Hybrid Ordered Hole-Random Hole Optical Fibers

Transcription

1 Advances in Science and Technology Online: ISSN: , Vol. 45, pp doi: / Trans Tech Publications, Switzerland Hybrid Ordered Hole-Random Hole Optical Fibers Gary R. Pickrell 1a, Evgenya S. Smirnova 2b, Stanton L. DeHaven 3c and Robert S. Rogowski 3d 1 Virginia Polytechnic Inst. and State University, Blacksburg, Virginia USA; 2 Los Alamos National Laboratory, Los Alamos, New Mexico USA; 3 NASA Langley Research Center, Hampton, Virginia USA a pickrell@vt.edu, b smirnova@lanl.gov, c stanton.l.dehaven@nasa.gov, d r.s.rogowski@larc.nasa.gov Keywords: Optical Fiber, Photonics, Optics, Photonic Crystal Fiber Abstract: Photonic band gap (PBG) fibers have generated significant interest over the last decade due to the unique set of properties these fibers exhibit. In general, these fibers have been made by drawing a series of glass tubes (which are stacked in an ordered array) into a fiber. These fibers consist of an ordered arrangement of holes or tubes in a glass matrix. In this invited paper we describe a novel type of fiber, called HORHOFs (hybrid ordered random hole optical fibers). In these fibers, the refractive index of the ordered-hole region is controlled by incorporation of very small tubes of glass produced in-situ during the fiber drawing process. The result is a region of controllable glass density inside the ordered hole. This allows tailoring of the refractive index of the hole region and of the matrix glass around the holes. Description of the process to produce these new types of fibers, micrographs of some of the fibers produced, some potential applications, and the results of some computer modeling to predict the properties of these fibers, are presented. Introduction Photonic crystal fibers have generated a significant amount of interest due to the unique properties exhibited by these fibers including single mode guidance over a very large wavelength range, guidance through a hollow core, and significantly diminished bend loss when compared to comparable solid core fibers [1-5]. The early optical fibers developed for the telecommunications industry consisted of a solid core with a higher refractive index, surrounded by a solid cladding region of lower refractive index. The difference in index of refractive between the core region and the cladding region causes the light propagating in the core region to be confined or guided along the fiber core. Step index, graded index, dispersion shifted, dispersion flattened, etc. fibers have been fabricated by controlling the radial refractive index profile of the fiber, which in turn controls the propagation parameters of that particular fiber. All of these fibers consist of solid silica glass containing various levels of dopants in either the core, the cladding, or both to control the refractive index profile of the fiber. Holey fibers have been developed within the last two decades which consist of only silica and holes, and thus the name holey fibers. These holes are actually tubes which can run the entire length of the fiber. These fibers have been fabricated by the tube stack and draw method. This method consists of stacking uniform fused silica tubes into the desired pattern, fixing the tubes in place, and then drawing this tube preform down to traditional fiber diameters using an optical fiber draw tower. The procedures when performed properly can yield optical fibers with an ordered All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-18/09/16,04:33:43)

