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Specialty optical fibers - revisited Ryszard S.Romaniuk Warsaw University of Technology, Institute of Electronic Systems, Poland ABSTRACT The paper contains description of chosen aspects of analysis and design of tailored optical fibers. By specialty optical fibers we understand here the fibers which have complex construction and which serve for the functional processing of optical signal rather than long distance transmission. Thus, they are called also instrumentation optical fibers. The following issues are considered: transmission properties, transformation of optical signal , fiber characteristics, fiber susceptibility to external reactions. The technology of tailored optical fibers offers a wider choice of the design tools for the fiber itself, and then various devices made from these fiber, than classical technology of communication optical fibers. The consequence is different fiber properties, nonstandard dimensions and different metrological problems. The price to be paid for wider design possibilities are bigger optical losses of these fibers and weaker mechanical properties, and worse chemical stability. These fibers find their applications outside the field of telecommunications. The applications of instrumentation optical fibers combine other techniques apart from the photonics ones like: electronic, chemical and mechatronic. Keywords: optical fibers, specialty fibers, non-telecom optical fibers, instrumentation optical fibers, sensory optical fibers
1. INTRODUCTION The first trials to manufacture low loss optical fibers for instrumentation applications from ultra-pure optical quality glasses were done by classical glass making methods. There were used specially processed glass components. Multicomponent glasses SLS – soda-lime-silica and SBS – soda-boron-silica were melted from the materials purified of transition metals to the level of single ppb (at high cost) using such methods as: ion exchange, electrolysis, recrystallization, extraction of solvents, zone melting. The output glasses obtained in such a way were melted again, clarified, pulled to the shape of a rod and introduced to the fiber pulling system of double crucible tower. The system of continuous fiber pulling consisted of the set of concentric platinum or pure silica glass crucibles. The stream of glass flowed from the upper crucible with the core glass, went through the bottom crucible which was a reservoir of the cladding glass, and was concentrically covered by the latter glass during the passage through the output nozzle in the bottom crucible. Time and temperature of molten glasses in contact is controlled to enable the diffusion of ions which modify the glass for higher or lower refraction and creation of the gradient refractive index in finally pulled optical fiber. Gradient index minimized the intermodal dispersion in multimode optical fiber. Despite its simplicity, there are many difficulties with this technology. Multicomponent glasses have larger internal losses (dissipation and absorption) and worse mechanical properties than pure silica glass. The main issue is with the recontamination of optical fiber glasses during their further processing and fiber pulling process. Recontamination is a source of losses in instrumental optical fibers. The level of impurities increased from the ppb in the substrates to the ppm in the fiber. In order to avoid the recontamination the process was closed in a clean room. Some progress was achieved by control of the valence and degree of oxidation for the impurity elements by keeping the partial pressure of oxygen in the atmosphere above the molten glass and particular oxygenation – reduction conditions inside the molten glass. Absorption in the area of NIR by Fe 3+ and Cu 2+ ions, which are the main two impurities, may be diminished by the change of their valence to Fe 2+ i Cu + , via oxidation. The same mechanism was applied for oxidation of other transition metals, which are constantly present in Multicomponent glasses like: Mn, Ni, Cr, V. The total losses of optical fibers from multicomponent glasses were lowered to a level below 5dB/km for λ=0,9µm. For the same wavelength, a telecom optical fibber has losses lower than 1,5dB/km. Other methods have to be applied to lower the losses of these fibers for longer wavelengths, associated with the transmission windows of 1,3µm and 1,5µm. Kinds of fundamental vibrations of the cation – oxygen set in the amorphous network of glass cause the shift in the long wavelength absorption edge in the direction of shorter wavelengths, in comparison with the pure silica glass, with exception for very heavy glasses. Width of the energy band
gap in light multicomponent glass of small refraction is smaller than in the pure silica glass. The window of transparency for light glasses is narrower from both sides. Contamination with hydroxyl ions OH- is associated with their strong bounding in the structure of multicomponent glass. Improvement of the conditions of glass processing and fiber pulling does not lower the fiber losses considerably in this spectral region. Low-loss optical fibers from multicomponent glasses were definitely shifted to applications outside the telecommunications. Optical fibers for communications are manufactured by CVD synthesis method of pure silica glass from SiCl 4 . Apart from the losses, an additional factor which has decided of telecom or non-telecom applications of optical fibers was how the optical fiber is immune to internal (with the transmitted wave) and external reactions, which may in any case distort the transmission of digital optical signal. It is possible, however, to induce such interactions purposefully, for example for sensing. Now, the optical