[1] [1] J. Knight, T. Birks, P. Russell and D. Atkin, Opt. Lett. 21,1547 (1996).

[2] [2] T. Birks, J. Knight and P. Russell, Opt. Lett. 22, 961 (1997).

[3] [3] J. Liu, T. H. Cheng, Y. Yeo, Y. Wang, L. Xue, D. Wanga and X. Yu, Opt. Commun. 281, 5415 (2008).

[4] [4] H. Li, X. Sang, J. Yuan, K. Wang and C. Yu, Optoelectron.Lett. 6, 172 (2010).

[5] [5] H. Zhang, Y. Wang, D. Xia, C. Cao, H. Cui and S. Chang,Journal of Optoelectronics Laser 21, 1621 (2010). (inChinese)

[6] [6] W. Wang, L. Hou, J. Song and G. Zhou, Opt. Commun. 282,3468 (2009).

[7] [7] W. Wang, L. Hou, Z. Liu and G. Zhou, Chin. Phys. Lett. 26,114202 (2009).

[8] [8] L. Zheng, X. Zhang, X. Ren, H. Ma and L. Shi, Chin. Opt.Lett. 9, 040601 (2011).

[9] [9] K. Saitoh, N. Florous and M. Koshiba, Opt. Express 13, 8365(2005).

[10] [10] K. Hansen, Opt. Express 11, 1503 (2003).

[11] [11] S. Haxha and H. Ademgil, Opt. Commun. 281, 278 (2008).

[12] [12] S. Razzak and Y. Namihira, IEEE Photon. Technol .Lett. 20,249 (2008).

[13] [13] H. Xu, J. Wu, Y. Dai, C. Xu and J. Lin, Chin. Opt. Lett. 9,050603 (2011).