2 Advances in Science and Technology Vol pattern of holes that matches the pattern of holes in the original preform. This process (which has been described only in simple terms above) has many small details in both the preform fabrication and the actual fiber drawing process that can be quite complex and numerous opportunities for deviations from the ideal fiber profile exist, including deviations from spherical for both fiber and holes; partial or total collapse of some or all of the holes; movement of the holes from the symmetric arrangement; and complete collapse of the holes to produce solid fibers. Nevertheless, many groups have successfully fabricated a wide variety of ordered hole fibers despite the many difficulties, and commercially produced fibers are now available. These ordered hole fibers generally fall into two groups, photonic crystal fibers and average index guiding fibers. In the photonic crystal fibers, the size and the spacing (including the geometrical arrangement in the fiber) are controlled such that based on the refractive index difference of the silica matrix and the hole region (usually taken to be air) a two dimensional photonic band gap exists. This photonic band gap is responsible for confining the light to the core region. In contrast, average index fibers while not meeting the necessary requirements to produce a photonic band gap, can still guide light. These fibers utilize the holes (also in an ordered arrangement) in the cladding region to lower the average refractive index and thus are more similar to the traditional solid core and cladding fibers in commercial telecommunications systems today. Recently, a new type of fiber has been developed which contains holes arranged in a random structure around the solid central core region [6]. These fibers have been named RHOFs (for random hole optical fibers). A scanning electron micrograph of one such fiber is shown in Fig. 1. Fig. 1. SEM micrograph of the fracture cross-section of a random hole optical fiber. These fibers are produced by introducing a gas forming agent into the preform, which produces gases in-situ (at the drawing temperature) during the fiber drawing process. The details of fabrication of these fibers has been published previously [6] and so will not be presented again in this paper. The RHOF, as seen in Fig. 1 contain a very large number of holes (which are the ends of tubes running parallel to the optical axis) in the cladding region. Holes as small as 25 nm in radius have been produced in these fibers. The holes present are random in size, location (within the cladding hole band) and spatial extent. Several different methods have been developed to produce the tubes in the cladding region [7,8,9]. Optical attenuation below 0.5 db/m at 1310nm and 1550nm have been measured in fibers produced with relatively impure starting materials [8]. It is anticipated that fibers

3 th International Ceramics Congress with much lower losses can be obtained by using higher purity starting materials and by controlling the maximum hole size. For the photonic crystal fibers as fabricated, the refractive index of the hole region is fixed by the gas within the holes. It has not been possible to vary or control the refractive index of the hole region and still maintain pure silica glass. This is due of course to the fact that the region is a hole, an absence of material. In modeling these fibers only the refractive index of the gas has been available, usually this is taken to be the refractive index of air. In this paper we discuss a novel type of fiber, the hybrid ordered hole-random hole optical fibers, or HORHOFs. Experimental Procedure Fibers were prepared from glass preforms using an optical fiber draw tower and standard drawing techniques (modified as necessary to achieve the appropriate diameter and fiber structure). The main components of the draw tower include a computer controlled water cooled graphite element furnace for heating of the preform; computer controlled mechanical feed systems to control the preform feed rate and center the preform in the furnace; systems for laser measurement of fiber diameter at key locations in the process; and a capstan and spooling system to allow the fiber to be wound onto spools. By controlling various parameters in the process including the furnace temperature, preform feed rate and fiber spooling rate, the diameter of the fiber can be controlled and the internal structure of the fiber can be affected. The transmission spectra were measured using a Micron Optics HR-SLI model V5.1 High Resolution Swept Laser Interrogator, with a published repeatability of 0.05 picometer and an accuracy of 1 picometer. The details of this system can be found on the Micron Optics Web site and so will not be repeated here. The swept laser interrogation system was used in conjunction with a set of mechanical alignment stages mounted on top of an optical breadboard. The transmission spectra were measured using two sets of x-y-z micro-positioning stages with approximately a one micron spatial resolution. The lead in single mode fiber from the swept laser interrogator output was aligned to the test fiber which cleaved on both ends at approximately one half meter in length. The lead out fiber was aligned to the other end of the test fiber and connected to the swept laser interrogator input. The reference level was set at 4% of the output signal strength, by an internal calibration routine. The far field pattern data was collected by using an argon ion laser in conjunction with mechanical alignment stages mounted on top of an optical breadboard. Eigenmode solutions for various structures were calculated using CSTMWS commercial software produced by Computer Simulation Technology, Inc. Results and Discussion In this paper we discuss a new type of fiber, the hybrid ordered hole-random hole fiber. In these fibers, regions of ordered holes exist and can be made in the same pattern as photonic crystal or microstructured fibers. These ordered hole regions are then filled with smaller holes. Fig. 2 shows a schematic representation of the concept of these new HORHOFs.

