Stimulated Brillouin Scattering: Fundamentals and Applications (Optics and Optoelectronics)
With the addition of two authors who bring 75 years of combined experience in fiber optic sensor technology, this edition is a significant update and an excellent resource for any engineer who has an interest in advanced sensing systems. Introduction 1 Fiber Optic Fundamentals 1. Preface Fiber optic sensor technology is not new, but is continuing to evolve after over 60 years of development and commercialization.
Brillouin Integrated Photonics
The sensing designs are not based on a single concept but on a variety of optical phenomena that can be used to measure a broad range of physical and chemical parameters. In early industrial applications, single point fiber optic sensors were used as an alarm to indicate the absence or presence of an object. As the technology evolved, the functionality increased to accurately determine the position of an object. Many of the sensing concepts that will be discussed throughout this book will be for single point sensors which operate by detecting changes in the intensity of light see Chapters 3, 8, and 9.
They operate by altering the transmitted or reflected light intensity in a manner proportional to the parameter being sensed such as temperature, strain, or displacement position. The sensing functionality can be expanded to monitor electric and magnetic field measurements using polarization concepts. As an example, certain materials exhibit Faraday rotation, which alters the plane of polarization and the resulting transmitted light intensity in the presence of a magnetic field. Polarization-based sensors are discussed in Chapter 7.
Interferometric sensors compare the phase of light in a sensing fiber to a reference fiber. Small phase shifts can be detected with extreme accuracy. This family of sensors has been especially useful in monitoring dynamic strain vibration see Chapter 4.
Also, a Sagnac interferometer is an interferometric sensor configured to be sensitive to rotation Chapter Two examples of successful commercialization of interfermetric-based sensors are hydrophones for submarine detection and fiber optic gyroscopes for advanced navigation systems. Both are for military applications primarily and have performed well for over 30 years with thousands of systems deployed.
Coherent Optics: Fundamentals and Applications
A wavelength or spectral shift is another sensing approach. By introducing coatings on the fiber or a target that fluoresces, under certain conditions usually related to chemical interaction or temperature fluctuation , a chemical reagent can be detected or temperature can be monitored.
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Hasebe, Y. Tadakoshi, H. Numata, N. Wu, K. Wu, Y. Hayashi, H. Raman lines are contributed by the interaction of the lightwave with molecular vibrations in the medium. Both Brillouin and Raman scattering are inelastic scattering because they are associated with some frequency shifts. The last mechanism that can be observed is the Rayleigh wing scattering attributed to fluctuations in the orientation of anisotropic molecules. Raman spectra usually contain many sharp bands with separations between bands corresponding to the electronic vibrations and each bandwidth results from molecular rotation or reorientation excitations.
As long as the input light is scattered without strongly altering the property of the medium, we will say that the scattering is spontaneous, which includes Rayleigh, Brillouin and Raman scattering. When the light intensity increases to a level such that the optical property of the medium is modified, and the scattered light is proportional to the power of the input light, then this regime becomes stimulated. In other words, the evolution from spontaneous to stimulated scattering corresponds to a transition of the medium behavior from a linear to a non-linear regime.
On the microscopic level the molecules making up any ordinary matters are immersed in a violent internal electromagnetic EM environment in spite of the macroscopic charge neutrality as is true for most macroscopic materials. Those violent EM environments are constantly causing the molecule to readjust its electron clouds.
By changing its own electron cloud configuration this molecule is contributing to the changing environment for other neighboring molecules in a perpetual cycle. Therefore on a relatively small spatial scale order of tens of molecular sizes one would observe fluctuations in terms of local charge density, local temperature or even strain values. Without incident light such short range fluctuations would not produce measurable macroscopic effects at a far distance, as they are mutually incoherent and thus cancelled out. In this case the macroscopic EM fields inside any material are zero.
However, with external light incident on a material, this EM field E will reorient the originally incoherent random fluctuating molecular clouds such that there is a tendency to respond collectively the same way on a small spatial scale covering a small fraction of the wavelength of the EM field. Some of the scattered Rayleigh light is re-captured by the waveguide and sent in the backward direction. This backward propagating Rayleigh scattered light has a time delay that can be used for distributed sensing. The Rayleigh scattering can be treated as a single scattering process.
Assuming simple linearity of the polarization for a non-magnetic material like the fiber, we write the electric displacement field vector [ 14 ]:. Using the Maxwell equation we can get following electric field E :. Hence the Maxwell equation can be further simplified:. For the case of optical fiber, we could neglect the lateral dependence and considering only the longitudinal dependence on z.