Silica-air double-clad optical fiber

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 8, AUGUST 2000,

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Silica–Air Double-Clad Optical Fiber V. A. Kozlov, J. Hernández-Cordero, R. L. Shubochkin, A. L. G. Carter, and T. F. Morse

Abstract—A new design of the silica–air double-clad optical fiber is proposed. The fiber was prepared from an MCVD preform inserted into a supporting silica tube with a fluorine-doped con3) nective element with a lower refractive index ( and air as the second cladding. A numerical aperture as high as 0.3 was measured for the first cladding and for fiber lengths up to 50 cm. Although it is realized that the results are preliminary, the optimization of the fiber parameters promises a new all-glass design for double- clad optical fibers for high-power fiber laser applications.

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Index Terms—Optical device fabrication, optical fiber applications , optical fiber cladding, optical fiber lasers, optical fiber materials, optical fibers.

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INGLE-MATERIAL low-loss single- and multimode optical fibers were first fabricated from pure fused silica in the early 1970’s [1]. Light was guided into a core of arbitrary shape which was supported by spoke-like membranes within a protective tubing. These fibers contained air regions in the cladding along the fiber axis and the fiber core numerical aperture (NA) was determined by the air-to-glass ratio, which can be significantly larger than that for doped all-glass fibers. During the past few years, silica–air optical fibers have attracted attention because of the great interest in the development of photonic crystal or holey fibers [2], [3], and silica–air microstructured fibers [4] that possess unique characteristics which are not possible with conventional optical fibers. The fabrication methods are similar for all fiber types. Fiber preforms are comprised of silica tubes and rods bundled in a specific arrangement to obtain the desired air structure in the fiber. Preforms can be drawn without collapsing the tubes. In this letter, we present preliminary experimental results of a silica–air double-clad optical fiber. Single-mode double-clad silica fibers doped with rare-earth ions are the most attractive design for high-power diode-pumped fiber lasers. The typical double-clad active fiber consists of a doped single-mode core, a silica cladding with a diameter of several hundred micrometers, and a second cladding of silicone resin or polymer with a refractive index lower than that of pure silica [Fig. 1(a)]. The NA between the first and second cladding is in the range of 0.35–0.47 depending on the refractive index of the coating material. The first cladding is used to couple pump radiation from low-brightness pump sources such as high-power diode lasers, diode laser bars/arrays, or fiber-cabled diode lasers (which are the most efManuscript received May 11, 1999; revised February 14, 2000. This work was supported by the Air Force Office of Scientific Research under Grant FQ8671-9900712. The work of J. Hernández-Cordero was supported by DGAPA-UNAM, México. The authors are with the Laboratory for Lightwave Technology, Department of Electrical and Computer Engineering, Boston University, Boston, MA 02215 USA. Publisher Item Identifier S 1041-1135(00)06277-7.

fective for such fiber laser applications) into the double-clad fiber geometry. The tradeoff for the relatively high coupling efficiency for multimode pump radiation that this type of geometry provides is a small overlap between the pump modes and the fundamental mode of the active core. To increase the absorption of the pump radiation (i.e., to increase the efficiency of the double-clad fiber lasers), fiber lengths on the order of tens of meters are used. Two practical approaches have been made to increase the overlapping of the pump radiation and the lasing mode: the shift of the single-mode core from the center of the circular cladding [5] and the use of a rectangular-shaped first cladding [6]. This type of arrangement has established a record brightness for fiber lasers to date [7], [8]. Raising the output power of diode-pumped double-clad fiber lasers requires an increase in pump power or an increase in NA. For diode laser sources with a fiber output, this implies an increase of the fiber core diameter and/or NA, which means that for a fiber laser with a double-clad geometry, the diameter of the first cladding and/or its NA must be increased. An increase of the NA is preferred because the core/cladding area ratio will remain constant. From another point of view, the material of the second cladding influences not only the NA, but it also affects the reliability of the double-clad fiber because any strong pump radiation that inadvertently interacts with the polymer outer cladding will destroy the fiber laser. The cross section of the fiber we consider is shown in Fig. 1(b). The main innovation is to use air or an inert gas as the second cladding of the fiber to produce a large NA. This is accomplished in the following manner. The first cladding and the doped core of the fiber are confined within a supporting tube by means of a connection element, which must have a lower refractive index than that of the first cladding. If the refractive between this element and the first cladding index difference is sufficiently high and the contact area between their surfaces is small, the effective NA of the first cladding will be larger than the one obtained when a polymer is used as a second cladding. Other advantages of an all-glass fiber are its reliability and its higher resistance to hazardous environmental conditions. A preliminary sample of the proposed fiber was fabricated using a standard multimode MCVD germanium-doped preform . A fluorine-doped silica rod with a reduced with ) was used as a connective refractive index ( element. This rod was softened, wrapped around a preform, and fused to the surface in a spiral with a pitch of approximately 5 mm. The preform was then inserted into a supporting silica tube and was fused to the inner wall surface. The resulting preform had a “candy cane” appearance. When drawn into fiber, the helix pitch increased to 20–30 cm. A schematic of the cross section is shown in Fig. 1(b). The drawn fiber cross section is shown in Fig. 2. The fibers had an outer diameter in the range of

1041–1135/00$10.00 © 2000 IEEE

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 8, AUGUST 2000,

Fig. 1. Cross section of double-cladding optical fiber (a) with a polymer as a second cladding and (b) with the proposed design using a silica–air structure.

