1. Q.-Y. Wang, M.-L. Hu, Y.-J. Song, et al., Chin. J. Lasers 34, 1603 (2007).
2. Y.-Y. Yuan and M.-L. Gong, Chin. J. Lasers 35, 1355 (2008).
3. K. Saitoh, T. Murao, L. Rosa, and M. Koshiba, Opt. Fiber Tech. 16, 409 (2010).
4. R. K. Varshney, A. K. Ghatak, I. C. Goyal, and S. C. Antony, Opt. Fiber Tech. 9, 189 (2003).
5. J. K. Dawson, R. J. Beach, and I. Jovanovic, in Proceedings of the Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference (Optical Society of America, 2003), CWD5.