Low Cost 1 x 2 Acrylic-based Plastic Optical Fiber Coupler with Hollow Taper Waveguide

PIERS ONLINE, VOL. 5, NO. 2, 2009
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  PIERS O NLINE , V OL . 5, N O . 2, 2009 129 Low Cost 1 × 2 Acrylic-based Plastic Optical Fiber Coupler withHollow Taper Waveguide Abang Annuar Ehsan 1 , Sahbudin Shaari 1 , and Mohd Kamil Abd. Rahman 21 Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia43600 UKM, Bangi, Selangor, Malaysia 2 Faculty of Applied Science, Universiti Teknologi MARA40450 Shah Alam, Selangor, Malaysia Abstract — A 1 × 2 Plastic optical fiber (POF) Y-coupler has been designed and fabricatedusing a simple acrylic (PMMA) mold insert. The device is composed of three segments: aninput POF fiber, an intermediate hollow taper waveguide and output POF fibers. The acrylicmold insert has been fabricated using EGX-400 desktop engraver machine with a spindle speedof 15Krpm and feed rate of 5mm/sec. The engraved regions which is a form of U-groove allows1mm core step index PMMA POF fibers to be slotted into the mold insert. The short POFfibers at the input and output ends are slotted inside the mold insert before the interfaces of thetaper waveguide. A top acrylic plate is then placed on top of the fabricated device and sealed.The final device has been tested for both splitter and combiner operations for an effective powerof 1mW. The device has an insertion loss of 10.48dB. In the splitter operation, the device has asplitting ratio of 52:48. In the combiner operation, the combined power is − 11 . 15dB for inputpower of  − 11 . 90dB and − 12 . 05dB. 1. INTRODUCTION Plastic optical Fiber (POF) is a well known medium for short distance communication applicationand it is finally making a way into the optical fiber market. It does not require any expensive orspecial tool, no special training and no long technical procedures to operate. POF is basically anoptical fiber with large-core size with multimode characteristics, low cost, and robust characteristics.In addition to short distance communication, POF is also being used in signaling, lighting anddecoration system. Other niche application of POF are in the automotive, entertainment, andsensor industries [1]. In all of these applications, it is necessary to split or combine the opticalsignals using passive optical components.Among the passive components for POF applications, optical coupler plays an important rolethat is borne out by the availability of a complete line of products. There have been many techniquesof assembling POF couplers. These techniques include (i) twisting and fusion (ii) side polishing(iii) chemical etching (iv) cutting and gluing (v) thermal deformation (vi) molding (vii) biconicalbody and (viii) reflective body [2].The 1 × 2 POF coupler which has been fabricated here may be an alternative to that of the optical1 × 2 POF coupler which was fabricated by IMM ( Institut f¨ ur Mikrotechnik Mainz  ) in Germany.The insertion loss of the device by IMM is about 6dB [3]. The fabrication technique requires severaladditional steps including laser machining (excimer laser) for PMMA resist patterning and injectionmolding for moulding. A similar device with circular cross section have also been fabricated byTakezawa etal. [4] which showed low excess loss (1.91dB). Nevertheless, the device requires the useof injection molding tool which can increase the cost of making these devices. Hollow waveguideson the other hand are waveguides where the inner section is hollowed. They have been previouslyused in laser light delivery system for medical application [5,6] where the radiation wavelengthsused are greater than 2 µ  m [6]. These devices are also being used for photonics integrated circuitswhere temperature insensitivity is required [7,8].A 1 × 2 POF coupler has been designed using a simple acrylic-based mold insert. The POFcoupler device is composed of three sections: an input POF fiber, an intermediate hollow taperwaveguide and output POF fibers. Finally, based on the CAD design, mold insert of the device isfabricated using a desktop engraver on an acrylic block. POF cable of 1mm core size is insertedat the input and output branches of the coupler. The POF coupler here is part of the passivecomponents that are being designed and fabricated to be used in a new portable optical access-card system [9,10]. This 1 × 2 POF coupler will be one part of the optical code generating device  PIERS O NLINE , V OL . 