A method of transmitting information in an optical transmission system which includes: an optical transmitter having at least one electrical input and an optical output arranged such that a signal applied at said electrical input is modulated onto an optical carrier and emitted at said optical output, and wherein there is a predetermined nonlinear relationship between input electrical signal amplitude and corresponding output optical field amplitude; an optical receiver having an optical input and an electrical output, and wherein there is a square law relationship between amplitude of an optical field applied to the optical input and a corresponding electrical output signal; and an optical channel disposed between the optical transmitter and the optical receiver, characterised in that the method includes the steps of: composing an electrical signal bearing said information, the signal having a corresponding frequency spectrum which includes a plurality of spectral components having the property that difference frequencies thereof do not substantially interfere with any of said spectral components; predistorting said signal in accordance with a predistortion characteristic which is substantially an inverse to said nonlinear relationship between input electrical signal amplitude and corresponding output optical field amplitude of the optical transmitter; applying the predistorted signal to the electrical input of the optical transmitter to generate a corresponding information-bearing signal at the optical output thereof; and transmitting the information-bearing optical signal over the optical channel along with an optical carrier component. an optical transmitter having at least one electrical input and an optical output arranged such that a signal applied at said electrical input is modulated onto an optical carrier and emitted at said optical output, and wherein there is a predetermined nonlinear relationship between input electrical signal amplitude and corresponding output optical field amplitude; an optical receiver having an optical input and an electrical output, and wherein there is a square law relationship between amplitude of an optical field applied to the optical input and a corresponding electrical output signal; and an optical channel disposed between the optical transmitter and the optical receiver, characterised in that the method includes the steps of: composing an electrical signal bearing said information, the signal having a corresponding frequency spectrum which includes a plurality of spectral components having the property that difference frequencies thereof do not substantially interfere with any of said spectral components; predistorting said signal in accordance with a predistortion characteristic which is substantially an inverse to said nonlinear relationship between input electrical signal amplitude and corresponding output optical field amplitude of the optical transmitter; applying the predistorted signal to the electrical input of the optical transmitter to generate a corresponding information-bearing signal at the optical output thereof; and transmitting the information-bearing optical signal over the optical channel along with an optical carrier component. composing an electrical signal bearing said information, the signal having a corresponding frequency spectrum which includes a plurality of spectral components having the property that difference frequencies thereof do not substantially interfere with any of said spectral components; predistorting said signal in accordance with a predistortion characteristic which is substantially an inverse to said nonlinear relationship between input electrical signal amplitude and corresponding output optical field amplitude of the optical transmitter; applying the predistorted signal to the electrical input of the optical transmitter to generate a corresponding information-bearing signal at the optical output thereof; and transmitting the information-bearing optical signal over the optical channel along with an optical carrier component.