1. Research Institute of Electronics, Shizuoka University, 3‐5‐1 Johoku, Hamamatsu 432‐8011, Japan
2. Electron Tube R&D Center, Hamamatsu Photonics K.K., 3145, Shimokanzo, Toyooka‐vilaage, Iwata‐country, Shizuoka Pref., 438‐0193, Japan
3. Department of Electronics and Information Engineering, Aichi University of Technology, 50‐2 Manori, Nishihazama‐cho, Gamagoori, 433‐0047, Japan