1. Graduate School of Science and Technology, Shizuoka University, Hamamatsu 432-8011, Japan
2. Research Institute of Electronics, Shizuoka University, Hamamatsu 432-8011, Japan
3. Graduate School of Information Science and Technology, Hokkaido University, Sapporo 060-0814, Japan
4. NTT Basic Research Laboratories, 3-1 Morinosato Wakamiya, Atsugi 243-0198, Japan