Polarization-Maintaining Mechanism of a Birefringence-Reduced

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Polarization-Maintaining Mechanism of a Birefringence-Reduced Plastic Optical Fiber Fabricated Using Poly(methyl methacrylate/benzyl methacrylate) Copolymer† Rei Furukawa,* Akihiro Tagaya, Shuichi Iwata, and Yasuhiro Koike Faculty of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan, and Koike Photonics Polymer Project, ERATO-SORST, Japan Science and Technology Agency, Shinkawasaki Town Campus Bldg E, 7-1 Shinkawasaki, Kawasaki 212-0032, Japan ReceiVed: December 17, 2007; ReVised Manuscript ReceiVed: January 15, 2008

In order to propose a multimode polarization-maintaining fiber in a large core diameter, polarization-maintaining graded-index plastic optical fiber (PMGIPOF) was fabricated using a birefringence-reduced copolymer of poly(methyl methacrylate/benzyl methacrylate). Successful reduction of orientational birefringence in the PMGIPOF was verified using polarized optical microscopy. The extinction ratio measured from the PMGIPOF was significantly higher than the value obtained for the step-index (SI) POF (14.3 and 0.5 dB for 1 m, respectively). Photoelastic birefringence in the PMGIPOF was studied using fiber macrobending. Retardation induced by the macrobending was observed in a manner of high regularity in the PMGIPOF, where as the other types of POFs did not show such regularity. Furthermore, the polarization-maintaining property of the PMGIPOF was effective at maximum 10 m propagation. 1. Introduction Polymeric materials are employed in the fabrication of optical devices due to their ease of processing, light weight, high transparency, and low cost. Polymer optical devices can be fabricated by simple procedures such as injection molding, extrusion, and drawing, but these procedures tend to result in the exhibition of birefringence in the final product. The birefringence caused by such procedures is called orientational birefringence. The principle of the orientational birefringence is as explained in Figure 1a. A polymer exhibits no birefringence when its chains are randomly oriented even if each monomer unit has an anisotropic polarizability. This is because smallscale anisotropies do not predominate in one particular direction; thus, the macroscopic refractive index remains isotropic. However, the polymer becomes birefringent when its chains are oriented, because the anisotropies no longer cancel each other out. Recently, a variety of polarized light sources have been studied1,2 and their application potentials are expected in many fields such as medical measurement and data storage.3,4 A flexible waveguide that enables optical pickup with easy and high-efficiency light-coupling will be a powerful contribution toward developing smarter device systems. Conventional polarization-maintaining optical fibers fabricated in silica are not sufficiently suitable for the purpose mentioned above. This is because the polarization-maintaining principle of these fibers is only effective for single-mode operation, which limits the core diameter to under 10 µm.5 These polarizationmaintaining (PM) fibers accomplish their purpose by having a large propagation constants difference between the two orthogonal modes. As a result, it prevents mutual mode coupling, which is the cause for disturbing initial polarization state. Typically, the large propagation constants difference in the PM fiber is created by the stress-induced birefringence.5,6 These fibers can † Part of the “Larry Dalton Festschrift”. * To whom correspondence should be addressed. Phone: +81-44-5801563. Fax: +81-44-580-1433. E-mail: [email protected].

Figure 1. Schematic explanation of orientational birefringence in polymeric material and its reduction by random copolymerization: (a) homopolymer made of negative birefringence monomeric units and (b) copolymer made of negative and positive birefringence monomeric units. Polymer chains are illustrated as the polarizability ellipsoids of monomeric units. The principal axes of the polarizability ellipsoid correspond to the polarizability in those directions. The representation ellipsoid of the polymer refractive index is the averaged property of each polarizability ellipsoid. The principal axes (n1 and n2) of the representation ellipsoid correspond to the refractive indices in those directions. In panel a, the directionality of the negative polarizability ellipsoids is isotropically averaged by polymer chain packing, which exhibits no birefringence (n1 ) n2). When drawn in x1 direction, the polymer chain is oriented in the direction of the polarizability ellipsoids, which induces negative birefringence (n1 > n2). In panel b, no birefringence exists before drawing for the same reason as in panel a (n1 ) n2). However, in the case of panel b, no birefringence is exhibited even after drawing, because positive and negative polarizability ellipsoids mutually compensate their anisotropy (n1 ) n2).

couple light only from a small area, which is less efficient for collecting a large amount of light. To the best of our knowledge, a multimode fiber that has a polarization-maintaining property has not yet been reported.

