Brillouin dynamic gratings in optical fibres for distributed sensing and advanced optical signal processing

Andrey Denisov
EPFL, 2015
EPFL thesis

Abstract

This thesis presents results of a research on applications of Brillouin dynamic gratings - distributed reflectors that can be dynamically created in an optical fibre by two optical waves. A basic theory of stimulated Brillouin scattering (SBS) is introduced, on a level enough for understanding the processes that govern SBS. A major part of this thesis is dedicated to studies of distributed Brillouin sensors based on phase correlation. First, the concept of correlation-based sensors is introduced - the commonly used Brillouin optical correlation-domain analysis (BOCDA) and the phase-correlation technique. It is described how the Brillouin interaction between two waves can be localised, creating a permanent reflector confined to a centimetre-scale section of the fibre. This allows creating a distributed sensing system with very a high spatial resolution. A detailed theoretical model for the phase-correlation technique is presented, showing how the gain response of a system can be calculated and how the system resolution can be determined. For an ideal case an analytical solution is derived, while for real experimental conditions the expected behaviour is found via numerical simulation. Results of numerical modelling are compared with experimentally obtained data, showing a good agreement. A spatial resolution of 1 cm is demonstrated over a 200 m distance representing 20000 separate points. The concept of time gating is introduced, extending the measurement distance from 200 m to 17.5 km while retaining a sub-centimetre spatial resolution. This technique allows for a two order of magnitude increase in the number of points that a system is capable to resolve. An absolute record for distributed fibre sensors is achieved, demonstrating a system capable to resolve 2100000 separate points. Limitations for a further increase in number of points are discussed as well as possible ways to overcome them. An issue related to the temperature dependency of the refractive index is discussed in details, since it can lead to significant errors in spatial accuracy. An algorithm is presented that is capable of using the measured temperature change to account for the change in the refractive index and correctly determine the positioning of measured data. In the last chapter potential applications of Bragg dynamic gratings in signal processing are investigated. A theoretical model of BDG's in polarisation-maintaining fibres is presented capable of calculating reflection of a probe wave - continuous or pulsed. The model includes the case of non-uniform birefringence along the fibre. Dynamic gratings are applied to create a flip-flop - an all-optical memory, turned on and off by a light pulse. A working system is demonstrated in a 1 m long fibre, corresponding to a 10 ns storage time. Using the theoretical model the birefringence variation is measured along the fibre. A preliminary study of spectral properties of dynamic gratings is presented, along with a model predicting the spectrum of a uniform BDG. It is demonstrated that the spectral properties of a BDG can be manipulated by changing the spectra of optical waves used in the generation process. This work is concluded by a discussion, summing up the above-mentioned theoretical the experimental work. Potential applications of the presented research are proposed along with the most promising direction of further research activities.

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Andrey Denisov
EPFL, 2015
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/212806?ln=en

About this result

Related concepts (24)

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Related publications (59)

