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Single-Mode Perfluorinated Polymer Optical Fibers With Refractive Index of 1.34 for Biomedical Applications Guiyao Zhou, Chi-Fung Jeff Pun, Hwa-yaw Tam, Senior Member, IEEE, Allan C. L. Wong, C. Lu, Member, IEEE, and P. K. A. Wai, Senior Member, IEEE Abstract—We demonstrate a technique for the fabrication of single-mode perfluorinated polymer optical fiber (PPOF). The PPOF preform is composed of poly-methyl-methacrylate (PMMA)-based outer cladding and a graded-index multimode PPOF as the core. A photosensitive graded-index single-mode PPOF with a core diameter of about 6.6 m and cladding diameter of 400 m was fabricated. The fiber has a cutoff wavelength of 854 nm and exhibits single-mode characteristics at wavelengths of 1310 and 1550 nm. The transmission loss is less than 0.2 dB/m in the wavelength range of 1410–1540 nm and less than 0.5 dB/m for wavelengths up to 1610 nm, significantly less than the typical transmission loss of 100 dB/m for PMMA fiber. Another important feature of the PPOF is its low refractive index of 1.34, close to aqueous solution of biomaterials, permitting strong optical coupling for biomedical applications. Index Terms—Low loss, perfluorinated polymer, polymer optical fiber (POF), single mode.
I. INTRODUCTION N recent years, polymer optical fibers (POFs) have attracted interest for sensing applications [1]–[3], particularly in biomedicals. This is because unlike silica optical fibers, POFs are rugged, biocompatible, and do not produce shards when broken. Single-mode poly-methyl-methacrylate (PMMA)-based POFs with fiber Bragg gratings (FBGs) written in them were reported in 1996 [4]. So far, virtually all reported polymeric FBGs were written at the 1550-nm telecommunication window [5], [6] because of the ready availability of fiber-optic components and FBG interrogators at that particular wavelength band. This limits their usable length to about 10 cm due to the extremely large attenuation of PMMA at 1550 nm. The transmission loss of PMMA-based POF mainly comes from the absorption of hydrocarbon (C–H) bonds because the overtones of C–H bond vibration occur in the wavelength range of 1300–1650 nm. PMMA contains C–H bonds, resulting in
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Manuscript received September 12, 2009; revised October 12, 2009; accepted October 30, 2009. First published November 20, 2009; current version published January 07, 2010. This work was supported by the University Grants Council’s Matching Grant of the Hong Kong Special Administrative Region Government under the Niche Areas project J-BB9J. G. Zhou is with the Photonics Research Centre, The Hong Kong Polytechnic University, Hong Kong, China, and also with Yanshan University, Hebei Province, 066004, China, (e-mail:
[email protected]). C.-F. J. Pun, H.-Y. Tam, A. C. L. Wong, C. Lu, and P. K. A. Wai are with the Photonics Research Centre, The Hong Kong Polytechnic University, Hong Kong, China. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2009.2036377
high intrinsic absorption loss at 1550 nm [7]. In this letter, we report our work in developing a low-loss, single-mode POF suitable for the fabrication of FBGs operating at the 1310- to 1550-nm wavelength range. The transmission loss of POFs is reduced significantly by using perfluorinated polymer as the core material of the POF. High photosensitivity, essential for writing Bragg gratings, of perfluorinated polymer in the UV region for FBG fabrication was also demonstrated in [8] and good thermal stability of Bragg grating written in perfluorinated polymer film was reported by the same group to be better than both PMMA and silica fibers [9]. Furthermore, perfluorinated polymer has a refractive index (RI) of 1.34, close to the RI of aqueous solutions of biomaterials, permitting strong optical coupling for biomaterial sensing. The low RI and low loss coupled with the enhanced photosensitivity of single-mode perfluorinated polymer optical fiber (PPOF) renders it an attractive candidate to realize practical POF grating biosensors. FBGs are usually fabricated in single-mode optical fibers so that the Bragg reflection spectrum contains a single peak of the fundamental mode for ease of demodulation of measurand-induced Bragg wavelength shift. However, fabrication of singlemode POFs is difficult