1. a Department of Electrical and Digital-System Engineering , Hiroshima Institute of Technology , Miyake, Saeki-ku, Hiroshima , 731-5193 , Japan
2. b Physics Laboratory , Meikai University , Urayasu, Chiba , 279-8550 , Japan
3. c Department of Applied Science , Yamaguchi University , Ube , 755-8611 , Japan