13/06/2011

Advanced fiber optics

gunnar3000100600580.jpgFew techniques have had an impact on society that can compare to that of fiber optics. These tiny hair-sized glass wires have deeply transformed, at the dawn of this third millennium, access to information, modes of communicate, and even the behavior of consumers. This tremendous impact was rightfully honored by the presentation of the 2009 Nobel Prize in Physics to charles K. Kao, who predicted in 1966, by careful calculations, that optical fibers could transmit a broadband signal over distances outperforming any existing transmission line. Since then, optical fibers have gradually occupied a more and more predominant place in the world of telecommunications, while deeply revolutionizing the field of optics by confining light in these small silica waveguides, transforming classical free-beam optics into a novel concept of wired optical circuits.


Today optical fibers are omnipresent in modern optical systems, so that a solid background in fiber optics is absolutely required by any engineer or scientist who has the ambition to become specialist in the design and the understanding of systems based on light signals. The tremendous development of optical communication has convinced a wide public that optical fibers can do much more than passively transmit a light signal. The research in this field remains extremely active as a consequence of the extraordinary properties of optical fibers, which allow them to confine light within a microscopic-sized surface over kilometric distances. This converts a minute effect into a large interaction that can lead to fantastic transformations on the light. The constant search for new functions entirely carried out within optical fibers has gradually led to the development of special fibers with specific properties, such as rare earth-doped fibers for amplification and laser emission, or photonic crystal fibers offering a larger control of the light confinement and dispersion properties.

An important field of applications of optical fibers is the domain of sensing that is currently experiencing economic growth that is proportionally larger than optical telecommunications. as a consequence of this new importance, the different aspects of fiber sensing are addressed in detail with three distinct chapters. One of the main assets of fiber sensing is its capability to realize distributed sensing, for which the measure of a quantity, such as temperature or strain, can be independently carried out at any point along the fiber. This offers the possibility of instantaneously obtaining a map of the distribution of the measured quantity along the fiber and to advantageously substitute a fiber for thousands of point sensors. Natural scattering processes in the fiber material are judiciously used for this purpose, and chapter 8 describes the most intense and most widely used type of scattering in fibers: rayleigh scattering. More advanced configurations use a weaker inelastic scattering (Brillouin and raman) that turn out to be more sensitive to the environmental quantities and thus more favorable for sensing. Chapter 9 is entirely dedicated to the study of inelastic scattering and their numerous applications ranging from sensing to amplification and the active control of the speed of a light signal (slow light). The possibility to realize sequential changes of the refractive index directly in the fiber core by high-energy illumination, with a periodicity in the range of the light wavelength, has offered a novel class of all-fiber optical devices to the community, designated as fiber Bragg gratings. These important devices are described in chapter 10 and offer a wavelength-selective function that makes excellent filters and very efficient miniature point sensors.

Finally there is a constant search for alternative materials to draw fibers from, which can potentially offer a better susceptibility to nonlinear optical interactions, in order to obtain certain advanced functions more efficiently; or a better sensitivity to the environment for more accurate sensing; or other advantages related to the economics and ease of implementation for particular applications. among these alternative materials, particular attention is currently paid to organic polymers, and chapter 11 addresses all recent progress and potential future applications using polymer optical fibers. chalcogenide and soft glasses are other types of materials that may lead to nonlinear optical effects enhanced by two or three orders of magnitude, while offering a better transparency in the infrared. although presenting their own limitations in terms of optical-power density, and despite difficulties in handling and coupling to standard fibers, these new materials have a promising future to turn optical fibers into an even more efficient and omnipresent tool for advanced photonics…

Optical fibers basics

Unquestionably, silica-based optical fibers are the waveguides of the present and the future. Currently, optical fibers carry the bulk of international and domestic telecommunications, and soon all homes will be connected to the central offices by optical fibers, providing telephone, internet and multi-channel high-definition television services. Optical fibers are revolutionizing distributed and point sensing, while fiber lasers can be made to emit extremely coherent and low noise beams, as well as kilowatts of optical power. Fibers are small in size, light in weight, and being dielectric (siO2), they do not emit electromagnetic radiation, nor are they affected by such radiation. Finally, they are low cost.