fiber sensor technology is a very well developed branch of optical fiber photonics. Such sensors are manufactured of telecom optical fibers, and a large variety of specialty optical fibers, alone or with additional components electronic, thermal, magnetic or mechanic. Also optical fiber components, active and passive, are manufactured of telecom and specialty optical fibers. These components include: modulators, polarizers, optical isolators, couplers, switches. There are constructed optical fiber sensors of considerable sensitivities in comparison with the ones made by classical or different techniques. Optical fiber sensors, which base on the changes of the fiber parameters induced by the external measured value, utilize selective properties of the sensitized glass, special dopant, or particular fiber construction, to activate the interaction processes between the fiber and the environment or the fiber and the propagated optical wave. The aim is to efficiently modulate the optical signal proportional to such interaction. A particularly attractive, in the case of optical fibers, are distributed measurements of such fields like strains and stresses, acoustic, magnetic, electrical, thermal, and building of MOEMS matrices. Initially, the sensors and functional components were manufactured of standard telecommunication fibers, mainly of the reason of their standardization and market availability. Telecom fibers have constant environmental sensitivities of their transmission parameters, guaranteed by the vendor. However, an ideal telecom optical fiber should be isotropic and insensitive to environmental reactions. Thus, in principle, its parameters are inappropriate for the design of optical fiber sensors and certain kinds of photonic functional components. Towards the end of eighties of the previous century, one may observe an increased interest in the technology and applications of specialty optical fibers of increased or decreased sensitivities to various measured values and of anisotropic optical fibers too. During this time such sensitized or anisotropic specialty optical fibers were not available commercially. One of the first kinds of specialty optical fibers manufactured by the CVD, were nonlinear optical fibers, birefringent polarization maintaining and single polarization fibers. Polarization maintaining optical fibers are used for building of optical fiber gyroscopes (Sagnac interferometers), and all kinds of other interferometers (in all optical fiber versions) like Michelson, Mach-Zehnder and Fabry-Perot, but also optical coherent transmission. A real, single mode, optical fiber for telecom is not ideally isotropic, due to technology or by the influence of the external conditions (like bending). The fundamental mode (degenerated in the isotropic conditions) consists of two orthogonal components of slightly different propagation constants. Local values of these two propagation constants change accidentally along the fiber, and, thus, the state of polarization of the composite transmitted wave in the fiber changes. The group of polarizing optical fibers includes fibers of different construction: fibers with a macro-hole located near the core and parallel to the core, fibers circularly birefringent, spun fibers, composite metal-glass fibers for Kerr modulators and fiber polarizers, and elliptical core fibers. Pure silica glass has relatively small value, in comparison with other photonic materials, of the nonlinear refractive index, and other coefficients like acousto-optic, magneto-optic, electro-optic. These values may be considerably increased by several main methods like: glass doping with sensitizing agents, like transition ions or rare earth ions, or making the fiber of particular glass which has large value of certain coefficient. Simultaneously the sensitized glass may have minimized other coefficients to make its sensitivity more selective to a single external reaction. The glasses considered for instrumentation optical fibers are: silica multicomponent, low-silica, fluoro-silikates, non-silica, heavy oxide, halogenide, chalcogenide, polymer. The increased losses do not eliminate these glasses totally as the instrumental applications of functional photonics components usually require short lengths of these fibers. The basic application of specialty optical fibers in telecommunications are active ones, with the ability to amplify the transmitted signal, as it passes a single time through the channel. These are ultra low loss high silica optical fibers, very similar to telecom ones, doped with Pr3+, Nd3+, Er3+ and other ions for the wavelengths 1060, 1080, 1300, 1536 nm and
longer, up to 1610 nm. The amplification band is used up to several µm in fluoride glass optical fibers. Active optical fibers build fiber lasers, photonic amplifiers, nonlinear components, but also sensors. Due to low losses of these fibers in the amplification region, additionally compensated by the signal amplification, they may be quite long, of the order of hundreds m or single km. Several configurations of the optical feedback in possible in optical fiber active components: fiber length is itself optical resonator with Fresnel reflection, Bragg grating mirrors or metal deposited mirrors at the faces; fiber length is positioned in external resonant cavity; fiber is configured as a resonant (standing wave) loop cavity. The fluorescence in active optical fibers is used for measuring of a number of physical and chemical values in the function of temperature, pressure, oxygen saturation, etc. In the nonlinear optical fiber, the output signal does not depend linearly on the input optical power for a set wavelength. Optical wave of the power over the nonlinearity threshold interacts with the fiber material