1. Faculty of EngineeringEhime UniversityMatsuyama 790‐8577Japan
2. Graduate School of Information SciencesHiroshima City UniversityHiroshima 731‐3194Japan
3. Department of PhysicsKumamoto UniversityKumamoto 860‐8555Japan
4. Faculty of EngineeringNagoya Institute of technologyGokiso‐cho, Showa‐kuNagoyaAichi 466‐8555Japan