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Since the invention of optical fiber cable data transmission using light has become ubiquitous in the past years. Nonlinear optical effects, which are hardly noticed in our daily life, have become crucial and highly influential for the design and performance of advance high-capacity systems. Future optical systems will definitely require signal processing operations to be performed on data signals “on the fly” solely in the optical domain. Brillouin scattering is currently one of the most prominent nonlinear effects in optical fibers, as it has found many applications in various fields of photonics, such as slow light, new high coherent sources, filters, optical fiber sensing, etc. In this thesis we investigate a novel phenomenon based on Stimulated Brillouin Scattering, which is called dynamic Brillouin grating (DBG). Investigating the process of DBG generation in optical fibers we propose several new approaches to realize new tools for signal processing of the optical signal by using phonon-photon interactions. First an analytical model is proposed to describe localized DBG generation in optical fibers using three coupled equations for Stimulated Brillouin Scattering. Based on this model we can show how the grating is generated and which shape it has in time when two pump pulses are used with optical frequency difference equal to the Brillouin shift of the fiber. Moreover optical phase conjugation is also theoretically predicted for the optical wave reflected from DBG. Several applications of Dynamic Brillouin grating are identified and investigated. An optical signal delay line or buffer is proposed and experimentally demonstrated using DBG principle. Storage of an input optical pulse is achieved by changing the position of the localized DBG inside the fiber. Here DBG acts as a Bragg reflector for the optical pulse and change in DBG position defines the time-of-flight of the input and reflected pulse. Single pulse delays can be dynamically controlled with a buffer capacity ranging from several picoseconds to more than one microsecond. Such delays show that DBG based signal delay lines can easily outperform slow light based delay lines in terms of delay-bandwidth product. Optical delay of pulse trains is also achieved, but with maximum limited amount of delay governed by the acoustic wave decay constant, which in standard silica fibers is around 10-12 ns. Moreover microwave photonic signals can be also delayed using DBG demonstrating true time delay. In this configuration, the reflection bandwidth of the DBG defines the maximum operational radio frequency. Relation of the DBG bandwidth as a function of its length was measured and it was determined that DBG exhibits similar properties as weak uniform fiber Bragg grating (FBG). Another application of the DBG gratings is demonstrated by exploiting this phenomenon to realize microwave photonic filters. For the first time, two-, three- and four-tap microwave photonic filter configurations are realized based on DBG. Free spectral range and IV tcartsbA stop band central frequencies of the proposed microwave filters can be easily tuned by adjusting the properties and positions of DBGs. Dynamic gratings generated by amplitude modulated pump waves are used to realize optical differentiation, integration and time reversal of input optical signals. Performed experiments demonstrate that a DBG based signal processor performs operations on the complex amplitude of the input optical wave. The DBG based integrator offers high flexibility in terms of operational wavelength and variable integration time, which can be controlled by changing the DBG length. For the first time, new operations of true time reversal of several bit sequences are performed using dynamic Brillouin grating. To overcome the problem of the decay of the localized acoustic grating, a novel method based on phase modulation by pseudo-random bit sequence (PRBS) is proposed, which allows generation of localized and stationary DBG at random positions inside the fiber. Such a method opens up new possibilities for signal control, and for optical fiber sensing. Advantages and limitations of the PRBS technique in terms of signal-to-noise ratio are discussed. It is experimentally verified that PRBS generation of DBG is highly attractive for the operations on optical signals where time averaging is possible, such as in the fields of microwave photonics and distributed fiber sensing. On the other hand, it was experimentally verified that realization of the optical buffer for continuous digital data using the PRBS technique is difficult due to the substantial amount of instantaneous noise. A separate chapter of this thesis is devoted to the application of DBG to the field of distributed fiber sensing. A DBG based distributed temperature and stress sensor in polarization maintaining fibers has been realized, showing 1 cm spatial resolution, which is - to the best of our knowledge - first time, demonstrated using a pulsed-based Brillouin sensor. The demonstration is performed in a 20 m fiber, but the sensing range of the proposed DBG based sensor can be expanded up to several hundred meters. In addition, it is shown that the demonstrated spatial resolution is not the ultimate limit, and a finer spatial resolution can be achieved by simply reducing the probe pulse duration. Another Brillouin distributed temperature and stress sensor is proposed based on the DBG principle. Here DBG is generated by modulating the phases of a continuous Brillouin pump and probe waves by common PRBS with unipolar encoding and modulation speeds faster than phonon lifetime. By choosing the proper parameters of PRBS, only one localized and permanent dynamic Brillouin grating is placed inside the sensing fiber. Distributed measurements of temperature or strain are accomplished by scanning the position of the DBG inside the fiber and determining the Brillouin resonance condition at each position. Using this technique, a random access sensor with 1 cm spatial resolution is demonstrated over a 40 m standard single mode fiber. In the proposed scheme the spatial resolution can be easily changed by changing the bit duration of the PRBS, while the measurement range is controlled by the length of the PRBS sequence.

