A method for measuring physical properties of optical signals as a function of wavelength and as a function of location in an optical link, the method comprising: generating a first modulated optical carrier at a first wavelength (λ1), the first modulated optical carrier carrying pump pulses (40,50); generating a second modulated optical carrier at a second wavelength (λ2) that differs from the first wavelength, the second modulated optical carrier carrying probe pulses (42,52); characterised in that the second modulated optical carrier carries pilot pulses (44,46,54); and the method comprises transmitting the first modulated optical carrier and the second modulated optical carrier on an optical fiber over the optical link, the optical fiber having non-zero dispersion, the optical link comprising multiple spans connected by one or more optical amplifiers, such that a probe pulse of the probe pulses is spatially overlapped in the optical fiber with a pump pulse of the pump pulses within at least one interaction region in the optical link, the probe pulse and the pump pulse having been launched into a same side of the optical link with a time delay (Δτ) between the pump pulse and the probe pulse; performing a measurement on the probe pulse beyond an end of the optical fiber to measure optical properties of the probe pulse relative to a coherent reference, the coherent reference including a component of at least one of the pilot pulses, the component coherent to the probe pulse; and calculating physical properties of the pump pulse from the measurement on the probe pulse, wherein the pilot pulses are arranged to be not spatially overlapped in the optical fiber with any of the pump pulses, or the pilot pulses are arranged so that any interaction between the pilot pulses and any of the pump pulses introduces a negligible error or a deterministic-yet-correctable error in measurement of the optical properties. generating a first modulated optical carrier at a first wavelength (λ1), the first modulated optical carrier carrying pump pulses (40,50); generating a second modulated optical carrier at a second wavelength (λ2) that differs from the first wavelength, the second modulated optical carrier carrying probe pulses (42,52); characterised in that the second modulated optical carrier carries pilot pulses (44,46,54); and the method comprises transmitting the first modulated optical carrier and the second modulated optical carrier on an optical fiber over the optical link, the optical fiber having non-zero dispersion, the optical link comprising multiple spans connected by one or more optical amplifiers, such that a probe pulse of the probe pulses is spatially overlapped in the optical fiber with a pump pulse of the pump pulses within at least one interaction region in the optical link, the probe pulse and the pump pulse having been launched into a same side of the optical link with a time delay (Δτ) between the pump pulse and the probe pulse; performing a measurement on the probe pulse beyond an end of the optical fiber to measure optical properties of the probe pulse relative to a coherent reference, the coherent reference including a component of at least one of the pilot pulses, the component coherent to the probe pulse; and calculating physical properties of the pump pulse from the measurement on the probe pulse, wherein the pilot pulses are arranged to be not spatially overlapped in the optical fiber with any of the pump pulses, or the pilot pulses are arranged so that any interaction between the pilot pulses and any of the pump pulses introduces a negligible error or a deterministic-yet-correctable error in measurement of the optical properties. transmitting the first modulated optical carrier and the second modulated optical carrier on an optical fiber over the optical link, the optical fiber having non-zero dispersion, the optical link comprising multiple spans connected by one or more optical amplifiers, such that a probe pulse of the probe pulses is spatially overlapped in the optical fiber with a pump pulse of the pump pulses within at least one interaction region in the optical link, the probe pulse and the pump pulse having been launched into a same side of the optical link with a time delay (Δτ) between the pump pulse and the probe pulse; performing a measurement on the probe pulse beyond an end of the optical fiber to measure optical properties of the probe pulse relative to a coherent reference, the coherent reference including a component of at least one of the pilot pulses, the component coherent to the probe pulse; and calculating physical properties of the pump pulse from the measurement on the probe pulse, wherein the pilot pulses are arranged to be not spatially overlapped in the optical fiber with any of the pump pulses, or the pilot pulses are arranged so that any interaction between the pilot pulses and any of the pump pulses introduces a negligible error or a deterministic-yet-correctable error in measurement of the optical properties.