An optical transmission system comprising: a transmission line including an optical fiber; a transmitter configured to output signal light; a mode exciter configured to excite a linearly polarized mode of the signal light output from the transmitter to an LP 02 mode and input the excited signal light to the transmission line; a first mode converter configured to convert the linearly polarized mode of the signal light, which is excited to the LP 02 mode and output from the transmission line, into a fundamental mode; and a receiver configured to receive the signal light converted into the fundamental mode, wherein the optical fiber is configured to propagate the signal light excited to the LP 02 mode; wherein the optical fiber has a core and a cladding that is provided on an outer periphery of the core and has a refractive index lower than a refractive index of the core, wherein the optical fiber has a refractive index distribution of step-index-type, the core being made of silica having a purity of 99.999999% or more and an impurity ratio of 10 -6 % by mass or less, the radius of the core being equal to 10.5 μm, the relative refractive index difference being 0.45%, wherein the receiver is a digital coherent receiver, and the signal light converted into the fundamental mode is demodulated by digital signal processing configured with a finite impulse response filter in the receiver, wherein the optical transmission system further comprises: an unnecessary mode removal device configured to remove a mode other than the LP02 mode, between the mode exciter and the first mode converter, characterised in that the unnecessary mode removal device comprises: a second mode converter configured to convert the signal light of the LP 02 mode into signal light of the fundamental mode, a third mode converter configured to convert the signal light of the fundamental mode into the signal light of the LP 02 mode, and wherein the optical fiber has a bent portion by one turn with a bending radius of 3 to 6 mm provided between the second mode converter and the third mode converter. a transmission line including an optical fiber; a transmitter configured to output signal light; a mode exciter configured to excite a linearly polarized mode of the signal light output from the transmitter to an LP 02 mode and input the excited signal light to the transmission line; a first mode converter configured to convert the linearly polarized mode of the signal light, which is excited to the LP 02 mode and output from the transmission line, into a fundamental mode; and a receiver configured to receive the signal light converted into the fundamental mode, wherein the optical fiber is configured to propagate the signal light excited to the LP 02 mode; wherein the optical fiber has a core and a cladding that is provided on an outer periphery of the core and has a refractive index lower than a refractive index of the core, wherein the optical fiber has a refractive index distribution of step-index-type, the core being made of silica having a purity of 99.999999% or more and an impurity ratio of 10 -6 % by mass or less, the radius of the core being equal to 10.5 μm, the relative refractive index difference being 0.45%, wherein the receiver is a digital coherent receiver, and the signal light converted into the fundamental mode is demodulated by digital signal processing configured with a finite impulse response filter in the receiver, wherein the optical transmission system further comprises: an unnecessary mode removal device configured to remove a mode other than the LP02 mode, between the mode exciter and the first mode converter, characterised in that the unnecessary mode removal device comprises: a second mode converter configured to convert the signal light of the LP 02 mode into signal light of the fundamental mode, a third mode converter configured to convert the signal light of the fundamental mode into the signal light of the LP 02 mode, and wherein the optical fiber has a bent portion by one turn with a bending radius of 3 to 6 mm provided between the second mode converter and the third mode converter. an unnecessary mode removal device configured to remove a mode other than the LP02 mode, between the mode exciter and the first mode converter, characterised in that the unnecessary mode removal device comprises: a second mode converter configured to convert the signal light of the LP 02 mode into signal light of the fundamental mode, a third mode converter configured to convert the signal light of the fundamental mode into the signal light of the LP 02 mode, and wherein the optical fiber has a bent portion by one turn with a bending radius of 3 to 6 mm provided between the second mode converter and the third mode converter. a second mode converter configured to convert the signal light of the LP 02 mode into signal light of the fundamental mode, a third mode converter configured to convert the signal light of the fundamental mode into the signal light of the LP 02 mode, and wherein the optical fiber has a bent portion by one turn with a bending radius of 3 to 6 mm provided between the second mode converter and the third mode converter.