[1] J. Li, J. Liu, and Z. Cao, J. Shandong Normal University (Nature Science) (in Chinese) 14, 152 (1999).

[2] H. Zhang, Y. Feng, G. Xu, A. Yao, Y. Bi, G. Wang, and Z. Xu, Chin. J. Lasers (in Chinese) 29, 666 (2002).

[3] L. Wang, G. Li, and W. Wang, Appl. Laser (in Chinese) 19, 101 (1999).

[4] Q. Wei, X. Ding, W. Wen, L. Fan, and J. Yao, Chin. Opt. Lett. 2, 34 (2004).

[5] L. Zou, X. Ding, Q. Wei, Y. Yu, W. Wen, B. Zhang, D. Xu, Y. Lu, P. Wang, and J. Yao, Chin. J. Lasers (in Chinese) 31, 1409 (2004).

[6] R. Scheps and J. F. Myers, IEEE Photon. Technol. Lett. 4, 1 (1992).

[7] A. Chen, J. Chongqing University 17, (6) 71 (1994).