[1] [1] C. Gomez-Reino, M. V. Perez, and C. Bao, Gradient-Index Optics: Fundamentals and Applications (Springer, Berlin, 2002).

[2] [2] F. Wang, D. Liu, X. Jiang, and S. Zhou, Chinese J. Lasers (in Chinese) 36, (S2) 48 (2009).

[3] [3] X. Li, X. Han, and B. Yan, Acta Opt. Sin. (in Chinese) 29, 1684 (2009).

[4] [4] S. Yuan, Y. Liu, and C. He, Acta Opt. Sin. (in Chinese) 29, (S2) 106 (2009).

[5] [5] M. Born and E. Wolf, Principles of Optics (Pergamon Press, New York, 1980).

[6] [6] R. W. Gilsdorf and J. C. Palais, Appl. Opt. 33, 3440 (1994).

[7] [7] S. Sinzinger, K.-H. Brenner, J. Moisel, T. Spick, and M. Testorf, Appl. Opt. 34, 6626 (1995).

[8] [8] G. J. Tearney, M. E. Brezinski, B. E. Bouma, S. A. Boppart, C. Pitris, J. F. Southern, and J. G. Fujimoto, Science 276, 2037 (1997).

[9] [9] A. M. Rollins, R. Ung-arunyawee, A. Chak, R. C. K. Wong, K. Kobayashi, M. V. Sivak, Jr., and J. A. Izatt, Opt. Lett. 24, 1358 (1999).

[10] [10] W. J. Smith, Modern Optical Engineering (McGraw-Hill, New York, 1990).

[11] [11] W. J. Tomlinson, Appl. Opt. 19, 1127 (1980).