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.