because it is not easy to control dopant diffusion from core to cladding during the polymerization of monomers in the preform making process and also during fiber drawing. Thus, it is not easy to maintain the RI profile of POF and, therefore, difficult to control the effective diameter of the fiber core—a necessary step to ensure single-mode operation. In this letter, we demonstrate a technique to fabricate single-mode PPOF through two processes, which begins by fabrication of a fiber preform using a graded-index multimode PPOF as the core of the eventual single-mode fiber to be drawn. The preform was then drawn into single-mode PPOF using our home-made POF drawing tower. To the best of our knowledge, this is the first reported work in the fabrication of single-mode PPOFs using the redrawing method. The multimode PPOF was used as the core of the single-mode fiber perform by inserting it into a PMMA tube which serves as the outer cladding, thus avoiding dopant diffusion in the preform-making process. The preform was then drawn into the appropriate diameter for single-mode operation at wavelengths longer than 1300 nm. The PMMA tube was designed to be drawn into fiber at low temperature to reduce dopant diffusion of the PPOF core. II. DESIGN OF THE SINGLE-MODE PPOF Perfluorinated polymer was used as the fiber core material to fabricate single-mode POFs. Perfluorinated polymers exhibit a
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ZHOU et al.: SINGLE-MODE PPOFs WITH RI OF 1.34 FOR BIOMEDICAL APPLICATIONS
much lower transmission loss than PMMA because the vibration frequency of a molecular harmonic oscillator is given by [10], where and are the bond force constant and the effective reduced mass, respectively. Fluorine atoms have much higher effective masses than that of hydrogen atoms in PMMA [11], and so the fluorocarbon (C–F) bond overtones are further away from the wavelength range of 600–1650 nm. The intrinsic absorption loss of C–F bonds is several orders of magnitude lower than that of C–H bonds in the wavelength range of 1300–1650 nm [12]. In 1996, Asahi Co., Ltd. fabricated low-loss multimode perfluorinated POFs for broadband optical communication with a loss of around 200 dB/km at 1550 nm [13]. Hitherto, perfluorinated polymer is the most transparent polymer at the 1550-nm window [14]. The graded-index multimode PPOF used in the experiment has core and cladding diameters of 130 m and 480 m, respectively. The index profile is approximately parabolic, with a core RI of 1.355, and a cladding RI of 1.342 [15]. Prior to the fabrication of the single-mode PPOF preform, we need to determine the amount of reduction, i.e., the draw-down ratio , needed to draw the multimode fiber down to become single-mode at around 1550 nm. For a graded-index fiber, the cutoff for the normalized freto support single-mode, is given by [16] quency (1) where is the profile parameter. For parabolic profile, and, therefore, for graded-index single-mode fibers. The normalized frequency is defined as [15] (2) where is the core radius and is the operating wavelength and are the RIs of the core and of the single-mode fiber. cladding, respectively. The fiber core radius was found to be 4.48 m by solving (1) and (2). Therefore, the core diameter of graded-index multimode PPOF needs to be drawn from 130 m to less than 8.96 m, giving a draw-down ratio of 14.5 (17.1), for the fiber to become single-mode at 1550 nm (1310 nm). III. FABRICATION OF PPOF The critical issue in designing the preform of the single-mode PPOF is to ensure the graded-index profile of the multimode fiber is maintained during the entire process from preform making to drawing fiber. The glass transition temperature of the perfluorinated polymer is 108 C and that for PMMA typically varies from 105 C to 120 C. It is, therefore, important to use PMMA with a lower glass transition temperature than that of perfluorinated polymer. We decreased the glass transition temperature of PMMA using the copolymerizing method, with methacrylate and butyl methacrylate (80%–19.65% molar ratio), the initiator is lauroyl peroxide (0.1% molar ratio), and chain-transfer reagent is 1-Butanethiol (0.25% molar ratio). The mixed solution was stirred at 80 C for about 1 h, until its
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Fig. 1. Actual and setting polymerization temperature.
