Microstructured optical fibers - fundamentals and applications

Microstructured optical fibers - fundamentals and applications

(Parte 1 de 5)

Microstructured Optical Fibers—Fundamentals and Applications

Jesper Lægsgaardw and Anders Bjarklev Research Center COM, Technical University of Denmark, DK-2800 Kgs., Lyngby, Denmark

In recent years optical fibers having a complex microstructure in the transverse plane have attracted much attention from both researchers and industry. Such fibers can either guide light through total internal reflection or the photonic bandgap effect. Among the many unique applications offered by these fibers are mode guidance in air, highly flexible dispersion engineering, and the use of very heterogeneous material combinations. In this paper, we review the different types and applications of microstructured optical fibers, with particular emphasis on recent advances in the field.

I. Introduction

CURRENTLY, microstructured optical (sometimes termed photonic crystal) waveguides are being widely explored in inte- grated optics as well as in fiber applications,1–4 and it is fair to say that it is one of the most active areas of research within the whole field of optics. It must be noted that whereas planar microstructured waveguides, which are typically considered for integrated optics, have the attractive potential of providing near loss-less transmission of light around sharp bends, and, therefore, may be the key to large-scale integrated photonics,5 microstructured optical fibers (MOFs) may, through a lifting of the usual core-cladding index requirements and restrictions, exhibit radically new properties. This may potentially push the conventional limits for e.g., high-power laser deliverance, spectral location of transmission windows, and data-transmission speeds. In contrast to the planar structures, microstructured waveguides of practically infinite vertical extent may readily be fabricated in fiber forms using traditional fiber manufacturing techniques with only modest modifications.6 This makes optical fibers appear to be the most mature technology today for exploring microstructured waveguides operating at optical wavelengths.

Optical fibers, in which air holes are introduced in the cladding region and extended in the axial direction of the fiber, have actually been known, since the early days of silica waveguide research.7 This early work demonstrated the first low-loss singlematerial fibers made entirely from silica, having very small silica cores held by thin bridges of silica in air. Because of the tech- nological development of doped-silica fibers—primarily within the area of optical communications—the so-called ‘‘single-material’’ (or more accurately ‘‘air-silica’’) fibers were not developed much further over a number of years. However, in 1996, Russell and co-workers demonstrated fibers having a microstructured cladding.8 Thus, the field of MOFs was founded, and the realization of microstructured silica fibers with a periodic array of several hundred air holes running down their length6,9 was demonstrated. The initial aim was to realize fibers that would be able to guide light using photonic bandgap effects.10 However, it was realized early that the operation of the first fibers was not based on the photonic bandgap effect, but a type of modified index guidance through a higher refractive index of the core region compared with the effective index of the cladding structure.1 Despite the resemblance with conventional optical fibers, these so-called index guiding MOFs were, however, found to show significant differences, and rapidly became the subject of significant research activities.1–14 While progress was made rapidly for index guiding MOFs, the realization of a truly photonic bandgap-guiding (PBG) fiber was initially hindered by the difficulties in realizing triangular photonic crystal cladding structures with sufficiently large air holes at the required dimensions. However, after a few years of intensive process improvements, together with the development of a much better understanding of the limiting properties, PBG fibers are at present easily fabricated and available on the commercial market.

The use of microstructuring in optical fibers has led to new developments in various areas of fiber applications, and it is interesting that each of these areas actually takes advantage of different aspects of the enhanced physical performance enabled by the location of microstructures in the fibers. The development of microstructured fibers has, from the initial stages, been divided among different players. On the one hand, we have experienced a strong driving force from academia searching for new exciting waveguiding principles, and on the other, large companies such as Lucent Technologies, Corning, and NTT have focused parts of their resources on new classes of specialty fibers represented by microstructured fibers. The academic activities over the past 5 years have led to establishment of ambitious start-up companies such as Crystal Fibres A/S and Blaze Photonics Ltd. (now a part of Crystal Fibres A/S, Bath, UK) manufacturing many new fiber products for the research market and establishing a strong patent portfolio. Given the presence of such different players, at present, it is difficult to predict which kind of enterprise will eventually dominate the market for microstructured fibers.