4 Advances in Science and Technology Vol RHOF Random Hole Region Photonic Crystal Fiber Solid Core Glass Region Hole Region Fig. 2. Schematic drawing of some possible designs of hybrid ordered hole-random hole optical fiber structures. The upper left hand drawing shows the recently demonstrated RHOF fibers. The bottom right hand drawing shows an ordered hole structure. Only one ring of holes is shown fro simplicity, but obviously the concept could be extended to multiple rings of holes. A large variety of fiber types could be produced in this manner. Although a great many designs can be envisioned, fabrication of these types of fibers presents a significant challenge. However, recent advances in fabrication techniques have allowed structures such as these to be realized in optical fibers. A few of these structures will be presented below. These fibers were fabricated at Virginia Tech and/or at the NASA Langley Research Optical Fiber Draw Tower facility. The first structure attempted for this work is a variation of the fiber shown in the top middle of Fig. 2. A more complex structure involves dividing the solid core region shown in this figure into a solid core surrounded by a random hole region. This adds a significant amount of complexity to the system in that the solid core now needs to be centered within a region of randomly oriented porosity. The results of attempts to fabricate this type of structure are shown in Fig. 3. Fig. 3. Optical micrographs of the fracture surface of a hybrid ordered hole-random hole fiber structure. The micrograph on the left shows a fiber fabricated at NASA and the fiber in the center and on the right shows the same structure fiber fabricated at Virginia Tech. The rightmost micrograph shows solid core centered within ring of random hole region. As seen in this figure, a well centered core within the ring of randomly arranged porosity can be see, which is in turn surrounded by 6 circular, symmetrically spaced regions of randomly arranged porosity. The higher magnifications show the high degree of control in the location of the random hole regions, and the centering of the central core region. This fiber is composed entirely of fused silica (and the hole regions), but could be fabricated in other glass compositions as well. The amount and size of the randomly arranged holes in ordered hole region can be varied by varying the fabrication procedures. So in effect, the random hole region can be filled with any selected percentage of holes and the size distribution

5 th International Ceramics Congress of these holes can be controlled. For example, for a selected hole size distribution, the randomly arranged hole region could be made to contain 10% holes or 50% holes (as measured for example by cross-sectional hole area relative to the total area). This would allow one to control the average (or effective) index of refraction in that region. As shown in the figure, the location of the random hole regions can be closely controlled, as can the spacing between the random regions. A larger or smaller size core can be fabricated and the inner and outer radius of the randomly arranged hole region surrounding the core can be selected. This is just one example of the virtually limitless new types of fibers which can now be fabricated utilizing these techniques. For example, one may want to prepare a fiber that consists of a porous core region surrounded by a ordered arrangement of random hole regions as shown in the top right fiber of Fig. 2. An example of such a fiber is shown in Fig. 4. Again this fiber is made from only fused silica, but any glass composition could conceivably be made. Fig. 4. Porous core and cladding hybrid ordered hole-random hole fibers at successively higher magnifications. As can be seen in this figure when compared to the last figure, the structures are almost identical except for the fact that one fiber has a solid core, and the other has a porous core. The tremendous flexibility in design and capability for control of the fiber properties should be evident from these fibers. Another example of a slightly different type of fiber is shown in Fig. 5. In this fiber, rings of random hole regions are shown (similar in design to the picture shown in the bottom center of Fig. 2). These fibers contain ordered rings (or bands) of porosity with randomly arranged holes. A very small ring of random porosity around the central core is also evident. Surrounding this region, is a solid ring surrounded by another hole band (ring of randomly arranged holes). This fiber rests inside a solid fused silica tube. It is also possible to mix ordered hole regions with random hole regions in any pattern desired, For example, in the previous figures, the 6 ordered holes that were filled with randomly arranged holes could be changed to have a randomly filled hole next to an open hole such that three open holes alternate with the three filled holes. By extension, any number of holes could be left open, and any holes selected could be filled with the desired amount of random porosity. An example of such a structure is shown in Fig. 6. Also shown in Fig. 6 (middle photo) is an example of a symmetric structure with a porous core and six hole porous ring (similar to Fig. 4), however, a central core rod has been added to one of the random hole regions in the ring.