10.1021/jp7118185 CCC: $40.75  2008 American Chemical Society Published on Web 04/19/2008

Birefringence-Reduced Plastic Optical Fiber POFs are suitable in carrying a large amount of light even better than the glass-based multimode fibers, because they can be fabricated in dramatically larger core diameters (up to 1000 µm) due to the flexibility of polymeric material. However, the large core diameter practically makes the fiber a multimode, which as mentioned earlier there is no known method to make a multimode fiber that has the polarization-maintaining function. This report demonstrates a polarization-maintaining gradedindex plastic optical fiber (PMGIPOF) that can transmit a large amount of light with a fairly high polarization-maintaining performance. Unlike the conventional PM fibers, the PMGIPOF is designed to have low birefringence in its core in order to transmit the initial polarization state without being disturbed. In this report, orientational birefringence in which POF is exhibited while being fabricated by heat-drawing was eliminated by a copolymerization.7 Copolymerization of methyl methacrylate (MMA) and benzyl methacrylate (BzMA) was formerly reported from our group as an effective method for reducing birefringence in the polymeric material. The reduction of orientational birefringence by the copolymerization is described in Figure 1b. By copolymerizing two monomers that each have a mutually opposite polarizability-ellipsoid, refractive index isotropy is maintained regardless of the polymer chain orientation. Reduced birefringence in such copolymer was previously demonstrated in films of 40-70 µm thickness.8 POF in which its core is fabricated using poly-(MMA/BzMA)(P(MMA/BzMA)) copolymer is presented here to explore how the polarization-maintaining property of the birefringence reduced copolymer exhibits in meter order waveguiding. In addition to the orientational birefringence, polymeric material is known to exhibit stress-induced birefringence, namely, photoelastic birefringence.7 In this report, we present the first study to employ the fiber macrobending to understand the effect of the photoelastic birefringence in the POFs. 2. Experimental Methods 2.1. Fabrication of the PMGIPOF. PMGIPOF was fabricated as follows. MMA (Mitsubishi Gas Chemical Company) was polymerized into a tubular geometry with a closed end (inner/outer diameter, 14.7/22.0 mm; length, 600 mm). Polymerization was performed at under 70 °C using t-butyl peroxy2-ethylhexanoate (Perbutyl-O, Wako Pure Chemical Industries, Ltd.) and 1-butanethiol (NOF Corporation) as the polymerization initiator and chain transfer agent, respectively. A poly-MMA (PMMA) tube was obtained by spinning the container while polymerizing. This PMMA tube is later used as the fiber cladding. A solution of MMA and BzMA (Wako Pure Chemical Industries, Ltd.) was prepared as the core material of the PMGIPOF. The composition used was adopted from Tagaya et al., which is MMA/BzMA ) 82/18 in weight.7 This is an optimized composition to eliminate orientational birefringence. The solution was placed in the center cavity of the PMMA tube prepared in advance and then polymerized at 120 °C in 0.6 MPa nitrogen atmosphere. Di-t-butyl peroxide (Perbutyl-D, Wako Pure Chemical Industries, Ltd.) and 1-dodecanethiol (NOF Corporation) were used as the polymerization initiator and chain transfer agent, respectively. The core refractive index was profiled during the process of polymerization. As shown in Figure 2, when a monomer is placed adjacent to a polymer, a gel effect9 occurs, which is known to accelerate polymerization. The gel effect is caused by the monomer diffusing into the polymer at their interface. Thus, in this case, the monomer solution polymerizes from the inner wall of the PMMA tube to the center. Since MMA has a

