Low-loss Polymer Optical Waveguides with High Thermal Stability
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Low-loss Polymer Optical Waveguides with High Thermal Stability Jae-Wook Kang, Jae-Pil Kim, Won-Young Lee, Joon-Sung Kim, Jae-Suk Lee, and Jang-Joo Kim K-JIST, Dept of Material Sciences and Engineering, Kwangju, Korea.
ABSTRACT
We synthesized novel cross-linkable fluorinated copoly(arylene ether sulfide)s for optical waveguide applications, which have high thermal stability, high optical transparency in the infrared communication region, and much smaller birefringence than other thermally stable fluorinated polyimides. The refractive index of the material can be easily controlled from 1.515 to 1.587 by changing the copolymer composition in the materials. The birefringence of the cured polymers were 0.0031~0.0039 at the wavelength of 1.55 µm. This is much lower than those of fluorinated polyimides for optical waveguides. The refractive index of fluorinated poly(arylene o ether sulfide) (FPAESI) after being stored at 100 C for 1000 hr remains almost constant demonstrating the thermal stability. The propagation loss of the channel waveguides fabricated using reactive ion etching was less than 0.4 dB/cm at the wavelength of 1.55 µm. INTRODUCTION
Polymer optical waveguides have attracted a lot of attention for their possible application as optical components in optical interconnects and optical communication systems because of their potential of easy, low temperature and low cost processibility. The key requirements of optical waveguide materials include low optical loss in the infrared region, high thermal stability, refractive index controllability, low birefringence and good adhesion with silicon substrates. However, hydrocarbon polymers have high optical loss in the infrared communication region due to vibrational absorption of the carbon-hydrogen (C-H) bond. Modifying a molecule by substitution of fluorine or deuterium for hydrogen in C-H bonds has the effect of greatly reducing the optical loss. Many polymers such as deuterated or fluorinated acrylate [1,2], benzocyclobutene [3,4], perfluorocyclobutane [5], fluorinated polyimide [6-8], deuterated polyfluoromethacrylate [9], deuterated polysiloxane [10], polycarbonate [11] and fluorinated poly(arylene ether) [12,13] have been synthesized for optical waveguide applications. In this paper, we describe single-mode optical waveguides using a newly synthesized crosslinkable fluorinated copoly(arylene ether sulfide)s. These polymers have high optical transparency in the infrared region because the replacement of C-H bonds with C-F bonds can reduce intrinsic vibrational absorption. In addition, these polymers have high thermal stability, good refractive index controllability and good processibility. These properties allow us to fabricate low-loss and thermally stable single-mode optical waveguides.
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EXPERIMENTAL We used a fluorinated copolymer of fluorinated poly(arylene ether sulfide) (FPAESI) [14] and fluorinated poly(arylene ether sulfide fluorene) (FPAESF), whose chemical structure is shown in figure 1. The sulfide group in the polymer was introduced to im
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