor based on coherent Rayleigh scattering is investigated to realise best accuracy measurements. Coherent Rayleigh scattering is found to be essentially assimilated to a random walk process, thus its statistical properties are theoretically analysed based on the mature theory on random walk. Then the spectral characteristics of the Rayleigh backscattered light is investigated for different incident pulse shapes. The spectral distribution of the trace amplitude turns out to be identical to the spectrum of the incident pulse. Meanwhile, a theoretical model is proposed to describe the coherent Rayleigh trace in time and frequency domains. Experiments are carried out to validate the theoretical analysis and the proposed model. Then the statistical properties and the visibility of the coherent Rayleigh traces obtained by experiment and simulation are investigated for different detection bandwidth. The minimum 3 dB bandwidth required to resolve all features of a coherent Rayleigh intensity trace is determined to be 3 times larger than the bandwidth of the incident rectangular pulse. Then coherent Rayleigh scattering is applied to distributed sensing using the optical time domain reflectometry technique. The working principle is explained by the restorability of the Rayleigh trace in time domain and by a spectral shift in frequency domain. The temperature sensitivity is calculated over a wide range from 300 K down to 4.5 K based on the thermo-optic effect and the thermal expansion of the silica fibre. The experimental results in a standard fibre with acrylate coating and a fibre specially coated with ORMOCER® confirm the high temperature sensitivity (over 1 GHz/K) and demonstrate a temperature resolution in the milliKelvin range even under cryogenic conditions. The fibre with the ORMOCER® coating exhibits a higher temperature sensitivity at room temperature than the acrylate coated fibre due to the extra strain induced by the temperate change in the coating and a more stable sensing behaviour because the polymers forming the coating stabilise the mechanical and thermal responses of the fibre. The sensors based on coherent Rayleigh scattering are also applied to retrieve the phase birefringence distribution along polarisation-maintaining and standard single mode fibres. The minimum detectable birefringence reaches 3×10¿7 when the spatial resolution is 2 m and even smaller value can be resolved using longer optical pulses. In addition, this birefringence measurement is capable of distributed temperature and strain sensing in a PANDA and an elliptical-core polarisation-maintaining fibres since the birefringence is sensitive to environmental variations. The experimental results demonstrate that this method is one order of magnitude more sensitive than a fibre sensor based on Brillouin scattering. Moreover, discriminated temperature and strain sensing is realised by a combination of this birefringence measurement and a standard COTDR sensor. This thesis is concluded by a discussion of further research directions on coherent Rayleigh scattering based distributed sensing.

EPFL2016

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

Investigating Novel Optical Fibres for More Advanced Distributed Optical Fibre Sensing

Malak Mohamed Hossameldeen Omar Mohamed Galal

The ever-growing need for distributed optical fibre sensors (DOFS) in numerous fields and applications demands continuous research towards the advancement of the existing sensing systems. The convenience in the field of DOFS lies in the several degrees of freedom that it offers, such as the scattering mechanism, the interrogation technique, the medium for propagation, and the sensing fibre itself. Even though a vast amount of research is oriented towards the fabrication of novel fibres, in many cases the real benefit of such fibres is not really exploited. Therefore, in this thesis, we use and characterise multiple novel optical fibres with the aim of harnessing their full potential by understanding their ultimate strengths and limitations. To that effect, we perform efficient distributed optical fibre sensing by investigating different scattering mechanisms in different scattering media by means of several interrogation techniques. The thesis is divided into two groups of chapters depending on the type of scattering mechanism and medium. In the first group of chapters, we work on the investigation of the most prominent scattering mechanism in silica-core single-mode optical fibres (SMFs), namely Rayleigh scattering. We interrogate a novel reflection-enhanced fibre (REF) based on fibre Bragg gratings using one of the most widely-utilised interrogation techniques for Rayleigh scattering, which is phase-sensitive optical time-domain reflectometry (f-OTDR). So far, no theoretical expression has been presented to relate the most crucial parameters of Rayleigh-based sensing systems. We, therefore, develop a model, as a figure-of-merit for Rayleigh-based systems, addressing this concern, and we confirm it with experimental results. By performing a distributed temperature measurement as a form of comparison between the REF and an SMF, we yield a 6× lower experimental uncertainty for the REF when compared to the SMF, and this lower uncertainty is directly attributed to the 6× higher signal-to-noise ratio that the REF offers. The model as well as the guidelines for utilising REFs to maximum effect will aid the scientific community in better understanding Rayleigh-based sensing systems and employing them more efficiently.The focus then shifts to the most prominent scattering mechanism in a gaseous medium of high density (at 1 atm for instance), namely Brillouin scattering. The analysis is carried out using several novel hollow-core optical fibres particularly anti-resonant fibres (HC-ARFs) which we fill with gas. We demonstrate for the first time to the best of our knowledge a Brillouin gain measurement in gas-filled HC-ARFs and highlight the square dependence of the Brillouin gain on the gas pressure. We perform a comparison between three hollow-core fibres of different dimensions which we fill with two different gases, and we indicate the trade-offs in terms of Brillouin gain coefficients, gas filling times as well as gas pressure limitations. Additionally, we conduct for the first time to our knowledge a distributed temperature measurement using a gas-filled hollow-core conjoined-tube anti-resonant fibre (HC-CAF) and show a higher sensitivity to temperature change which is about 2× greater than the sensitivity of conventional silica fibres. The results indicate the great potential of gas-filled hollow-core fibres and pave the way for researchers to employ them as promising candidates for lasing,sensing and imaging applications.

EPFL2022