Fig. 2. Cross section of the PPOF and its approximate RI profile.
viscosity thickened. This is an important step to minimize the dissolution of the grade-index multimode PPOF in the mixed solution. The solution is then poured into a glass test tube with the graded-index multimode PPOF kept taut along the central axis of the glass test tube. To draw a single-mode PPOF with a cladding diameter of 400 m, the preform diameter must be larger than 5.8 mm (0.4 mm ) for 1550 nm and 6.84 mm for 1310 nm. The value of was calculated in the previous section to be 14.5 and 17.1 for 1550 and 1310 nm, respectively. The inner diameter of the glass test tube is 7 mm and, therefore, the drawn fiber should support single-mode operation at both 1310 and 1550 nm. The glass test tube with the multimode PPOF and monomer solution was placed in a programmable oven for further polymerization. The polymerization process includes four steps as shown in Fig. 1. The first step is the polymerization initiation process that took about 2 h at 75 C. The second step is an important polymerization process that lowers the temperature of the solution to 60 C for about 8 h to prevent the occurrence of explosive polymerization so that bubbles would not form in the preform. This was followed by a normal polymerization process at 70 C for 30 h and then the temperature was increased linearly from 70 C to 110 C over a period of 50 h to complete the polymerization process. The glass transition temperature of the PMMA was estimated to be 85 C (determined by the differential scanning calorimetry technique), lower than that of the perfluorinated polymer (108 C). To reduce the diffusion effect, the preform was drawn into a fiber with an outer diameter of about 0.4 mm at 180 C and relatively high drawing speed of 10 cm/s. The cross section and the approximate RI profile of the PPOF are shown in Fig. 2. The fiber was illuminated with a light source and the guided light in the fiber core is clearly observed. The core diameter was measured to be 6.6 m.
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Fig. 4. Measured transmission loss of single-mode PPOF.
Fig. 3. Measured cutoff wavelength (top) and measured output intensity distribution (bottom) of the single-mode PPOF at 1310 and 1550 nm.
IV. OPTICAL CHARACTERISTICS The cut-off wavelength of the fiber was determined by using the bending method described in the TIA standard [17]. An 820-nm superluminescent diode was used to illuminate the PPOF. The cut-off wavelength of the fiber was measured to be 854 nm (top figure of Fig. 3). The output intensity distribution of the fiber was measured using a far-field profile analyzer (Photon Inc., Model: LD8900 HDR). Two laser sources centered at 1310 and 1550 nm were used. The end face of PPOF was cut with a razor blade, and both end faces are perpendicular to the fiber axis of PPOF. The length of the PPOF was 53 cm. Fig. 3 (bottom figures) show that the fiber supported single-mode propagation at the wavelengths of 1310 and 1550 nm. The transmission loss of the single-mode PPOF was measured using the cut-back method. A broadband light source with a standard single-mode fiber output was used as the input light. The PPOF was placed on a high-precision three-axis translation stage to allow for precision alignment to the standard singlemode fiber, and the output power of the PPOF was measured with an optical power meter. The coupling loss was measured to be 13 dB. The position of the PPOF was then firmly secured for the entire experiment. The other end of the PPOF was connected to an optical spectrum analyser via a 400- m diameter fiber adapter. The initial length of the PPOF was 4.0 m. The end-face of the output fiber was cut using a razor blade and the output spectrum was measured and recorded. A short 0.5-mm length of the fiber from the output fiber end-face was cut again and the output spectrum was measured and recorded. This cutting of the fiber by 0.5 mm was repeated 45 times (see inset in Fig. 4); the spectrum for each cutting was recorded to obtain the average loss spectrum of the PPOF. The fiber attenuation measurement errors due to fiber end-face quality and cladding-modes were reduced by the averaging of a large number of output spectra and by the relatively long fiber length ( 4 m). Fig. 4 shows the measured attenuation of the PPOF from about 1410–1650 nm.
