[2] [2] Ghatak A K, Goyal IC, Jindal R. Design of waveguide refrac. tive index profile to obtain flat modal field[J]. Society of Pho. to-optical Instrumentation Engineers, 1999: 40-44.

[3] [3] Dawson J W, Beach R J, Jovanovic I, et al. Large flattenedmode optical fiber for high output energy pulsed fiber lasers[J]. Optical Society of America, 2003.

[4] [4] Wang C, Zhang F, Lu Y, et al. Single-mode operations inthe large flattened mode optical fiber lasers and amplifiers[J]. Pure and Applied Optics, 2009: 065402.

[5] [5] Noda J, Okamoto K, Sasaki Y. Polarization-maintaining fi. bers and their applications[J]. Lightwave Technology, 1986, 4(8): 1071-1089.

[6] [6] Kubota H, Kawanishi S, Koyanagi S,et al. Absolutely singlepolarization photonic crystal fiber[J]. Photonics TechnologyLetters, IEEE, 2004, 16(1): 182-184.

[7] [7] Wang C C, Gong L, Li J L, et al. Single-polarizationin-phase supermode operation in multicore fiber lasers[J].Optics Communications, 2011, 284(18): 4215-4219.

[8] [8] Simpson J, Stolen R, Sears F, et al. Single-polarization fi. ber[J]. Lightwave Technology, 1983, 1(2): 370-374.

[9] [9] NolanDA,BerkeyGE,LiMJ, et al. Single-polarization fi. ber with a high extinction ratio[J]. Optics Letters,2004, 29(16): 1855-1857.

[12] [12] Gong M, Yuan Y, Li C, et al. Numerical modeling of trans. verse mode competition in strongly pumped multimode fi. ber lasers and amplifiers[J]. Optics Express, 2007, 15(6): 3236-3246.