An optical transmission system (301; 302) comprising an optical fiber , wherein the optical fiber is a multimode optical fiber has a core with a graded refractive index profile, arranged for the propagation of at least Z modes, where Z is an integer of 2 or more, wherein the multimode optical fiber comprises a core having an ath-power refractive-index distribution represented by Formula and a clad provided on an outside of the core, wherein Formula states: n 2 r = n 1 2 1 − 2 Δ 1 r / a 1 α 0 ≤ r ≤ a 1 n 2 r = n 1 2 1 − 2 Δ 1 a 1 ≤ r where n(r) denotes a refractive index at a position r in a radial direction from a center, n 1 denotes a refractive index at a core center, a 1 is the core radius, and α denotes an alpha parameter; said alpha parameter α satisfies 1.67 - 0.31exp(-(M-3)/1.80) ≤ α ≤ 2.37 + 0.63exp(-(M-3)/1.25) such that a mutual difference between propagation constants is 1000 rad/m or less in a mode group M that the multimode optical fiber can propagate, where M is a mode group including LPlp modes satisfying M = 2p + I -1, where M is 3 or more; and characterized in that said optical transmission system (301; 302) is adapted to perform Raman amplification; and in that the optical transmission system further comprises : a mode converter arranged to convert pump light to perform Raman amplification in the multimode optical fiber into a single propagation mode included in the mode group M, and causes the converted pump light to enter the multimode optical fiber ; and a mode multiplexer arranged to multiplex signal light from two or more and Z or less transmitters , as mutually-different propagation modes, and arranged to couple the multiplexed signal light to one end of the multimode optical fiber , at least two of the propagation modes of the signal light being propagation modes included in the mode group M. said alpha parameter α satisfies 1.67 - 0.31exp(-(M-3)/1.80) ≤ α ≤ 2.37 + 0.63exp(-(M-3)/1.25) such that a mutual difference between propagation constants is 1000 rad/m or less in a mode group M that the multimode optical fiber can propagate, where M is a mode group including LPlp modes satisfying M = 2p + I -1, where M is 3 or more; and characterized in that said optical transmission system (301; 302) is adapted to perform Raman amplification; and in that the optical transmission system further comprises : a mode converter arranged to convert pump light to perform Raman amplification in the multimode optical fiber into a single propagation mode included in the mode group M, and causes the converted pump light to enter the multimode optical fiber ; and a mode multiplexer arranged to multiplex signal light from two or more and Z or less transmitters , as mutually-different propagation modes, and arranged to couple the multiplexed signal light to one end of the multimode optical fiber , at least two of the propagation modes of the signal light being propagation modes included in the mode group M. a mode converter arranged to convert pump light to perform Raman amplification in the multimode optical fiber into a single propagation mode included in the mode group M, and causes the converted pump light to enter the multimode optical fiber ; and a mode multiplexer arranged to multiplex signal light from two or more and Z or less transmitters , as mutually-different propagation modes, and arranged to couple the multiplexed signal light to one end of the multimode optical fiber , at least two of the propagation modes of the signal light being propagation modes included in the mode group M. <EOS>