6 Advances in Science and Technology Vol Fig. 5. Optical micrograph of the fracture surface of a concentric ring ordered holerandom hole fiber. The rightmost photo in Fig. 6 shows the same fiber as in the middle photo, however, only transmitted light is used. As can be seen, light is still effectively transmitted through the sold core (which now is centered in one of the random hole regions within the 6 hole ring). Fig. 6. Non-symmetric structures -alternating structure random hole-ordered hole regions with defects (left) and non-centered core (middle) and transmitted light showing bright core (right) within one of the ordered hole regions where the core resides. These are only a few examples of the numerous combinations which can now be fabricated with this technique. Application of computer modeling may be the most efficient way to arrive at the best structure of these HORHOFs. In the past, the computer modeling has been limited due the fact that it was not feasible to make the hole region anything but a hole, and therefore only the refractive index of the air within the hole regions has been used. Now with these new types of fibers, it is evident that the effective refractive index of the hole region could be changed over a very wide range. Although mathematical modeling of the modal propagation in these fibers to arrive at optimum designs may take significant funding and many years, some simple modeling has been performed, but a large amount of work is anticipated in this area. As one example, we begin with the results from modeling the propagation of light in the structure as shown in Fig. 6 (center and rightmost micrographs). This was performed with a commercial software package for computing the eigenmodes in electromagnetic structures (CST Microwave Studio, 2005 CST - Computer Simulation Technology, Wellesley Hills, MA, USA, The results from this structure are shown in Fig. 7. As can be seen, the light is confined to the core region within the upper right hole where the core was designed to reside. This result matches well with what is experimentally observed in these fibers for the transmitted light micrographs shown in Fig. 6 (right).

7 th International Ceramics Congress Fig. 7. Modeling results of eigenmode calculations for the center and rightmost micrograph of Fig. 6. Fig. 8 shows the results of modeling for the first three modes in a structure such as the one shown in Fig. 5. The pictures marked with "x component" show the x-component (perpendicular component) of the electric field. The other pictures (without the label x-component ) show the corresponding magnitude of the electric field of that mode - this is essentially what one would see with an eye in the fiber. Since the magnitude pictures do not really show the structure of the eigenmodes, the pictures with components have been included to give a better understanding of the modal structure. The transmission spectra were measured using a Micron Optics HR-SLI model V5.1 High Resolution Swept Laser Interrogator. The transmission spectra for some of the structures above are shown in Fig. 9. The fibers were measured as described in the experimental procedure section. The measured transmission intensity is with respect to the internal reference level. The x-y-z coordinates of the alignment stages were adjusted to obtain the maximum transmission. The peaks and valleys in the spectrum are primarily the result of the Fabry- Perot cavities occurring within the system. The transmission spectra measured are only shown to indicate that for these fibers the optical signal a) b) c) d) e) f) Fig. 8. Results of Eigenmode calculations a) mode 1; b) mode 1 x-component; c) mode 2; d) mode 2 x-component; e) mode 3; f) mode 3 x-component

8 Advances in Science and Technology Vol a) b) c) d) Fig. 9. Transmission spectra for a) structure shown in Fig. 3; b) structure shown in Fig. 5; c) structure shown in Fig. 6 (left); d) structure shown in Fig. 6 (center). transmitted is of sufficient strength for use in a variety of applications including fiber sensors, and not to demonstrate how low the loss can be in any particular spectral region. These fibers and structures have not been optimized in any fashion. They are shown only as a few examples of the many new structures that are now possible with these HORHOF fiber techniques. Since the reported dynamic range of the Micron Optics interrogator is on the order of -80dB, a sufficiently strong optical signal is observed and may have some interesting sensor applications, such as gas sensing which has already been demonstrated with the RHOF fibers [10]. Different applications may require different values of the hole size, hole density, length of the tubes, and arrangement of the ordered and random hole regions, in addition to the number of modes present, the optical loss, etc. The far field patterns of these fibers were measured with an argon ion laser with central wavelength at around 514 nm. The laser was focused onto the end of a 50/125 multimode optical fiber which was aligned with the end of the test fiber. The far field pattern of the fibers were measured. The speckle pattern which results gives an indication of the number of modes present in the optical signal. The far field patterns obtained at a wavelength of 514 nm indicates that each of the fibers shown above has many modes as expected. In addition to fabrication of symmetric structures, non-symmetric structures can also be fabricated. This could be used to induce the desired birefringence properties in the fiber for a particular application. This may be valuable when fabricating fibers where it is desired to have a difference in the propagation constants of the different polarization components to be different, such as in polarization maintaining fibers and in fiber optic sensors for example. This may be accomplished by the alternating ordered hole, random hole regions shown in Fig. 6. Instead of a symmetric alternation of the open and filled regions, a non-symmetric alternating pattern would induce birefringence in the fiber.