EPFL2013

Modulation instability (MI) as the main limit to the sensing distance of distributed fiber sensors is thoroughly investigated in this thesis in order to obtain a model for predicting its characteristics and alleviating its effects. Starting from Maxwell's equations in optical fibers, the nonlinear Schrödinger equation (NLSE) describing the propagation of wave envelope in nonlinear dispersive media is derived. As the main tool for analyzing modulation instability, the NLSE is numerically evaluated using the split-step Fourier method and its analytical closed-form solutions such as solitons are utilized to validate the numerical algorithms. As the direct consequence of the NLSE, self-phase modulation is utilized to measure the nonlinear coefficient of optical fibers via a self-aligned interferometer. The modulation instability gain is obtained by applying a linear stability analysis to the NLSE assuming a white background noise as the seeding for the nonlinear interaction. The MI gain spectrum is expressed by hyperbolic functions for lossless fibers and by Bessel functions with complex orders for fibers with attenuation. An approximate gain spectrum is presented for lossy fibers based on the gain in lossless optical fibers. The accuracy of the analytical results and approximate formulas is evaluated by performing Monte Carlo simulations on the NLSE. The impact of background noise on the onset and evolution of modulation instability is analytically investigated and experimentally demonstrated. Power depletion due to the nonlinear process of modulation instability is modeled by integrating its gain spectrum using Laplace's method. Based on that, a critical power for MI is proposed by introducing the notion of depletion ratio. The model is verified by numerical simulation and experimental measurement. An optimal input power for distributed fiber sensors is proposed to maximize the output optical power and thus, the far end signal-to-noise ratio. Furthermore, the recurrence phenomenon of Fermi-Pasta-Ulam is experimentally observed and numerically simulated, validating the utilized numerical techniques. A standard Brillouin optical time-domain analyser serves as the experimental test bench for the proposed model. As the physical phenomenon behind the experiment, stimulated Brillouin scattering is described based on a pump-probe interaction mechanism through an acoustic wave. A 25 km single-mode fiber is employed as a nonlinear medium with anomalous dispersion at the pump wavelength 1550 nm. The evolution of pump power propagating along the fiber is mapped using the Brillouin interaction with the probe lightwave. The measured longitudinal power traces are processed to extract the impact of MI on the pump power. It is experimentally demonstrated that distributed fiber sensors with orthogonally-polarized pumps suffer less from modulation instability. As the scalar modulation instability of each pump reduces, vector modulation instability occurs because of interaction between the pumps; however, the overall performance improves. A version of the coupled nonlinear Schrödinger equations known as the Manakov system is shown to describe the behavior of two-pump distributed fiber sensors employing optical fibers with random birefringence. The excellent agreement between the experimental and numerical results indicates that the performance limit of two-pump distributed fiber sensors is determined by polarization modulation instability.

EPFL2016

Developed societies require more and more information for their safety and for their economic development. Recent disasters due to landslides, fires in tunnels and collapses of bridges are a source of serious concern on the part of the public which seeks for more safety and for an efficient prevention of these frequent recurrences of dangers. Sensing in adverse environment and extreme conditions requires dedicated techniques. Quite recent technologies may offer novel valuable solutions and give rise to a strenuous research effort. Optical technologies are an essential actor owing to their tremendous capability to transmit and process a high density of information. Optical fibre is a key component for these technologies and its potential for optical processing and for collecting information as sensing probe is still widely unexplored. The development and the popularity of telecommunications have entirely screened the fact that optical fibres may be efficiently used for sensing purposes. In this paper we present applications of a novel technique using optical fibres to monitor large structures for safety purpose. The fibre is used as sensing element and can provide distributed measurements of quantities like temperature or strain. In other words a value of temperature and/or strain can be obtained for any point along the fibre. This is made possible by using a nonlinear optical effect in the fibre called Stimulated Brillouin Scattering (SBS). Optical fibre sensors based on stimulated Brillouin scattering have now clearly demonstrated their excellent capability for long-range distributed strain and temperature measurements. The Brillouin interaction causes the coupling between optical and acoustic waves when a resonance condition is fulfilled. It turns out that this resonance condition is strain and temperature-dependent, so that determining the resonance frequency directly provides a measure of temperature or strain. The resonance frequency is an intrinsic property of the material that may be observed in any silica fibre. This is very attractive since the bare fibre itself acts as sensing element without any special processing or preparation on the fibre. Standard optical cables may thus be used, resulting in a low-cost sensing element that may be left in the structure. Since the optical effect only depends on the fibre material, it is absolutely stable in time and independent of the instrument. Different measurements performed over a long-term period are thus fully comparable. The latest developments in this class of sensors will be shown, such as the possibility to measure with a spatial resolution of 10 cm and below, while preserving the full accuracy on the determination of temperature and strain. Illustrative examples of site measurements will be presented and future prospects will be discussed.

Materials Research Society of Singapore2009

EPFL2013

EPFL2016