Method for determining an optical signal-to-noise ratio of a dual-polarization optical signal comprising at least one time interval dedicated to a modulation, on at least one lower amplitude level substantially equal to zero and a higher amplitude level, of said dual-polarization optical signal by a modulation signal, comprising the following steps: - detection, in said dual-polarization optical signal, of the modulation signal modulating said dual-polarization optical signal on at least one lower amplitude level substantially equal to zero and a higher amplitude level, and - determination of said optical signal-to-noise ratio on the basis of a measurement of the power of said modulation signal, - the method being characterized in that said at least one time interval comprises at least three successive and separate time zones: ∘ a first time zone comprising information signalling the end of transmission of a portion of useful data that is contained in said optical signal before said time interval, said first zone enabling the synchronization of a reception device in a synchronization mode internal to said reception device, ∘a second time zone over which said optical signal is modulated by said modulation signal at said amplitude levels, ∘ a third time zone comprising learning information enabling the synchronization mode of said reception device to be changed in order to move from said internal synchronization mode to a synchronization mode associated with a portion of useful data that is contained in said optical signal after said time interval. - detection, in said dual-polarization optical signal, of the modulation signal modulating said dual-polarization optical signal on at least one lower amplitude level substantially equal to zero and a higher amplitude level, and - determination of said optical signal-to-noise ratio on the basis of a measurement of the power of said modulation signal, - the method being characterized in that said at least one time interval comprises at least three successive and separate time zones: ∘ a first time zone comprising information signalling the end of transmission of a portion of useful data that is contained in said optical signal before said time interval, said first zone enabling the synchronization of a reception device in a synchronization mode internal to said reception device, ∘a second time zone over which said optical signal is modulated by said modulation signal at said amplitude levels, ∘ a third time zone comprising learning information enabling the synchronization mode of said reception device to be changed in order to move from said internal synchronization mode to a synchronization mode associated with a portion of useful data that is contained in said optical signal after said time interval. ∘ a first time zone comprising information signalling the end of transmission of a portion of useful data that is contained in said optical signal before said time interval, said first zone enabling the synchronization of a reception device in a synchronization mode internal to said reception device, ∘a second time zone over which said optical signal is modulated by said modulation signal at said amplitude levels, ∘ a third time zone comprising learning information enabling the synchronization mode of said reception device to be changed in order to move from said internal synchronization mode to a synchronization mode associated with a portion of useful data that is contained in said optical signal after said time interval.