While the subject of optical fibers has been covered in great detail by many excellent books, this chapter introduces only the basic physical processes that govern light propagation in these unique waveguides, with emphasis on the characteristics of silica-based (glass) single-mode optical fibers (microstructured and photonic crystal fibers are briefly mentioned towards the end of this chapter. they are discussed in great detail in Chap.3, while plastic fibers are described in Chap.11).

Most common optical fibers comprise a core, a cladding and a jacket (Fig. 1.1). light propagates mainly in the core, where it is trapped, and therefore guided, due to the difference in refractive index between core and cladding. the jacket has two main functions: (i) to absorb all cladding light which is not properly guided; and (ii) to protect the otherwise naked glass from atmospheric interactions, which can compromise the mechanical strength of the fiber. Jacket materials include: simple acrylate, easy-to-strip coating (normally 250 micron in diameter), a bit thinner (∼140 micron) polyimide coating for high temperature (180°C) applications, including embedding in composite structures, as well as more exotic compounds such as carbon or aluminum for special applications…

Polarization effects in optical fibers

After defining the notion of polarization state, the most important mathematical formalisms (Jones and Stokes formalisms) used to describe the polarization effects in materials are introduced.

Single-mode optical fibers which enable the propagation of only one transversal mode (the lp01 mode) in fact guide two polarization modes. these modes are degenerate if the circular symmetry of the fiber is perfect. in practice, perfect circular symmetry cannot be realized, the two modes are no longer degenerate and are characterized by different propagation constants, which leads to the phenomenon of birefringence. the random nature of the asymmetry distribution along the fiber leads to another important polarization feature: polarization mode coupling, which is related to the variation of the birefringence properties along the fiber length. after explaining the notions of birefringence and polarization mode coupling, a model of the polarization properties of an optical fiber is presented. the combined effect of birefringence and polarization mode coupling results in a dispersion phenomenon (broadening of the optical pulses launched in the fiber): polarization mode dispersion. this important feature for optical communication systems is described in the last part of this chapter.

Polarized and unpolarized light

In homogeneous media, as well as in optical fibers under the quite practical condition of the weakly guiding approximation, light is transversely polarized so that the electric and magnetic fields are perpendicular to the direction of propagation. to describe the polarization properties of light, the electric field is commonly chosen since it is the key physical quantity when studying light-matter interactions.

Defining the polarization state of an optical wave consists in answering the following question: how does the electric field vector vary with time at a fixed position in space? the state of polarization is indeed the shape drawn by the tip of the electric field vector as a function of time at a given location.

If the light source emits a monochromatic wave (a single frequency), the light is fully polarized. however, in many cases it is not possible to clearly define a polarization state for an optical wave: the electric field vector can randomly vary in the transverse plane and the tip does not describe a well defined pattern. in these cases, the light is said to be unpolarized. this is typically the case of a broadband light such as sunlight: the polarization behavior of each frequency is different and it is therefore not possible to deduce a well defined polarization state. between the cases of unpolarized and polarized light we find partially polarized light, for which the electric field vector is still characterized by random orientations but stays around a particular polarization state. this is typically the case when a source emits a non-monochromatic wave with a relatively small but non-zero spectral width…

Photonic crystal fibers

The photonic crystal fiber, a new paradigm in fiber optics, was devised by P. st. J. Russell almost twenty years ago. The propagation of light beams in air for kilometers, the emission of multi-kilowatt continuous wave radiation in a single transverse mode, the generation of octave-spanning light continuum or frequency combs are some breakthroughs brought by researchers in the field of photonic crystal fibers (PCf). The power of the photonic crystal fiber concept resides in its versatility in terms of morphology, application and material used. As a consequence, this phrase includes so many kinds of optical fibers that this chapter cannot give an exhaustive overview. it is possible, however, to derive two generic classes of PCfs according to the mechanism on which propagation relies.