causing, for example, a transfer of part of optical power to other wavelengths or causing dissipation (or vice versa, strong focusing). Nonlinear optical fibers are used in DWDM systems. Nonlinear effects are confining the allowed maximum power which is possible for carrying in a singlemode telecom optical fiber. Nonlinear fibers are used in distributed optical telemetry, telecommunications and to build nonlinear photonic functional components and sensors. Summarizing the classification, the tailored optical fibers embrace a lot of various classes of technologically and constructionally designed fibers. These are fibers: with increased resistance to macrobendings and microbendings, or vice versa very sensitive to microbendings; two-mode; conical single and multiple conical; of increased resistance to ionizing radiation; fibers of special covers – metal, hard soft or soft hard; glass ceramic fibers; with Langmuir-Blodgett layer; with sensitizing polymer layers; magnetostrictive, piezoelectric; fluorescence; chemo-optical; optrodes – optodes; fibers of liquid cores; fibers with liquid crystal cores; photonic fibers impregnated with liquid crystal, fibers with liquid cladding; single material fibers; pulled from spun perform; nearly isotropic – of very small birefringence; of complex refractive index; HB type; polarizing, polarization maintaining, with Bragg grating; polymer; twin-core; two-core; hetero-core, multiple core; of helical core; holey, holey with macro holes, with micro-holes; photonic-photonic, photonic-refractive; elliptical; square; of complex cross section of core; doped with rare earth; active; doped with transition glass sensitizing ions; nonlinear; resonant; with photonic meta-material; optical fiber lasers; sensitized or desensitized technologically; D type; of increased mechanical strength; from specialist multicomponent glasses, and many other types. The technology of specialty optical fibers enables design, construction and manufacturing of sensors, photonic functional components and the whole all optical, photonic systems of information processing.
2. ISOTROPIC OPTICAL FIBERS Making an ideal isotropic optical fiber is practically not possible. An isotropic fiber is a theoretical model in order to differentiate from real fibers. The factors which cause anisotropy in an optical fiber are: optical losses and caused by them forward, skew and backward dissipation, fluctuations of dimensions and shape of the core (ellipticity), induced stress by bending, twist and longitudinal stretching, inbuilt and externally induced mechanical stresses of all kinds, fluctuations of refraction, thermal irregularities. A real fiber displays an anisotropy of accidental and systematic distribution along its length. The internal component of anisotropy depends essentially on the method of fiber fabrication. Application of the fiber to measure the electrical values, via the Faraday effect, requires nearly ideal isotropy of the light guide. Lowering of the birefringence in the fiber depends on the precision and advancement of the technology. Lowering of the stresses on the core-cladding boundary requires a choice of glasses of nearly the same thermal expansions in the region of thermal work of the photonic system. For a fiber of small numerical aperture NA≈0,1, lowering the relative polarization delay (retardance), to the value necessary in the Faraday sensor which is 3o/m, requires the core centricity better than 0,05%. Such fibers of ultra-low birefringence were manufactured experimentally as well as commercially. They possessed no birefringence only when straight. In a classical single mode optical fiber, which was not optimized for polarization properties, the phase of optical wave may change accidentally of the order 10π/m.
3. OPTICAL FIBERS OF COMPLEX REFRACTION Construction of a specialty optical fiber of very complex refractive profile requires precise consideration of the diffusion conditions in the technology determined set of coordinates – geometrical, thermal, and ion, during the perform manufacturing and fiber pulling. The modified multi-crucible method allows for fiber manufacturing of gradient and step index profiles, essentially of three groups of complex RIPs: multistep, ring (M, W or double W), and mixed. The process starts with the choice of appropriate pair of glasses, so as to enable mutual exchange of modifying ions. The
best dopant in the core is such that increases the refraction and relatively easy is subject to diffusion. When the distribution of ions is irregular, with the local concentration gradient, then in favorable conditions, there exists an associated and directed stream of diffusion. The condition to generate this diffusion stream is, in a general case, the existence of the gradient of chemical potential. The diffusion process exists only in a strictly confined area, where both glasses are fluid. It is a region between the crucible nozzles. The system is described by the second Fick law, for axial symmetry.
4. RING INDEX OPTICAL FIBERS Ring index optical fibers are characterized by minimal value of the refractive index on the fiber axis. The basic parameters of ring-index fiber are: magnitude of the central refraction dip in the geometric and refractive coordinates, relative thickness and radius of the high index ring against the wavelength λ. The refractive profile is expressed by n(r)=n1 dla a≤r≤b and n2 outside this region. This there are three refraction regions: inside central axial refraction valley (internal cladding), high refraction ring core and external cladding. Actually the central refractive valley is a part of the complex optical core. Let us introduce a parameter η=b/a which is a normalized thickness of the ring index core. We assume ∆≈1-n2/n1