In this paper, we will begin by giving an overview of MOF fabrication issues, with a particular focus on silica MOFs,

Journal

Feature

J. Ballato—contributing editor

Jesper Laegsgaard is financially supported by the Danish Technical Research Council (STVF).Author to whom correspondence should be addressed. e-mail: jl@com.dtu.dk

Manuscript No. 20393. Received April 8, 2005; approved September 14, 2005.

although other base materials will also be discussed briefly. Subsequently, we will discuss some of the most interesting classes of MOFs, and present the specific properties and possible applications of the different designs. One of the highly interesting possibilities of the microstructured fibers lies in the ability to confine light in a very small cross-section area through the use of a high-index contrast between holes and glass. This is used in the so-called highly non-linear MOFs, HNL-MOFs. One might say that the opposite possibility is explored in the case of so-called large-mode-area MOFs, LMA-MOFs, which shift the non-linear limitations to higher power levels by spreading light to larger areas than possible in conventional fibers. The possibility of microstructuring does, however, hold even further possibilities, and one is to obtain very high numerical apertures. When these possibilities are combined in an optimal manner, one of the most promising applications of microstructured fibers lies in the rapidly developing area of high-power fiber lasers. As representatives of the many new technological possibilities offered by microstructured hollow fibers, we will finally present the interesting possibilities of fiber devices with filled holes. One class of such fiber applications are the microstructured fiber sensors, and another is the actively controlled liquid-crystal-filled microstructured fiber devices.

I. Production of Microstructured Fibers

(1) Fabrication of MOF Preforms

The traditional method of manufacturing optical fibers involves two main steps: fabrication of a fiber preform and drawing of this using a high-temperature furnace in a tower set up.15 For conventional silica-based optical fibers, both techniques, which have been developed over the past two-to-three decades may today be considered very mature (see e.g., Kao16). Various vapor deposition techniques have been developed for the fabrication of fiber preforms (including, among others, modified chemical vapor deposition (MCVD), vapor axial deposition (VAD), and outside vapor deposition (OVD)17). These techniques allow for fabrication of preforms with silica glasses having very low, unintentional impurity levels, and very precisely controlled doping levels. A common characteristic of these perform fabrication techniques is, however, that they are tailored for fabrication of circular-symmetric preforms, and although a very accurate control over the deposition is obtainable, this may only be achieved in radial direction without significant modifications of the methods. Therefore, for fabrication of preforms for microstructured optical fibers, where the desired preform structure is not circular symmetric, the above-mentioned techniques are not directly applicable. Preforms for conventional optical fibers having a non-circular symmetry have, however, been fabricated for several years, e.g., for polarization-preserving fibers, where typically a mechanical post-processing of the circular preforms is performed. This may be done, e.g., by drilling a very limited number of holes down the fiber perform and feeding additional material components into these holes (and provide, e.g., stress-induced refractive index changes in the core region) or milling/cutting the outer surface of the preform to provide an outer asymmetry that, during drawing, gives rise to an asymmetric core.18

Althoughdrilling of several tens to hundredsof holes in a periodic arrangement into one final preform may be utilized for fabrication of MOFs, a different—andrelativelysimple—method has become the preferred method of fabrication over the past years. The method, commonly known as the stack-and-draw procedure, typically involves hand stacking of silica capillary tubes and solid silica rods into the desired pattern. After stacking, the capillaries and rods may be held together by thin wires and fused together during an intermediate drawing process, where the preform is drawn into preform canes. Such an intermediate step may be introduced to provide a large number of preformcanes for the development and optimization of the later drawing of the PCFs to their final dimensions. During the draw- ing process, the outer lying tubes/rods may experience some distortion, but the coreregionand its nearestsurroundings generally retain the desired morphology to a large degree. This technique allows relatively fast, clean, low-cost, and flexible preform manufacture.This method was firstpresentedby Birkset al.19 in 1996, who described the fabricationof a 2D photonic crystal cladding structureusing a method where a central hole was drilled into a silica rod, which was subsequently polishedto have a hexagonal outer shape, to facilitatecompact stacking.Recently,the method has been developed further (or simplified) in the sense that the fiber preforms are often manufactured by directstackingof silica tubes and rods with a circular outer shape (i.e., the hexagonal milling of the rods has been omitted).The small interstitial holes appearing between different rods can be eliminated during the initial fusion and/or drawing of the preform.