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Figure 2. Schematic explanation of graded-index formation during MMA/BzMA copolymerization: (a) structures of MMA and BzMA and (b) relationship between core composition and refractive index profile. Parameters n and panels a in b respectively are the refractive index and the radial distance of the PMGIPOF preform. Gel effect from the PMMA cladding involves simpler monomer (MMA) dominantly; thus, BzMA is mainly contained in the guide center, as shown in panel b. The gradual change of composition results in the graded-index profile since the two monomers have different refractive indices.

smaller molecular volume than BzMA, it is more diffusive and contributes more to the gel effect. A large number of BzMA monomers are displaced into the center while MMA mainly polymerizes from the interface. This results in the final polymer containing a higher number of BzMA monomeric units in the center. Since poly-BzMA (PBzMA) has a higher refractive index than the PMMA, the polymerized rod has a graded refractive index. The obtained preform was heat-drawn in a furnace with a maximum internal temperature of 210 °C. As it entered the furnace, the preform was simultaneously pulled in the longitudinal direction by the bottom take-up roll. Entering and takeup speeds were controlled for the drawn fiber to be 750/1125 µm at the core/fiber diameter. The fiber was cut out from an area where the diameter fluctuation was under 20 µm. 2.2. Measurements of Refractive Index Profile and Basic Fiber-Optic Properties. The refractive index profile of the PMGIPOF core was measured by transverse interferometric technique developed by Ohtsuka and Koike10 using an interference microscope (Interphako; Carl Zeiss). For comparison to the PMGIPOF, a typical GIPOF prepared using a method adopted from Koike and Ishigure11 and a SIPOF purchased from Mitsubishi Rayon (EskaMEGA M-30) were tested in this study. As for the basic fiber-optic properties, transmission spectrum, transmission loss, and bandwidth were obtained for each test POF. Transmission spectra of the three test fibers were measured using an optical spectrum analyzer (AQ6315; Ando Electric). For the PMGIPOF and the GIPOF, transmission losses at 650 nm were estimated from fiber cutback (5 × 5 m), and bandwidths at 50 m were measured using a setup of a pulse generator, a laser, and a sampling optical oscilloscope (C8188, M8903-36, and C8898; Hamamatsu Photonics). The information stated for the SIPOF including the transmission loss and the bandwidth were provided by Mitsubishi Rayon. 2.3. Polarized Light Microscopy. Orientational birefringence in each test POF was observed by polarized light microscopy. Vertical (along the fiber axis) and horizontal (along the fiber cross section) slices of each POF were viewed in a

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polarized optical microscope (BX51, Olympus) with a 10× magnification lens (UMPlanFL, Olympus) and a sensitive tint plate (U-TP530, Olympus) insertion. Vertical and horizontal fiber slices were prepared with thicknesses of 2.0 and 0.5 mm, respectively. The birefringence was estimated from the photomicrographs using a interference color chart provided by Olympus. 2.4. Extinction Ratio Measurement Combined with Fiber Macrobending. Figure 3 shows the extinction ratio (ER) measurement apparatus. Light from a He-Ne laser was aligned to pass through a polarizer, a half-wave plate, and a pinhole (diameter, 400 µm) before coupling to a 1 m test POF laid either straight or in a loop(s). A half-wave plate was inserted in order to control the input polarization angle (θpol) referring to the plane that a test fiber was bent in a loop(s) (x1-x3 plane in Figure 3). The 633-nm-output He-Ne laser was adopted because its wavelength was acceptably close to that of the common transmission window and it had sufficiently high output power. The pinhole was in physical contact to the fiber butt and positioned at the core center in order to avoid cladding mode launch. Output light from the test fiber was aligned to pass through another 400-µm-diameter pinhole, and an analyzer then was detected by a power meter. Power was plotted at intervals of 10° at each analyzer angle (θA). An intensity curve was obtained over a total 180° rotation of θA. A quarter-wave plate was used in an experiment discussed later in this section. θpol, θQWP, and θA rotates in the x1-x2 plane, and the x1 axis is their 0 position. ER was calculated by eq 1. Imin and Imax were taken from the intensity curve of 180° rotation of θA.

