1100 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 8, AUGUST 2003 Refractive Microlens on Fiber Using UV-Curable Fluorinated Acrylate Polymer by Surface-Tension Kyung-Rok Kim, Selee Chang, and K. Oh, Member, IEEE Abstract—A new novel fabrication method for lensed fiber has been proposed utilizing a viscosity-optimized UV-curable fluorinated acrylate polymer. The structure is composed of three segments; single-mode fiber, coreless silica fiber and UV-curable polymer lens-tip. Flexible control of the curvature of lens-tip was realized by control of deposited volume and surface tension of the liquid polymer. Free-space interconnection performances such as working distance, spot size at beam waist, and misalignment tolerances are characterized. Index Terms—Integrated optics, lenses, optical coupling, optical fiber devices, optical fibers, polymers. I. INTRODUCTION O PTICAL fiber devices based on free-space intercon- nection such as in-line isolator, microelectromechanical systems (MEMS) switch, pigtailed laser diode (LD), and ar- rayed waveguide grating (AWG) requires advanced techniques for fiber termination and alignment [1], [2] to make them reliable building blocks in optical communication systems. To serve those purposes, various types of optical field transformers using tapered waveguide, refractive lens, inhomogeneous medium lens, diffractive lens or long-period fiber grating (LPFG) were developed to realize efficient all-fiber solutions [3]–[7]. Among these, refractive lensed fibers are being pre- ferred to provide compact, robust structure, as well as high coupling efficiencies. Conventional refractive lensed fibers, however, have been based on polishing and arc-discharge techniques, which require sophisticated mechanical process. Flexibility to adjust the radius of curvature of the lens has been rather restricted, especially in the case of arc-discharge tech- niques the surface-tension of melted glass sets a fundamental upper-limit for the radius of curvature. In this letter, we report a new technique that utilizes a novel UV-curable liquid polymer to form a lens on the cleaved end of a hybrid fiber consisted of conventional single mode fiber (SMF) and coreless silica fiber (CSF). Flexible control of radius of the curvature using liquid polymer, and mode field expansion in the CSF segment were experimentally demonstrated for the first time, which will provide a new solution to fiber-based free- space interconnection. Characteristics of the proposed lensed fibers were analyzed in terms of coupling efficiencies. Manuscript received February 11, 2003; revised April 15, 2003. This work was supported in part by the KOSEF through the UFON Research Center, in part by the MOE through the BK21 Program, and in part by the ITRC-CHOAN Program. K.-R. Kim and K. Oh are with the Department of Information and Com- munications, Kwangju Institute of Science and Technology (K-JIST), Kwangju 500-712, Korea (e-mail:koh@kjist.ac.kr). S. Chang was with Luvantix. Co., Ltd., Ansan, Kyounggi 425-100, Korea. Digital Object Identifier 10.1109/LPT.2003.815321 Fig. 1. Fabrication process of a lensed fiber using coreless silica fiber (CSF) and UV-curable polymer. II. FABRICATION METHODOLOGY The proposed fabrication process of a lensed fiber is schemat- ically illustrated in Fig. 1. Appropriate length of the CSF is first spliced to the SMF and then UV-curable liquid polymer is dropped on the cleaved end of CSF by a precise syringe. The droplet forms a spherical shape by the surface-tension to mini- mize its surface energy. The liquid polymer is, then, solidified by UV-irradiation under nitrogen environment. Note that CSF is used to control distance from SMF to lens, which provides an efficient method to control the mode field expansion. The radius of curvature of the lens can be calculated from the volume of the deposited liquid polymer. (1) (2) Here, and are the height and volume of the lens, respec- tively. is the radius of CSF, 62.5 m. Through above equa- tions, can be eliminated and becomes a function of only the volume of the cured polymer, . Note that is determined by the surface tension coefficient of liquid polymer and the initial liquid volume from the syringe . The surface tension coef- ficient can be controlled either by temperature or the viscosity of the liquid polymer. UV-curable liquid polymers with various viscosities were synthesized. Five samples with different vis- cosities and ’s were prepared and radii of the curvature of the lenses were measured and the results are summarized in Table I. As expected in the (1) and (2), a larger result in a smaller radius of curvature. The variation of radius become more sensi- tive to the change of and error are increased, as the viscosity is higher. Fig. 2(a) shows pictures of fabricated lens-tips with various radii of curvature on fibers. Fig. 2(b) shows the com- posite lensed fiber. 1041-1135/03$17.00 © 2003 IEEE