5, N O . 2, 2009 130 where the other part will be the 1 × 2 asymmetric couplers. By using low-cost arcylic materialwhich can be easily obtained in large bulk sheet and low-cost desktop engraver system, we are ableto fabricate POF coupler with very minimum cost. The design and fabrication processes shownin this paper illustrate the potential of low-cost acrylic material and a simple engraving techniquefor producing POF coupler. Nevertheless, some modifications are required in order to improve theoptical performance of the devices which will be illustrated also along in this paper. 2. POF COUPLER DESIGN The 1 × 2 coupler is the simplest coupler design where the input optical power is split into two.The basic coupler design will utilize a simple 1 × 2 Y-coupler which is shown in Fig. 1. In thiscoupler design, the splitting angle is set large at an angle of 53 ◦ , as shown in this figure. Figure 1: CAD design of 1 × 2 POF Y-coupler with large splitting angle. The input and output waveguides are constructed using POF fibers which are slotted into aY-shaped mold insert, shown in Fig. 2. The POF fibers are slotted until the fibers are positioned just before the taper waveguide region as shown in this figure. POF fibersHollow taper region Figure 2: 1 × 2 POF Y-coupler block layout. The slot width of the mold insert has been set at 1mm which allow the POF fiber to fit infirmly. The POF fibers used are standard SI POF with NA of 0.5, core size of 980 µ  m, refractiveindices of the core and cladding are 1.49 and 1.42 respectively. Due to the hollow structure of thetaper waveguide region, non-sequential ray tracing will not give a good result of the device opticalcharacteristics based simply on the acrylic material itself. However, the optical characteristics of the hollow taper waveguide region can be modified by simply filling this space with low-cost UVcurable glue, normally used for connecting fibers and are easily available. The ray tracing resultsfor the UV-glue filled taper waveguide region will be shown briefly to illustrate how the simpleoptical device can be constructed and ray traced easily. Fig. 3 shows the device construction of the1 × 2 POF coupler with the UV-glue filled taper waveguide region and the ray tracing results forthis device is shown in Fig. 4. Input POFn=1.49Output POFn=1.49Cladding n=1.0Cladding n=1.49Taper waveguide n=1.56Claddingn=1.0 Figure 3: 1 × 2 POF Y-coupler block layout with UV-glue filled taper waveguide region.  PIERS O NLINE , V OL . 5, N O . 2, 2009 131 Figure 4: 1 × 2 POF Y-coupler ray tracing diagram. The output power for the POF couplers have been obtained from the ray tracing plot. Theoutput signal measured at the output ports of the 1 × 2 POF coupler is 0.24mW and 0.25mW.The insertion loss of this device is about 6dB whereas the excess loss of this device is about 3dB. 3. POF COUPLER FABRICATION AND MEASUREMENT In this project, a rigid mold insert is designed and fabricated using a micro engraving tool. We haveused acrylic material as the main substrate and cladding region for the taper waveguide. The RIof the acrylic material used is about 1.49. The waveguide design is then engraved onto the acrylicsubstrate using EGX-400 desktop engraver tool.The fabrication start with the acrylic sample positioned onto the workpiece of the machine. Themachine parameters are configured using a CAD/CAM machining software. Milling tool size of 0.5mm is used and spindle speed of 15,000rpm and feed rate of 5mm/sec have been utilized. Afterthe mold insert has been fabricated, short SI POF fibers (10cm) are inserted into the engravedslots (input and output ports) until the fibers are positioned just before the taper waveguide region.Fig. 5 illustrates the steps taken in the fabrication of the 1 × 2 POF coupler. POF fibersHollowtaper region (a) (b)(c) (d) Figure 5: 1 × 2 POF Y-coupler fabrication step, (a) acrylic mold inert with engraved U-grooves, (b) POFfibers insertion, (c) POF fibers alignment, (d) enclosed POF coupler. The insertion and excess loss of this device has been tested using a light emitting diode (LED)at a wavelength of 650nm using Advanced Fiber Solution FF-OS417 and optical meter OM210.The effective input power P  in is 0dBm. The output power detected at both output ports are P  1 = − 10 . 