( )

ER(dB) ) -10 log

Imin Imax

Figure 3. Schematic diagram of the experimental apparatus used to measure the polarization maintaining property of the POFs. θpol, θQWP, and θA are defined as the angles of the input light polarization, the quarter-wave plate, and the analyzer, respectively. θpol, θQWP, and θA rotates in the x1-x2, plane and the x1 axis is their 0 position.

(1)

First, ER was measured for the different macro-bending conditions that were created on the bending rods. The fiber was bent along the bending plane (x1-x3 plane in Figure 3) in order to induce the local photoelastic birefringence in the POF core symmetrically along the bending plane. Rod diameter and number of loop(s) of the three employed macrobending conditions were 10 cm(rod diameter) × 1(loop), 5 cm × 2, and 4 cm × 3. Each condition had similar fiber length involved in the macrobending. ER was measured for three input polarization angles, namely, horizontal (θpol ) 0), diagonal (θpol ) 45), and vertical (θpol ) 90). The quarter-wave plate was removed. The measurement was done for the three macrobending conditions and also when the fiber was straightened. Length of the three test POFs was 1 m. Next, a quarter-wave plate was inserted in the position shown in Figure 3. ER measurement was done in a condition of θpol ) 45° and θQWP ) 0°. Here, θQWP was in the orientation of 45 ° to θpol to match the optical axis of the quarter-wave plate to the input polarization. 2.5. Extinction Ratio Measurement in Different Fiber Lengths. ER for different fiber lengths was measured by cutback. The longest and the shortest lengths were 20 and 0.6 m, respectively. In order to minimize the effect of photoelastic birefringence, fiber at the long lengths (5 m and longer) was winded in large loops (diameter ≈32 cm) and laid on the bending plane defined in Figure 3. At the lengths shorter than 5 m, fiber was straight. The input polarization was vertical to the bending plane (θpol ) 90) and the quarter-wave-plate was removed.

Figure 4. Measured refractive index profile of thePMGIPOF core (solid line). The left and right axes are the refractive index and the corresponding P(MMA/BzMA) composition, respectively. P(MMA/ BzMA) compositions for zero-orientational birefringence (82/18 wt%) and zero-photoelastic birefringence (92/8 wt%) are shown in broken lines. 0 and 1 of the normalized radius correspond to the guide center and core-cladding boundary, respectively. The fiber is considered to have an effective elimination for the orientational birefringence than the photoelastic birefringence.

3. Results 3.1. Refractive Index Profile and Fiber-Optic Properties of the PMGIPOF. Figure 4 shows the refractive index profile of the PMGIPOF core. The left and right ordinates in the figure are the refractive index and its corresponding P(MMA/BzMA) composition, respectively. P(MMA/BzMA) composition is estimated from the refractive index of each homopolymer, which are nPBzMA ) 1.568 and nPMMA ) 1.492. Optimum composition for eliminating the orientational birefringence (82/18 wt%) and photoelastic birefringence (92/8 wt%)7 are also marked in the figure. A large portion of the core does not exactly match the zero-orientational birefringence composition. However, it is considered to be ranging close enough to have a sufficient reduction of orientational birefringence. On the other hand, photoelastic birefringence is expected to exhibit when the fiber is deformed, because the profile lies relatively far from the zerophotoelastic birefringence composition. Transmission characteristics of the test POFs are shown in Table 1. From the spectral attenuation shown in Figure 5, the

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Figure 7. Normalized intensity for a 180° rotation of θA (θpol ) 0°, -90°