Optical Properties of Fluorinated Polyimides and Their Applications ...

(dn/dT) of refractive indices of polymeric materialsincluding PIs is much larger than silica, thermo-optic(TO) switch can be a good target of polymerwaveguide circuits. Kobayashi et al. [71] installed ametal heater on the both sides of a Y-branchedwaveguide fabricated using copolymers of 3 and 5,and examined its digital TO switching performance.By using a FPI exhibiting a large TO coefficient, therequired electric power for switching can be effectivelyreduced. Moreover, Ido et al. [72] produced a1x8 digital TO switch by combining three steps of1x2 TO switches fabricated by FPIs. On the otherhand, Fujita et al. [73] produced a 2x2 TO switchusing FPIs by combining a TO switch and an opticalattenuator. This circuit can be useful as an add-dropcircuit in the WDM systems.3.6.4 Polarization SplittersAs stated in Section 2.3, aromatic PIs having rigidmolecular structures exhibit large birefringence comparedwith quartz or other organic polymers. Usingthis character, Oh et al. [74] fabricated a Y-branchoptical waveguides by combining waveguides of nonbirefringentpolymeric materials (polyetherketone,perfluorocyclobutane) and highly birefringent FPIs.For a branch circuit of one side including PIwaveguide (TE-branch), the light propagating conditionis attained only for the TE mode. Hence, thiscircuit functions as a polarization splitter which dividesthe incidence light into the TE polarization andthe TM polarization. The insertion loss of this splitterincluding the connection loss between thewaveguides and the input / output-fibers was 3.8 dBat 1.55 µm, and the cross talk was below −20 dB.3.6.5 Optical Networking Unit (ONU)With the progress of optical-communication technologyand the Internet technology, the Fiber To TheHome (FTTH) projects was started in the last decade.This project aims to introduce optical-fiber networksto every places of working offices and homes in Japan.An opto-electronic mixed-loading components(Optical Networking Unit, ONU, Fig. 15), which performtransmission and detection of optical signals andinter-conversion of optical and electric signals, areindispensable at all subscribers in the FTTH system.Ido et al. [72, 75] produced optical transceiver modulesusing FPI waveguides at a low cost, and reportedthat good fundamental properties (optical output >2mW @ 60mA, detection sensitivity >0.39 A/W) wereobtained. Moreover, since the refractive index of thecladding of FPI is higher than that of silica, manyadvantages are given. For example, the cladding layerof FPI can be thin because the waveguides are fabricatedon Si substrate covered by SiO 2 thin layer, andthe alignment between the core of waveguide and thesemiconductor optical elements (LD, PD) in the verticaldirection becomes easy.4. Summary and Future ProspectsIn this article, the outline and circumstances onthe basic optical properties and the applications ofFPI optical materials are widely reviewed, in particularfor aiming at the application to optical components,opto-electronic devices, and optical interconnection.The optical transparency in the visible andNIR regions was greatly improved by partial fluorinationof polyimides, but much further progress inthe optical transparency at the optical-communicationswavelengths was attained by the developmentof perfluorinated polyimides. The controls in refractiveindices, birefringence, and thermo-optic coefficientswere also attained by the progress in advancedmolecular design, copolymerization, and uniaxialdrawing technology. Moreover, the progress in thefine-patterning and processing technology using variousmethods supports the fine processing of opticalwaveguides with high precision. Many optical componentsand optical devices have been already developed,and optical applications of FPIs on the basis ofsuch components are coming to extensive use in nearfuture. Although further examinations are requiredfor the reliability and the optical mounting technology,FPI optical material is promising due to its veryattractive characteristics. More active researches areexpected to be continued for further progress.AckowledgementsThe author thanks Dr. F. Yamamoto, Dr. S. Sasaki,Dr. T. Matsuura, Dr. Y. Inoue, Mr. T. Sawada, Mr. S.Matsuda, and Mr. Y. Terui for their helpful discusssionsand fruiteful collaborations.References1) Y Imai, R. Yokota, Eds. “Novel Polyimide - Basicsand Applications”, (Japanese) Japan PolyimideConference, NTS, p.601 (2002).a) S. Ando, “Chemical Structures and Optical Propertiesof Polyimides” (Basics Part, Chap.5),b) S. Ando, T. Matsuuta, “Fluorinated Polyimides”(Application Part, Chap.4).2) Y. Imai, M. Kakimoto, R. Yokota, Eds. “The LatestProgress in Polyimides, Annual Reports of JapanPolyimide Conference”, (Japanese) Japan PolyimideConference, Vol.1 (1993)〜 Vol.12 (2004).3) K. Ihara, S. Kojiya “Fluoropolymers”, (Japanese)Polymeric New Materials Series-23, KyoritsuPublisher (1990).230