As mentioned above, PCfs allow light to be guided in air along kilometric lengths. This outstanding result is quite astonishing if we remember the classical fiber optics textbooks in which the very first chapters, in general devoted to the snellDescartes law and total internal reflection, tell us that the core refractive index must be higher than that of the cladding for the fiber to efficiently guide light. obviously, low-loss1 guiding in an air-core, with index close to unity, violates this iron law. for this law to be circumvented, P. st. J. Russell, as early as 1991, devised a novel and very elegant guidance mechanism in optical fibers using solid-state physics and the concept of photonic bandgap.

In a single atom, only discrete energy levels are allowed for the electrons. When two atoms are close enough, and according to Pauli’s Exclusion Principle, their outermost electrons (the so-called valence electrons with highest energy and therefore weakest bond to the atom) must orbit with very slightly different energies. instead of a single discrete energy level, we are left with two levels very close to each other. in a semiconductor material the periodicity of the semiconductor atoms (tens of angstroms), set at the nodes of a network with a period of the order of the atom radius, is responsible for the existence of the energy distribution in the form of bands. Between two allowed bands, which originate from two discrete energy levels through Pauli’s Exclusion Principle, lies a forbidden band: the bandgap.

back to optics, the periodic variable is the refractive index, the period is of the order of the optical wavelength (micron), and, although a material with a periodic dielectric permittivity does exist in nature2, a photonic bandgap is generally obtained in an engineered photonic material. Thus, a photonic material periodic along three dimensions does not allow propagation of photons with energy or wavelength within the bandgap. if a defect is put in the periodic material and a source of photons, whose wavelength lies within the bandgap, is set in the defect, the photons’ propagation through the material is forbidden: photons are trapped in the defect. in a waveguide, an optical fiber for instance, the material must be periodic in the transverse plane and invariant along the third dimension, the propagation axis. The defect is the fiber core, while the periodic material is the fiber cladding. obtaining a full bandgap, and therefore a low level of confinement loss, was theoretically demonstrated in 1995 by T. A. birks et al. in an air-silica fiber. The periodic material consists of a silica background in which air holes are embedded, while the defect could be a larger air hole. since then, huge efforts were made to manufacture such a hollow-core photonic bandgap fiber (HC-PbGf). The first demonstration of photonic bandgap guidance in optical fiber, one of the two guidance mechanisms according to which the various kinds of PCfs are classified, was reported by J. C. Knight et al. in 1998, rapidly followed by the very first propagation of light in air-core PCf. since then, HC-PbGfs have been used in many applications in various fields of applied physics as well as fundamental research.

Meanwhile, researchers realized that surrounding a solid core by a photonic crystal cladding might be interesting in many respects. As the surrounding air holes are embedded in a glassy material, in most cases silica, the average index of the PCf cladding is lower than that of the solid, all-glass, core, thereby allowing guidance in the core by total internal reflection. This is the second guidance mechanism at work in PCf, radically different from the first one. The heterogeneous nature of the cladding material allows some previously unattainable features to be achieved in index-guiding PCfs. for instance, the fiber reported in exhibits a single transverse mode from the blue region of the spectrum (458 nm) up to 1550 nm, which is almost impossible to obtain in a standard optical fiber. These investigations opened the way to kilowatt-class continuous-wave fiber lasers as well as yielding octave-spanning light continuum.

To sum up, two main classes of PCfs, photonic-bandgap-guiding and indexguiding PCfs, may be derived. in the following, their particularities are explained, keeping in mind the underlying physics. some outstanding applications are depicted. Although the first good was to create photonic bandgap guiding fibers, the underlying physics is somewhat hard to understand. Thus this chapter begins with the description of index-guiding fibers as well as reviewing basic definitions. The second chapter is devoted to photonic bandgap-guiding fibers…

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