Many different fiber structures may, in principle, be obtained using the stacking of rods and tubes with equal outer dimensions. As a stack of tubes naturally aligns in a close-packed arrangement, the triangular close-packed hole structure, the honeycomb structure, or the Kagome structure20 are among the easiest to obtain, and in fact, most MOFs fabricated have a triangular lattice of airholes in the cladding. However, by using a suitable stacking pattern of tubes and solid rods quadratic structures, or ring geometries, like the one shown in Fig. 1, may also be fabricated using the hand-stacking technique.

An interesting alternative to hand stacking, described by Bise et al.,21 is the sol–gel casting technique, in which a silica gel is

Fig.1. A ring-structured microstructured optical fiber fabricated by the hand-stacking technique. In the figure on the right, some length scales are shown. The bridges between individual airholes are down to 50 nm wide. The structural tolerances are about 720 nm. Both pictures have been kindly provided by Crystal Fibre A/S.

January 2006 Microstructured Optical Fibers 3 formed within a mold containing an array of mandrel elements in the desired pattern. Removal of the mandrels leaves airholes in the gel, which may be retained during drying and sintering, so that a useful MOF preform is finally obtained. The possibility of fabricating a variety of MOF geometries with this method has already been demonstrated, but more data on losses, tolerances, etc. will be needed for a detailed comparison with the stack-anddraw method.

(2) Drawing of Microstructured Fibers

The drawing of MOFs is generally performed in a conventional drawing tower operating at a relatively low temperature of around 19001C. For comparison, conventional optical fibers are typically drawn at temperatures around 21001C. The reason for drawing MOFs at a lower temperature level is that surface tension otherwise may be found to lead to collapse of the air holes. To control the size of the airholes, a slight overpressure relative to the surroundings is often applied to the inside of the preform during the drawing process. Also, the drawing speed can influence the size of the holes—Eom et al.2 reported that as the drawing time increased, increased opening of air holes was seen, a result that is in line with the theoretical and experimental studies done by Fitt et al.23 This indicates that time dynamics, temperature, and pressure variations become significant properties in the accurate control of MOFs. It also shows that the final MOF structure need not be identical to that of the preform—a number of ‘‘handles’’ for structural control are available during the final drawing step.

To fully reap the benefits of the increased design flexibility offered by MOFs, a key objective in the drawing process is to maintain the highly regular structure of the preform all the way down to fiber dimensions. The typical outer fiber dimensions of ‘‘state-of-the-art’’ PCFs are 80–200 m micron diameters. The drawing transition from preform to fiber dimension is illustrated in Fig. 2(a).

An example of the cross section of an index-guiding PCF fabricated under accurate control of the drawing conditions is shown in Fig. 2(b). We note that the scanning electron micrograph in Fig. 2(b) demonstrates a highly regular distribution of cladding holes in a triangular pattern. It is noteworthy, however, that the presence of the core (consisting of a missing airhole) leads to a distortion of the nearest airholes, a phenomenon that is most pronounced when the airholes are large. This effect is also visible in the twin-core structure shown in Fig. 3(a). The fabrication of multi-core structures is one of the very interesting possibilities offered by MOF technology. In Fig. 3(b), a 37-core MOF fabricated by Crystal Fibre A/S in collaboration with QinetiQ Ltd. (Great Malvern, UK) is shown. It is evident that very large and complex structures may be fabricated with a high degree of regularity using the hand-stacking procedure. It should be noted that numerous possibilities appear in the application of such designs, as the required number of cores may readily be added, and by controlling the hole dimensions and the core spacing, coupling of optical power between the individual cores may be enhanced or reduced depending on the specific application.

(Parte 1 de 5)

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