1. Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso‐cho, Showa‐ku, Nagoya 466, Japan
2. Instrument and Analysis Center, Nagoya Institute of Technology, Gokiso‐cho, Showa‐ku, Nagoya 466, Japan
3. Nagoya Industrial Science Research Institute, Yotsuya‐dori, Chikusa‐ku, Nagoya 464, Japan