1. Graduate School of Science and Engineering , Yamagata University , 4-3-16, Jonan, Yonezawa , Yamagata 992-8510 , Japan
2. Development Department , Japan Communication Accessories Manufacturing Company Co., Ltd. , 1-226, Higashi, Komaki , Aichi 485-0831 , Japan
3. Boron Laboratory Co., Ltd. , 4-31-5, Nippori, Arakawa-ku , Tokyo 116-0014 , Japan
4. Optical Communications R&D Laboratories, Sumitomo Electric Industries, Ltd. , 1 Taya-cho, Sakae-ku , Yokohama 244-8588 , Japan