The fiber attenuation is less than 0.2 dB/m from 1410 nm up to 1540 nm and less than 0.5 dB/m from 1540 nm up to 1610 nm. These fiber losses are substantially lower than that of PMMA fibers. The ultralow loss of the PPOF together with the other important features such as RI of 1.34 close to water and high photosensitivity would open up a new opportunity for the realization of practical biomedical photonic sensors. REFERENCES [1] S. Kiesel, K. Peters, T. Hassan, and M. Kowalsky, “Behaviour of intrinsic polymer optical fibre sensor for large-strain applications,” Meas. Sci. Technol., vol. 18, pp. 3144–3154, 2007. [2] T. Kaino, “Plastic optical fibers for near-infrared transmission,” Appl. Phys. Lett., vol. 48, pp. 757–758, 1986. [3] J. M. Yu, X. M. Tao, and H. Y. Tam, “Trans-4-stilbenemethanol-doped photosensitive polymer fibers and gratings,” Opt. Lett., vol. 29, pp. 156–158, 2004. [4] G. D. Peng, P. L. Chu, Z. J. Xiong, T. W. Whitbread, and R. P. Chaplin, “Dye-doped step- index polymer optical fiber for broadband optical amplification,” J. Lightw. Technol., vol. 14, pp. 2215–2223, 1996. [5] K. Kalli, H. L. Dobb, D. J. Webb, K. Carroll, M. Komodromos, C. Themistos, G. D. Peng, Q. Fang, and I. W. Boyd, “Electrically tunable Bragg gratings in single-mode polymer optical fiber,” Opt. Lett., vol. 32, pp. 214–216, 2007. [6] K. J. Kim, A. Bar-Cohen, and B. Han, “Thermo-optical modeling of an intrinsically heated polymer fiber Bragg grating,” Appl. Opt., vol. 46, pp. 4357–4370, 2007. [7] A. Yeniay, R. Gao, K. Takayama, R. Gao, and A. F. Garito, “Ultra-lowloss polymer waveguides,” J. Lightw. Technol., vol. 22, pp. 154–158, 2004. [8] H. Y. Liu, G. D. Peng, P. L. Chu, Y. Koike, and Y. Watanabe, “Photosensitivity in low-loss perfluoropolymer (CYTOP) fiber material,” Electron. Lett., vol. 37, pp. 347–348, 2001. [9] H. Y. Liu, G. D. Peng, and P. L. Chu, “Thermal stability of gratings in PMMA and CYTOP polymer fibers,” Opt. Commun., vol. 204, pp. 151–156, 2004. [10] S. Califano, Vibrational States. Hoboken, NJ: Wiley, 1976. [11] C. Pitois, A. Hult, and D. Wiesmann, “Absorption and scattering in low-loss polymer optical waveguides,” J. Opt. Soc. Amer. B, vol. 18, pp. 908–912, 2001. [12] R. Yoshimura, M. Hikita, S. Tomaru, and S. Imamura, “Low-loss polymeric optical waveguides fabricated with deuterated polyfluoromethacrylate,” J. Lightw. Technol., vol. 16, pp. 1030–1037, 1998. [13] M. Murofushi, “Low loss perfluorinated POF,” in Polymer Optical Fibre Conf., Paris, 1996, pp. 22–24. [14] N. Taino and Y. Koike, “What is the most transparent polymer?,” Polymer, vol. 32, pp. 43–50, 2000. [15] W. R. White, M. Dueser, W. A. Reed, and T. Onishi, “Intermodal dispersion and mode coupling in perfluorinated graded-index plastic optical fiber,” IEEE Photon. Technol. Lett., vol. 11, pp. 997–999, 1999. [16] J. Senior, Optical Fiber Communications: Principles and Practice, 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, 1992. [17] Measuring Cutoff Wavelength of Uncabled Single Mode Fiber by Transmitted Power, TIA/EIA Standard, FOTP-80, Feb. 1996.