1dB and P  2 = − 10 . 48dB. The insertion loss of this device is about 10.48dB. The effectivetap-off ratio or the splitting ratio is 52:48. The high loss is expected due to the hollow structureof this device. Similarly in the combiner operation, the device exhibit high loss as expected due tothe hollow taper waveguide region. The output power detected for each input port individually are P  out(1) = − 11 . 90dB and P  out(2) = − 12 . 05dB. The output power detected when both input portsare activated are P  out(combine) = − 11 . 15dB. These values correspond to the expected result for acombiner where the combined output power will be halved.The fabricated devices are shown in Fig. 6. The acrylic-based mold insert for 1 × 2 POF couplers  PIERS O NLINE , V OL . 5, N O . 2, 2009 132 are shown in Fig. 6(a) and a close up view in Fig. 6(b). The connected 1 × 2 POF coupler is shownin Fig. 6(c). (a) (b) (c) Figure 6: Fabricated 1 × 2 POF coupler, (a) Y-Coupler mold insert, (b) close view of the taper region, (c)Y-coupler with POF fibers. 4. CONCLUSIONS Low cost 1 × 2 POF coupler with hollow taper waveguide has been designed and fabricated. Thisdevice is part of our initial work on an optical code generating device for a portable optical access-card system. The POF coupler has been fabricated using cheap acrylic material and fabricatedusing a desktop engraving system. The device structure is composed of input POF fiber, middlehollow taper waveguide and output POF fibers where the hollow structure can be modified by fillingthis area with cheap UV curable glue. The insertion loss of the hollow structure device is still highat 10.48dB but the device has been shown to work in both splitter and combiner operation. ACKNOWLEDGMENT The authors would like to thank Universiti Teknologi MARA for the financial support on thisproject and machining facilities, and to the Institute of Microengineering and Nanoelectronics(IMEN), Universiti Kebangsaan Malaysia for providing the simulator tool Zemax. REFERENCES 1. Kawase, L. R. and H. S. Nalwa, Passive Optical Fiber Devices — Polymer Optical Fibers  ,American Scientific Publishers, California, 2004.2. Ziemann, O., J. Krauser, P. E. Zamzow, and W. Daum, Passive Components for Optical Fiber — POF Handbook  , 2nd Edition, Springer, Berlin, 2008.3. Klotzbuecher, T., T. Braune, D. Dadic, M. Sprzagala, and A. Koch, “Fabrication of optical1 × 2 POF couplers using the laser-LIGA technique,” Proc. SPIE Laser Micromachining for Optoelectronic Device Fabrication  , Vol. 4941, 121–132, Brugge, Belgium, October 2003.4. Takezawa, Y., S. Akasaka, S. Ohara, T. Ishibashi, H. Asano, and N. Taketani, “Low excesslosses in a Y-branching plastic optical waveguide formed through injection molding,” Appl.Opt. , Vol. 33, No. 12, 2307–2312, 1994.5. Hongo, A., T. Koike, and T. Suzuki, “Infrared hollow fibers for medical applications,” Hitachi Cable Review  , Vol. 23, 1–5, 2004.6. Verdaasdonk, R. M. and C. F. P. Swol, “Laser light delivery systems for medical applications,” Phys. Med. Biol. , Vol. 42, 869–894, 1997.7. Miura, T., F. Koyama, Y. Aoki, A. Matsutani, and K. Iga, “Hollow optical waveguide fortemperature-insensitive photonic integrated circuit,” Jpn. J. Appl. Phys. , Vol. 40, Part 2,No. 7A, L688–L690, 2001.8. Miura, T., F. Koyama, and A. Matsutani, “Modeling and fabrication of hollow optical waveg-uide for photonic integrated circuits,” Jpn. J. Appl. Phys. , Vol. 41, Part 1, No. 7B, L4785–L4789, 2002.9. Ehsan, A. A., S. Shaari, and M. K. A. Rahman, “Portable optical security card system,” Malaysia Patent  , File No. PI 20071163, July 2007.10. Ehsan, A. A., S. Shaari, M. K. A. Rahman, and K. M. R. Kee Zainal Abidin, “Optical codegenerating device using 1 × N  asymmetric hollow waveguide couplers,” Acta Photonica Sinica  ,Vol. 37, No. 5, 849–854, 2008.
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