[1] NTT Opto-electronics Laboratories, Nippon Telegraph and Tel. Corp., Ibaraki 319-1193, Japan
[2] NTT Opto-electronics Laboratories, Ibaraki, Japan
[3] NTT Elec. Commun. Laboratories, Ibaraki, Japan
[4] NTT Elec. Commun. Laboratories, Nippon Telegraph and Tel. Corp., Ibaraki, Japan