9 th International Ceramics Congress Fig. 10. Schematic representation of a few potentially birefringent HORHOFs fiber designs. Some of the possible variations are shown schematically in Fig. 10. It is obvious that there are a very large number of derivative structures that can be fabricated using these newly developed techniques. Obviously, this could have many potential applications in optical fiber and optical fiber related devices. In addition, research has been conducted in depositing materials inside of the holes of holey fibers in order to fabricate electronic or other related devices inside of the optical fiber [11,12,13]. The results presented in this paper can serve as a platform for tailoring different regions of the fiber to accommodate a variety of different in-fiber devices. Conclusions A new type of optical fiber has been devised and demonstrated called HORHOFs, the hybrid ordered random hole optical fiber. A few examples of fabrication of some of the extremely large number of potential variations of these new fibers has been presented for the first time. This greatly expands the opportunity for design and fabrication of new types of fibers whereby the average refractive index of the hole region (of ordered hole fibers) can be modified to the desired value. This may have significant potential applications in optical fibers, optical fiber related devices and in the fabrication of integrated electronic and other devices inside of optical fibers. Acknowledgments: This work was partially funded by an award from the Aspires program. The contributions of the NASA Langley Research Center is also thankfully acknowledged. References [1] T.A. Birks, J.C. Knight, P.St.J. Russell, Opt. Lett. 22 (1997) 961. [2] J.C. Knight, T.A. Birks, P.St.J. Russell, J.P. de Sandro, J. Opt. Soc. Am. A 15 (1998) 748. [3] J.C. Knight, T.A. Birks, P.St.J. Russell, D.M. Atkin, Opt. Lett. 21 (1996) [4] J.K. Ranka, R.S. Windeler, A.J. Stentz, Opt. Lett. 25 (2000) 25. [5] DiGiovanni, et al. Patent Number: (September 1, 1998). [6] G. Pickrell, D. Kominsky, R. Stolen, F. Ellis, J. Kim, A. Saffaai-Jazi, A. Wang, Photonics Technol. Lett. 16 (2004) 491. [7] G. Pickrell, N. Manjooran, N. Goel, Proc. SPIE 5589 (2004) 257.

10 Advances in Science and Technology Vol [8] D. Kominsky, G. Pickrell, R. Stolen, J. Opt. Lett. 28 (2003) [9] G.R. Pickrell, D. Kominsky, R.H. Stolen, A. Safaai-Jazi, R.G. May, A. Wang, Proc. SPIE 4578 (2001) 271. [10] G. Pickrell, W. Peng, A. Wang, Opt. Lett. 29 (2004) [11] Incorporation of Biological Agents in Random Hole Optical Fibers, Gary Pickrell and Navin Manjooran, Proceedings of the 107 th Annual Meeting of the American Ceramic Society, Baltimore, MD, 2005 [12] Fabrication, Characterization and Deposition of Materials from Liquids in Random Hole Optical Fibers, Navin Manjooran and Gary Pickrell, Proceedings of the 107 th Annual Meeting of the American Ceramic Society, Baltimore, MD, 2005 [13] Incorporation and Characterization of Carbon Nano Tubes in Random Hole Optical Fibers, Navin Manjooran and Gary Pickrell, Proceedings of the 107 th Annual Meeting of the American Ceramic Society, Baltimore, MD, 2005