0.4–1.0 mm with a maximum length of approximately 2 m and no polymer jacket. The fiber length was limited to short sections, since the fiber could not be pulled with an ordinary capstan, and a “cane puller” or “tractor” is required to distribute the normal stress over a large area to prevent cracking. One fiber with a 700- m diameter had a first cladding diameter of 300 m and a core diameter of 40 m. The size of the connection element m with a contact length of m and the was m. For comsupporting silica tube had a wall thickness of parison, a piece of the original MCVD preform was drawn into a regular double-cladding fiber with the core and first cladding diameters of 40 m and 300 m, respectively, and a silicone resin was used as a second cladding. Since this was not an active fiber, no comparison with the efficiency of a typical double-clad fiber can be made. Fig. 3 presents the measured values of the NA versus fiber length for the first cladding of the “candy cane” (curve 1) and the silicone resin (curve 2) double-cladding fibers. For this measurement, a white light source with an NA close to 0.47 was coupled into one end of the fiber and the output light from the other end was observed on a screen. The NA of the silicone resin double-cladding fiber is independent of the length of the fiber, and it is equal to the expected value of 0.4. For the same fiber, however, the NA of the cladding without the resin jacket was only 0.2 (curve 3). For the “candy cane” fiber, the NA of the cladding dropped from a value of 0.33 (fiber length of 15 cm) to approximately the same aforementioned level of 0.2 for pieces of fiber longer than 80 cm. As shown with these results, the maximum NA for the “candy cane” fiber is smaller than expected, and the reduction in NA for longer pieces of fiber indicates considerable optical losses in the cladding. Because of the relatively large contact area between the cladding and the connection element, and the small between these two elements, some portion of the coupled light will leak from the first cladding. Surface losses are also large because the internal part of the “candy cane” fiber was not isolated from the atmosphere after the drawing process. The NA of the first cladding in the silicone resin double-clad fiber without a jacket was the same as that for the first cladding of the “candy cane” fiber but for shorter lengths. A contributing factor for this could be that the silicon resin jacket was removed mechanically and the surface losses have to be larger in this case and the effective NA has to be smaller for a shorter length of fiber.

Fig. 2. Cross section of the fiber drawn from the silica–air double-cladding preform.

Fig. 3. NA for the first cladding of the designed silica–air fiber (1), silicone resin double-cladding fiber (2), and the same fiber without silicone resin coating (3).

To estimate the light leakage from the first cladding of the “candy cane” fiber into the connective element, numerical calculations have been made. A “beam propagation method” was used to calculate the amplitude of the electric field vector of a wave traveling along the “candy cane” fiber. To facilitate the use of a cylindrical coordinate system, we used the geometrical cross section shown in Fig. 4(a) with parameters similar to those of the real fiber. The cladding diameter was taken as 300 m and the m. Both elements supporting tube wall thickness was had a refractive index equal to that of fused silica. The connective element of width and angle had a lower refractive index, with . The input wave had a rectangular amplitude distribution uniformly covering the surface of the first cladding. The fiberparameters for the numerical calculations approximated the experimental parameters (Fig. 4(a): the width ( ) of the conand its NA . Fig. nective element was 150 m, 4(b) shows the electrical field amplitude distribution for a fiber length of 1.5cm. The incident wavewas parallel to the fiberaxis. A substantialpartoftheincidentwavepenetratedintotheconnective element indicating that the guided wave will be attenuated. Fig. 4(c)showstheamplitudedistributionforthesecondmodeledfiber m, andNA ( ), with which was used to simulate a fiber with better confinement characteristics. The remaining fiber parameters and the incident wave

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use this approach to fabricate all-glass double-clad active fibers. A limitation of this geometry is associated with pump light coupled into the first cladding with an NA larger than that of the index difference between the first cladding and the connective element. In this case, light can leak out from the first cladding through the connective element; however, a smaller connective element and a higher between the element and the first cladding can be used to minimize this loss. Efforts to reduce the area of connection are in progress. In conclusion, a new design of silica–air double-cladding optical fiber is proposed. The “candy cane” type fiber was prepared from a MCVD preform inserted into a supporting silica tube ) with a fluorine-doped connection element ( and air as the second cladding. A numerical aperture as high as 0.3 was measured for the first cladding and for fiber lengths up to 50 cm. Although these first experiments were performed with passive optical fibers, an active fiber with the “candy cane” geometry can be easily fabricated if a rare-earth-doped preform is used. Pump absorption efficiencies comparable to those observed in regular double-cladding fibers should be obtained with this new geometry. The optimization of the fiber parameters promises new reliable double-cladding optical fibers for highpower fiber laser applications [9]. Future efforts will focus on the use of a lower refractive index connection rod, and the use of a “cane puller” or “tractor” to pull the fiber to a smaller diameter. It is clear that, for such a design to be of potential use, the losses associated with “leakage” of photons through the connective section must be less than the increase of the number of photons absorbed by the core as a consequence of the higher effective numerical aperture. REFERENCES

Fig. 4. (a) Cross section used to model the proposed fiber with the first cladding diameter d , supporting tube wall thickness t , and connection element width h and angle ; n1–n2 —fused silica and fluorine-doped fused silica refractive index, respectively. (b) Electrical field amplitude distribution for a fiber with d 300 m, h = 150 m, t = 50 m,  = 54 , n1–n2 = 4 10 and fiber length of 1.5 cm. (c) Same calculations for a fiber with d = 300 m, h = 50 m, t = 50 m,  = 15 , n1–n2 = 17 10 and fiber length of 50 cm.

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were the same and the fiber length for this case was 50 cm. It is worthwhile to notice that such parameters are realistic and could beeasilyattainablewiththerightchoiceofglassfortheconnective element. The light penetration into the connective element and into the supporting tube is smaller, which then suggests that the effective NA for the first cladding will be larger. For nonzero incident angles, the difference between the two model fibers was even larger. used for this first “candy cane” The values of NA and fiber are low. Nonetheless, our results show that it is possible to

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